The McGill Physiology Virtual Lab

Compound Action Potential

Conduction velocity> Difference and Absolute methods
 

The objective of this part of the lab is to measure the conduction velocity of several groups of fibres in the nerve, using two different methods.


 

 
 

Background

The variation in conduction velocity in the various fibres in the nerve is an important factor in determining the shape of the Compound Action Potential, since the conduction velocity of each fibre determines the latency of its contribution to the CAP.

Conduction velocity is systematically related to fibre diameter in fibres of a given type.  In myelinated frog nerve fibres, the relationship is approximately:

Velocity (m/s) = Diameter (Ámeters) x 2.5

The conduction velocity of nerve is normally reported in meters per second.  It is more easily recorded as millimeters per millisecond, which yields the same result.
 

Procedure: the difference method

The stimulus duration is set to 0.2 ms, and two pair of  recording leads are connected similar to the example below:
 
 

A maximal CAP is elicited, and the latencies (in ms) to the peaks of the responses are measured. This latency signifies the time it took for average fibres in the nerve to transmit their APs from the stimulating electrodes to the first recording electrode.

Recording from Channel A

(proximal electrode R1)

Recording from Channel B

(distal electrode R7)

In order to calculate a velocity measurement, we need a time and a distance measurement. Now if we subtract the latency determined proximally from the latency determined distally, we are getting a value for the time it took for the CAP to travel from electrode R1 to electrode R7.

Conduction velocity (difference method)

= (d2-d1)/(latencydistal - latencyproximal)

In summary, to calculate the conduction velocity by the Difference Method, we must take latency measurements using two different recording positions. The conduction distance is the distance between the first recording electrode for each position, and the conduction time is the difference between the latencies at the first recording electrode for each position. 
 

Procedure: the absolute method

The conduction velocity can also be calculated by the absolute method, which means that the velocity is calculated using a single latency and distance measurement.  Here we will calculate the absolute velocity at electrode R1.
In this case, we will need to measure the absolute distance travelled.  In the sample bath to the right, we measure from the cathode (the stimulating electrode nearest the recording electrodes) to the first recording electrode R1

Latency = latency at electrode R1 (ms)
d = distance (mm) from stimulating electrode to recording electrode R1

Velocity = d / latency (m/s) or (mm/ms)

One can also calculate the velocity for electrode R7: the same formula applies except that the distance d and the latency are of course longer.
 

Questions and answers

Q: How do the absolute and difference methods for calculating velocity compare?  What errors are involved in determining velocity with the frog sciatic nerve preparation?
A:  By using the difference method, you subtract out any "uncertainties" involved in the measurement of latencies.  For example, if we are uncertain as to where the AP's are actually originating within the vicinity of the stimulating electrodes, this "error" will be introduced into both latency measurements, and therefore subtracted out when performing a difference method calculation.  However, the difference method is only experimentally sound when one is dealing with the same population of nerve fibres over the recording electrodes used, which is not the case with the sciatic nerve, as it is a short nerve, and thin at one end.
     The non-uniformity of the nerve, and the difficulty in making accurate measurements of very small distances and latencies are principal points to consider when making conduction velocity measurements. Naturally if the nerve studied were longer and more uniform, we would improve the accuracy of our calculations.

Q:  Could we just as easily have measured velocities by taking a latency measurement to the peak of the negative phase of the CAP (instead of to the peak of the positive phase)?
A:  We could do the measurement using the peak of the negative phase of the CAP, but the negative phase is often smaller, overlapping with the positive phase, thus making measurements more difficult. If we did measure the latency at the negative phase, it would be a latency corresponding to the second recording electrode, and distance measurements would then have to be taken accordingly.

Click here to continue with the topic of conduction velocity (a-alpha and A-beta fibres)