1.
PERIPHERAL NERVES
NAME: NAINA JOSHI
MPT 2ND YEAR

2.
BASIC STRUCTURE OF PERIPHERAL NERVOUS SYSTEM
• The peripheral nervous system refers to the part of the nervous system outside of the brain
and spinal cord.
• Functionally, peripheral nerves are categorized into motor, sensory, and autonomic nerves.
• The cell bodies (soma) of motor neurons reside in the ventral gray matter of the spinal
cord and are called anterior horn cells.
• Motor fibers often have very long axons that extend all the way to the neuromuscular
junction

3.
• A motor unit consists of an anterior horn cell, its motor axon and all the muscle fibers it innervates,
forming a synapse at the neuromuscular junction.
• Sensory neurons are bipolar with an afferent axon receiving sensory input from the periphery and an
efferent axon entering the central nervous system via the dorsal root.
• The cell body of the sensory neuron resides in the dorsal root ganglion or one of the sensory ganglia of
sensory cranial nerves.
• The autonomic nervous system is classified into sympathetic and parasympathetic nerves.
• The neurons of sympathetic nerves are located in the lateral horn of the spinal cord from T1 to L2,
whereas the neurons of parasympathetic nerves are located in the brain stem and sacral spinal cord (S2,
S3, and S4).

4.
• Peripheral nerves have multiple layers of connective tissue surrounding axons; epineurium contains blood
vessels and other connective tissues that surround multiple fascicles of nerves.
• Each fascicle is encased in perineurial connective tissue. Inside of each fascicle; individual myelinated
and/or unmyelinated axons are surrounded by endoneurial connective tissue. Blood vessels (vasa
vasorum) and nerves (nervi nervorum) are also contained within the nerve.

5.
ELECTROPHYSIOLOGY IN PERIPHERAL NEUROPATHY
• TYPES OF NEUROPATHIC DISORDERS: AxonalDegeneration
• Results from injury or mechanical compression of the nerve. Other possible causes include
application of toxic substances causing death of the cell body.
• Nerve ischemia, if sufficiently prolonged, also induces axonal degeneration, affecting large
myelinated fibers first followed by smaller myelinated fibers and unmyelinated axons.
• Neuropathies with this type of abnormality include those associated with alcoholism, uremia, some
cases of diabetes and carcinoma, and most cases of toxicity and nutritional deficiency

6.
• Most axonal neuropathies affect both sensory and motor fibers as exemplified by uremic neuropathies and
amyloidosis.
• Motor abnormalities prevail in hereditary motor sensory neuropathy (HMSN) Type II or neuronal type of
Charcot-Marie-Tooth disease (CMT).
• Sensory changes predominate in the majority of toxic or metabolic polyneuropathies, Friedreich’s ataxia, and
some cases of carcinomatous neuropathies

7.
SEGMENTAL DEMYELINATION
• Demyelinative process affects the nerve throughout its length uniformly in most hereditary
neuropathies, delaying conduction at all the nodes of Ranvier.
• The pathophysiology of demyelination and its clinical consequences include:
• axonal excitability changes and conduction block resulting in clinical weakness and sensory loss
• exaggerated hyperpolarization after the passage of impulse, inducing conduction block even at low
firing frequencies causing fatigue after mild but sustained effort.
• prolonged refractory periods with frequency-dependent conduction block especially at very high
firing rates, accounting for reduced strength during maximal voluntary muscle contraction effort.

8.
NCV (NERVE CONDUCTION VELOCITY)
• Nerve conduction studies (NCSs) have become a simple and reliable test of peripheral nerve
function.
• With adequate standardization, the method now provides a means of not only objectively
identifying the lesion but also precisely localizing the site of maximal involvement.
• Electrical stimulation of the nerve initiates an impulse that travels along motor or sensory nerve
fibers.
• The assessment of conduction characteristics depends on the analysis of compound evoked
potentials recorded from the muscle in the study of motor fibers and from the nerve itself in the case
of sensory fiber

9.
ELECTRICAL STIMULATION OF THE NERVE
• CATHODE AND ANODE: Surface electrodes, usually made of silver plate, come in different
sizes, commonly in the range of 0.5 to 1.0 cm in diameter.
• Stimulating electrodes consist of a cathode, or negative pole, and an anode, or positive pole, so
called because they attract cations and anions.
• As the current flows between them, negative charges that accumulate under the cathode, by making
inside the axon relatively more positive than outside, depolarize the nerve or cathodal
depolarization.
• Conversely, positive charges under the anode hyperpolarize the nerve

10.
TYPES OF STIMULATOR
• Most commercially available stimulators provide a probe that mounts the cathode and the anode at a
fixed distance, usually 2 to 3 cm apart.
• The intensity control located in the insulated handle, though bulky, simplifies the operation for a
single examiner.

11.
• The ordinary banana plugs connected by shielded cable also serve well as stimulating electrodes.
• The use of a large diameter electrode for stimulation lowers current density in the skin, causing less pain,
although the exact site of nerve activation becomes uncertain.
• A monopolar stimulation with a small cathode placed on the nerve trunk and a large anode over the opposite
surface in the same limb.
• The use of a subcutaneously inserted needle as the cathode reduces the current necessary to excite the nerve
compared to surface stimulation.
• A surface electrode located on the skin nearby or a second needle electrode inserted in the vicinity of the
cathode serves as the anode.
• The maximum current during such stimulation causes neither electric nor heat damage to the tissue.

12.
Banana Plugs
Monopolar electrodes

13.
RECORDING OF MUSCLE AND NERVE POTENTIAL
• Surface and Needle Electrodes:
• Surface electrodes with a larger recording radius serve better than needle electrodes in assessing a
compound muscle action potential (CMAP).
• Its onset latency indicates the conduction time of the fastest motor fibers, and amplitude, the
number of available motor axons.
• A needle electrode, despite its small recording radius has its place in identifying the activity from a
small muscle when surface recording fails.
• Its use also improves segregation of a target activity from neighboring discharges after proximal
stimulation, which tends to excite unwanted neighboring nerves simultaneously.

14.
AMPLIFIER SYSTEM
• The electrodes convert bioelectric signal resulting from muscle or nerve depolarization
into an electrical potential capable of being processed by AMPLIFIER.
• The difference of electric potential between the 2 recording electrodes is processed.
• The potential difference is measured in volts.
• The amplitude of potential is measured in microvolts.
• Before the potential can be visualised, it is necessary to amplify the small myoelectric
signals.
• An amplifier converts the electric signal large enough to be displayed.

15.
• Differential amplifier:
• The electric potential composed of signal from the nerves and unwanted noise from the static electricity in the
air and power lines.
• To control for the unwanted part of the signal, the differential amplifier is used, as noise is transmitted to the
amplifier as a common mode signal when the difference of potential is reduced at both the ends, the noise
being cancelled out both the ends of amplifier.
• common mode rejection ratio: Noise is not eliminated completely in differential amplifier.
• The common mode rejection ratio is a measure of how much desired signal voltage is amplified related to
unwanted signal.
• A CMRR of 1000:1 indicates that the wanted signal is amplified 1000 times more than noise.
• Expressed in decibels (DB)

16.
• SIGnaL TO NOISE RATIO:
• Noise can be generated internally by the components of the amplifier system such as resistors, transistors, or the
circuit.
• The factor that reflects the ability of the amplifier to limit this noise relative to the amplified signal is the signal to
noise ratio.
• This ratio can also be described as the wanted signal to the unwanted signal.
• GAIN: refers to the ratio of the output level of the signal to the input level of signal.
• This characteristics refers to the amplifier’s ability to amplify the signals.

17.
• Input Impedance: A resistive property present in the alternating current circuits.
• Present at the input of the amplifier and as well as at the output of the electrodes and are directly related to
voltage.
• As per the law, if the impedance at the amplifier is more than the impedance at the electrodes, the voltage drop
will be less.
• FREQUENCY BAND WIDTH: Waveforms are processed by the amplifiers are actually the summation of signals of
varying frequencies.
• Frequency is measured in Hertz (Hz).

18.
DISPLAY AND STORAGE OF RECORDED SIGNALS
• Modern oscilloscopes provide a very stable time base requiring no marking of calibration signals on
the second beam.
• The use of an oscilloscope with a digital storage capacity can display a series of responses with a
stepwise vertical shift of the baseline.

19.
MOTOR NERVE CONDUCTION
• Motor nerve conduction velocity is calculated measuring the distance between two points of
stimulation in mm which is divided by the latency difference in ms.
• The nerve conduction velocity is expressed as m/sec.
• Conduction velocity = D/PL-DL
• Where,
• D= distance between proximal and distal stimulation in mm
• DL= Distal latency in m/sec
• PL= Proximal latency in m/sec

20.
• Motor nerve is stimulated at least at 2 points along its course.
• Onset latency is the time in ms from the stimulus artifact to the first negative deflection of CMAP.
• The amplitude of CMAP is measured from baseline to the negative peak (base to peak).

21.
• Stimulating electrode is typically a two pronged bipolar electrode with cathode and anode.
• A ground electrode is placed between the stimulating and recording electrodes.
• For accurate motor nerve conduction velocity measurement, the distance between two points of stimulation
should be at least 10 cm.
• Stimulation at shorter segments of nerve, however, is necessary in the evaluation of focal compressive
neuropathies Example: Carpal Tunnel Syndrome.
• In a diseased nerve, the excitability is reduced and current requirements may be much higher than normal.
• The measurement for motor nerve conduction study includes the onset latency, duration and amplitude of
compound muscle action potential (CMAP).

22.
SENSORY NERVE CONDUCTION
• Can be measured orthodromically or antidromically.
• In orthodromic conduction, a distal portion of nerve is stimulated and SNAP is recorded at a
proximal point along the nerve.
• In antidromic conduction, the nerve is stimulated at a proximal point and nerve action potential is
recorded distally.
• In antidromic conduction measurement, the action potential may be obscured by superimposed
muscle action, which is elicited due to simultaneous stimulation of motor axon in mixed nerve.
• Ring electrodes are preferred to stimulate digital nerve (orthodromic).
• Surafce stimulating electrodes are commonly used for antidromic stimulation.

23.
• Sensory nerve conduction measurement includes onset latency, amplitude, duration of SNAP and nerve conduction velocity.
• Latency of orthodromic potential is measured from the stimulus artifact to the initial positive or subsequent negative peak.
• The latency following orthodromic stimulation is shorter compared to antidromic.
• The SNAP amplitude is measured from baseline to negative peak or from positive to negative peak
• The SNAP recorded with a surface electrode is of higher amplitude in antidromic recording compared to orthodromic because
nerves are closer to recording electrode specially in digital nerves.

24.
The Duration of SNAP is measured from the
initial positive peak to the intersection
between the descending phase and the
baseline or to the negative or subsequent
positive peak or return to the baseline.

25.
VARIABLES AFFECTING THE NERVE CONDUCTION STUDY
• Physiological Variables: Age: NCV in a full term infant is nearly half of the adult value. As the
myelination progress, NCV attains the adult value by 3-5 years of age. The conduction velocity
begins to decline after 30-40 years of age.
• Upper v/s lower limb: The median and ulnar nerve conduction velocity is higher compared to tibial
and peroneal nerve. Height is inversely proportion to NCV that is longer nerves conduct slower than
shorter nerves.
• Temperature: Low temperature results in slowing down NCV and increase amplitude. For each
degree fall in the temperature, velocity increases by 0.3 ms. Laboratory temperature, should be
maintained between 21-23 degree Celsius.

26.
TECHNICAL VARIABLES
• Stimulating system: Failure of stimulating system may result in unexpectedly small responses.
The nerve may be stimulated submaximally or the applied current may not reach the intended target.
• Recording System: Faulty connection in the recording system may results in errors in spite of
optimal stimulation.
• Inadvertent stimulation of unintended nerves: Spread of stimulating current to an adjacent
nerve or root not under study is frequent and failure to recognize, results in errors in latency
measurement.

27.
Median motor
palmar study.
Stimulation site
over the median
nerve at the wrist,
recording the
abductor pollicis
brevis muscle
Median sensory
study.
Stimulation site
over the median
nerve at the
wrist, recording
the index finger.

28.
Peroneal motor study: Distal stimulation site over the
anterior ankle, slightly lateral to the tibialis anterior
tendon, recording the extensor digitorum brevis muscle.

29.
F WAVES
• F waves (F for foot where they were first described) are a type of late motor response.
• When a motor nerve axon is electrically stimulated at any point an action potential is propagated in
both directions away from the initial stimulation site.
• The distally propagated impulse gives rise to the CMAP. However, an impulse also conducts
proximally to the anterior horn cell, depolarising the axon hillock and causing the axon to backfire.
This leads to a small additional muscle depolarisation (F wave) at a longer latency.
• F waves vary in latency and shape because different populations of neurones normally backfire with
each stimulus. The most reliable measure of the F wave is the minimum latency of 10–20 firings.

30.
WHY ARE F WAVES USEFUL?
• F waves allow testing of proximal segments of nerves that would otherwise be inaccessible to
routine nerve conduction studies.
• F waves test long lengths of nerves whereas motor studies test shorter segments. Therefore F wave
abnormalities can be a sensitive indicator of peripheral nerve pathology, particularly if sited
proximally.
• The F wave ratio which compares the conduction in the proximal half of the total pathway with the
distal may be used to determine the site of conduction slowing—for example, to distinguish a root
lesion from a patient with a distal generalised neuropathy

31.
REFERENCES
• Electromyography and Neuromuscular Disorders: Clinical–Electrophysiologic
Correlations (3rd edition).
• Textbook of electrotherapy (3rd edition).
• Malik et al. NERVE CONDUCTION STUDIES: ESSENTIALS AND PITFALLS IN
PRACTICE. J Neurol Neurosurg Psychiatry 2005;76(Suppl II):ii23–ii31