U.S. patent application number 14/265013 was filed with the patent office on 2014-10-30 for materials and approaches for optical stimulation of the peripheral nervous system.
This patent application is currently assigned to The Board of Trustees of the Leland Stanford Junior University. The applicant listed for this patent is The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Karl A. Deisseroth, Scott L. Delp, Michael E. Llewellyn, Christine A. McLeavey Payne.
Application Number | 20140324133 14/265013 |
Document ID | / |
Family ID | 41507417 |
Filed Date | 2014-10-30 |
United States Patent
Application |
20140324133 |
Kind Code |
A1 |
Deisseroth; Karl A. ; et
al. |
October 30, 2014 |
Materials and Approaches for Optical Stimulation of the Peripheral
Nervous System
Abstract
A variety of methods, devices, systems and arrangements are
implemented for stimulation of the peripheral nervous system.
Consistent with one embodiment of the present invention, method is
implemented in which light-responsive channels or pumps are
engineered in a set of motor units that includes motor units of
differing physical volumes. Optical stimuli are also provided to
the light-responsive channels or pumps at an optical intensity that
is a function of the size of motor units to be recruited. In
certain implementations, the intensity of the optical stimuli is
increased so as to recruit increasingly larger motor units.
Inventors: |
Deisseroth; Karl A.;
(Stanford, CA) ; Delp; Scott L.; (Stanford,
CA) ; Llewellyn; Michael E.; (Menlo Park, CA)
; McLeavey Payne; Christine A.; (Portola Valley,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of the Leland Stanford Junior
University |
Palo Alto |
CA |
US |
|
|
Assignee: |
The Board of Trustees of the Leland
Stanford Junior University
Palo Alto
CA
|
Family ID: |
41507417 |
Appl. No.: |
14/265013 |
Filed: |
April 29, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12996753 |
Mar 10, 2011 |
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PCT/US2009/049936 |
Jul 8, 2009 |
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14265013 |
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61079035 |
Jul 8, 2008 |
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Current U.S.
Class: |
607/88 |
Current CPC
Class: |
A61N 2005/0652 20130101;
A61N 5/0622 20130101; A61N 5/0601 20130101; A61N 2005/0626
20130101; A61N 5/0613 20130101 |
Class at
Publication: |
607/88 |
International
Class: |
A61N 5/06 20060101
A61N005/06 |
Claims
1. A method comprising: engineering light-responsive channels or
pumps in a set of motor units that includes motor units having
respective motor neurons of different physical sizes; determining
an optical stimulus profile as a function of a recruitment order of
the set of motor units that corresponds to the different physical
sizes of the motor neurons; and providing the optical stimulus
profile to the light-responsive channels or pumps.
2. The method of claim 1, wherein the step of determining an
optical stimulus profile includes correlating an activation of
increasingly larger physical sizes of the motor neurons to the
optical stimulus profile.
3. The method of claim 1, wherein the step of determining an
optical stimulus profile includes correlating a muscle fatigue
factor to the optical stimulus profile.
4. The method of claim 1, wherein the step of providing optical
stimulus profile includes increasing an intensity of providing
optical stimulus profile to recruit motor units having increasingly
larger motor neurons.
5. The method of claim 1, wherein the step of providing the optical
stimulus profile is implemented using a curved optical delivery
device that at least partially surrounds the motor neurons of the
set of motor units.
6. The method of claim 1, wherein the step of engineering includes
engineering, in fast twitch motor units, first light-responsive
channels having a first optical response profile; and engineering,
in slow twitch motor units, second light-responsive channels having
a first optical response profile, wherein the first optical
response profile and the second optical profile have respective
peak sensitivities to different wavelengths of light.
7. The method of claim 6, wherein the step of providing the optical
stimulus profile includes providing an optical stimulus at a
wavelength sufficient to activate the slow twitch motor units
without activating the fast twitch motor units.
8. The method of claim 1, further including the step of sensing a
neural activation signal and wherein the step of providing optical
stimuli is responsive to the sensed neural activation signal.
9. The method of claim 1, further including the step of calibrating
the optical stimulus profile as a function of contractile strength
of the motor units in response to the optical stimulus profile.
10. The method of claim 1, further including the step of sensing
spastic motion of a portion of the body and wherein the step of
providing the optical stimulus profile is responsive to the sensed
spastic motion and mitigates the spastic motion.
11. The method of claim 1, wherein the light-responsive channels or
pumps are one of ChR2, NpHR and variants thereof.
12. A method comprising: engineering light-responsive channels or
pumps in a set of motor units that includes motor units having
respective motor neurons of different physical sizes; and providing
optical stimuli to the light-responsive channels or pumps at an
optical intensity that is a function of the different physical
sizes of the motor neurons to be recruited.
13. A method comprising: engineering light-responsive channels or
pumps in a set of peripheral afferent nerves; and providing optical
stimuli to the light-responsive channels or pumps to mitigate
pain.
14. The method of claim 13, wherein the step of engineering
includes engineering NpHR in the peripheral afferent nerves and the
step of providing optical stimuli is responsive to an external
control signal indicating a desired stimulus profile.
15. The method of claim 13, wherein the step of providing optical
stimuli includes providing an optical stimulus pattern to modify
pain recognition in the central nervous system.
16. A method comprising: engineering light-responsive channels or
pumps in a set of vagal fibers associated with the gastrointestinal
system; and providing optical stimuli to the light-responsive
channels or pumps to mitigate appetite.
17. The method of claim 16, wherein the step of engineering
includes engineering NpHR in the set of vagal fibers and wherein
the step of providing optical stimuli is responsive to an
indication of meal time.
18. A method comprising: engineering light-responsive channels or
pumps in a set of skeletal muscle stem cells; implanting the set of
skeletal muscle stem cells at a target location within a muscle to
repopulate the muscle; and providing optical stimuli to the
light-responsive channels or pumps to cause activation of the
muscle at the target location.
19. The method of claim 18, wherein the step of implanting includes
implanting the set of skeletal muscle stem cells for myocardial
repair.
Description
RELATED PATENT DOCUMENTS
[0001] This patent document claims the benefit, under 35 U.S.C.
.sctn.119(e), of U.S. Provisional Patent Application Ser. No.
61/079,035 filed on Jul. 8, 2008, and entitled "Materials and
Approaches for Optical Stimulation of the Peripheral Nervous
System;" the underlying provisional application is fully
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to stimulation of
the peripheral nervous system, and more particularly to
arrangements and approaches involving optical stimulus to affect
the cells of the peripheral nervous system.
BACKGROUND
[0003] The peripheral nervous system extends from the brain and
spinal cord to various portions of the body. Two of the main
functions of the peripheral nervous system are muscle control and
sensory feedback. Peripheral nerves carry signals between the
various portions of the body and the central nervous system using
electrical signals.
[0004] A typical muscle is composed of many thousands of fibers,
which contain the contractile machinery of the muscle. Rather than
individually controlling each fiber, a single motor neuron can
control groups of fibers that form motor units. Motor units vary in
size from 100 to several hundred fibers, and also vary in
composition of muscle fiber type. Small motor units are typically
composed of slow type muscle fibers that are fatigue-resistant,
while larger motor units are generally composed of fast type fibers
that are easily fatigable and medium sized motor units consist of a
mixture of slow and fast fiber types. Motor units are recruited, or
turned on, in a specific order that generally begins with the
smallest group and progresses to the largest group. In this way,
the smaller, fatigue-resistant motor units are used more often, and
thus allow for fine force control for longer periods of use. The
larger motor units, with larger capacity for generating force, are
conserved for brief periods of time when they are most needed,
e.g., during a reflex, emergency or other strenuous activities. The
size of a motor neuron is correlated to the size of the motor unit
that the motor neuron controls, so that a large motor neuron will
control a large motor unit.
[0005] The normal physiologic recruitment order refers to a typical
(healthy) order of motor neuron recruitment, where the size of the
motor axons and the motor neuron cell bodies define the sequence of
recruitment. For a given synaptic input of current, a smaller motor
neuron will be recruited before a larger motor neuron, thus
determining the order, small to large.
[0006] External electrical stimulation of motor neurons has been
attempted. One such attempt stimulates the axon of a motor neuron.
This, however, results in a recruitment order that is reversed when
compared to the normal physiologic order (the larger motor units
are recruited before smaller ones). The implication of this
recruitment reversal is that large, fatigable motor units are
recruited first, resulting in the loss of fine motor control and
sustained motor function. Thus, fatigue has become a limiting
factor in limb reanimation projects that have attempted to use
electrical stimulation.
[0007] The other type main function of the peripheral nervous
system, sensory feedback, is responsible for pain, touch, appetite
and a variety of other aspects. When problems with arise with
sensory feedback mechanisms, the results are often drastic and
sometimes even life threatening. For example, chronic pain is a
serious health issue that affects many individuals, seriously
degrading their quality of life and often having long-term
psychological impact. Another issue addressable through sensory
feedback relates to appetite suppression.
[0008] Aspects of the present invention relate to control and/or
stimulation of peripheral nervous system using optical
stimulus.
SUMMARY
[0009] Aspects of the claimed invention relate generally to
stimulation of the peripheral nervous system, and more particularly
to arrangements and approaches involving optical stimulus to affect
the cells of the peripheral nervous system.
[0010] Consistent with an embodiment of the present invention, a
method is implemented in which light-responsive channels or pumps
are engineered in a set of motor units that includes motor neurons
of differing physical volumes. Optical stimuli are also provided to
the light-responsive channels or pumps at an optical intensity that
is a function of the size of motor units to be recruited. In
certain implementations, the intensity of the optical stimuli is
increased so as to recruit motor units having increasingly larger
motor neurons.
[0011] Embodiments of the present invention relate to a method
where light-responsive channels or pumps are engineered in a set of
peripheral afferent nerves. Optical stimuli are provided to the
light-responsive channels or pumps to mitigate pain. Specific
implementations relate to the expression of NpHR in the peripheral
afferent nerves while providing optical stimuli to modify pain
recognition in the central nervous system.
[0012] An embodiment of the present invention relates to a method
in which light-responsive channels or pumps are engineered in a set
of vagal fibers associated with the gastrointestinal system.
Optical stimuli are provided to the light-responsive channels or
pumps.
[0013] Consistent with other embodiments of the present invention,
light-responsive channels or pumps are engineered in a set of stem
cells. The set of stem cells are implanted at a target location,
and optical stimuli are provided to the light-responsive channels
or pumps to cause activation of muscle at the target location.
Specific embodiments relate to the use of skeletal muscle stem
cells to repopulate muscles or implanting the set of stem cells for
myocardial repair.
[0014] Other embodiments of the present invention relate to a
device, kit or system having delivery component for expression of
light-responsive channels or pumps in the peripheral nervous system
and having an optical component for providing optical stimulus to
the light-responsive channels or pumps in the peripheral nervous
system. In a particular implementation, the delivery component
includes a nucleic acid molecule capable of transporting the
light-responsive channels or pumps to which it has been operatively
linked.
[0015] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention may be more completely understood in
consideration of the detailed description of various embodiments of
the invention that follows in connection with the accompanying
drawings, in which:
[0017] FIG. 1A depicts a peripheral nerve stimulated by optical
stimulus devices, consistent with an embodiment of the present
invention;
[0018] FIG. 1B shows muscle fibers controlled by a set of
light-responsive motor neurons, according to an example embodiment
of the present invention;
[0019] FIG. 2 shows a light stimulation device for placement around
peripheral nerves, according to an example embodiment of the
present invention;
[0020] FIG. 3 shows a system for stimulating motor neurons,
according to an example embodiment of the present invention;
[0021] FIG. 4A shows a block diagram of an implantable device,
according to an example embodiment of the present invention;
[0022] FIG. 4B shows a circuit diagram corresponding to the block
diagram of FIG. 4A, according to an example embodiment of the
present invention;
[0023] FIG. 5A shows a stimulation cuff for placement around a
peripheral nerve, consistent with an embodiment of the present
invention;
[0024] FIG. 5B shows the muscles electrical response (M-wave) as
measured by fine wire electrodes placed in the muscle belly and
near the Achilles tendon, consistent with an embodiment of the
present invention;
[0025] FIG. 5C shows the contractile force output as measured by a
force transducer attached to the Achilles tendon, consistent with
an embodiment of the present invention;
[0026] FIG. 5D depicts electromyography (EMG) and force traces from
twitches elicited by optical and electrical stimulations in both
Thy1-ChR2 animals and control C57bl/6 animals, consistent with an
embodiment of the present invention;
[0027] FIG. 6A shows peak force during a single twitch vs.
rectified integrated EMG for both electrical and optical
stimulations, consistent with an embodiment of the present
invention;
[0028] FIG. 6B depicts average latency measured from initiation of
stimuli to detection of EMG, consistent with an embodiment of the
present invention;
[0029] FIG. 6C shows the average contraction time measured from 10%
of peak force to peak force, consistent with an embodiment of the
present invention;
[0030] FIG. 6D depicts average relaxation time measured from peak
force to 10% of peak force, consistent with an embodiment of the
present invention;
[0031] FIG. 7A shows rectified-integrated EMG (iEMG) vs. estimated
optical intensity at the surface of the sciatic nerve for soleus
(SOL) and lateral gastrocnemius (LG), consistent with an embodiment
of the present invention;
[0032] FIG. 7B shows rectified-integrated EMG vs. electrical
stimulation voltage applied to the sciatic nerve, consistent with
an embodiment of the present invention;
[0033] FIG. 7C shows optical intensity required to achieve maximum
iEMG in SOL and LG, consistent with an embodiment of the present
invention;
[0034] FIG. 7D shows electrical stimulation required to achieve 95%
of maximum iEMG in SOL and LG, consistent with an embodiment of the
present invention;
[0035] FIG. 7E shows an example cross-section of the sciatic nerve
where retrograde dye was injected into the LG only, scale
bar=100.mu. consistent with an embodiment of the present
invention;
[0036] FIG. 7F shows distribution of motor axon diameters for SOL
and LG found in cross-section of the sciatic nerve, consistent with
an embodiment of the present invention;
[0037] FIG. 7G shows depth of motor axons for SOL and LG found in
cross-section of the sciatic nerve, consistent with an embodiment
of the present invention;
[0038] FIG. 8A depicts a confocal image of sciatic nerve in
cross-section, consistent with an embodiment of the present
invention;
[0039] FIG. 8B depicts a confocal image of sciatic nerve in a
longitudinal section with the same staining as in FIG. 8A,
illustrating several nodes of Ranvier (gaps formed between myelin
sheaths of cells), scale bar is 50 .mu.m, consistent with an
embodiment of the present invention;
[0040] FIG. 8C shows YFP fluorescence intensity versus motor axon
size in cross-section (n=4), consistent with an embodiment of the
present invention;
[0041] FIG. 8D shows the average fluorescence intensity parallel to
the long axis of sampled axons, where the origin indicates the
center of the node of Ranvier (n=15, shaded region is standard
deviation (s.d.)), consistent with an embodiment of the present
invention;
[0042] FIG. 9A shows the average tetanic tension over two minutes
in muscle being stimulated with 250 ms trains at 1 Hz using
electrical and optical stimulation (n=7, shaded region is standard
error (s.e.)), consistent with an embodiment of the present
invention;
[0043] FIG. 9B shows the average fatigue index for electrical and
optical stimulation, measured as decline in tetanic tension over
two minutes (n=7, error bars are s.e., * indicates p<0.01),
consistent with an embodiment of the present invention; and
[0044] FIG. 9C shows an example of tetanic tension taken from a
single mouse using both optical and electrical stimulation in
contralateral hindlimbs over 20 minutes, consistent with an
embodiment of the present invention.
[0045] While the invention is amenable to various modifications and
alternative forms, examples thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments shown and/or described. On
the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and scope
of the invention.
DETAILED DESCRIPTION
[0046] The present invention is believed to be applicable to a
variety of different types of processes, devices and arrangements
relating to stimulation of peripheral nerves. While the present
invention is not necessarily so limited, various aspects of the
invention may be appreciated through a discussion of examples using
this context.
[0047] According to one embodiment of the present invention,
peripheral nerves are optically stimulated to activate
light-responsive molecules therein. The light responsive molecules
can inhibit and/or facilitate electrical signaling (e.g., action
potentials) within the peripheral nerves. For instance, many
peripheral nerve bundles include mixed types of nerves (e.g., motor
and sensory). One or both of the nerve types can be affected by
optical stimulation. In specific instances, each of the nerve types
can be selectively stimulated.
[0048] As used herein, stimulation can include either activation or
deactivation of electrical signaling in the nerves. For instance,
nerve cells are stimulated by adjusting the membrane voltage level
of the nerve cell to facilitate action potentials or to inhibit
action potentials. Moreover, for various embodiments of the present
invention, the temporal precision of various light responsive
molecules allow for control of individual action potentials,
whether the control is via facilitation or inhibition.
[0049] According to an embodiment of the present invention, a
cuff-shaped optical delivery device allows for stimulation of both
types of nerves, or for selective stimulation. These and other
implementations can be used for treatment of various conditions,
such as muscle spasticity, among other things.
[0050] According to an example embodiment of the present invention,
motor neurons are optically stimulated. The optical stimulus
activates ion channels and/or pumps in the motor neurons to excite
or inhibit neural activation and thereby affect contractions and/or
relaxation of muscle tissue. Properties of light stimulus can be
modified to allow for variations in the effect on the muscle
tissue.
[0051] A specific embodiment of the present invention uses
variation of the intensity of the optical stimulus to control
activation of motor neurons engineered with light responsive ion
channels or pumps. It is believed that different motor neurons will
respond differently to light of varying intensities. The differing
responses can be particularly useful for selectively producing
coarse and fine contractions. Other properties of light that can be
used to control responsiveness of motor neurons include, but are
not limited to, wavelength, spatial location and temporal
properties (e.g., pulse duration or pulse separation).
[0052] Motor neurons use electrical signaling to transmit control
signals between portions of the nervous system and muscle fibers.
The electrical signals take the form of electrical pulses or action
potentials. An action potential is a voltage pulse that travels
along the membrane of the motor neuron. An action potential is
generated when the membrane voltage reaches a threshold voltage
level. An action potential of the motor neuron results in the
release of chemicals (neurotransmitters). These chemicals cause the
muscle fiber to contract.
[0053] One embodiment of the present invention uses light to
activate light-responsive cation channels in the motor neuron.
Light of sufficient intensity and wavelength activates the cation
channels, which induces a current in the motor neuron. The induced
current moves the membrane voltage toward the threshold voltage
necessary to produce an action potential. If sufficient current is
induced, an action potential is generated and the muscle fibers of
the corresponding motor unit are activated.
[0054] One embodiment of the present invention involves introducing
light-activated cation channels in one or more motor neurons. One
mechanism for introducing the cation channels involves the use of
vectors, such as lentiviruses, retroviruses, adenoviruses and
phages. The vectors are introduced to the motor neurons and result
in expression of the gene for the light-activated cation
channels.
[0055] Surprisingly and consistent with embodiments of the present
invention, it has been discovered that optical stimulation can be
used to recruit motor units in a largely normal physiologic order.
The number of light-activated channels opened is proportional to
the intensity of the light that is applied. Although not bounded by
theory, it is believed that the density of light-activated channels
(e.g., using vectors) is relatively uniform between different sized
motor neurons. As the size of a motor axon increases, the membrane
area increases by the power of two, while the motor axon volume
increases by the power of three. The number of light-activated
channels relates more directly to the membrane area as opposed to
the motor axon volume. Therefore, as a motor axon increases in
size, the volume increases at a rate such that larger motor axons
have fewer light-activated channels per volume. This implies that
for a given light intensity, motor neurons of smaller motor units
have a faster change in voltage due to the light-activated
channels. Accordingly, smaller motor neurons exhibit larger changes
in membrane potential than larger motor neurons. Thus with
increasing light intensity, the size of motor units recruited also
increases, matching the normal physiologic order.
[0056] Embodiments of the present invention are implemented with
knowledge of these unexpected results. For instance, the optical
stimulus profile (e.g., optical intensity, optical frequency or
spatial location) can be set as a function of the size of motor
unit/neuron to be recruited. A lookup-table or an algorithm can be
used to associate a desired muscle response with a particular
optical profile. According to one such implementation, the optical
stimulus profile can be set according to a muscle fatigue factor.
Due to the activation of smaller motor neurons before larger motor
neurons, measurements of the muscle fatigue can be used to
determine the point at which motor neurons of increasingly larger
size are recruited. The experimental results presented herein
provide examples of fatigue-based determinations that are
consistent with embodiments of the present invention.
[0057] Another factor that can be used includes the contractile
response (strength and/or speed) of recruited motor units. The
contractile response can be correlated to the size of the motor
units/neurons that were recruited under a specific optical stimulus
profile. The optical stimulus is then determined as a function of
the desired contractile response of the muscle. The contractile
response can be measured by a variety of different mechanism.
Non-limiting examples include force and/or speed measurements
caused by muscle contraction or monitoring of muscle activation
(e.g., electromyography (EMG) measurements).
[0058] Such factors are but a few of the possible mechanisms for
determining an optical stimulus profile. Other factors can include,
for example, the location, size and/or type of muscle tissue under
stimulus. Moreover, different species may require different
stimulus profiles and/or different stimulus devices. The age,
physical size and fitness of the patient can be used as factors in
determining an optical stimulus profile.
[0059] Various other determinable factors are contemplated for
determining the optical stimulus profile, many of which are
facilitated by knowledge of the orderly recruitment of motor units
that can be provided by embodiments of the present invention.
Accordingly, while the invention is not limited to orderly
recruitment of motor units, various embodiments are facilitated by
this aspect.
[0060] Turning now to the figures, FIG. 1A depicts a peripheral
nerve stimulated by optical stimulus devices, consistent with an
embodiment of the present invention. Nerve trunk 101 includes nerve
bundles 111 and 113. The nerve bundles 111 and 113 are engineered
to include light responsive channels/pumps. Optical stimulus
devices 103 and 107 provide optical stimulus to the engineered
light-responsive channels/pumps. Although not limited thereto,
optical stimulus devices 103 and 107 are depicted as light-emitting
diodes (LEDs) controlled by optical control circuits 105 and 109,
respectively.
[0061] In one implementation, a single optical stimulus device
103/107 can be used. In other implementations, multiple optical
stimulus devices 103/107 are possible. The optical stimulus devices
can operate at the same wavelength of light or at different
wavelengths of light. When operating at the same wavelength of
light, the use of multiple optical stimulus devices can increase
the intensity of the provided optical stimulus, increase the area
of optical stimulus and/or provide spatially controllable optical
stimulus. For instance, the physical location of the optical
stimulus devices can be used as a factor in the stimulation of the
light-responsive channels/pumps. Due to relative position of the
devices, morphology of the nerve trunk or other factors, the
devices can provide different responsiveness of the nerves and
associated functions.
[0062] Optical control circuits 105 and 109 can also be implemented
so that individual control of the optical stimulus devices is
possible. This can be particularly useful for implementations where
the optical stimulus devices operate at different wavelengths.
Different types of light responsive channels/pumps can be designed
to have a different wavelength for the optimal responsiveness. In a
particular embodiment of the present invention, the differences in
the wavelengths are sufficient to allow for activation of one type
of light responsive channel/pump without activating the other type
of light responsive channel/pump. In this manner, a first type
(e.g., ChR2) of channel/pump can be used to facilitate activation
(action potentials) in a nerve and a second type (e.g., NpHR) of
channel/pump can be used to inhibit activation (action potentials)
in a nerve. Other possibilities include the targeting of the first
type of channel/pump to a first type of cell (e.g., slow twitch
motor unit) and the second type of channel/pump to a second type of
cell (e.g., fast twitch motor unit).
[0063] These examples show the wide variety of applications and
possible applications for embodiments of the present invention. A
number of such embodiments, including those discussed in connection
with the various figures, are directed to control of muscle fibers
through optical stimulation of motor neurons. Other embodiments,
some of which are expressly discussed herein, are also
contemplated. For instance, peripheral nerves also provide sensory
responses (e.g., pain, touch or appetite). A number of disorders
are associated with abnormal sensory responses. Accordingly,
various embodiments relate to treatment or characterization of
various sensory-related disorders.
[0064] FIG. 1B shows muscle fibers controlled by a set of
light-responsive motor neurons, according to an example embodiment
of the present invention. Muscle fibers 100 are responsive to motor
units 104, 106 and 108. Each motor unit responds to a different
motor neuron. Motor unit 104 has a motor neuron with a relatively
low threshold, meaning that the motor neuron is responsive to a
lower amount of stimulus. Motor unit 106 has a motor neuron with a
relatively high threshold, and motor unit 108 has a motor neuron
with threshold between the other two motor neurons. Under normal
physiologic recruitment, this would allow for the activation of
motor unit 104 without activating motor units 106 or 108 and for
the activation of motor unit 108 without activation motor unit
106.
[0065] In a specific implementation, the motor neurons include
proteins/molecules that function as light-activated ion channels or
pumps. Light source 102 provides light to the light-activated ion
channels or pumps. If the light is sufficient to activate the
light-responsive molecules, ion flow across the membrane modifies
the membrane voltage of the motor neurons. As the intensity of the
light increases, the percentage of light-responsive molecules that
are activated also increases. Thus, light intensity can be used to
activate the smaller motor units without activation of the larger
motor units.
[0066] It should be noted that factors other than light intensity
can play a role in the activation of the light-responsive
molecules. For example, the wavelength of light can also have an
effect on activation of motor units. For example, increasing the
intensity of light at a specific wavelength may have little or no
effect when the wavelength is outside of an effective absorption
band (i.e., wavelengths that the molecules respond to) for
light-responsive molecules. In another example, shifting the
wavelength of the light relative to the effective absorption band
can change the percentage of light-responsive molecules that
respond without modifying intensity of the light. Other examples
involve the duration of the light and/or the spatial location of
the delivered light relative to the motor neurons.
[0067] In one implementation, the application of the optical
stimulus is responsive to a sensed neural activation. For instance,
a damaged portion of a nerve can be effectively bypassed by sensing
neural activation signals and providing responsive optical stimuli
at a point beyond the damaged portion of the nerve. In one
implementation, the sensed neural activation can be neural
activation within the nerve, but prior to the damaged portion. In
another implementation, the sensed neural activation could be from
an otherwise unassociated portion of the nervous system. For this
second type of implementation, the patient can retrain the neural
pathways to control the damaged nerve using the previously
unassociated portion of the nervous system. Another example of
sensed activation includes sensing muscle activation more directly
(e.g., using an EMG). In response to sensed activation, optical
stimulus can be provided to recruit additional motor units. In this
manner, the muscle activation can be increased by the application
of an optical stimulus profile.
[0068] FIG. 2 shows a light stimulation device for placement around
peripheral nerves, according to an example embodiment of the
present invention. Light stimulation device 202 surrounds motor
neurons 204 and 206. Motor neurons 204 and 206 can be of different
sizes, allowing for selective activation thereof. In one
implementation, stimulation device 202 can vary the intensity of
the generated light as desired so as to allow for selective
activation of certain ones of the motor neurons.
[0069] Embodiments of the present invention include implementations
of light stimulation device 202 that do not surround motor neurons
204 and 206. For example, light stimulation device 202 can be
implemented using a U-shaped cuff that is designed to be placed
proximate to the motor neurons. Other shapes are possible,
including point light sources, such as an optical fiber.
[0070] In certain implementations light stimulation device 202 is
attached to an arm (not shown) that can be used to guide the light
stimulation device 202 near the motor neurons 204 and 206. If
desired, the arm can be subsequently removed. Alternatively, the
arm can be left in place and used the help fix the position of
light stimulation device 202, provide adjustment of the position of
light stimulation device 202 and/or provide power/control signals
to light stimulation device 202.
[0071] FIG. 3 shows a system for stimulating motor neurons,
according to an example embodiment of the present invention. Motor
units (motor neurons and muscle fibers) 302 include light
responsive ion channels/pumps. Light generator 308 includes one or
more light sources 304 that stimulate the channels/pumps within the
motor units. Parameter controls 306 allow for control of light
sources 304 by modifying the light properties. The modification of
parameters can be implemented so as to allow for activation of some
motor units without activating others. For example, the intensity
of the light can be set at a level that activates some motor units
and not others. In another instance, the wavelength of light can be
changed, thereby achieving much the same effect as modifying the
intensity of the light. Other possibilities include activating only
some of the light sources 304, or using different light parameters
for certain light sources 304.
[0072] Motor unit monitor 310 provides feedback on the activation
of the motor units. The feedback can be implemented using a number
of different measurements. For example, motion associated with the
muscle can be monitored to determine the strength of the
contraction using a pressure sensor, speed of the movement using
image capture and/or the preciseness of the movement (e.g., smooth
or jerky). In some instances it may also be possible to measure the
electrical responsiveness of the motor units. As the stimulus is
optical and not electrical, the electrical signals represent the
results of the stimulus without separating the (optical) stimulus
signals from the (electrical) results thereof. These and other
results can be stored in a results database 316.
[0073] Control unit 312 can be used to generate stimulus profiles
that are used to control the light generator 308. These profiles
can be stored within a stimulation profile database 314. In one
implementation, a sequence of profiles are implemented and
correlated to the results stored in results database 316. The
desired muscle response can then be implemented by providing a
stimulation profile that is correlated to the desired result.
[0074] According to a specific embodiment of the present invention,
both inhibitory and excitation molecules are implemented to provide
control of the motor units. In certain instances this can provide
further delineation between activation of different motor units by,
for example, enabling both the inhibitory and excitation molecules.
This can effectively reduce the likelihood of a motor neuron action
potential (relative to enabling the excitation molecules without
enabling the inhibitory molecules). In certain instances,
stimulation for inhibition and excitation can be provided at
different spatial locations. This can allow for each of the
inhibition and excitation stimulus to more strongly affect
different motor neurons, respectively.
[0075] In one embodiment of the present invention, an implantable
device includes a control portion that responds to magnetic fields.
This control portion can be implemented as an electrical wire,
resistive element or other responsive element. In such an
embodiment, the intensity, duration and frequency of light
generated would be controlled by the current generated from an
introduced magnetic field. This can be particularly useful for
creating inexpensive, long lasting and small devices. An example of
such an embodiment is discussed further in connection with FIG. 4A
and FIG. 4B.
[0076] In another embodiment of the present invention, the control
portion can be implemented as a more complex circuit. For instance
the control circuit may include and otherwise implement different
rectifier circuits, batteries, pulse timings, comparator circuits
and the like. In a particular example, the control circuit includes
an integrated circuit (IC) produced using complementary
metal-oxide-semiconductor (CMOS) or other processes. Integrated
circuit technology allows for the use of a large number of circuit
elements in a very small area, and thus, a relatively complex
control circuit can be implemented for some applications.
[0077] In a particular embodiment of the present invention, the
light generating portion is a blue LED, such as LEDs in 0603 or
0805 package sizes. A particular example is a blue surface mount
LED having part number SML0805, available from LEDtronics, Inc
(Torrance, Calif.).
[0078] FIG. 4A shows a block diagram of an implantable device,
according to an example embodiment of the present invention. FIG.
4A shows an inductor comprising coils 402 and core 404 connected to
LED 408 using conductive paths shown by 406. FIG. 4B shows a
circuit diagram corresponding to the block diagram of FIG. 4A.
Inductor 412 is connected in parallel to LED 410. Thus, current and
voltage generated by changing a magnetic field seen at inductor 412
causes LED 410 to produce light. The frequency and strength of the
changing magnetic field can be varied to produce the desired amount
and periodicity of light from LED 410.
[0079] Examples of light stimulation devices are taught by
International Application No. PCT/US08/50628, entitled System for
Optical Stimulation of Target Cells, to Schneider et al., and filed
Jan. 9, 2008. The patent document teaches a variety of devices and
delivery devices for use with light-responsive molecules. As such,
the document is hereby incorporated by reference in its
entirety.
[0080] There are a number of suitable light-responsive molecules
that can be used to modify nerve cells so that the cells are
optically responsive. One class of molecules facilitates action
potentials in the nerve cells by inducing ionic current that moves
the membrane voltage toward the voltage threshold of the cell. In
one embodiment of such a molecule, the light-responsive molecule is
one of the proteins ChR2, Chop2, ChR2-310, or Chop2-310. In another
embodiment, the light-responsive molecule is a 7-transmembrane
protein. In another embodiment, the light-responsive molecule is a
single-component protein. In yet another embodiment, the
light-responsive molecule covalently binds retinal. For further
details on light responsive molecules reference can be made to the
aforementioned System for Optical Stimulation of Target Cells, to
The Board of Trustees of the Leland Stanford Junior University,
which is fully incorporated herein by reference.
[0081] Another class of molecules discourages action potentials in
the nerve cells by inducing ionic current that moves the membrane
voltage away from the voltage threshold of the cell. In one
embodiment, the light responsive molecule is an archaeal
light-driven chloride pump (NpHR) from Natronomonas pharaonis. For
further details on such light responsive molecules, reference can
be made to Zhang et al., (2007) Multimodal Fast Optical
Interrogation of Neural Circuitry, Nature 2007 Apr. 5;
446(7136):617-9, which is fully incorporated herein by reference.
These and other molecules can be used alone or in conjunction with
one another.
[0082] A few specific examples of light responsive molecules, their
use and stimulation devices and techniques (e.g., ChR2 or NpHR) are
provided in U.S. patent application Ser. No. 11/459,636, entitled
Light-Activated Cation Channel and Uses Thereof, to Boyden et al.,
and filed Jul. 24, 2006; in International Application No.
PCT/US2008/050628, entitled System for Optical Stimulation of
Target Cells, to The Board of Trustees of the Leland Stanford
Junior University and filed on Jan. 9, 2008; and also in U.S.
patent application Ser. No. 12/041,628, entitled Systems, Methods
and Compositions for Optical Stimulation of Target Cells, to Zhang
et al., and filed on Mar. 3, 2008. These documents teach a number
of different light responsive molecules (including, but not limited
to, specific sequence listings) as well as variants thereof. These
documents include numerous discussions of example molecules as well
as delivery and stimulation techniques. As such, these documents
are hereby incorporated by reference in their entirety.
[0083] A particular embodiment uses a two-part approach: expression
of ChR2 (or NpHR in other cases) in the neurons of interest,
followed by implantation of a light source to illuminate the nerve
at the specified frequency. ChR2 expression can be achieved through
"projection targeting", whereby opsin vectors are injected not at
the site of eventual illumination, but at a distant site where the
cell bodies of the target neurons lie. Alternately, target muscles
can be infused with a retrograde virus; in this approach, one does
not need to know cell type-specific promoters, and only the axons
of the targeted cells are optically modulated even though they may
be intermixed with other cell types in the nerve. Unlike other
optically-responsive channels that have been developed, although
ChR2 and NpHR require an all-trans-retinal (ATR) chromophore as a
cofactor, retinoids naturally present in mammalian cells are
sufficient.
[0084] A specific implementation uses an LED-based nerve cuff,
where several micro LEDs are embedded in a solid, optically
transparent cuff, and surgically placed around the desired nerve.
This cuff provides high intensity light source for stimulating the
desired nerve. A specific example light intensity for ChR2
stimulation is >1.0 mW/mm.sup.2 light power density. Embodiments
of the present invention allow for alternatives to LEDs, such as
solid state laser diodes, or some future technology. Considerations
for selection of the light source can include efficiency concerns
in terms of size, expense, heating, and battery life.
[0085] The following description provides details relating to an
experimental mouse model for such therapies, as well as evidence
that optical stimulation recruits muscle fibers in a normal
(healthy) physiologic order, thereby avoiding the problem faced by
electrical stimulation. Human nerves are generally larger and the
technology is therefore scaled accordingly, however, the same
principles of operation can be used.
[0086] Muscle parameters were measured in vivo to characterize
motor unit recruitment. Motor unit recruitment is often
characterized by stimulating individual motor axons from the
peripheral nerve at the ventral root. In small animals such as
mice, however, this technique is impractical.
[0087] FIG. 5A shows a stimulation cuff (e.g., synthesized optical
light source) for placement around a peripheral nerve, consistent
with an embodiment of the present invention. The stimulation cuff
502 is depicted as having a curved portion designed to at least
partially surround the peripheral nerve, however, other embodiments
allow for variations including, but not limited to, a cuff that
predominantly surrounds a peripheral nerve and point light sources.
In the experiment, both electrical and optical stimuli were
provided to an anesthetized Thy1-ChR2 mouse by way of such a cuff.
The experiment was carried out using an optical cuff (and compared
to an electrical cuff) around the sciatic nerve of an adult
Thy1-ChR2 or control (C57bl/6) mouse. Stimuli were provided by the
cuff to evoke an electrical and contractile response of the
muscle.
[0088] FIG. 5B shows the muscles electrical response (M-wave) as
measured by fine wire electrodes placed in the muscle belly and
near the Achilles tendon, consistent with an embodiment of the
present invention. The waveform depicts an electromyography (EMG)
plot of typical twitch from optical stimulation.
[0089] FIG. 5C shows the contractile force output as measured by a
force transducer attached to the Achilles tendon, consistent with
an embodiment of the present invention. In a mouse, the medial
gastrocnemius (MG), the lateral gastrocnemius (LG) and the soleus
(SOL) have free tendons that attach to the distal end of the
Achilles tendon. To measure muscle forces of an individual muscle,
the free tendons of the muscles not being measured are detached.
The detached Achilles tendon was fixed to a force transducer to
measure muscle contractions. The force traces show typical titanic
contractions at various frequencies using optical stimulation.
[0090] FIG. 5D depicts EMG and force traces from twitches elicited
by optical and electrical stimulations in both Thy1-ChR2 animals
and control C57bl/6 animals, consistent with an embodiment of the
present invention. These traces show that typical twitch response
generated by optical stimulation from the MG does not have a
significantly different shape than twitches evoked by electrical
stimulation in either Thy1-ChR2 or control animals. The exception
to this observation is the absence of a stimulation artifact in the
EMG response just prior to the initiation of depolarization under
optical stimulation, but seen in all cases of electrical
stimulation. There was no response to optical stimulation in
control animals, which implies that optical stimulation occurs by
photostimulation of the ChR2 channels and not by heat or other
electrical means.
[0091] To compare electrical and optical stimulation intensities
the rectified integrated EMG (iEMG) was used over the time of
non-zero activity in each trial. To verify that measurement of iEMG
represents a common response of the muscle under both electrical
and optical stimulation and also to verify that optical stimulation
can elicit contractile forces comparable to electrical stimulation,
the average peak force during a twitch was compared to the iEMG
response. FIG. 6A shows peak force during a single twitch vs.
rectified integrated EMG for both electrical and optical
stimulations, consistent with an embodiment of the present
invention. For a given iEMG, both optical and electrical
stimulations produce similar trends, but the peak twitch forces
were on average 15.4% lower using optical stimulation. Average peak
twitch forces using electrical stimulation (0.32.+-.0.05 N) were
significantly higher (p<0.01) than average twitch forces with
optical stimulation (0.29.+-.0.01 N), possibly indicating that most
but not all motor neurons are stimulated under optical stimulation.
Twitch forces produced by electrical and optical stimulation are
consistent with previous measurements.
[0092] Measurement of motor axon conduction latency is the most
common method used to estimate motor unit recruitment. Smaller
axons have slower conduction speeds, and therefore have longer
latencies for a given distance. FIG. 6B depicts average latency
measured from initiation of stimuli to detection of EMG, consistent
with an embodiment of the present invention. Latency represents the
time difference between the initiation of the stimuli and the
depolarization measured on EMG (M-wave). Latencies measured under
optical stimulation for all intensities (2.18.+-.0.02-1.72.+-.0.13
ms) were significantly longer than those under electrical
stimulation (1.15.+-.0.05-0.99.+-.0.01 ms, p<0.01 in all cases).
This difference is possibly due to lower cation conductance of ChR2
channels, which delays the formation of an action potential. The
conduction velocity was estimated (32.2-40.4 m s-1), due to
significant uncertainty in the path length of the axon from the
site of stimulation to site of measurement and found to be
consistent with expected values. At the lowest levels of activity,
the drop in latency from 1 mV ms to 2 mV ms under optical
stimulation was significant (p<0.01) while the difference under
electrical stimulation was not (p=0.11). This implies that smaller
axons are recruited preferentially at the lowest levels of optical
stimulation but not under electrical stimulation.
[0093] Other measures of motor unit recruitment, such as the
contraction and relaxation times, were found to suggest orderly
recruitment with optical stimulation. FIG. 6C shows the average
contraction time measured from 10% of peak force to peak force,
consistent with an embodiment of the present invention. Under
optical stimulation (11.1.+-.0.08 ms), the contraction time was
significantly longer at the lowest levels of muscle activity than
electrical stimulation (8.79.+-.1.01 ms, p<0.01). While at the
highest levels of muscle activity, contraction time under optical
and electrical stimulation was not found to be significantly
different (8.34.+-.0.07 ms, p=0.60).
[0094] FIG. 6D depicts average relaxation time measured from peak
force to 10% of peak force, consistent with an embodiment of the
present invention. This relaxation time was significantly longer at
the lowest level of muscle activity with optical stimulation
(21.73.+-.0.39 ms) than electrical stimulation (17.46.+-.0.68 ms,
p<0.01), whereas relaxation time at the highest levels of muscle
activity were not significantly different between the different
types of stimulation (14.54.+-.0.09 ms, p=0.10). The measurements
of contraction and relaxation time, which are consistent with other
in vitro data, both imply that at the lowest levels of muscle
activity, optical stimulation preferentially recruits slower motor
units than electrical stimulation.
[0095] To further examine differential motor unit recruitment, the
recruitment of two different muscles, soleus (SOL) and lateral
gastrocnemius (LG), were compared. FIG. 7A shows
rectified-integrated EMG (iEMG) vs. estimated optical intensity at
surface of the sciatic nerve for soleus (SOL) and lateral
gastrocnemius (LG), consistent with an embodiment of the present
invention. Whereas FIG. 7B shows rectified-integrated EMG vs.
electrical stimulation voltage applied to the sciatic nerve,
consistent with an embodiment of the present invention.
[0096] SOL contains 58.+-.2% slow oxidative (SO) fibers and 0% fast
glycolytic (FG) fibers, while LG has 69.+-.13% FG fibers and
1.+-.3% SO fibers. It has been reported that smaller motor units
tend to have higher compositions of SO fibers, and therefore, it
was expected that SOL motor units would be recruited prior to the
faster motor units of LG, an observation that has been reported in
physiological recruitment studies. FIG. 7C shows optical intensity
required to achieve maximum iEMG in SOL and LG, consistent with an
embodiment of the present invention. Under optical stimulation SOL
(14.9.+-.1.9 mW mm-2) reaches 95% peak activity at a significantly
lower optical intensity than LG (FIG. 7C, 24.4.+-.1.9 mW mm-2,
p<0.01). At the lower levels of optical stimulation, LG and SOL
have similar levels of activity. This observation can be attributed
to the possibility that LG contains small motor units composed of
fast muscle fibers.
[0097] FIG. 7D shows electrical stimulation required to achieve 95%
of maximum iEMG in SOL and LG, consistent with an embodiment of the
present invention. The electrical stimulation used to evoke 95% of
peak activity in SOL (0.64.+-.0.15 V) and LG (0.64.+-.0.09V) was
not significantly different (FIG. 7D, p<0.01). These findings
suggest that slower muscle fibers are preferentially recruited by
optical stimulation before faster fibers; however, the order of
motor unit recruitment would be more certain given knowledge of the
size distribution of the motor axons innervating each muscle.
[0098] To analyze axon size distribution, and to determine if there
was bias in the location of the axons innervating each muscle
within the cross-section of the peripheral nerve, retrograde dye
(Fast Blue) was intramuscularly injected into the muscles of
interest to backfill only the axons innervating the muscle in which
it was injected. FIG. 7E shows an example cross-section of the
sciatic nerve where retrograde dye was injected into the LG only,
scale bar=100.mu., consistent with an embodiment of the present
invention.
[0099] FIG. 7F shows distribution of motor axon diameters for SOL
and LG found in cross-section of the sciatic nerve, consistent with
an embodiment of the present invention. FIG. 7G shows depth of
motor axons for SOL and LG found in cross-section of the sciatic
nerve, consistent with an embodiment of the present invention.
[0100] In cross-sections of the sciatic nerve (FIG. 7E) SOL and LG
do not contain significantly different numbers of motor axons (FIG.
7F, 53.5.+-.4.9, 55.+-.3.7, p=0.71). However, the average motor
axon innervating LG had a significantly larger Feret's diameter
than those innervating SOL (6.7.+-.0.16 .mu.m, 4.5.+-.0.17 .mu.m,
p<0.01). No bias was observed as to the location of either set
of axons within the peripheral nerve so that one set would be
exposed to significantly higher light intensities than another.
These observations support the premise that small motor units are
preferentially recruited under optical stimulation, and that the
observed difference in optical intensity required for peak muscle
activity in SOL and LG is not influenced by either number of axons
or position of those axons in the peripheral nerve.
[0101] To determine the location of the ChR2 channels within the
motor axons and whether there were any differences in expression
levels in relation to the size of the motor axons, cross-sections
of the sciatic nerve were made both parallel and perpendicular to
the long axis of the motor axons. FIG. 8A depicts a confocal image
of sciatic nerve in cross-section, consistent with an embodiment of
the present invention. The first channel is due to anti-laminin
labeling basal lamina of the peripheral nerve. The second channel
is due to YFP fluorescence expressed natively in the transgenic
neurons, scale bar is 50 .mu.m. FIG. 8B depicts a confocal image of
sciatic nerve in a longitudinal section with the same staining as
in FIG. 8A, illustrating several nodes of Ranvier, scale bar is 50
.mu.m. The ChR2 channels are labeled with yellow fluorescent
protein (YFP), so the average relative YFP fluorescence intensity
of axons was compared to the diameter of those axons using confocal
microscopy.
[0102] FIG. 8C shows YFP fluorescence intensity versus motor axon
size in cross-section (n=4), consistent with an embodiment of the
present invention. No correlation was found between axon size and
fluorescent intensity in the transverse sections (FIG. 8C,
R2=0.0021, p=0.88). Additionally it was possible to locate nodes of
Ranvier within the cross-sections (FIG. 8B).
[0103] FIG. 8D shows the average fluorescence intensity parallel to
the long axis of sampled axons, where the origin indicates the
center of the node of Ranvier (n=15, shaded region is s.d.),
consistent with an embodiment of the present invention. The
fluorescence, and presumably the ChR2 channel density, varies along
the axolemma. Fluorescence intensity at center of the nodal region
is at a minimum, while fluorescence intensity in the peri-nodal
region is at a maximum. It is known from immuno-localization
studies that the center of the nodal region contains high
concentrations of Na+ channels, which is likely the cause of the
lower fluorescent signal is this region. Additionally, the nodal
and internodal regional morphology appears normal, giving no
indication for abnormal behavior of the transgenic motor
neurons.
[0104] The ability to preferentially recruit slower motor units
with optical stimulation has potentially enormous functional
significance. Functional Electrical Stimulation (FES) systems have
been developed to serve as neuro-prosthetics for patients with
paralysis, but have not been adopted widely because of early onset
fatigue possibly due to reverse recruitment by electrical
stimulation. To test whether optical stimulation of muscle elicits
less fatigue than electrical stimulation, measurements were taken
of tetanic tension generated by the plantar flexor group of
Thy1-ChR2 mice using both stimulation types. FIG. 9A shows the
average tetanic tension over two minutes in muscle being stimulated
with 250 ms trains at 1 Hz using electrical and optical stimulation
(n=7, shaded region is s.e.), consistent with an embodiment of the
present invention. FIG. 9B shows the average fatigue index for
electrical and optical stimulation, measured as decline in tetanic
tension over two minutes (n=7, error bars are s.e., * indicates
p<0.01), consistent with an embodiment of the present invention.
Using stimulation intensities in each modality that elicited
2.times. body weight for each unfatigued mouse, 1 Hz stimulation
trains were used for 2 minutes, with each train lasting 250 ms. The
average fatigue index, measured as the average tetanic tension of
the last train divided by the average tetanic tension in the first
train, declined significantly lower in trials using electrical
stimulation (0.11.+-.0.09), than those using optical stimulation
(0.56.+-.0.09, p<0.01). Additionally, when this fatigue protocol
is extended to 20 minutes in an individual mouse using
contralateral hindlimbs, electrical stimulation diminishes tetanic
tension to .about.0% after just 4 minutes, while optical
stimulation continues to elicit 31.6% of its initial tension after
the entire 20 minute trial. FIG. 9C shows an example of tetanic
tension taken from a single mouse using both optical and electrical
stimulation in contralateral hindlimbs over 20 minutes, consistent
with an embodiment of the present invention.
[0105] Physiological measurements were taken according to the
following methodology. Normal appearing, 9-12 week old Thy1-ChR2 or
C57bl/6 control mice were anesthetized and the hindlimb was shaved
and fixed in a frame. The Achilles tendon was freed by cuffing the
distal end of the calcaneous to a force transducer (Aurora
Scientific, 300CLR) by thin steel wire. An optical cuff, made of 16
LEDs (Rohm, SMLP12BC7T, 465 nm) arranged in a concentric perimeter
facing the peripheral nerve center, or a bipolar hook electrical
cuff was inserted around the exposed sciatic nerve that is cut
proximal to the site of stimulation. In most cases optical and
electrical stimulation were conducted in the same leg at different
times. Stainless steel hook electrodes were inserted for
differential EMG recordings. EMG recordings were filtered in
hardware only (BP 3-3000 Hz). All force, EMG, and stimuli data were
sampled at 100 kHz.
[0106] Imaging was implemented consistent with the following steps.
Fresh sciatic nerve was fixed in 4% PFA for 30 min and washed in
PBS. The samples were then embedded in 5% low-melting point agarose
and cut (50 .mu.m) with a vibratome. The sections were labeled with
anti-tau and anti-lamin. The sections are imaged on a confocal
microscope (Leica, DM6000). The number, size and fluorescence
intensity of the motor axons (.gtoreq.3 .mu.m and G-ratio
.gtoreq.0.5)33, 34 were determined by manual analysis in
ImageJ.
[0107] All other data analysis was conducted in Matlab. All data
reported for the MG was broken in arbitrarily defined bins based on
iEMG value. To determine stimuli needed for 95% maximum iEMG in SOL
and LG, a Weibull cumulative distribution function was fit to data
points. The confidence interval (c.i.) generated by the curve fit
was used to define the 99% c.i. of the required stimuli.
[0108] Samples tested for statistically significant differences
were first tested for normality using Lilliefors test
(.alpha.=0.05), then tested using unpaired two-tailed Student's
t-test (.alpha.=0.05). All sample groups tested were found to be of
normal distribution, except for the axon size data which was tested
using the Mann-Whitney U-test. All data points listed are
mean.+-.s.e.m. or data.+-.99% confidence interval (c.i.) when
referring to FIGS. 7C and 7D.
[0109] The specific implementations of the above-mentioned
experiment, while instructive, are not meant to be limiting. The
results, however, show the versatility and broad-applicability of
related methods, devices and treatments, some of which are
discussed hereafter.
[0110] In a specific implementation, the desired physiologic order
is exaggerated by expressing inhibitory NpHR in the fast-twitch
fiber motor neurons, providing a method to reduce or prevent
fatigable muscle usage when not desired. This, however, might
require long periods of yellow light production, causing possible
heating and reduction of battery life. Mutant forms of
channelrhodopsin that respond to different light frequencies can be
used by expressing these different forms of ChR2 in slow and fast
twitch fiber motor neurons, thereby creating a definitive system
where each could be controlled separately using the different light
frequencies. Variations and combinations are contemplated
including, but not limited to, multiple forms of ChR2 used in
combination with NpHR.
[0111] Targeted expression can be accomplished using a cell
specific promoter. Examples of cell specific promoters are
promoters for somatostatin, parvalbumin, GABA.alpha.6, L7, and
calbindin. Other cell specific promoters are promoters for kinases
such as PKC, PKA, and CaMKII; promoters for other ligand receptors
such as NMDAR1, NMDAR2B, GluR2; promoters for ion channels
including calcium channels, potassium channels, chloride channels,
and sodium channels; and promoters for other markers that label
classical mature and dividing cell types, such as calretinin,
nestin, and beta3-tubulin.
[0112] Other aspects of the present invention use spatial
properties of the light stimulus to control the activation of the
desired motor neurons. For instance, the use of a LED-based nerve
cuff, e.g., a stimulation device 202, includes controllable light
sources that provide illumination from respective portions of the
cuff. The light sources in the different portions of the
illumination device are designed to be separately addressable.
Calibration techniques can be used to determine the optimal
stimulus profile. For example, it may be determined that optical
stimulation from a first portion of the illumination device
activates higher percentage of fast twitch fiber motor neurons with
respect to optical stimulation from another portion of the
illumination device. Optical stimulus from this portion can be
reduced or avoided altogether when fine motor control is desired.
Similarly, when NpHR is used, yellow light can be used from such a
portion to reduce activation of fast twitch motor neurons.
[0113] According to embodiments of the present invention, an
optical stimulation system is configured to provide graduated
levels of optical stimulation according to the desired muscle
contraction strength. For fine motor control, the optical
stimulation is relatively low and/or targeted at the slow twitch
motor neurons. For increasingly strong and/or rapid contractions,
additional motor neurons are recruited including the fast twitch
motor neurons. The graduated levels can be implemented as a
function of the respective ratio of fast to slow twitch motor
neurons that are responsive to a particular aspect of the optical
stimulation. Example aspects include the optical wavelength,
optical intensity, duration of optical stimulus and/or the location
of the optical stimulus. A stimulation profile can then be
determined to provide the desired responsiveness ranging from
slow/fine control to fast/coarse control of the particular muscle
group. This can be implemented using an algorithm that takes a
desired response and determines the optical parameters
corresponding to the response. Alternatively, a look-up table,
having stored optical parameters that are indexed according to the
desired motor response, can be used.
[0114] Optogenetic techniques offer novel therapies in several
areas. Example applications include, but are not limited to, muscle
stimulation, spasticity, tremor, chorea suppression, pain
management, vagus, phrenic, and sacral nerve stimulation, cardiac
arrhythmia management, and stem cell therapies.
[0115] Embodiments of the present invention relate to treatment or
characterization of a patient suffering from spasticity. Spasticity
is a devastating and common human clinical condition that arises as
a result of neonatal injury (e.g., cerebral palsy), genetic disease
(e.g., Niemann-Pick disease) and postnatal injury (e.g., spinal
cord injury and stroke), and is characterized by hyperreflexia
which leads to involuntary muscle contraction in response to
movement. Spasticity limits the daily activities of more than
millions worldwide, and is estimated to cost billions of dollars in
the United States alone. There are a number of medical and surgical
treatments for spasticity, including botulinum toxin, intrathecal
baclofen, selective dorsal rhizotomy, and gene therapy; however
most, if not all, can cause significant side-effects, have limited
efficacy, and can be prohibitively expensive. It is extremely
difficult to "turn down" the overactive nerves that cause severe
muscle contraction, abnormal posture, and pain, since drug
approaches are slow and nonspecific with regard to cell type, and
electrodes cannot effectively or precisely turn down neural
activity, or attain cell type specificity. In many cases physicians
will resort to risky surgeries like dorsal rhizotomy, but this can
only be done in a minority of spastic patients and has inconsistent
efficacy with the potential for serious adverse events.
[0116] Consistent with an embodiment of the present invention, both
ChR2 and NpHR channels/pumps are expressed in the affected motor
neurons, and a nerve cuff, or other light source capable of
producing both yellow and blue light, is placed around the nerve.
The combination of both ChR2 and NpHR allows for spasticity to be
reduced while maintaining muscle function and strength. The
stimulation pattern could either be based on a learned feedback
pattern (such as those used to control prosthetic limbs) or could
be controlled more explicitly by the user (deciding, for example,
to leave the muscle limp for a period of time in order to
rest).
[0117] For instance, the spastic motor control can be effectively
overridden using a combination of ChR2 and NpHR stimulation.
Electromyography devices can be used to detect the activation
signal of muscles, e.g., a sensor/microchip can be implanted in
muscles to detect electrical signals from the brain. These signals
are transformed into corresponding optical stimulus. A trained
technician or biomedical engineer can configure an initial (coarse)
response relative to the optical stimulus device. Over time the
patient can learn to finely control the optical stimulus device so
as to perform the desired movements.
[0118] In a similar manner, optogenetic therapy is used to control
tremors or various forms of chorea. Motion detection coupled in a
feedback manner to ChR2 and NpHR stimulation could effectively
provide a low-pass filter for muscle activation, or one in which
certain patterns of activation (the specific tremor or choreic
motions) were dampened. For example, accelerometers or gyros can be
used to provide motion-detection associated with the muscle
responsive to the stimulation. In response to motion exceeding a
threshold level of forcefulness, speed and/or repeated motions,
dampening stimulation can be provided. The threshold level can be
adjusted to allow for suitable movement by the patient while also
providing sufficient dampening functionality. The optical
stimulation device can have an adjustable setting for this
dampening functionality.
[0119] In one implementation, one or more additional accelerometers
or gyros can be placed at the core of the patient. These
accelerometers or gyros detect motions associated with the entire
individual rather than a specific limb or other body part. When
such motion is detected, it can be used to distinguish between
motions caused by external forces (e.g., riding in a vehicle) from
unwanted spastic motions.
[0120] Other embodiments of the present invention relate to
treatment or characterization of chronic pain. Millions of people
are adversely affected by chronic pain. Chronic pain causes
billions of dollars a year in medical costs, lost working days, and
workers compensation, and is a major risk factor for depression and
suicide.
[0121] Pain can be divided into two general categories: nociceptive
and neuropathic. In the former, mechanical, thermal, or chemical
damage to tissue causes nociceptor response and initiates action
potentials in nerve fibers. Afferent fibers terminate directly or
indirectly on transmission cells in the spinal cord that convey
information to the brainstem and midbrain. Neuropathic pain, in
contrast, involves a miscoding of afferent input; mild inputs yield
dramatic pain responses, through mechanisms that are not well
understood. Often this is the result of an initial nociceptive pain
that, instead of resolving with healing of the initial stimulus,
proceeds to spontaneous pain and low-threshold for light touch to
evoke pain. It is believed that increased sodium channel and
decreased potassium channel expression in dorsal root ganglia, the
development of "cross-talk" between adjacent afferents, or an
increase of glutamate release in spinal cord neurons are among the
possible mechanisms for this increased pain sensitivity.
[0122] Treatment of pain depends on many factors, including type,
cause, and location. There are myriad options, most notably topical
agents, acetaminophen and NSAIDs, antidepressants, anticonvulsant
drugs, sodium and calcium channel antagonists, opioids, epidural
and intrathecal analgesia, acupuncture and other alternative
techniques, botulinum toxin injections, neurolysis, cryoneurolysis,
spinal cord stimulation, neurosurgical techniques, radiofrequency
ablation, peripheral nerve stimulation, transcutaneous electrical
nerve stimulation, and rehabilitation therapy.
[0123] So many treatments exist, however, because each has
limitations. For example, local anesthetic drugs block sodium
channels, preventing neurons from achieving action potentials.
However, effectiveness of this treatment is limited by the degree
to which specificity for pain neurons can be maintained, avoiding
the side effects of numbness or paralysis from blocking other
sensory or motor fibers (as well as potential cardiac effects
should the drug travel further through the circulatory system). In
order to achieve this, low dosages are needed, requiring frequent
administration of the drug. Additionally, not all kinds of pain
react to local anesthetic treatment, and some cases become
refractory over time, or require ever increasing doses.
[0124] Surgical treatments, including dorsal or cranial nerve
rhizotomy, ganglionectomy, sympathectomy, or thalomatomy, are more
drastic options, appropriate in certain severe cases. However,
relief from these is unpredictable; notably, it is sometimes only
temporary, and may involve complications. Spinal cord stimulation
(SCS) is also used in some cases, attempting to limit chronic pain
through placement of electrodes in the epidural space adjacent to a
targeted spinal cord area thought to be causing pain; however, a
recent review found limited evidence of the effectiveness of this
technique.
[0125] Alternately, pain can be addressed in the brain. As it is
correlated with depression and anxiety, pain is sometimes
responsive to antidepressant and anti-anxiety medications such as
the tricyclics. Recent promising research suggests the effect of
real-time fMRI biofeedback, where patients learn to decrease
activation of the rostral anterior cingulate cortex, with resultant
reduction in perceived pain. While each of these methods is
effective in some cases, chronic pain remains a largely intractable
problem. NpHR and ChR2 expression in peripheral afferent nerves is
therefore used to influence pain signals.
[0126] Control of the peripheral afferent fibers with the high
temporal precision of optogenetic techniques offers the ability to
inhibit pain signals at a given moment, as with local anesthetic
treatment. For instance, NpHR can be engineered in afferent nerves
and optical stimulus can be provided to the NpHR to provide
anesthetic treatment. The optical stimulus can be relatively
constant or responsive to an external control. For instance, a
doctor or patient can control the delivery of the optical stimulus
in terms of frequency of stimulus, intensity of stimulus or simply
turn the stimulus on or off. The temporal properties of ChR2 and
NpHR can also be used to interface with and reprogram pain
recognition in the CNS. Reprogramming can be implemented as
suggested by electrical stimulation, antidepressant medication and
biofeedback mechanisms.
[0127] The temporal precision and nerve specificity of optogenetic
stimulation is particularly useful for reprogramming pain
recognition circuits. Response to pain can be "turned up" in
neuropathic conditions, creating hypersensitivity to afferent
stimulation, suggesting that it is possible to reverse this through
other patterns of stimulation. Particular embodiments relate to
stimulating pain fibers and larger sensory fibers separately, as
larger sensory fiber messages tend to overwhelm and turn down pain
fiber recognition.
[0128] Embodiments of the present invention also relate to vagus
nerve stimulation. The vagus nerve is composed of both afferent and
efferent pathways. In the peripheral nervous system, vagal afferent
fibers innervate the heart, vocal cords, and other laryngeal and
pharyngeal muscles, and also provide parasympathetic input to the
gastrointestinal viscera. Afferent fibers project mainly to the
brain, in such regions as the pontine and midbrain nuclei, the
cerebellum, thalamus, and cortex.
[0129] Given this variety of nerve function, vagus nerve
stimulation is used for a wide range of treatments, including
appetite management, cardiac rate suppression, depression, and
epilepsy. In the latter two, effectiveness of the treatment is not
well understood. The right vagus nerve provides more innervation to
the cardiac atria than the left vagus nerve does, so in situations
where cardiac effects are not desirable, electrical stimulation is
generally performed on the left side. However, even with these
precautions, side effects such as hoarseness, throat pain,
coughing, shortness of breath, tingling, and muscle pain are
relatively common in patients receiving vagus nerve stimulation.
Even more dangerous, bradycardia followed by transient asystole is
reported in association with tests during stimulator implantation
and there is one case report of bradycardia and asystole with
syncope in a patient after two years of wearing the device.
[0130] Optogenetic techniques are particularly useful for parsing
through the various functions of the vagus nerve and to stimulating
only the particular neurons that are of interest. Selective
stimulation mitigates the unwanted side effects, particularly the
potentially life-threatening cardiac events.
[0131] Aspects of the present invention are also particularly
useful for studying the effects of vagus nerve stimulation. Little
is known about why vagus nerve stimulation is effective in treating
epilepsy and depression. Optogenetic techniques provide a means of
studying and improving these treatments. In animal models, ChR2
expression in various types of vagal fibers allows for the
identification of the specific fibers best suited for stimulation.
Once these fibers of interest are identified, therapies could be
modulated so that only these fibers are stimulated, thus avoiding
unwanted side effects.
[0132] In certain embodiments, inhibition is desired, rather than
stimulation. Vagus nerve techniques for appetite suppression
involve either severing the nerve or over-stimulating it so that it
no longer has meaningful effect on the gastrointestinal system.
However, damage to the vagus nerve can cause gastroparesis, where
the stomach no longer propels food forward through the digestive
system, causing nausea, vomiting, and dangerous fluctuations in
blood sugar levels.
[0133] In one such embodiment, NpHR is used to provide a more
targeted technique to depress vagus nerve firing. The optical
stimulation of the NpHR can be provided to specific times, such as
during meals, to more closely mimic natural physiology. For
example, optical stimulation could be responsive to patient input
indicating consumption of food. Alternately, one could bypass the
vagus nerve and instead express ChR2 in the muscles of the proximal
stomach. These muscles relax to allow the stomach to expand while
eating; blocking or mitigating this expansion through ChR2
stimulation would create a premature sense of satiety, similar to
the effects of gastric bypass surgery. Selection of one treatment
method over the other can be determined as a function of the
ability to directly control stomach muscle movement against the
complexity of needing a light source for the entire stomach muscle
region rather than a small cuff for the vagus nerve.
[0134] Other aspects of the present invention relate to cardiac
applications. Abnormal heart rhythms, such as atrial fibrillation
or atrial flutter, are often treated using defibrillation and
cardioversion. These treatments are based on the concept of
creating a large electrical field to interrupt the abnormal heart
rhythms, thereby allowing the heart to return to normal rhythm.
This is also the basis for external defibrillation, used to
resuscitate patients that otherwise would die using an external
shock system. Implantable defibrillators have become the standard
of care in patients felt to be at high risk for life-threatening
rhythm abnormalities called ventricular tachycardia or ventricular
fibrillation. These devices have several major limitations--one of
the greatest limitations is the need for a painful shock and
potential for symptoms prior to conversion due to the inability of
the device to prevent the rhythm from occurring or progressing
prior to the shock. The shock is not desirable since it is painful
and creates patient anxiety, resulting in an impairment of quality
of life.
[0135] For atrial arrhythmias such as atrial fibrillation,
techniques such as catheter ablation often do not completely
eliminate atrial fibrillation. Since there are estimates of
millions of patients currently with atrial fibrillation, a
substantial numbers of patients may remain in atrial fibrillation.
Atrial fibrillation results in an increased risk of stroke in most
patients and may be highly symptomatic. For many of these patients,
electrical cardioversion is possible but requires anesthesia to be
administered with its resulting inconvenience and cost. Implantable
defibrillators, although approved for this indication, have not
been utilized for atrial fibrillation: the discomfort of the shock
is not well tolerated, and only about 50% of atrial fibrillation
episodes are converted by maximum energies in current implantable
defibrillators.
[0136] Consistent with a specific implementation, local activation
of cells in specific regions or with specific cell types is used to
assess the role of the cells in the genesis of arrhythmias. There
are two fundamental mechanisms that might be employed to convert
atrial or ventricular arrhythmias. The first mechanism brings
localized regions of heart tissue in specific geometric
relationships to reach subthreshold potential so that the abnormal
rhythm stops. The second mechanism controls afferent sympathetic
and parasympathetic nerves with optical stimulation. In the first
mechanism, the subthreshold regions create firewalls around the
regions of initiation of the ventricular tachycardia or ventricular
fibrillation so that the rhythm would not actually start, while in
the second mechanism the vagus nerve itself (for example) can be
accessed at a position where the cardiac fibers are still
embedded.
[0137] Embodiments of the present invention are particularly useful
for replacement of supplementation of electrical stimulation
therapies that target phrenic and sacral nerves. The phrenic nerve
controls the diaphragm, and implantable electrodes can be used as
an alternative to mechanical ventilators for long term
ventilation-support needs. Because the phrenic nerve is relatively
isolated and has few functions beyond diaphragm control, electrical
stimulation is generally an effective technique. Side effects,
however, come from the initial surgery to implant the electrodes,
which may include thoracotomy. There is also report of chest pain
with stimulation at high intensity, due to simultaneous stimulation
of phrenic nerve afferent fibers, though this is generally fixed by
lowering the stimulation levels.
[0138] Optogenetic techniques can be useful for avoiding the need
for thoracotomy to implant the electrodes. With the specificity
provided by genetic targeting, accidental stimulation of unwanted
nerves can be mitigated or completely prevented. Therefore an LED
cuff (or alternate light source) could be installed above the rib
cage, where the nerve first leaves the spinal cord. This would
avoid the need for a potentially dangerous thoracotomy, and would
hasten post-operative recovery time.
[0139] The sacral nerve influences bladder and bowel control, and
may be damaged either in paraplegia, or as a side effect of radical
prostatectomy. Correct bladder control is the product of careful
coordination of the detrusor and sphincter muscles, as controlled
by sacral nerve parasympathetics and thoracic nerve sympathetics.
While filling, the sphincter muscles must remain strongly
activated, while the detrusor muscles relax to allow the bladder to
stretch, as monitored by stretch receptors. Bladder release
requires coordination of these same muscles in the opposite
fashion: sphincter muscles release followed by detrusor muscle
contraction. Failure to synchronize these events is known as
detrusor-sphincter dysenergia (DSD).
[0140] Electrical stimulation suppresses hyperreflexia of the
detrusor muscle, allowing for increased bladder filling and
increased time between voiding, however DSD is sometimes a side
effect of stimulation. Alternately, dorsal rhizotomy is sometimes
performed to increase bladder capacity and provide urinary
continence. However, both of these techniques are clearly limited;
one can either stimulate or cut innervation, but not do both in
carefully timed succession as would be needed for true restoration
of function. With the genetic targeting techniques, and light cuffs
implanted around the sacral and thoracic nerves, optogenetic
techniques allow control of the sphincter and detrusor muscles, so
that stimulation and inhibition of each could be achieved with high
synchrony, effectively recreating normal bladder physiology.
[0141] Other embodiments of the present invention relate to uses of
stem cells. The success of bone marrow transplantation demonstrates
the great promise of stem cell research. However, in many areas of
stem cell research, potential therapies still face major technical
hurdles. While injected stem cells will often successfully
repopulate cells in the needed area, it is difficult to guarantee
that these new cells will perform the needed function.
[0142] One promising field of research is the use of skeletal
myoblasts and stem cells to treat myocardial infarctions and heart
failure. Intravenous injection of these cells does improve cardiac
function, but there is significant concern that the treatment may
be arrythmogenic, either because of the electrical properties of
the injected cells, or because of damage or increased nerve
sprouting from the injection.
[0143] A second major research area is the use of stem cell
injections to treat spinal cord injury. Here it is difficult to
establish what types of cells are most appropriate in order to
bridge the injured area and to restore function. A cell that is
less differentiated has more potential to react to the
environmental cues to produce the needed variety of cells; however,
it also has more potential to differentiate to produce unwanted
cell types, creating the danger of teratomas and other cancerous
growths. Also, even once the cells are in place, they may or may
not integrate into the neural circuit and become functional.
[0144] Optogenetic techniques can be used to solve some of the
problems faced by stem cell therapies, particularly in cases such
as the cardiac, muscular, and nervous systems, where the cells need
to perform specific electrical tasks. Stem cells are often
genetically modified prior to injection; the inclusion of ChR2 and
NpHR would allow direct control of the electrical properties of the
transplanted cells, insuring that cells will be functional.
[0145] In the case of myocardial repair, tonic NpHR inhibition
could be used to prevent arrhythmia, as discussed previously. A
detector noting changes in the potential fields around cardiac
pacemaker cells could trigger stimulating light pulses, so that the
new cells would fire in synchrony with the native myocardial
cells.
[0146] With spinal cord injury, it may be possible to use
optogenetically modified stem cells directly at the site of injury.
Knowledge of the complex circuitry can be used to determine and
provide the needed light stimulation patterns. An alternative
implementation uses modified skeletal muscle stem cells ("satellite
cells") to repopulate muscles with cells that are responsive to
optical control. For example, after spinal cord or nerve injury,
denervated muscle begins to atrophy from lack of use. Rather than
attempt to reinstitute peripheral nerve supply, one could use
optical stimulation to control a newly grown population of skeletal
muscle cells.
[0147] Muscle fibers evolve and change type from slow to fast
twitch and the reverse, according to patterns of stimulation.
Accordingly coordination of stimulation patterns to specific muscle
types is implemented. Establishment of different populations of
satellite cells that respond to different frequencies of light
allows for independent control of slow and fast twitch muscle
fibers.
[0148] For the various embodiments of the present invention
discussed herein, one concern is the ability to effectively use
gene therapy without significant side-effects. Animal studies have
thus far shown that the expression of these types of foreign
proteins in neuron cell membranes does cause an immune response.
Notwithstanding, inflammatory effects can be countered using oral
peptide-tolerization strategies or mild oral immunosuppression
strategies, which can specifically reduce inflammatory responses.
Further advances in gene therapy and immune suppression techniques
will help to minimize these risks.
[0149] Moreover, these types of side-effects are relatively minor
when compared to many of the severe ailments that can be treated,
such as for intense chronic pain and severe cardiac arrhythmias.
With progressively safer genetic techniques, the therapies proposed
herein become increasingly viable, and optogenetic therapies may be
the preferred approach even when they offer only slight benefits
over traditional techniques.
[0150] The invasiveness of implantation surgery, scarring around
electrodes or light sources, longevity of electronics and power
supplies, and battery requirements in the case of power supply
implantation, or possible infection risks if wire leads are needed
to connect to a power supply outside the body can be mitigated
using biocompatible materials and/or power supplies.
[0151] While the present invention has been described above, the
skilled in the artisan will recognize that many changes may be made
thereto without departing from the spirit and scope of the present
invention. Such changes may include, for example, the
implementation of one or more approaches involving variations of
optically responsive ion channels. These and other approaches as
described in the contemplated claims below characterize aspects of
the present invention.
* * * * *