6.2 Neuronal Membranes and Saltatory Conduction

A neuron has dendrites that receive signals, a cell body that integrates them and an axon specialized for transmission. At rest its interior is about −70 mV relative to outside. The Na⁺/K⁺ pump establishes unequal concentrations, while selective leak channels—especially for K⁺—allow charge separation. The potential is an electrical difference across an extremely thin membrane, not a uniform charge throughout all cytoplasm.

A sufficient depolarization opens voltage-gated Na⁺ channels. Na⁺ enters down its electrochemical gradient and depolarizes the next membrane region, creating a self-propagating action potential. Na⁺ channels then inactivate and voltage-gated K⁺ channels open; K⁺ leaves and repolarizes the membrane. The pump maintains long-term gradients but does not move ions quickly enough to create each millisecond voltage change.

An action potential is all-or-none once threshold is crossed. A stronger stimulus does not make one action potential taller; it can increase firing frequency or recruit more neurons. Refractory periods caused by channel states prevent immediate re-excitation and strongly favour one-way propagation.

Schwann cells in peripheral nerves wrap axons in multiple lipid-rich membrane layers. Myelin increases electrical resistance and reduces charge loss across internodes. Voltage-gated channels cluster at gaps called nodes of Ranvier, so action potentials are regenerated mainly at nodes. Local current spreads rapidly beneath myelin and the impulse appears to jump node to node: saltatory conduction.

Larger axon diameter reduces internal resistance, and myelination reduces the membrane area that must repeatedly depolarize. Together they can raise conduction speed from around metres per second to tens or more than one hundred metres per second. Damage to myelin slows or blocks transmission even when the axon itself remains present.