Axon

The axon is the transmission line of the nervous system. While dendrites receive signals and the cell body integrates them, it is the axon that carries the neuron's output — the action potential — over distances ranging from a fraction of a millimeter in local interneurons to over a meter in motor neurons projecting from the spinal cord to the foot. Understanding axonal structure and function is essential for explaining how the brain communicates with itself and with the rest of the body, and why neurological damage can have such devastating consequences.

Key Structures

• Neurons (throughout nervous system) — The electrically excitable cells that process and transmit information through electrical and chemical signaling.
• Dendrites — The branching extensions of a neuron that receive electrochemical signals from other neurons and conduct them toward the cell body for integration.
• Myelin — The fatty insulating sheath that wraps around axons, dramatically increasing the speed of neural signal conduction and enabling the rapid information processing that complex cognition requires.

Key Functions

Conducts action potentials from the soma to the axon terminals, enabling rapid long-distance neural communication.

Structure and the Action Potential

An axon emerges from the cell body at a specialized region called the axon hillock, where the threshold for firing an action potential is lowest. Once initiated, the action potential — a brief reversal of electrical polarity driven by the sequential opening of voltage-gated sodium and potassium channels — propagates down the axon in an all-or-none fashion. The signal does not diminish with distance, a remarkable property that allows reliable communication across long neural pathways. At the axon terminal, the electrical signal triggers the release of neurotransmitters into the synaptic cleft.

Myelination and Saltatory Conduction

Many axons are wrapped in myelin, a fatty insulating sheath produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Myelin dramatically increases conduction velocity by forcing the action potential to "jump" between gaps in the sheath called nodes of Ranvier — a process known as saltatory conduction. Unmyelinated axons conduct at roughly 0.5–2 meters per second, while myelinated axons can reach 120 meters per second. This hundred-fold speed increase is critical for time-sensitive functions such as motor coordination and sensory processing.

Axonal Damage and Regeneration

When axons are severed, the portion disconnected from the cell body undergoes Wallerian degeneration. In the peripheral nervous system, Schwann cells can guide regrowth at approximately 1 millimeter per day, allowing partial recovery from nerve injuries. In the central nervous system, however, inhibitory molecules in myelin debris and glial scar tissue largely prevent regeneration — explaining why spinal cord injuries typically cause permanent paralysis. Overcoming these barriers to central axonal regeneration is one of the major goals of contemporary neuroscience research.

Disorders

• Multiple sclerosis (myelin sheath destruction) — Autoimmune demyelinating disease causing varied neurological symptoms; cognitive deficits in processing speed, memory, and executive function.
• ALS (motor axon degeneration)
• Peripheral neuropathy