Essay on Compound Action Potential

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The basis of life on earth is formed by neurons and nerve impulses. In this case, the internal processes of the body and the sense of the environment are all involved in complex steps of output and input of the nerve cells (Agrawal, 2016). By definition, neurons refer to the specialized cells that help in communicating with each other, as well as organs and muscle, through electrical synapses throughout the entire body. The neuro is made of the cell body (soma), dendrites, and the axon. On the other, nerves consist of a bundle of axons and nerve fibers. A phenomenon called an action potential can produce by each of the axon located in the nerve (Lollo, 2016).

Practically, it is possible to elicit an action potential in each axon simultaneously using electrical stimulation. The process of electrical simulation is called CAP (compound action potential) which involves a collection of the artificial response of nerve. It is expected that the action potential’s amplitude for each of the individual axon does not change with the intensity of stimulus but rises with the CAP’s amplitude at higher stimulus voltages. Remember, other axons are incorporated in the amplitude of the CAP within the same nerve that reaches their threshold (Mishra & Stringer, 2010).

This experiment primarily aims at dissecting out and examine the sciatic nerve of a frog. This largest nerve in the human being will be taken and measured based on the readings from extracellular. The lab also helps in getting a better understanding of the way of interpreting compound action potentials. Another objective of this lab is to assist in exploring how sodium/ potassium pump causes the action potentials. Overall, this experiment purposely aims to investigate the stimulus voltage’s effects on a sciatic nerve frog for observing the temporal summation, curves of the strength-duration, threshold phenomena, velocity of conduction, as well as the refractory periods.


This exercise’s primary goal is to apply a brief stimulus at the nerve’s proximal end as well as record a compound action potential from the end of the distal. This procedure was carried out in four exercises. In the first exercise, the effects of the voltage of stimulus on the compound action potential were investigated. Here, the slider bar was used to change the stimulus amplitude to 0.00 Volts (zero). Afterward, “start” was clicked to record a 30ms sweep before observing a flat line. Moreover, a marker that identifies the sweep was added. The stimulus amplitude was then changed to 0.05 Volts after which the “Start” was clicked to change the title to 0.05V. The stimulus amplitude was increased continuously in 0.05V increments before dropping the fresh solution of Ringer on the nerve to prevent desiccation.

In the second exercise, the primary task involved investigating the behavior of the saline soaked string versus the CAP. The negative (white) recording electrode was moved back one or two pins before stimulating the nerve by the use of voltage that produces the maximum CAP. After that, the lead cable was transferred to the negative (white) recording electrode one or two pins closer to the negative stimulating electrode. In the same way, the positive (red) recording electrode was moved in a similar direction for two pins to maintain the distance between the recording electrodes. At this point, a new sweep was recorded before removing the nerve.

The third exercise involved investigating the effect of temperature on CAP properties’ conduction velocity. In this exercise, the stimulus voltage was still set to the amount that produces the maximum CAP. For the first warm assay, the nerve was soaked for about 2 minutes in the warm Ringer that was taken from the water bath on the room’s side. Subsequently, the nerve was soaked in chilled saline solution from the refrigerator for about 2 minutes before placing it back in the chamber.

Finally, the sciatic nerve for investigated whether it is axon directional. Here, the orientation of the nerve was reversed in the chamber to make the stimulating electrodes be on the end of the distal of the nerve. “Start” was clicked to stimulate the nerve. Also, the sweep was renamed for indicating the direction of stimulation. Again, the orientation of the nerve was flipped several times to help in recording multiple CAPs.


Table 1: Exercise 1 – The Action is none potential because as stimulus increases so does the compound action potential


CAP (peak to peak) amplitude - Dependent













Table 2: Exercise 2 - Measured time from stimulation to peak

Delta D

Delta T



0.01 m

0.001 s

10 m/s


0.01 m

0.00002 s

500 m/s

Delta T for String = 0.02ms (given by TA because change in time is so fast it's almost immeasurable)

Table 3: Exercised 3: Conducting Velocities

Delta D

Delta T



37 C

0.01 m

0.6 x 10-4 s

23.154 mV

166.67 m/s


0 .01 m

7.8 x 10-4 s

11.728 mV

12.82 m/s

Room temp

0.01 m

1.2 x 10-4 s

14.24 mV

83.33 m/s

Table 4: Exercise 4 at Room Temperature



Reversed nerve 1

0.00082 s

15.686 mV

Reversed nerve 2

0.00078 s

15.77 mV

Normal 1

0.00088 s

23.50 mV

Normal 2

0.00096 s

21.93 mV

Figure 1: Amplitude of Stimulus and Action Potential

Figure 2: Relationship between the Amplitude and Stimulus

            As already noted, the action is none potential because as stimulus increases so does the compound action potential. Also, the action potentials are not propagated at the same speed as electrical current travels through the salty liquid. In reality, the action potential increases to about 35mV as more sodium ions leave. As a result, the sodium pump is closed (Nordqvist, 2009). In the same way, the potassium ions leave the axon as the pump get closed. In effect, the potential is brought back to the resting state. An increase in temperature results in a rise in the speed of conduction of the CAP.

In general, the firing rates of the action potential are very sensitive to temperature. In this case, the channels take time to open to allow the ions to cool neuron as their travel across the membrane thus reducing the speed of the action potentials. However, the action potential speeds increase as the channels take less time to open. Evidently, the axon can conduct action potential in the direction of the cell to cell and postsynaptic to the postsynaptic neuron. In the cell to cell neuron, the action potential is conducted through the potential differences that are maintained across the cell membrane (Lollo, 2016). In the same way, inside the sending cell's axon terminal, several synaptic vesicles are filled with the neurotransmitter molecules.


Figure 1 above shows that the action potential in a single axon increases with an increase in amplitude as more sodium leaves the system. A biphasic recording of a compound has both negative and positive deflections but for different reasons. Conversely, the intracellular recording is simply a technique of electrophysiology that uses a microelectrode that is usually inserted in a single cell such as neuron to measure its electrical activity.

Based on the calculated conduction velocity, a nerve impulse from the big toe can travel to the spinal cord of 1.64 m tall person at a speed of 280 miles per hour (448 kilometers per hour. In the Nernst equation, the T increases with a decrease in E-cell (Agrawal, 2016). In the experimental findings, the temperature also increases velocity and amplitude as shown in Table 3 above. The electrical properties of ion channels change with the temperature. Nothing will happen if a nerve is stimulated in the middle of an axon because the action potential goes to the neuromuscular junction (Lollo, 2016).


This lab achieved its primary goal of investigating the stimulus voltage's effects on a sciatic nerve frog for observing the temporal summation, curves of the strength-duration, threshold phenomena, velocity of conduction, as well as the refractory periods. It is noted that an action potential is "none potential" because fails to deteriorate as it travels down the axon. Either the potassium or sodium pump helps in governing the action potential. In the first place, the channels of sodium open up allowing for an outflow of its ions into the cell thus causing depolarization. The potential increases to about 35mV as more sodium ions leave. As a result, the sodium pump is closed. In the same way, the potassium ions leave the axon as the pump get closed. In effect, the potential is brought back to the resting state. Often, the potassium pump causes an overshoot because it does not close quickly enough. Such hyperpolarization of overshoot makes brief drop beneath the resting potential.


Agrawal, K. (2016). A Rare Cause of Sciatic Pain: Multiple Schwannoma of Sciatic Nerve. Journal of Medical Science and Clinical Research, 04(11), 13683-13385.

Lollo, L. (2016). Combined Saphenous-Sciatic Nerve Blockade Superior to Femoral-Sciatic Nerve Blockade for Postoperative Analgesia Following Foot and Ankle Surgery. Journal of Anesthesia & Critical Care: Open Access, 3(2).

Mishra, P., & Stringer, M. (2010). Sciatic nerve injury from intramuscular injection: a persistent and global problem. International Journal of Clinical Practice, 64(11), 1573-1579.

Nordqvist, P. (2009). The Action of Hyaluronidase on Frog Sciatic Nerve, with Special Reference to Penetration of Procaine. Acta Pharmacologica Et Toxicologica, 8(3), 195-206.

August 09, 2023
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Cell Membrane

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