Science in Christian Perspective
Electrical Stimulation of the Nervous System for the Management of Neurological
Disorders
CURTIS A. GLEASON
Nenrosciences Department
Mount Zion Hospital and Medical Center
San Francisco, California
From: JASA 28 (December 1976): 151-156.
The approach that this paper takes to the theme, "What is Man'?", is
to look at man as a machine. By understanding the mechanistic characteristics,
particularly of the nervous system, we can learn how to compensate
for disorders
related to the nervous system. With our increasing knowledge of
neuroanatomy and
neurophysinlngy and the advances in electronic technrilogy, we are
understanding
and practicing nondestructive approaches to the long-term management of these
disorders.
A Long History
An electrical approach to pain management was used in Rome around 46
AD, according
to medical historians. The agent was the black torpedo fish that was
found along
the shores of the Mediterranean Sea. The pain of gout was eliminated when the
afflicted stood in a pool with these fish, and headache was relieved
by the application
of the fish to the head. More than 200 years ago in England, John Wesley, the
founder of many
social reforms including free clinics and founder of the Methodist Societies,
used electrical stimulation as a "natural and easy method of curing most
disorders". When his brother, Charles, was critically ill he
sent the following
instructions to a friend:
"1. Carry Dr. Whitchead to him, whether my brother consents or not;
2. get him outdoor exercise if possible;
3. let him he electrified-not shocked but filled with electric tire; and
4. inquire if he has made his will."
As long as people have been able to generate electricity it has been applied to
people in an attempt to cure their ailments. John Wesley, as well as
others, used
the spinning discs of the electrostatic generator, at that time the
most readily
available source of electricity.
Later, around 1800, a new source of electricity became available.
Volta discovered
how to use two dissimilar metals to produce electricity, the beginning of the
electric battery. At the same time, Galvani discovered what was later referred
to as "intrinsic animal electricity" (in species other than
the electric
fish). Hence the discovery and significant realization that people
might be running
on electricity. Since that time there have been many examples of
using electricity
to alleviate the physical (and even mental) disorders of people. The extent to
which electricity was used in medicine is seen in the book
Electricity in Medicine,
published in 1919. The authors, Jacoby and Jacoby, described a
variety of methods
of applying electricity to the body to cure just about every known
human ailment.
Electrification of the whole body was easily achieved by placing the electrodes
of the electrical generator in the bath water, for example. However,
not all procedures
proved to be effective and soon the use of electricity as described
by the Jacobys
subsided. The continued use of electricity on people was limited primarily to
muscle stimulation, diathermy and electrocoagulation.
However, studies of the effects of electrical stimulation continued,
particularly
with animal preparations. In 1956, researchers found that the
reactions of animals
to painful stimuli of the limbs were eliminated when the spinal cord
was electrically
stimulated. These experimenters were seeking an understanding of the
physiological
basis for pain and its control and proposed a theory that a
"gate" mechanism
in the spinal cord could account for their observations. Others
investigated this
theory and participated in the development of implantable electrical
stimulators
for the management of chronic pain. The implantable stimulator consisted of a
small radio receiver that was attached to small wires that penetrated
the spinal
cord. The receiver was tuned to a special battery-powered
transmitter, which the
patient was able to control.
The sensation experienced by the patient is usually a tingling in the area of
the pain. In general, about ten minutes of stimulation produced several hours
of relief of a previously intractable chronic pain. However, not all patients
responded favorably to the implanted system. Either the electrical stimulation
was ineffective or the tingling sensation was too unpleasant. It
became apparent
that the patients should he screened to determine whether a stimulator should
be implanted or not. To do this, electrodes (EKG patches) were placed on the skin over or
near the painful
areas and then connected to an external pulse generator. Most often
it was found
that this external application was sufficient to give pain relief
while the stimulator
was on. In addition, in some cases several hours of relief were obtained after
a few minutes of stimulation. Hence, electrical stimulation on the
skin for pain
management was rediscovered. In contrast to the equipment that was used earlier
in history, the modern pulse generators contain means for very
carefully controlling
the amount of current delivered. The controls for the amount of
current used are
available to the patient. The sensations on the skin are varied.
Examples of sensations
that have been reported are massage-like, vibrations, and pins and needles.
With the latest achievements in technology, including the
miniaturization of electronic
systems, sophisticated applications of electricity to the body can be effective
for many neurological disorders.
Neuroanatomy and Neurophysiology
Nerve Cells-The nervous system is composed of specialized cellular
units or nerve
cells called neurons that are linked together by special junctions to
form pathways
for the nerve impulses. The longest neurons (single cells) extend from the toes
to the brain stem. These cells make up the &ectrical circuits and produce
the electricity that was discovered in the 1800's. Of course this is
not the same
form of electricity as that produced by the utility companies for
heat and light.
Although electricity is the movement of electric charges, utility
company electricity
is the movement of free electrons in a metallic conductor whereas
body or nervous
system electricity is produced by the ions of the chemicals in and around the
nerve cells. Electrical impulses in a wire are produced by the electrons moving
momentarily in one direction but electrical impulses in nerves are
much more complex.
A crude analogy would he the chemical change along a firecracker fuse after it
is ignited. The nerve, however, recovers from its chemical changes and is able
to propagate another impulse very soon after the preceding impulse.
The electrical
impulse in a wire travels instantaneously, for all practical
purposes. In contrast,
the nerve impulse travels quite slowly, about twenty meters in one second.
The nerve impulse can be initiated in a nonphysiological way by an electrical
or a mechanical disturbance of the chemical or structural environment
of the nerve
cell. Some examples of uncontrolled stimulation of the nervous system
are: striking
the ulnar nerve in the elbow, which produces a sharp tingling sensation in the
lower arm and hand, and sticking the fingers in an empty, energized
lamp socket,
which produces tingling in the fingers.
Nervous System-The nervous system, which contains billions of neurons, consists
of the central nervous system and the peripheral nervous system.
Certain specialized
anatomical structures, namely the cerebrum, cerebellum, brain stem and spinal
cord make up the central nervous system. The whole system can be divided also
into the sensory and the motor systems. Both of these systems contain ascending
and descending pathways, and they perform facilitatory and inhibitory
functions.
Sensory System-The sensory system contains the general senses and the special
senses. The general senses include temperature, pain (nociception),
simple touch
and stereogliosis. The special senses are vision, hearing, taste,
smell and balance.
There are many types of receptors in the body that respond each in a
special way
to a variety 0f stimuli. Some are sensitive to stimuli originating
some distance
away. Others are sensitive to stimuli affecting the skin, and others to stimuli
originating within the body. However, perception or recognition of a sensation
takes place only when the nerve impulses reach certain higher centers, such as
the cerebral cortex. It follows, then, that disruption of any part of
the system
by disease or trauma would produce a sensory deficit.
The visual system includes the eyes, optic nerve, optic track, brain stem and
the visual cortex of the cerebrum. Blindness can occur if any part of
the system
ceases to function properly. With the present stateof-the-art the visual system
has not been replaced by electrical stimulation of any point along the visual
system. People have reported "seeing" flashes of light
during stimulation
of the visual cortex, but meaningful patterns have not been elicited.
However, electrical stimulation has been used to enable blind people to sense
the presence of objects. A large array of many small stimulating electrodes are
placed on the surface of the skin, for example on the hack, and
activated according
to the patterns produced by images from a small television camera. A
person without
vision is able to learn the meaning of patterns of stimulation on the skin just
as a person with vision learns the meaning of visual patterns. The dimension of
color, however, is lost by this method.
Failure of any of the components of the auditory system affects the ability to
hear. In this case stimulation of the auditory or associated pathways
and cortex
elicits noise. Tests are being carried out in several centers to
determine effective
ways by which sounds can be modulated or processed so that the
electrical stimulation
of the coehlear nerve, for example, can produce meaningful information.
The sensation of smell by stimulation of the olfactory bulbs has also
been reported.
The suppression of vertigo by stimulation of the vestibular system has not been
reported. But vertigo has been elicited during electrical stimulation
of the brain
stem and the cerebellum. The neurological pathways for taste are quite deep in
the brain stem and they would be difficult to sCmulate effectively in people,
but presently there does not seem to he a need for eliciting taste by
electrical
stimulation.
As for the general senses, electrical stimulation at any point along
the sensory
system, from the skin to the cortex, has resulted in such sensations
a tingling,
warmth, vibrations and pain. The latter is sensed as the amplitude of
the electrical
pulses is increased higher than that required to elicit a tingling sensation.
The sensation of pain, from a cut or abrasion of the skin for example, in the
periphery is transmitted by nerve cells that are smaller in diameter than those
transmitting other sensory information. As a result, electrical
stimulation will
initiate activity in the other sensory nerves at an amplitude lower than that
required to elicit pain. Also, it has been found that some mechanism
"We should never forget that it is not the electricity as such that cures, but that it is the entire procedure of electrization with all the physical and psychic effects thereby produced."
that blocks the information in the pain pathways to the brain when a
great amount
of activity is present in the other sensory nerves. Hence, low
amplitude stimulation
of the skin, peripheral or cord nerves can block the sensation of pain from a
peripheral injury.
However, the sensation or perception of pain presumably is very
complex and certainly
not understood. A leg could he in pain as the result of a stroke.
Electrical stimulation
of the nerves from the leg will not provide any benefit because the
pain is being
generated in the brain, probably in the higher levels of the brain
stem, but not
in the leg. In this example it is easy to understand why such
destructive procedures
as cutting of peripheral and cord nerves would he ineffective in stopping the
pain. However, the idea of destroying the brain center that perceived
pain aroused
some interest which resulted in the development of a special
procedure and instrumentation
that made it posishle to selectively destroy nerve cells in the bran stem. This
procedure is discussed after a quick review of the motor system.
Motor System-A special area of the cerebral cortex is the site of the
nerve cells
that are used to initiate voluntary motor control. Some of these
cells have very
long axons that descend to the spinal cord while other cortical cells terminate
in the brain stem. The spinal cord is the site of the cells (motor
neurons) that
directly activate the skeletal muscles. Activity of these cells
causes the muscles
to contract or relax in response to the information that comes from the brain
stem and cerebral cortex. In between the anatomical extremes of the
cortical cells
and the spinal motor cells is a complex array of interconnections
within and among
the cerebrum, cerebellum, brain stem and cord that are required to
perform purposeful
movements.
The proper execution of voluntary movements depends on a properly functioning
involuntary system, also. For example, simply raising an arm requires complex
activity of the central nervous system. In addition to the muscles
that contract
to raise it, other muscles (antagonists) must relax. If one is standing, then
leg and trunk muscles must respond to maintain posture and balance as
the raising
arm shifts the center of gravity. Also, when the arm is raised to a
desired position,
it is expected to reach that position without hunting and to remain steady. To
achieve proper limb control, information on limb position and muscle tension is
sent to the spinal cord, the brain stem and the cerebellum. The information in
the cord is processed to assist in the relaxing of the antagonists as
other muscles
are contracting (prime movers) to effect coordinated synergistic movements. The
cerebellum coordinates the action of muscle groups and times their contractions
so that limb movements are performed smoothly and accurately. It is understood
that the signals that leave the cerebellum are primarily inhibitory and tend to
provide a braking action to the motor control circuits in the brain stem. The
brain stem
is reciprocally connected to the cerebrum, cerebellum and spinal cord
and contains
many groups of cells that are extensively interconnected. Some of these groups
are facilitatory in their function and others are inhibitory. Our righting and
antigravity reflexes are controlled by cell groups in the brain stem.
The controls
for the complex coordination of muscle groups for the swallowing
reflex, for eye
movements and for eye focusing are found in the brain stem. In
addition, the primary
control of blood pressure, cardiac activity, respiration and
alimentary movements
originate in the brain stem.
There are many clinical signs associated with disease or damage of
the motor system
depending on the location and extent of the lesion. Flaccid paralysis
occurs when
the motor neurons in the cord or the associated peripheral nerves are damaged
thereby removing all control to the muscles. Damage to motor pathways
in the cord
or to structures in the brain stem associated with the motor system or to the
motor cortex also pro duces paralysis of skeletal muscles. However,
in this case,
the motor neurons are activating the muscles but in an uncontrolled
manner, hence
producing spastic paralysis. This type of paralysis of the muscles is the most
common clinical characteristic of cerebral palsy. Volitional control
of the muscles
is difficult due to the increased tension of the muscles because of
the inability
of the antagonists to relax.
A common disease that is associated with degeneration of certain parts of the
brain stem is Parkinson's disease. The most obvious clinical sign is
the tremor,
which is caused by damage to a part of the involuntary muscle control system.
The tremor is more pronounced during rest than during intended movements. There
is also a lack of swing in the arms during walking, which itself is difficult
to initiate but once in progress also is difficult to terminate.
Disease or damage to the cerebellum produces a number of characteristic signs
involving the motor system. Some examples are intention tremor, which
is evident
during intended movements but not present during rest; disturbances of gait and
posture; and inability to stop a movement at a desired point, that
is, overshooting
or undershooting.
Multiple sclerosis is usually a diffuse, chronic, slowly progressive neurologic
disease that degenerates the white matter of the nervous system, resulting in
the breakdown of the insulating qualities of the cell's long fibers.
The resulting
clinical signs of course depend upon the site and extent of the disease. Some
common signs are intention tremor and spastic paralysis.
Stroke, or a cerebral vascular accident, can produce many
neurological disorders
if not death. Common signs are pain and spastic paralysis, either together or
separate.
Epilepsy is characterized by sudden, transient alterations of brain function,
usually with motor or sensory involvement and often accompanied by alterations
in consciousness. It is the result of abnormally active brain cells caused by
injury, infection, genetic factors or unknown factors. Increased
nerve cell activity
in the cerebrum can produce sensation of vision, sound, smell or uncontrolled
muscle activity, or a combination of these depending on the extent of
the abnormal
activity.
Electrical Treatment of Motor System Disorders
The most obvious, and simplest, application of electrical stimulation is to the
muscles of a paralyzed limb. Healthy muscles will contract either with direct
stimulation or through intact nerves that attach to the muscle. Electrodes that
are placed either on the surface of the skin or implanted on the nerve can be
activated appropriately to produce purposeful movements. Spastic paralysis may
also he approached by this technique. As an example, some victims of stroke are
left with spastic paralysis of a foot, resulting in "foot drop", an
extension of the foot due to the greater strength of the extensors than of the
flexors. Electrical stimulation of the nerve going to the foot flexor muscles
will increase the tension in those muscles but, because of the cord
interneuronal
connections, also will decrease the tension in the extensors. A switch in the
heel of the shoe activates the electronics at the correct part of the walking
cycle.
The cerebellum, which produces an overall inhibitory effect on the
motor system,
is also a logical candidate for the reduction of the overactive
muscles in spastic
paralysis. If the output of the cerebellum could be increased, then
possibly the
action of the motor system could be decreased. Hence, stimulation of
the cerebellum
was investigated and has been successful in reducing spasticity.
Intractable epilepsy, which does not respond to
medication, is another candidate for electrical stimulation of the
motor inhibitory
system. An epileptic attack has been described as an electrical storm
of the cerebral
cortex because of the characteristics of the brain waves that are
recorded during
a seizure. The inhibitory output of the cerebellum was found to be effective in
suppressing neuronal activity in the cerebral cortex. Hence,
electrical stimulation
of the cerebellum is being investigated for the control of epilepsy
in people.
The discovery that electrical stimulation of the spinal cord could
reduce spasticity
was made when a patient with intractable chronic pain was being
treated with electrical
stimulation of the spinal cord. This person had muscle spasticity,
too, resulting
from multiple sclerosis. After a few sessions of cord stimulation it was noted
that the spasticity as well as the pain was reduced.
Electrical stimulation of the phrenic nerve, which is involved in our breathing
process, has also been effective. The nerve is stimulated
automatically to produce
periodic contractions of the diaphragm. This technique gives the person freedom
of movement that is not available from an iron lung.
The victim of a broken back or neck can experience not only paralysis
of the skeletal
muscles but also of the bladder muscles. Bladder contraction has been effected
by electrical stimulation either of the cord, near where the nerves leave it to
go to the bladder, or of the bladder wall muscles directly.
An area in the brain stem that has been electrically stimulated
enabled a partially
paralyzed arm to respond to a desired movement, That is, when the patient tried
to raise his arm he was unable to until the stimulator was turned on. If he did
not try to raise his arm then it remained at rest even if the
stimulator was turned
on. The stimulation was effective only in augmenting volitional
movement in this
ease.
Stereotaxic Surgery
Even though certain areas or sites can be electrically stimulated to overcome
certain neurological disorders, electrodes must be placed in the desired sites.
In the simplest cases the electrodes are attached to peripheral
nerves by relatively
common surgical procedures. For placing electrodes in deep brain
targets a stereotaxic
surgical procedure is required. The first stereotaxic apparatus for
reaching deep
into the human brain was described in 1947. The stereotaxic procedure
was developed
for the purpose of placing a wire or small tube accurately into a
desired suhcortical
area with minimal injury to the cerebral cortex or to the white
matter. The purpose
of stereotaxic surgery was to produce lesions (by thermocoagulation)
or to remove
or inject fluids in deep brain structures.
The apparatus in use today consists of a light, rigid metal frame that contains
millimeter scales on the three axes. Skull x-rays are taken with air or x-ray
opaque dye injected into the brain and with the frame mounted on the skull. The
scales on the frame provide the information that is needed to compute
the coordinates
of the desired brain targets in terms of the frame coordinate system.
A standard
brain atlas provides the relative coordinates of various deep brain structures.
The skull x-rays show the relationship of the patient's brain
landmarks and brain
size with the scales of the attached frame. Computation of the frame
coordinates
of desired deep brain targets are based on the standard atlas
coordinates of these
targets. The apparatus is designed so that the tip of the electrode, which is
attached to a long, one millimeter diameter tube, is always at the center of a
sphere that is scribed by the electrode holder, which is attached to the frame.
The electrode holder is attached to the frame so that the center of the sphere
is at the x, y, and z coordinates that are determined for a
particular target.
After electrode implantation, skull x-rays are used to verify the
electrode placement.
A few days after implantation, with the patient awake and alert, a laboratory
pulse generator is attached to the electrode wires that are protruding through
the scalp. Low amplitude electrical impulses are then used to provide
a physiological
test of the placement. As an example, an electrode placed in a motor
facilitatory
area will increase the tremor in a person with Parkinson's disease. A
heat lesion
that is made with this electrode would result in a reduction or elimination of
the tremor. For pain it was found that electrical stimulation of pain
perceiving
areas reduced the sensation of pain, but also that destruction of
tissue reduced
the pain.
Although the motor and sensory systems are anatomically separate in the brain
stem there is still the possibility of undesired side effects from lesions. For
example, destruction of tissue to stop tremor might also produce a
sensory deficit
if the electrode were too close to the sensory fibers. Similarly, the sensation
of pain could be reduced by destroying tissue, but an area of the involved limb
or side could be left with either a chronic tingling sensation or a numbness.
These possible side effects had to be considered in early stereotaxic surgery
because the effects of a lesion are irreversible. Brain cells are unique cells
in that they do not reproduce; once destroyed there is no replacement.
Because electrical stimulation of the deep electrodes was used to provide a physiological test of electrode placement, records were
obtained of the effects of stimulation in the human brain. However, even though
the results of the stimulation may have been beneficial, no means
were available
to permit continued periods of stimulation over long periods of time. In order
to provide a means of chronic electrical stimulation of selected
targets, electronic
devices had to be designed for chronic implantation.
The suppression of chronic, intractable pain was the first use of the
implantable
systems. Then the investigation, in animals and humans, of chronic stimulation
for other purposes became more intense. Now with the implantable
hardware available
and further knowledge of the human nervous system the possibility of
nondestructive
means for reducing the clinical manifestations of neurological disorders can be
realized.
Summary
The recorded use of electricity for the management of certain
neurological disorders
dates back almost two thousand years. Now, new applications of
electrical stimulation
are possible with the development of miniaturized electronic hardware and with
increased understanding of the nervous system. Special characteristics of nerve
cells permit their activation by electrical stimulation. In addition,
the anatomical
separation of the sensory and motor systems as well as separate
facilitatory and
inhibitory centers permit selective control of certain neurological processes.
Sensory modalities can be augmented and muscle contractions can be initiated or
suppressed to compensate for certain neurological disabilities. The stereotaxic
procedure, which allows the placement of electrodes into selected
deep brain targets,
and the development of sophisticated electronic stimulating systems provide a
minimal destruction of the nervous system and therefore offer new possibilities
for the management of neurological disorders in people.
In their book, Electricity in Medicine, 1919, Jacoby and Jacoby list
seven rules
that should he followed when applying electricity to people. The
rules point out
the usual precautions that should be followed when using electricity,
for example,
the first rule is ". . . turn off the power before applying the
electrodes".
However, I think that the seventh rule is most appropriate. Perhaps it provides
us with a better understanding of how electricity cures our ailments.
Their seventh
rule is, "We should never forget that it is not the electricity
as such that
cures, but that it is the entire procedure of electrization with all
the physical
and psychic effects thereby produced".
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