7.2 Action Potentials and Impulse Conduction

An unstimulated neuron typically has a membrane potential near −70 mV: the cytoplasmic side is negative relative to the outside. The value varies among cell types. Unequal ion distributions, selective resting permeability and intracellular negative molecules all contribute. The sodium–potassium pump uses ATP to export three Na⁺ for every two K⁺ imported, maintaining the concentration gradients on which repeated signalling depends.

At rest, K⁺ leak channels make the membrane more permeable to K⁺ than to Na⁺. K⁺ tends to leave down its concentration gradient, leaving unbalanced negative charge behind, while the growing electrical attraction opposes further loss. Membrane voltage reflects this balance of chemical and electrical forces. The pump is essential over time, but the rapid phases of one action potential are produced mainly by ions moving through voltage-gated channels.

Myelinated axons conduct impulses much faster than non-myelinated fibres of similar diameter. Myelin reduces current loss between exposed nodes, so the signal can cover longer distances between regions where the action potential is regenerated. Increasing axon diameter also increases speed by reducing internal resistance.

Giant squid achieve relatively rapid conduction with exceptionally wide unmyelinated axons, but myelinated vertebrate axons achieve higher speeds with far less space. Conduction speed is positively correlated with axon diameter, and myelinated fibres are faster than non-myelinated fibres of comparable size. A correlation describes the relationship; it does not by itself identify every causal mechanism.