Three questions about action potentials (in an experiment)

[rosetyler]

Hi!

 

In our physiology class, we were working with the Simnerv program, where you experiment with the n. ischiadicus of a frog. In the first experiment, we were stimulating the nerve with different amplitudes: 20, 40, 60 ... mV. The amplitude of the action potential was increasing with the greater amplitude of the stimulus. My question is why? Isn't it so that the amplitude of the action potential is not dependent on the amplitude of the stimulus? That it's all-or-nothing?

 

My second question is why do we need two stimulating and two recording electrodes on the nerve? Isn't one of each enough? I imagine that you put one stimulating electrode on one end of the nerve and activate it, and the signal gets to the other end of the nerve where the recording electrode is. Am I being too simplistic?

And why does the shape of the AP change if we change the distance between the electrodes?

 

Thank you!

[Vignesh Loganathan]

i too have same questions regarding tis....

[OneHalfGiraffe]

1. Amplitude

The intensity of a stimulus affects the frequency of the action potential. So, a weak stimulus like 20 mV will cause a small frequency, or a low number of action potentials whereas a strong stimulus like 60 mV will cause a large frequency, or a high number of action potentials. Note that the size of the action potential does not become larger with a strong stimulus; rather, the

intensity of the stimulus is related to the frequency of action potentials. This is known as the rate law.

Yes, action potentials follow the "all-or-nothing" principle where the signal is either too weak or strong enough to cause a neuron to fire. If the stimulus is strong enough to reach the threshold, then a neuron will fire. The frequency of the action potential is not dependent on the stimulus, you are correct, and this is because the amount of excitation beyond the threshold does not cause a neuron to fire any differently than it would if there were just enough excitability to reach the threshold. It's the number of times a neuron fires in a specific time period that's important, not the intensity of the stimulus.

So if you were to measure the number of action potentials that occurred during a 3 second period at 20, 40 and 60 mV, you would want to count the number of times the neuron fired which would determine if the stimulus was weak or strong.

In summary, the intensity of a stimulus affects the frequency of the action potential up to the point where it meets the threshold (all-or-nothing), causes the neuron to fire, and the weaker/stronger the stimulus the less/more times the neuron will fire (rate law).

2. Electrode

This site may answer your questions regarding number and placement of electrodes: http://www.medicine.mcgill.ca/physio/vlab/CAP/recording.htm

Hope this helps!

Edited December 23, 2012 by OneHalfGiraffe

[rosetyler]

Thanks for the answer! But I then realized that the increased amplitude because of the stimulus meant the increased amplitude for the entire n. ischiadicus and not only one neuron. So problem solved!