Human Nerve Conduction Experiment

Introduction

 Nerve conduction is a way of determining the strength and speed of the electrical impulse of a nerve. This can be used to determine the functionality of said nerve¹. Nerve fibers are capable of producing action potentials which are carried through the length of the nerve to cause some response. In this case, a muscle contraction was the response. These action potentials begin from some stimulus like a mild shock. As the strength of the stimulus is increased, the action potential is increased as well, generating a larger response. This can ultimately be used to determine if the nerve is damaged or be used as an indication of various conditions and diseases when compared to normal nerve function². The test is carried out by placing electrodes on the skin. Two distinct types of electrodes are used: stimulating and recording. The stimulating electrodes provide a mild shock to “excite” the nerve and generate an action potential to allow the muscle to contract. The recording electrode measures the response³. Recordings are then gathered on a computer software and used to form a conclusion regarding the functionality of the nerve.

Methods

 The equipment was set up by connecting the IXTA to the computer via a provided USB cable, and also connecting it to a power source with the power supply cable. The amplifier was connected to the IXTA. To this, a set of three electrodes were connected. These three electrodes were used for data collection. A separate set of stimulating electrodes were connected to the IXTA. These two electrodes were used to provide a mild shock to stimulate muscle contraction. Electrodes were placed on the subjects in a similar manner.  The set of three were placed around the wrist and the pinky of the right hand. The first electrode (red) was placed on the right pinky just above the first knuckle. The second electrode (black) was placed at the base of the pinky finger. The third electrode (green) was placed at the crease of the wrist. The stimulating electrodes were placed further up the arm about halfway between the elbow and the wrist. The two sets were placed adjacent to each other to form somewhat of a tetrad shape on the ulnar edge of the arm. Two sets were used, because different exercises required different placement of the electrodes. This will be discussed in each individual exercise below. For all three exercises the same two subjects were used. Both subjects cleaned the areas where the electrodes would be placed prior to placement of the electrodes. Subjects were asked to remain calm with their arm resting on the lab bench.

Exercise 1

 This exercise was to determine at what minimum stimulus the muscle would respond and what maximum stimulus the muscle would respond. This was done by placing the electrodes on the subjects as discussed previously. The stimulating electrodes were placed on the set of pads closest to the wrist with the red electrode on the top and the black electrode on the bottom. This left the pads closer to the elbow to be blank. Initially, the stimulus was set at 0mA and no response was observed. For the remaining trials, for both subjects, the amplitude was increased by 0.5mA. The amplitude was continuously increased for each trial until a maximum point was reached. At this point, the graph was beginning to level out indicating that the muscle was contracting at its full potential and no greater stimulus would cause it to contract more.

Exercise 2

 The purpose of Exercise 2 was to determine if the time of the muscle contraction would be delayed if there was a reverse in the polarity of the electrodes. This was done by placing the electrodes as discussed in Exercise 1. The results from Exercise 1 were used to determine the maximum amplitude at which the muscle would contract. This amplitude was used in Exercise 2. After the first trial, the positive and negative electrodes were switched. This put the red electrode on the bottom and the black electrode on the top. This second trial was ran with the same maximum amplitude as the first trial. Note: The maximum amplitude was different for both subjects.

Exercise 3

 This exercise was to determine the conduction velocity of the muscle. This was done by placing the electrodes as discussed in Exercise 1. The maximum amplitude was used to record the first trial. For the second trial, the electrodes were pushed back to the blank set. Normal polarity was kept for this exercise. The second trial used the maximum amplitude as well. Afterward the distance between the distal and proximal stimulating electrodes was recorded in order to calculate the conduction velocity.

Results

Exercise 1

Figure 1: Raw data from Exercise 1 for Subject 1. This demonstrates the maximum value for muscle contraction. Maximum value: 5mA (trial 11.1).

Figure 2: Raw data from Exercise 1 for Subject 2. This demonstrates the maximum value for muscle contraction. Maximum value: 6mA (trial 13.1).

Graph 1: Graphical comparison between both subjects in Exercise 1.

Exercise 2

Graph 2: Graphical comparison between both subjects in Exercise 2. For Subject 1, the delay was 0.8ms. For Subject 2, the delay was 0.45ms.

Exercise 3

Graph 3: Graphical comparison between both subjects in Exercise 3. For Subject 1, the change in velocity was 13.4 m/s. For Subject 2, the change in velocity was 31.8m/s.

Discussion

 Discussion of the results follows. For Exercise 1, the minimum muscle response and the maximum muscle response was to be recorded for both subjects. This information was then used for the following exercises. In looking at the raw data for Subject 1 (Figure1), it was clear where the maximum response occurred. The data has a positive slop until trial ten. Trial eleven is at a similar point, and then the data begins to drift downward. Trial eleven was said to be the maximum response, as this is where there was no change, and this was also the highest placement of the data. This value corresponds to a stimulus value of 5mA and a response value of 0.722mV. For subject 2, the highest data point was recorded as the maximum value, although the raw data did not level out as with Subject 1. This occurred at trial thirteen (Figure 2) which corresponds to a stimulus value of 6mA and a response value of 0.288mV. In looking at Graph 1, the comparison shows that both subjects follow a linear trend indicating that the response does in fact increase with an increase in stimulus. For Subject 1 (shown in blue), there is somewhat of a plateau at the top of the graph before the response values begin to drift downwards. This indicates that the muscle reached a maximum contraction point. For Subject 2 (shown in red), the data seems to have a hill-like shape, where the response values seem to increase and then decrease and then increase again, with two rather large values at the very end of the graph. However, between stimulus points (shown on the x-axis) 4.5 and 6mA, the response values seem to be closer together and begin to level out. This is something that was not viewed in the raw data (Figure 2). Still, at 6mA the maximum value is reached, with the exception of the final two values, which are much higher than other data values. Also at 6mA, the graph is somewhat level, with no major dramatic changes before this value. The fluctuation in the response values for Subject 2 could be due to a number of things. However, most likely, not enough time was allotted between trials. The small amount of time between trials probably did not give the muscle long enough to fully relax before the next trial began. Whereas Subject 1, possibly had a few more seconds between trials which would allow for the muscle to fully relax before continuing on. The amplitude of the action potential increases as the stimulus increases until a maximum response is achieved. The response of each muscle fiber increases as well, again until a maximum is reached.

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 The purpose of Exercise 2 was to determine if changing the polarity of the electrodes changes the nerve conduction time. Graph 2 shows a comparison between both subjects for this exercise. Both subjects show an increase in latency time with reversed polarity. Subject 1 (shown in blue) increased from 1.6ms for normal polarity to 2.4ms for reversed polarity. This is a 0.8ms change. Subject 2 (shown in red) increased from 0.55ms for normal polarity to 1.0ms for reversed polarity, a 0.45ms change. In comparing the two subjects, Subject 1 had almost a doubled increase compared to Subject 2. The latency of the muscle response increases when there is a reverse in polarity. This causes a delay, and the positive and negative charges of the electrodes are reversed and do not connect as well with the charges of the cells.

 Exercise 3 was used to determine the conduction velocity. Proper nerve function has a conduction velocity between 20 m/s and 130 m/s. In viewing Graph 3, both subjects have an increase in conduction velocity when the electrodes were moved to the distal location. Subject 1 (shown in blue) increased from 53.3m/s to 66.7m/s. This is an increase of 13.4m/s. Subject 2 (shown in red) increased from 57.1m/s to 88.9m/s. This is an increase of 31.8m/s. This increase is due to an increased distance between the two trials. Also, the signal had to cover more distance in almost the same amount of time. Although not shown, change in time between the two trials were nearly negligible. Subject 1 had a decrease in time of 0.15ms. Subject 2 had a decrease in time of 0.25ms.

 Both subjects seemed to have followed similar trends for each exercise. However, some of the data varied between the two subjects. With any experiment, some error is associated. It is important to note that the two subjects were two different people. Everyone is different, and this could account for some of the variance. More specifically, perhaps the electrodes were not exactly placed the same for both subjects. Electrodes were placed in a similar manner, but no exact measurements were taken. Stress levels and general fatigue of each subject could have affected the data. The amount of muscle mass for each individual is another factor. There is a possibility that the ulnar nerve was not stimulated in one subject, but was stimulated in the other. There is a possibility that the ulnar nerve was not stimulated in either subject. A more probable cause would be errors in data analysis. It is possible that data analysis was not done precisely and this could skew the data, resulting in the variances seen in the exercises.

Conclusion

 Human nerve conduction is a way of determining the functionality of a human nerve. For this report, the ulnar nerve was tested. Exercise 1 tested how many mA would elicit an initial response and a maximum response. Results showed that the initial was the same for both subjects, but the stimulus was different for the maximum response in both subjects. Exercise 2 demonstrated how polarity of stimulus electrodes affects muscle response. In both subjects, a slight delay in muscle response was shown with reversed polarity. Exercise 3 determined the conduction velocity. For both subjects, conduction velocity increased with a more distal placement of the electrodes. This experiment ultimately demonstrated that muscles reach a maximum point of contraction, because the action potential also has a maximum threshold. There is an average range in which conduction velocity lies for most people. Anything outside of this range can be used as an indication of nerve damage or some other issue with the nerve. Overall, both subjects followed similar trends for all three exercises. Both subjects seemed to have fell within the average values for nerve conduction, indicating functional nerves.

Literature Cited

1.)   Lava, N. 2016. What is an EMG and Nerve Conduction Study [Internet]? WebMD; [cited 2018 Oct 14]. Available from https://www.webmd.com/brain/emg-and-nerve-conduction-study#1

2.)   No Author. Experiment HN-3: Human Nerve Conduction. Lab Handout.

3.)   No author. Nerve Conduction Studies [Internet]. Baltimore (MD): Johns Hopkins; [cited 2018 Oct 18]. Available from https://www.hopkinsmedicine.org/healthlibrary/test_procedures/neurological/nerve_conduction_velocity_ncv_92,P07657