Axon: Definition, Structure & Function in Neural Signaling

An **axon** (nerve fiber) is the single, typically long projection of a neuron that conducts electrical impulses (action potentials) away from the cell body (soma) to target cells.

Key Structural Features

• Axon Hillock: Integration zone; lowest threshold for action potential initiation (high Naᵥ channel density).
• Initial Segment: Adjacent to hillock; action potential generation.
• Myelin Sheath: Schwann cells (PNS) / oligodendrocytes (CNS) wrap axon in lipid-rich layers → saltatory conduction.
• Nodes of Ranvier: Gaps between myelin segments; voltage-gated Na⁺/K⁺ channels concentrated → action potential "jumps."
• Axon Terminals (Terminal Boutons): Presynaptic endings containing synaptic vesicles with neurotransmitter.

Functional Properties

• Conduction Velocity: Myelinated: 50–100 m/s; Unmyelinated: 0.5–2 m/s.
• Diameter: Larger diameter → faster conduction (lower axial resistance).
• All-or-None: Action potential amplitude constant; frequency codes intensity.
• Directionality: Orthodromic (soma → terminal); antidromic possible experimentally.

Axonal Transport

• Anterograde (kinesin): Vesicles, mitochondria, proteins from soma → terminal.
• Retrograde (dynein): Endosomes, signaling molecules (NGF, viruses) from terminal → soma.

Clinical Relevance

• Demyelination (MS, Guillain-Barré) → conduction block, slowing.
• Axonopathy (diabetes, chemotherapy, trauma) → Wallerian degeneration.
• Nerve conduction studies (NCS) assess axonal integrity.

The axon is the neuron's output cable; its integrity defines neural circuit function.