3. An action potential will propagate from its site of initiation in the cell bo

Question

3. An action potential will propagate from its site of initiation in the cell body of a spinal cord motor neuron to the presynaptic terminal at the neuromuscular endplate (i.e. anterograde propagation), but upon depolarizing the presynaptic terminal the action potential does not repropagate back to the cell body (i.e., retrograde propagation) Why is this? A. Not enough time is available to restore the loss of intracellular Na that occurs during the anterograde action B. Nerve axons are incapable of propagating an action .C. Distal regions of nerve axons are incapable of initiating D. Na channel inactivation prevents the depolarized nerve potential to allow for immediate retrograde propagation. potential in a retrograde direction. action potentials. action from immediately propagating in a retrograde direction E. Distal regions of nerve axons lack K' channels, and therefore cannot repolarize to a potential sufficient to be re excited again.

Explanation / Answer

The correct answer is: E. Distal region of the nerve exons lack K+ ion channels, and therefore cannot repolarize to a potential sufficient to be re-excited again.

The retrograde propagation can also be understood in terms of backfire of action neurons are there, the depolarization of previous neurons occur due to voltage gated ion channels open and the concentration of K+ ions make a movement towards the pre-synaptic neuron and initialize the action potential again, which is retrograde propagation.

But in case of neuromuscular junction, where a presynaptic motor neuron and a muscle interact, due to absence of threshold concentration of potassium ions at the muscular junction part, the depolarization is not able to initiate again and thus retrograde propagation is not initiated in the neuromuscular junction. Only anterograde propagation occurs.

The main reason lies in the concentration of potassium ions.

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