axondiameter

Axon diameter is the width of a neuron's axon, usually measured in micrometers (μm). It is a key structural parameter that influences how quickly electrical signals travel along the axon and can also reflect metabolic and developmental state. Across vertebrates, axon diameters span a broad range, from roughly 0.2 μm in small unmyelinated fibers to over 20 μm in large myelinated fibers. In peripheral nerves, larger-diameter axons are typically associated with faster conduction and greater reliance on rapid reflexes.

Conduction velocity, the speed at which action potentials propagate, is strongly linked to axon diameter. In unmyelinated fibers, velocity increases approximately in proportion to diameter. In myelinated fibers, saltatory conduction dramatically raises speed, and velocity also rises with diameter, though the relationship is more complex because myelination and internodal length play major roles. Consequently, thicker axons generally support faster signaling, which is important for motor control, sensory timing, and rapid neural communication.

Axon diameter varies by neuron type and developmental stage. Large-diameter, heavily myelinated motor and somatosensory axons (for example, Aα and Aβ fibers) transmit signals rapidly, whereas small-diameter, lightly myelinated or unmyelinated fibers (such as Aδ and C fibers) conduct more slowly. Diameter also changes with aging, injury, and disease, where axonal atrophy or degeneration can reduce conduction speed and alter neural function.

Measurement methods include histology and electron microscopy for fixed tissue, and in vivo estimates from nerve conduction studies or diffusion-based imaging techniques.axon diameter remains a fundamental determinant of neural timing and network performance, shaping how information is integrated and acted upon by the nervous system.