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enter image description here

A diagram is presented as such above. The question given states What would be the effect of stimulation to cause a nerve impulse with a microelectrode at the middle of the axon?

I thought the nerve impulse only travels in one direction to the muscle fibres, but the book says "A nerve impulse would pass in both directions."

Why is that the case?

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Allow me to do a silly analogy: think about the electric wire going from your outlet to your computer, conducting electricity. If you make two cuts in this wire, rotate it 180 degrees and weld it again at the cutting points, what will happen? It will conduct electricity just as it did before.

The same happens to an axon (but please have in mind that this is just an analogy, an action potential is not an electric current). That is, the axon doesn't determine or influence the direction of the action potential. If you cut a piece of that axon, rotate it 180 degrees and join it back in the cutting points, it will conduct the action potential the same way.

That being said, imagine that you stimulate that axon at a given point. There will be two action potentials, going to opposite ways:

enter image description here unmyelinated (A) and myelinated (B) nerve cells

In your figure (given that is a motor neuron), if you stimulate that axon in the middle, the action potential going to the neuromuscular junction, which is the normal direction, is called orthodromic (from the greek orthos, "proper", and edramon, past of "run"), while the one going to the soma (the perikaryon) is called antidromic (from the greek anti, "against").

According to Oh (2003):

The sensory nerve conduction study measures the conduction of the nerve impulse along the sensory nerves. The routine method measures the conduction velocity of the large diameter sensory nerve fibers of the nerve being tested. There are two methods of obtaining sensory nerve action potentials (SNAP), orthodromic and antidromic. The orthodromic method includes recording of the sensory nerve action potential proximally and stimulating the nerve distally whereas in the antidromic method the location of the stimulating and recording electrodes is reversed. The latency and conduction velocities are identical with the orthodromic and antidromic methods if the recording and stimulating electrode positions are constant. (emphasis mine)

In a simplified scenario, the orthodromic impulse will reach the neuromuscular junction and stimulate the muscle contraction, while the antidromic impulse will be canceled by another orthodromic impulse or, if it can reach the dendrites, it will simply end, since there will be no neurotransmitter release.

Source: Oh, Shin J., 2002. Clinical Electromyography: Nerve Conduction Studies. Third. LWW.

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I think Gerardo has given a very nice answer, there is just one think I would like to add:

The only reason it usually DOESN'T travel antidromically is because it is initiated at the axon initial segment at the soma. Once the action potential starts travelling (orthodromically) down the axon, it is directly followed by a section of the AP that is in the absolute refractory state. Thus, the action potential cannot turn around mid-way or start spreading into both directions.

However, when you stimulate the middle of an axon, there is (initially) no part of the axon in a refractory state which would keep the action potential from travelling in one direction or another. Both sides of the axon are ready to propagate the action potential, which is why it travels in both directions.

From the Wikipedia page on Action Potentials (under Propagation > Refractory period)

The absolute refractory period is largely responsible for the unidirectional propagation of action potentials along axons. At any given moment, the patch of axon behind the actively spiking part is refractory, but the patch in front, not having been activated recently, is capable of being stimulated by the depolarization from the action potential.

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