CONDUCTION VELOCITY

The velocity with which the sciatic nerve conducts the compound nerve action potential along its length can be determined by measuring the time taken for the CNAP to be conducted over a certain distance.

Thus, if the negative stimulating electrode is set at a distance (d mm) from the positive recording electrode, the latency (x ms) of the CNAP gives the time for the CNAP to be conducted over that distance. The latency is the time from the start of the stimulus artefact to the start of the CNAP.

The velocity can then be calculated from the equation:

Conduction velocity (mm/ms) = d / x

Stimulate the nerve moving the positive recording electrode either closer to or further away from the negative stimulating electrode to make a number of calculations of conduction velocity. For each separation distance measure the latency and time to peak of the CNAP.

Movement of the positive recording electrode, either closer to or further away from the negative stimulating electrode, allows a number of velocity calculations to be made so that a mean value can be determined.

Since the individual fibres within the sciatic nerve will have different conduction velocities, the CNAP could be regarded as representing a simple distribution of the conduction velocities of these fibres.

Thus, the latency of the CNAP gives a measure of the time for the fastest conducting fibres to conduct action potentials over the distance (d). The time taken to the end of the CNAP gives the time taken by the slowest fibres and the time to the peak gives a measure of the mean time.

After the experiment...

1. Calculate the conduction velocities in:
   1. the fastest conducting fibres,
   2. the sciatic nerve as a whole,
   3. the slowest conducting fibres.
   
   (Express velocities in metres/second.)
   
   
2. List the factors which determine the conduction velocity of a nerve fibre.
3. How does the conduction velocity of the frog sciatic nerve compare to that of mammals? Give reasons for any differences.