Question

I have some questions that I need answe for them

1) Action potentials are said to be “all or none” responses. Why does the CAP that you measure in the frog sciatic nerve give a graded response?

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2) Briefly describe the absolute refractory period in relation to V-gated sodium channels, V-gated potassium channels, and the membrane permeability to those ions.

3) When you determined the relative refractory period, you actually recorded the interval when a few neurons entered the relative refractory period. Explain why this interval signifies the beginning of the relative refractory period and not the absolute refractory period.

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Answer 1

Answer 1:

Action potential is generated in a single neuron after a threshold is reached, whereas CAP or compound action potential is generated in a bundle of neurons (like the sciatic nerve) in response to a stimuli. Action potential is "all or none" - this means that a neuron will give a fixed response when a threshold is reached, or not give any response if threshold is not reached. The magnitude of response does not depend on magnitude of stimuli. A graded response means that the magnitude of response to a stimuli, depends on the strength of stimuli : if the stimuli is strong, response will be higher, and if the stimuli is weak, then response will be low.

Now, the CAP of a frog sciatic nerve gives a graded response because sciatic nerve is made up of a number of neurons, and the response shown by the sciatic nerve depends on how many neurons of the nerve are firing action potential at a given point of time.

If fewer neurons are generating action potential, then response shown by sciatic nerve will be of lower intensity. However, when a larger number of neurons are generating action potential, then the nerve will show a response of higher intensity. Thus, CAP of frog sciatic nerve gives a graded response.