ELECTRICAL STIMULATION OF NERVE AND MUSCLE:
A motor neuron can be activated by a
current that varies in magnitude to cause contraction of the muscles it
supplies, or an appropriate current can directly stimulate the muscle fibers in
the absence of a motor nerve. In both situations, intermittent currents are
employed, and a wide variety of these currents are accessible. There is a range
of 0.01 milliseconds (ms) to 3 seconds for the current used. Durations of
0.01–0.03, 0.1–0.3, 1, 3, 10, 30, 100, and 300 ms are frequently offered by the
apparatus. Short-duration
impulses, defined as those lasting less than 10 milliseconds, are utilized to
activate normal (innervated) muscles. These impulses are classified as faradic
impulses. The impulses repeat at a high rate, typically between 50 and 100
times per second.
Long-duration impulses, defined as those lasting longer than 10 milliseconds,
are used to the stimulation of paralyzed muscles. They are frequently referred
to as modified or interrupted direct current. The impulses are repeated less
often than those with short durations; for example, 30 repetitions per minute
is typical for impulses lasting 100 ms apiece.
FARADIC-TYPE CURRENT:
A short-duration interrupted direct current with a pulse duration of 0.1–1 ms and a frequency of 50–100 Hz (hertz) is known as a faradic-type current. Originally, the term "faradaism" was used to describe the kind of current generated by an induction coil known as a faradic coil. The initial faradic coils produced an unevenly alternating current, with each cycle having two unequal phases: a low-intensity, long-duration phase and a high-intensity, brief-duration phase. The second phase, the effective one, lasted for roughly one millisecond, with a frequency of about 50 Hz. A current graph electronic stimulators have now supplanted faradic coils. These supply currents, despite frequently having quite different wave forms, have the same physiological consequences as the initial faradic current. In order to produce these physiological effects, it is necessary to repeat impulses of a duration of 0.1 to 1 ms 50-100 times per second. The rules governing the duration of the impulses and the intervals between them are controlled by very low resistances in the electronic stimulator for producing the faradic-type current, yet they nonetheless produce the necessary duration and repetition rate.
Modified faradic currents:
In
order to induce a muscle contraction and relaxation that is almost identical to
a tetanic one, faradic-type currents are always increased for therapeutic
purposes. The current is'surged' in such a way that the intensity of subsequent
impulses rises gradually, with each impulse peaking at a higher value than the
one before it and then either falling drastically or gradually. The current was
manually pushed in the first faradic coils, but an electronic mechanism is employed
in stimulators nowadays. It is possible to adjust the circuit to produce surges
with different wave shapes, frequencies, and durations. To achieve the best
possible muscular contractions and rest intervals, it is ideal for the
durations of the surges and the intervals between them to be controlled by
independent controllers.
- (a) In original form.
- (b) During spikes.
- (b) Various-duration cravings.
- (d) Differing durations between spikes.
- (e) Wave-form variations in surges.
The Nerves' Electrical Activity
Transfer of nerves:
A nerve's internal and external
potentials differ from one another due to variations in the concentration of
ions within and outside the plasma membrane. The resting nerve has a positive
exterior and a negative inside, and sodium ions cannot pass through the plasma
membrane. This is referred to as the membrane's polarized stage. Stimulation of
a nerve results in a decrease in the potential difference across the plasma
membraneThe membrane's ability to permeabilize sodium ions is altered when this
fall reaches a particular critical point. As a result, the ion concentrations
inside and outside change, and the PD continues to decrease until the
membrane's polarity reverses, becoming negative on the outside and positive on
the inside. Following this action, the sodium ions are pushed out once more,
causing that portion of the nerve to go back to rest. Local electron flow
occurs between the active and neighboring sections of the nerve due to the
potential difference between the active and resting regions of the neuron. In
contrast to the potential difference (PD) across the fiber, the current passes
through the membrane in the other direction Because the fiber resists the flow
of current, the PD is lowered and the membrane becomes permeable to sodium
ions, which causes the PD to reverse once more. The nerve fiber's entire length
then experiences these alterations. The impulse traveling through the nerve is
represented by this wave of change in the polarized state.
Electrical nerve stimulation:
An electrical stimulation can start a
nerve impulse. Applying a changing current with sufficient intensity is
necessary to do this. As the current passes through the nerve fiber's plasma
membrane, which creates a resistance in series with the other tissues, a
potential difference is created across it. In comparison to the other surface,
the membrane's surface closer to the cathode becomes negative. The plasma
membrane surfaces with the letter "n" are the closest to the cathode
and hence become more negative, whilst the surfaces with the letter
"p," which are closer to the anode, become more positiveThis raises
the resting potential difference (PD) across the membrane on the side of the
neuron closest to the anode; however, the extra charges on the side of the nerve closest to the cathode (B) are of the opposite polarity from those on the
resting membrane, which lowers the PD across it. Sodium ions start to enter the
axon and start the sequence of events outlined above if the PD drops below the
point at which the membrane becomes permeable to them, resulting in the
initiation of a nerve impulse.
Polarity Effects on Nerve Stimulation:
If the PD crosses any portion of the
nerve cell's or fiber's plasma membrane sufficiently, an impulse is started.
The side of a superficial nerve closest to the cathode is activated when the
cathode is applied to it; however, the anode can also initiate a nerve impulse.
The portion of the nerve that is more distant from the anode gets activated in
this instance. The anode is less successful than the cathode at starting an
impulse because the current diffuses across the tissues, resulting in a current
density that is somewhat lower on the distant surface of the nerve fiber than
on the closer one. The polarity of the terminals is marked on several kinds of
electronic equipment. This is the polarity at the effective stimulus, or high
peak of the current. The cathode, which causes an innervated muscle contraction
with less current needed at the anode, should be connected to the active
electrode.
Accommodation:
When a steady current flows, the nerve
adjusts to the changed environment by a process that is still unclear. We call
this effect accommodation. Consequently, an impulse cannot be started by an
unvarying current. As previously mentioned, an impulse is started when the
current rises, but it can also be started by a decline in current. The neuron
accommodates itself while the current flowing continuously, and the PD that
results from this flow no longer influences the excitability of the nerve fiber
since the nerve fiber has adapted to the new circumstances. The PD that the current created across the plasma membrane
abruptly vanishes when it stops, changing the overall PD throughout the
membrane. The applied PD was boosting that across the resting membrane on the
part of the neuron closer to the anode, and its abrupt disappearance results in
a decline in the PD. An impulse is started if this fall reaches the point where
the membrane starts to let in sodium ions. To start an impulse, a decrease in
current works less well than an increase in current. Since the afflicted side
of the nerve is closer to the anode, the anode stimulates the nerve more than
the cathode doesA current that rises or falls abruptly is more effective at
starting an impulse than one that varies slowly because the nerve has the
ability to accommodate. A larger current is required to be successful in a
gradual current change as there is more time for accommodation to occur than in
a sudden one. A very slow-changing current does not at all start a nerve
impulse.
Consequences of nerve stimulation:
A nerve impulse can only go in one
direction down the axon when it is initiated at a nerve cell or eud organ;
however, if it is begun somewhere along the nerve fiber, it is simultaneously
sent in both directions from the place of stimulation. The upward traveling
impulse is felt when it reaches the conscious levels of the brain, whereas the
downward traveling impulse is inactive when a sensory neuron is triggered. It
is discovered that the sensory stimulation received changes with the duration
of the impulse when impulses with varying durations are administered while
utilizing the same current for each. Long-length impulses cause an unpleasant,
stabbing sensation, but this subsides as the impulses' duration is shortened
until, at impulses of 1 ms and below, just a slight prickling sensation is
felt.
Motor Nerve Stimulation:
When a
motor nerve is stimulated, the downward-traveling impulse reaches the muscles
the nerve supplies, causing them to contract, whereas the upward-traveling
impulse is unable to cross the first synapse because it is traveling in the
incorrect direction. All of the muscles that a motor nerve trunk feeds below
the point at which it is stimulated receive impulses from the stimulus, which causes
the muscles to contract. The nerve fibers in an innervated muscle are activated
in an identical manner when an electrical current is put directly over it. Either stimulation at the motor point,
where the main nerve enters the muscle, or, in the case of deeply situated
muscles, at the point where the muscle emerges from beneath the more
superficial ones, will result in the largest response.
Impact of stimulation frequency:
A single stimulus causes impulses to
go to several motor units at once, resulting in a rapid, vigorous contraction
that is quickly followed by relaxation in normal conditions. When a series of
stimuli are presented at relatively long intervals—one stimulus every second,
for example—each one causes a separate muscle contraction, and there is a gap
of time during which the muscles can fully relax. The duration of the
relaxation phases shortens as the stimulus frequency increases, until at
frequencies higher than 20 Hz there is insufficient time for total relaxation
in between contractions, leading to partial tetany. Increases in frequency
cause the relaxation to decrease even more until, at frequencies higher than 60
Hz, the contraction is entirely tetanic and there is no discernible relaxation.
Contractional strength:
This is dependent on both the rate of change of current and the number of motor units that are activated, which in turn depends on the strength of the applied current. Muscle contraction occurs when there is insufficient time for accommodation to occur when the current intensity increases abruptly. A higher current intensity is required to cause a contraction if the current rises more slowly, as is the case with the sawtooth, trapezoidal, and triangular impulses.
IMPACT OF FARADIC-TYPE CURRENT ON PHYSIOLOGY
Because the bodily fluids that make up the tissues are conductors and contain ions, the tissues can convey an electric current. As a result, the ions moving through the body in both directions make up the current, and the conductivity of the various tissues changes based on the volume of fluid they contain. For instance, muscle is an excellent conductor due to its good blood supply, whereas fat has a poor conductivityAlthough it isn't always possible for the current to avoid the layers of high resistance, it generally moves through tissues with low resistance. Because the epidermis does not hold much fluid and its outer layers do not easily absorb moisture, it has a high resistance of 1000 ohms or more. The epidermis must allow current to flow through it, and suitable steps are taken to lessen resistance. When using electrical therapies: Current flow can cause chemical alterations that could be dangerous for some therapies.
Stimulation of sensory nerves:
A
slight prickling sensation is felt when the body is exposed to a faradic
current. This results from the activation of the sensory nerves, and because
the stimuli are brief, it is not very noticeable. The skin somewhat reddens
(erythema) as a result of the reactive vasodilatation of the superficial blood
vessels brought on by the sensory stimulus. Vasodilatation is primarily limited
to the superficial tissues and is not very useful. Activation of the motor
neurons
Faradic Stimulation for Muscle Contraction: A faradic current stimulates the motor neurons, which in turn causes the muscles they supply to contract if the current is strong enough. The contraction is tetanic because the stimuli are repeated at least fifty times per second. Muscle exhaustion results from maintaining this kind of contraction for extended periods of time, hence the current is frequently increased to promote muscle relaxation. Similar to a voluntary contraction, the contraction progressively gains and loses strength when the current is surged.
Consequences of contracting muscles:
The alterations that occur within a muscle when it contracts due to electrical stimulation are comparable to those that occur during voluntary contraction. There is a higher rate of metabolism, which raises the need for food and oxygen as well as the production of waste products like metabolites. The metabolites lead to arterioles and capillaries dilation, which significantly increases the muscle's blood supply.
The veins and lymphatic vessels that are located within and surrounding the muscles are pumped by the contraction and relaxation of the muscles. These veins' valves make sure that the fluid inside is directed toward the heart. A pumping effect occurs if the muscle contractions are powerful enough to move the joint. Increased lymphatic and venous return results from this.
Activation of paralyzed muscle:
Usually, the current needed to cause a denervated muscular contraction with an impulse lasting one millisecond is too high to be tolerated for therapeutic reasons. For the purpose of stimulating muscles that have been denervated, the faradic kind of current is consequently inadequate.Effects of faradic-type current on chemicals:
Chemical reactions occur at the electrodes of an electrolyte when a direct
current is applied. There is a risk of electrolytic burns if the chemicals
produced come into touch with the tissues, albeit this risk is noticeably lower
with intermittent direct current than with continuous direct current. When
there is an alternating current, the ions travel in one direction during one
phase and the opposite direction during the other. If the two phases are equal,
the chemicals generated in one phase are neutralized in the subsequent phase.
However, if the current's reverse wave differs from its forward wave, chemical changes may occur that could lead to an electrolyte burn. Electronic devices may produce current that flows more in one way than the other, but if the impulses are brief, there shouldn't be enough chemical production to pose a significant risk of burns. Nonetheless, it is wise to take the necessary safety measures.
When to employ currents of the faradic type
Muscular contraction facilitation
- Contraction of the intrafusal muscle fibers results from the stimulation of the tiny (fusimotor) efferent fibers.
- The activation of the extrafusal muscle fibers is caused by the muscle spindle being stretched, which activates the primary nerve endings and transmits information to the big anterior horn cells.
- Blocking the anterior horn cells that supply the opposing muscle group.
A Therapeutic Method for Pain Management and Rehabilitation:
The big anterior horn cells are inhibited by pain, which prevents impulses from
reaching the motor units. By electrically stimulating the motor neurones, the
inhibition should be reduced, allowing voluntary impulses to travel to the
muscle and causing its antagonists to relax. Electrical stimulation may help
produce voluntary contraction in cases when muscle contraction is restricted by
pain or recent damage, such as when rheumatoid arthritis of the knee joint
prevents active contraction of the quadriceps or after a meniscectomy.
In order to prevent the discharge from
the big anterior horn cells, the treatment must be set up so that the part is
in a pain-free position and that no pain-producing movement occurs. Electrical
stimulation should be tried concurrently with voluntary contraction; it's
necessary only until a decent voluntary contraction can be executed without
assistance.
Retraining the muscles to contract:
A muscle's inability to contract freely can be caused by improper use, as in the case of the abductor hallucis in hallux valgus, or by long-term inactivity, as in the intrinsic foot muscles in a persistent flat foot. In certain situations, contractions can be induced by faradic stimulation, which can aid in regaining movement sensation. The current should be administered in a way that creates the movement that the patient is unable to do since the brain responds better to motions than to muscular actions. It is best to try active contractions concurrently with electrical stimulation, since therapy is a prerequisite for physical exercise. Establishing a voluntary contraction may likely take longer than in situations when inhibition is brought on by pain or injury, but. When a contraction is achieved that is satisfactory, electrical stimulation ought to be stopped.
Training a new muscle action:
A muscle may need to execute an action different from what it used to do after undergoing tendon transplantation or other reconstructive procedures. One must establish a new movement pattern. The patient must focus on the movement and make an effort to help with voluntary contractions as the muscle is stimulated with the faradic-type current to perform its new activity. This allows for the teaching of the new muscular action, but at a slower rate than re-educating an action the muscle has already performed.Motor nerve neuropathy:
In this instance, the lesion prevents brain impulses from passing through to the muscles that the injured nerve supplies. As a result, voluntary power is diminished or eliminated. However, the nerve does not degenerate; therefore, impulses passing to the muscles below the lesion site cause them to contract when the nerve is stimulated with faradism. In most cases, electrical stimulation is not required during neurapraxis since the muscle tissue heals without noticeably changing.
Damaged motor nerve:
Axon degeneration occurs once a nerve is severed, and short-duration stimuli no longer elicit a sufficient response. Degeneration takes several days, and a faradic-type current may be used to produce a muscle contraction for a few days following the injury. If this is the case, faradism can be utilized to exercise the muscle as long as there is a strong reaction; if not, modified DC must be applied as soon as the response starts to wane.Enhanced lymphatic and vein drainage:
The pumping action of alternating muscular contraction and relaxation as well as joint movement on the veins and lymphatics results in increased venous and lymphatic return. Applying the current using the "faradism under pressure" method yields the best results from the treatment. It may be applied to treat gravity ulcers and oedema in certain cases.
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