Axon structure, signaling, and development

Axon function and information flow

An axon is a long, slender projection of a neuron that typically carries information away from the cell body by transmitting electrical impulses called action potentials. Its overall job is to deliver signals to other neurons, muscles, or glands. In sensory pathways, some neurons are pseudounipolar, where the impulse travels from the periphery to the cell body and then to the spinal cord along branches of the same axon. Axons are distinct from dendrites: dendrites mainly receive signals, while axons mainly transmit them. Axons are covered by the axolemma, and their internal cytoplasm (axoplasm) supports branching into telodendria and ending in axon terminals that form synapses with target cells. Information flow along an axon depends on specialized axonal regions and structures. The axonal region includes the axon hillock and the axonal initial segment (AIS), where action potentials are initiated. The AIS is unmyelinated and highly specialized, containing a high density of voltage-gated sodium channels and scaffold proteins that organize these channels; its length and position can change, tuning neuronal excitability. Myelination by glial cells (Schwann cells in the PNS, oligodendrocytes in the CNS) speeds conduction via saltatory conduction, where action potentials effectively “jump” between nodes of Ranvier—short unmyelinated gaps that can regenerate the signal. Once an action potential reaches the presynaptic terminal, it triggers neurotransmitter release through calcium-dependent vesicle fusion, allowing electrochemical communication across the synaptic cleft to excite, inhibit, or otherwise modulate the postsynaptic target. Axonal transport also supports this flow by moving materials: anterograde transport delivers components from the cell body to terminals, while retrograde transport returns waste to the cell body.

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Axon versus dendrite distinctions

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