You may have recalled from the lectures that action potentials are only produced if the membrane of the cell is depolarised to a 'threshold' value of membrane potential.

You will have found that below a certain stimulus voltage no EMG response was recorded (and the subject remained calm and motionless). Above this so-called 'threshold' voltage the subject's hand will twitch and the computer screen will show a 'biphasic' deflection.

As the stimulus strength was increased you will have found that the size of the response becomes larger until a maximum EMG response size is attained. Any further increase in stimulus size will not result in a larger EMG.

A graph of stimulus voltage against EMG amplitude might look something like this:

Figure 1 - Plot of stimulus strength (measured in volts) against EMG response size (measured in mV)

You can see that for this experiment the threshold voltage was between 30 and 32 volts. The maximum EMG response occurs with stimulus voltages above 45 V.

In the remaining sections I used a stimulus strength of 70 V which is clearly above the voltage required to elicit a maximal EMG response

An action potential is actively propagated along an excitable membrane. The speed or velocity at which an action potential travels can be measured simply be measuring how long it takes the action potential to cover a known distance (remember velocity = distance/time).

In this case the recording electrodes were in a fixed position and the position of the stimulating electrode was altered. At each position you were asked to record:

• the distance from the stimulating electrode to the recording electrode (in cm)
• the delay (latency) between the stimulus being delivered and the EMG response of the muscle

In fact you had to record the latency to the beginning of the EMG (onset) response and the peak of the EMG response - more on why later.

In the figure below you can see how, as the distance between the electrodes increases, the EMG lies increasingly to the right (longer latency)

Figure 2 - Data traces showing increasing latency for longer conduction distances

From these traces you measured the latency to the onset of the EMG and the latency to the peak of the EMG response. These data were to be plotted against the distance between the electrodes. The graph should look something like this:

Figure 3 - Plot of latency (onset & peak) against distance between the stimulating and recording electrodes

The normal range for conduction velocity in ulnar nerve is 50 to 60 meters per second.

You will recall that for a short period of time after the passage of an action potential the membrane is not excitable; it is said to be 'refractory'. The explanation for this is that the voltage-gated Na⁺ channels, whose opening is responsible for the upstroke of the action potential, become 'inactivated' during the action potential and must recover from this 'inactivated' state before they can open again. This takes time and only happens once the membrane has been returned (repolarised) to close to the resting potential.

Until the Na⁺ channels have recovered from inactivation the membrane will not be able to generate an action potential.

At some point in time enough Na⁺ channels will have recovered from inactivation to enable the membrane to generate an action potential. BUT the stimulus must be large enough to activate all the recovered Na⁺ channels and is in reality must be much larger than that required to stimulate a axon that has not recently conducted an action potential. This is usually explained in terms of the action potential 'threshold' being higher (requiring a greater depolarization). Click here if you want to remind yourself of thresholds, depolarization and excitability.

Thus, if 2 stimuli are applied to a nerve the second stimulus will elicit an action potential only:

• if the stimulus strength is very high (I used 70 volts when the maximum response to a single stimulus occurred at 45 V)
• after the absolute refractory period has past

You were instructed to apply pairs of pulses separated by 20, 15, 10, 5, 3, 2 and 1 ms. At 20 ms all the axons in the nerve should have recovered. At shorter times, however there may be evidence that not all the axons generated an action potential in response to the second pulse. Your data probably looked something like this:

Figure 4 - Data traces: paired pulses separated by different intervals

The compound action potential makes it difficult to see how large the second EMG responses are because each may start from a different level. That is to say that the first EMG starts from close to zero mV but subsequent EMGs may begin when the voltage is still quite negative or positive.

There is a way to deal with this problem and it is explained in your practical handbook notes.

Essentially what must be done is to subtract the first EMG response from the trace, leaving the 2nd response in isolation. The result of this subtraction is shown in the next figure:

Figure 5 - Data traces: EMG response to second stimulus shown in isolation after subtraction of first response

If you look at figure 5, particularly the traces for intervals of 2 and 1 ms (red & black lines) you will see that the second EMG response becomes smaller as the interval between the first and second pulses is reduced. The next figure shows these data graphically:

Figure 6 - Plot of the amplitude of the second EMG response (as a percentage of the first pulse) against the interval between the two stimuli

Hopefully, what you can see is that for intervals longer than 5 ms the response is almost the same amplitude as the first (within experimental error). At intervals shorter than 5 ms, however, the amplitude of the EMG response falls off sharply. This reflects the fact that fewer and fewer axons are able to generate an action potential as the interval shortens.

At 1 ms there is effectively no response. No action potentials - all the axons are in the period of absolute refractoriness.