Compound action potentials (CAPs) recorded from mixed peripheral nerves display distinct peaks—labeled A, B, and C—which correspond to different populations of nerve fibers based on their size and conduction velocity. These peaks are sensitive to mechanical pressure applied to the nerve, affecting the ability of corresponding fibers to conduct signals.

Understanding the A, B, and C Peaks of CAPs

• A Peak: Represents large diameter, heavily myelinated fibers (Group A fibers) with fast conduction velocity.

• B Peak: Represents intermediate diameter myelinated fibers (Group B fibers), which include autonomic preganglionic fibers.

• C Peak: Represents small diameter, unmyelinated fibers (Group C fibers) with slow conduction velocity, typically responsible for pain and temperature sensation.

Effects of Mechanical Pressure on CAP Peaks

Mechanical pressure affects nerve fibers differently depending on their diameter and myelination:

• Large diameter A fibers tend to be more sensitive to pressure; thus, the A peak can be inhibited at lower intensities of pressure.

• Small diameter C fibers are more resistant to pressure and require higher intensity of pressure for inhibition to occur.

• Intermediate B fibers require pressure levels between those affecting A and C fibers but typically are less sensitive to low-pressure effects than large A fibers.

Evaluating the Statements:

• A. ‘A’ peak was inhibited by lower intensity of pressure: Correct. Large fibers (A) are more vulnerable to mild compression.

• B. ‘C’ peak was inhibited by higher intensity of pressure: Correct. Small fibers (C) are more resistant and inhibited only at higher pressures.

• C. ‘B’ peak was inhibited by lower intensity of pressure: Incorrect. Intermediate fibers require medium or higher pressure, not lower intensity.

• D. ‘C’ peak was inhibited by lower intensity of pressure: Incorrect, as C fibers are inhibited only by higher pressure.

• E. ‘A’ peak was inhibited by higher intensity of pressure: Incorrect because the A peak is inhibited at lower pressure intensities.

Correct Combination of Statements

The correct statements are:

(1) A and B

Conclusion

In summary, increasing mechanical pressure on a mammalian mixed peripheral nerve first inhibits the conduction of large diameter myelinated fibers, reflected in the loss of the A peak at lower pressures. The small diameter unmyelinated fibers, reflected in the C peak, are inhibited only at higher pressure intensities. Intermediate B fibers show effects at pressure intensities higher than those affecting A fibers but lower than those affecting C fibers.