Human movement

Human movement

Science of Human Movement, Practical 2, Human Motor Nerve Conduction

Results section

Table 1: Elbow

Amplitude (mV)

Latency (s)

Magnitude (mV)

10

0.0087

4.139

0.0088

3.929

0.0088

4.032

0.0087

4.024

The above table shows the data for latency and magnitude obtained after the recording of the elbow at 10 mV amplitude. The mean values for latency and magnitude are 0.00875(s) and 4.031(mV) respectively.

Table 2: Wrist

Amplitude (mV)

Latency (s)

Magnitude (mV)

10

0.0034

0.0037

0.0035

0.0037

The above table shows data for latency on the wrist at 10 mV. The mean value is 0.003575(s)

Table 3: Elbow

Amplitude (mV)

Latency (s)

Magnitude (mV)

7.5

0.0091

1.527

0.0090

1.657

0.0090

1.533

0.0090

1.758

The above table shows data for latency and magnitude obtained after the recording of the elbow at 7.5 mV

The mean values for latency and magnitude are 0.009025(s) and 1.62(mV) respectively

Comparing the graphs obtained from the recordings of the elbow and the wrist, the magnitude on the elbow at 10 mV current is higher than in 7.5 mV but the latency is lower. The latency on the wrist is a lot smaller than the latency on the elbow. The magnitude is also a lot higher.

Specific Questions

Question1.

In order for the test to be done correctly, the subject doing this test needs to keep the arm still on a surface without moving it at all otherwise the unwanted movement on the muscle fibres will affect the data. Also the electrodes once attached on the arm and connected to the system must not be disturbed in any way. An unwanted movement could affect both the magnitude and latency on the graph.

Question 2.

From the data for the elbow (Table 1):

we calculate Mean Latency for the Elbow: Lelbow= 8.75 ms

From the data for the wrist (Table 2):

we calculate Mean Latency for the Wrist: Lwrist = 3.575 ms

As the distance between stimulation sites equal D= 32 cm applying data into the formula:

V = D / (Lelbow - Lwrist ) we obtain Nerve Conduction Velocity as follows:

V = 320 mm / (8.75 - 3.575) ms = 61.84 m/s

MEDIAN MOTOR NERVE CONDUCTION

NCV (m/sec)

CMAP

Parameter

Terminal Latency

(msec)

Elbow-Wrist

Axille-Elbow

Duration

(msec)

Amplitude

(mV)

Mean +/-SD

2.7 +/- 0.41

58.78 +/- 4.41

65.76 +/- 4.90

12.58 +/- 1.68

14.62 +/- 8.45

Normal Limit

3.60

49.96

55.96

15.94

5.00

[Shin J. Oh (2003) Clinical Electromyography: Nerve Conduction Studies 3rd Edition]

Table of Normal Values for Median Motor Nerve Conduction

Authors

Normal Conduction Velocity

(m/s)

Burnham and Steadward (1994)

59.6 +/- 3.7

Jackson and Clifford (1989)

--

Kimura (1989)

48.8 +/- 5.3

Kimura (1979)

49.0 +/- 5.7

Melvin et al. (1973)

56.7 +/- 3.8

Thomas et al. (1967)

59.1 +/- 5.2

Johnson and Olsen (1960)

53.0 +/- 6.4

Average

54.7

Comparing the normal values with the subject's, the subject's latency, magnitude and conduction velocity values are higher than normal. This can be because the subject is a heavily trained athlete.

Question 3.

Using data calculated as in previous question 2 we calculate

V'= 320 mm / 8.75 ms = 36.57 m/s

Conduction time from the distal point to the muscle is usually longer than what is expected from the conduction time over the proximal segment of the same nerve. This delay is caused by slowing of impulses in the terminal fibers, synaptic delay, and conduction time of an action potential through muscle tissue (residual latency). Because of the delay, it is not possible calculate the conduction velocity over the most distal segment of the motor nerves. The NCV could be accurately calculated by stimulating at two different points, along the nerve, and measuring the latency for each response.

The time during the stimulation and the appearance of the CMAP (latency) shows three components: 1) time for action potentials to travel through the nerve, 2) the time it takes to cross the neuromuscular junction, 3) time for the muscle action potentials to disperse. Only the first component is relevant when calculating the nerve conduction velocity. If we use only the latency instead of the difference between latencies we assume that the time taken to stimulate the nerve is included within the latency and thus does not reflex the genuine conduction time. If the latency is used, the distance between the stimulating and recording electrode should be strictly standardised.

Question 4.

a) The H-wave is an indicator of the monosynaptic stretch reflex and is normally obtained in only a few muscles. The stimulus travels along the Ia fibers, and is transmitted across the central synapse to the horn cell which fires it down along the alpha motor axon to the muscle. The result is a motor response, between 0.5 and 5 mv in amplitude, occurring at low stimulation strength. The latency of this reflex is much longer than that of the M response, and a sweep of 5-10 ms/division is necessary in order to see it. Abnormal latency on one side of the EMG could indicate possible problem with on the muscle. Observing the F- wave we can see that it has a long latency muscle action potential, always after suprarmaximal stimulation to a nerve. Although it is eligible for a big variety of muscles, it is best obtained in the small muscles of the foot and hand. The F-wave response varies and is obtained after nerve stimulation

b) Stimulus artifact is known to be caused by detection of the electrical stimulus pulse by the recording electrodes. Spread of excess current along the skin and deeper tissues resulting in a stimulus artifact. For reducing stimulus artifact we have to place the ground electrode in between the recording and stimulating electrodes. This way, a low-impedance pathway for excess current to flow trough is forming.

c) In general, a ground electrode is necessary for providing a common reference for measurement.

A ground electrode should be attached to the limb under test and is ideally located between the stimulating and recording electrodes. This is recommended to avoid any possible electrocution by a transthoracic current pathway. Such a possibility exists if a ground electrode is placed on the right arm when NCV is been tested on the left arm. To reduce the stimulation artifact to a minimum, a ground electrode should be placed (ideally) between the stimulating and recording electrodes. However, it is not right to change the ground electrode during the NCV studies over the different segments of the nerves, and so a ground electrode should be positioned on the limb being tested. Usually this is adequate for the nerve conduction studies. However, when there is difficulty obtaining compound nerve or muscle action potentials (CNAPs or CMAPs) clearly because of stimulation artifacts, do not hesitate to place a ground electrode between the stimulating and recording electrodes.

Conduction time from the distal point to the muscle is usually longer than what is expected from the conduction time over the proximal segment of the same nerve. This delay is caused by slowing of impulses in the terminal fibers, synaptic delay, and conduction time of an action potential through muscle tissue (residual latency). Because of the delay, we are unable to calculate the conduction velocity over the most distal segment of the motor nerves. The NCV can be accurately calculated by stimulating at two different points along the nerve and measuring the latency for each response.

The time during the shock and the appearance of the CMAP (latency) shows three components: 1) time for action potentials to travel through the nerve, 2) the time it takes to cross the neuromuscular junction, 3) time for the muscle action potentials to disperse. Only the first component is relevant when calculating the nerve conduction velocity. Components (2) and (3) would introduce a small systematic error if included, but these are just constants and could be removed by subtracting the distal site latency from the proximal site.

Question 5.

Theoretically, the motor, sensory, and mixed NCVs represent the maximal conduction velocities of the fastest conducting motor , sensory, and mixed nerve fibres, respectively. Normally, the conduction velocity varies with age, skin temperature, each segment of nerve, and different nerves.

There are two types of conduction: salutatory or continuous. The former occurs in unmyelinated nerve fibers and is continuous and bidirectional. Conduction in a myelinated fiber occurs in a somewhat different manner, however. The myelin sheath, is covered by a Schwann cell. The cell process is not continuous at intervals up to 2 mm along the length of the fiber, and the axon is uncovered. This is known as the node of Ranvier. The circuit flow is also bidirectional from one node of Ranvier to the next. The impulse goes from one node to another (saltatory), resulting in faster conduction. Therefore, large-diameter fibers have faster conduction times (Table ).

Types of Nerve Fibres

Fiber Type & Diameter

Conduction Speed (m/sec)

A fibers: myelinated fibers of somatic nerves

muscle nerve

afferent group

I (12-21μ)

II (6-12μ)

III (1-6 μ)

IV (C fiber)

efferent group

α motor neuron

γ motor neuron

cutaneous nerve

afferent group

α (6-17μ)

γ (1-6μ)

5-120

B fibers: myelinated preganglionic fibers of autonomic nerve

3-15

C fibers: unmyelinated fibers of

somatic or autonomic nerve

sC fibers: efferent postganglionic fibers

of autonomic nerve

0.7-2.3

drC fibers: afferent fibers of the dorsal

root & peripheral nerve

0.6-2.0

Richard M. Lehman, M.D.- A review of neurophysiological testing Neurosurg. Focus / Volume 16 / April, 2004

Physiological factors such as skin temperature, age, height, sex have direct effects on action potential propagation. The only one that can be modified is the temperature. When lowering the temperature of the nerve the amount of current required to generate an action potential will increase. Decreasing the temperature affects the protein components. Also it causes a delay in opening and closing of the gates. In general, this means an increase in the AP (action potential) latency, amplitude and CV.

Question 6.

Voluntary contraction of a muscle is accompanied by electrical activity in all the contracting muscle fibres. Each time a motor unit (MU) is activated and its constituent muscle fibres contract,'' a complex electrical signal, the “motor unit potential” (MUP), can be detected by a needle electrode placed in the midst of the contacting muscle fibres''. MUP = Σ Muscle AP's

Surface electrodes, in general, are better than intracellular electrodes because its onset latency indicates the conduction time of the fastest fibers. The use of a needle electrode improves the recordings from small muscles because the needle electrode registers only a small portion of the muscle action potential with less interference from neighboring discharges. Needle electrodes yield a more sensitive signal. Routine recording of the sensory nerve action potential, in general, is performed with surface ring electrodes, which provide adequate and reproducible information non-invasively. The main difference is that when using intracellular electrodes the graph will start recording at minus values. This is because this type or testing is very specific and enters the muscle cells and the electricity in the cells is minus. Also the F wave between the two types of recording data is very different. This is because of the number of the fibres that have been stimulated. When using extracellular electrodes the area tested is very generic compared with the one tested when using intracellular electrodes, which is extremely specific.

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