Figure 1.

Diagram of a myelinated axon illustrating ion channels, pumps and exchangers responsible for determining axonal excitability. Transient Na⁺ channels (Na_t) are clustered at high density at the node of Ranvier, with persistent Na⁺ channels (Na_p) and slow K⁺ channels (K_s) contributing to excitability and resting membrane potential. Fast K⁺ channels (K_f) are located at highest density at the juxtaparanode, acting to limit re-excitation of the node following an action potential. Internodal conductances include voltage-independent ‘leak’ conductances (Lk) and hyperpolarization-activated cation conductance (I_H). The Na⁺–K⁺ pump (Na⁺/K⁺-ATPase) utilises energy to maintain the electrochemical gradient necessary for impulse conduction by removing 3 Na⁺ ions for every 2 K⁺ ions pumped into the axon. The Na⁺–Ca2⁺ exchanger exports Ca2⁺ ions and imports Na⁺, driven by the electrochemical Na⁺ gradient. Paranodal myelin terminal loops are depicted with anchoring proteins to form paranodal junctions at the paranodal region.