Pathophysiology
Clinical meaning
Neuronal signaling is the fundamental electrochemical communication process of the nervous system, involving generation, propagation, and synaptic transmission of electrical impulses. At rest, neurons maintain a resting membrane potential of approximately -70 mV, established by the Na+/K+-ATPase pump (3 Na+ out, 2 K+ in per cycle) and the selective permeability of the membrane to K+ through leak channels (described by the Goldman equation). When a stimulus depolarizes the membrane to threshold (approximately -55 mV), voltage-gated sodium channels undergo rapid conformational change, opening and allowing Na+ influx that further depolarizes the membrane to approximately +30 mV (the rising phase of the action potential). Sodium channels then inactivate (ball-and-chain mechanism), and voltage-gated potassium channels open with a slight delay, allowing K+ efflux that repolarizes the membrane (falling phase). Brief hyperpolarization (undershoot to approximately -80 mV) occurs as K+ channels close slowly, creating the absolute and relative refractory periods that ensure unidirectional propagation. Action potential propagation along myelinated axons occurs via saltatory conduction: the myelin sheath (produced by Schwann cells in the PNS, oligodendrocytes in the CNS) insulates the axon between nodes of...