Question

An action potential can be propagated in either direction along the surface of an axon. The direction it follows under physiological conditions is determined by a consistent polarity at most synapses

What does polarity mean in this context? Is it an electrical polarity or a molecular one?

Answer 1

Polarity in this context would mean both structural and electrical.

With respect to structural polarity, The structure of the nerve cell determines which direction the action potential would take. Although technically an action potential could travel through an axon both ways, there are mechanisms (discussed below) which in most cases only allow the action potential to move in one direction, from the synapse to the dendrites,the soma, the axon hillock, the axonal body and finally the axon terminal and then through the synapse of the next neuron. This is the direction of the action potential in most cases.

An action potential happens when the voltage in the dendrite becomes less negative, this is called as depolarization. This causes an influx of sodium due to the opening of voltage gated channels. the sodium moves like wave down the length of the axon, because the depolarization caused by the sodium opens subsequent gates to let in more sodium and the depolarization spreads down the axon to the next synapse, at which point potassium channels open and potassium rushes out and the voltage becomes more negative, this is called hyperpolarization.

The sodium channels once opened cannot be reopened unless they are reset by hyperpolarization at the end of an action potential. This is called a refractory period and this prevents bidirectional propogation of action potential. this would answer the question about polarity and direction of action potential.

There is a phenomenon called neuronal backpropogation where the action potential travels in either direction of the axon. But that is still poorly understood. The above mentioned sequence still holds good in most cases of neuronal transmission.