16.18: Propagation of Action Potentials

The propagation of an action potential refers to the process by which a nerve impulse, or "action potential," travels along a neuron.

Neurons (nerve cells) have a resting membrane potential, with a slightly negative charge inside compared to outside. This is maintained by ion channels, such as sodium (Na⁺) and potassium (K⁺⁾ channels, which control the flow of ions. When a stimulus, like a touch or a signal from another neuron, triggers the neuron, sodium channels open, allowing sodium ions to rush into the neuron, causing depolarization.

If the depolarization is strong enough and reaches a certain threshold, it triggers an action potential. The initiation of an action potential occurs at the axon's beginning, or the initial segment, where a high concentration of voltage-gated Na⁺ channels allows a swift depolarization. As the depolarization advances along the axon, more Na⁺ channels open, facilitating the spread of the action potential. This is achieved as Na⁺ ions flow inwards, progressively depolarizing the cell membrane.

However, the Na⁺ channels become inactivated at peak depolarization, rendering them unopenable for a brief period, known as the absolute refractory period. As a result, any depolarization attempting to reverse direction is null, ensuring that the action potential's propagation is towards the axon terminals, thereby preserving neuronal polarity.

This propagation method applies to unmyelinated axons. In myelinated axons, the process differs. The depolarization spreads optimally due to the absence of constant Na⁺ channel opening along the axon segment. The precise placement of nodes ensures the membrane remains sufficiently depolarized at the next node.

Propagation in unmyelinated axons, known as continuous conduction, is slower due to the constant influx of Na⁺. In contrast, myelinated axons exhibit saltatory conduction - a faster method as the action potential leaps node to node, renewing the depolarized membrane. Furthermore, the speed of conduction can be influenced by the axon's diameter, a concept known as resistance.

The Propagation of an action potential is the process by which a nerve impulse is transmitted along the axon. It is a self-propagating process; once initiated, an action potential propagates with constant velocity.

When the membrane at the trigger zone is depolarized to a threshold, the voltage-gated sodium channels open to initiate an action potential.

These sodium ions move down the axon, depolarizing the adjacent region. This triggers the opening of sodium channels in that part of the membrane, propagating the action potential.

This type of propagation is called continuous propagation because the depolarization and repolarization of each segment happen continuously in a series of small steps. This mode of propagation is seen in unmyelinated axons.

In contrast, in the myelinated axons, the action potential propagates by saltatory propagation. Here, the first action potential at a node creates an ionic current that opens the ion channels at the next node.

Since the action potential can jump from one node to another, saltatory conduction allows the impulse to propagate faster in a myelinated axon than in an unmyelinated axon.