Propagation of Action Potentials

When the membrane at the trigger zone is depolarized to threshold, voltage-gated sodium channels open to initiate an action potential. This depolarization occurs at the axon's initial segment, where a high concentration of sodium channels allows swift depolarization. Once threshold is reached, sodium ions rush inward, progressively depolarizing the cell membrane and propagating the nerve impulse.

Continuous propagation occurs in unmyelinated axons, where depolarization and repolarization happen continuously in small steps as sodium channels open sequentially along the entire membrane. Saltatory conduction in myelinated axons is faster because the action potential jumps from one node to the next, skipping the myelinated segments. This jumping mechanism allows impulses to propagate much faster in myelinated axons than unmyelinated ones.

Sodium channels become inactivated at peak depolarization, creating an absolute refractory period during which they cannot reopen. This inactivation prevents depolarization from reversing direction, ensuring the action potential propagates only toward the axon terminals. This directional propagation preserves neuronal polarity and maintains proper signal transmission through the nervous system.

In myelinated axons, nodes of Ranvier are gaps where the myelin sheath is absent and voltage-gated ion channels are concentrated. The first action potential at a node creates an ionic current that opens ion channels at the next node, allowing the action potential to jump between nodes. The precise placement of nodes ensures the membrane remains sufficiently depolarized for the impulse to continue propagating rapidly.

The absolute refractory period occurs when sodium channels become inactivated at peak depolarization, preventing them from reopening briefly. This ensures that once an action potential is initiated, it propagates forward along the axon as a self-propagating process with constant velocity. The refractory period prevents backward propagation and guarantees unidirectional transmission of the nerve impulse.

The speed of conduction is influenced by whether the axon is myelinated or unmyelinated, and by the axon's diameter. Myelinated axons conduct faster via saltatory conduction than unmyelinated axons using continuous propagation. Larger diameter axons offer less resistance to ionic current flow, allowing faster propagation of the action potential along the nerve fiber.

When voltage-gated sodium channels open, sodium ions move down the electrochemical gradient and flow inward through the membrane. These sodium ions depolarize the adjacent region of the axon, triggering the opening of sodium channels in that part of the membrane. This sequential opening of channels propagates the action potential along the axon as depolarization spreads progressively.