The axon is the long, cable-like extension of a nerve cell that carries electrical signals away from the cell body and delivers them to other neurons, muscles, or glands. Some axons are microscopic, while others stretch up to one meter in length, like those running from the base of the spine down to the foot. It is the part of the neuron responsible for long-distance communication throughout the nervous system.
Basic Structure of the Axon
A neuron has three main parts: the cell body (which houses the nucleus), the dendrites (short branches that receive incoming signals), and the axon (a single long fiber that sends signals outward). The axon begins at a small mound on the cell body called the axon hillock, which transitions into the axon initial segment. This initial segment is where the neuron “decides” whether an incoming signal is strong enough to fire.
At the far end, the axon splits into smaller branches that end in tiny swellings called axon terminals. These terminals sit extremely close to the next cell, separated by a microscopic gap called the synapse. When an electrical signal reaches the terminal, it triggers the release of chemical messengers called neurotransmitters, converting the electrical signal into a chemical one that crosses the gap and stimulates the neighboring cell.
How Signals Travel Along the Axon
The electrical signal that moves down an axon is called an action potential. It works through a rapid chain reaction involving charged particles, primarily sodium and potassium. At rest, the inside of the axon is negatively charged compared to the outside. When a signal arrives that’s strong enough, channels in the membrane snap open and allow positively charged sodium to rush in. This flips the local charge from negative to positive in about one millisecond.
That burst of positive charge triggers the next stretch of membrane to open its own sodium channels, and the signal rolls forward like a wave. Almost immediately behind it, potassium channels open and potassium flows out, restoring the negative charge. The potassium channels are slightly slower to close than the sodium channels are, so the membrane briefly dips below its normal resting charge before settling back. This brief dip, called hyperpolarization, prevents the signal from traveling backward.
The Myelin Sheath and Signal Speed
Many axons are wrapped in a fatty insulating layer called the myelin sheath. In the brain and spinal cord, myelin is produced by cells called oligodendrocytes. In the rest of the body, cells called Schwann cells do the job. The myelin doesn’t cover the axon continuously. Instead, it wraps around it in segments, leaving tiny exposed gaps called nodes of Ranvier spaced along the length.
This arrangement dramatically speeds up signal transmission. Instead of traveling as a slow, continuous wave, the electrical signal jumps from one node to the next, a process called saltatory conduction. At each node, the action potential is regenerated at full strength, so the signal doesn’t weaken over distance. The result is striking: unmyelinated axons conduct signals at roughly 0.5 to 10 meters per second, while myelinated axons can reach speeds up to 150 meters per second. That’s the difference between a signal taking a full second to travel from your spine to your toe versus arriving almost instantly.
Two physical properties control how fast an axon conducts. The first is whether it’s myelinated. The second is its diameter. Thicker axons conduct faster because they offer less internal resistance to electrical flow. Research using advanced MRI techniques has found that axon diameter and the ratio of axon thickness to total fiber thickness (including myelin) account for about 85% of the variation in conduction speed.
The Internal Scaffolding
An axon that stretches a meter long but measures only a fraction of a millimeter wide needs serious structural support. Three types of protein filaments form an internal skeleton that keeps the axon intact and functional. Microtubules are hollow tubes built from protein subunits that run the length of the axon, all oriented in the same direction with their growing ends pointing toward the axon tip. This uniform orientation is unique to axons (in dendrites, microtubules point in mixed directions) and is essential for organized transport.
Neurofilaments are the most abundant structural filaments in axons. Their primary job is controlling axon diameter, which directly influences conduction speed. The third component, actin, forms evenly spaced rings beneath the axon’s outer membrane, connected by a protein called spectrin at intervals of roughly 180 to 190 nanometers. These rings act like the hoops of a barrel, maintaining the axon’s cylindrical shape.
How the Axon Moves Cargo
Because the cell body manufactures most of the proteins and organelles a neuron needs, these materials must be shipped down the full length of the axon to reach the terminals. This is no small feat in a motor neuron whose axon is a meter long. The transport system relies on motor proteins that walk along microtubules carrying cargo.
Proteins in the kinesin family carry cargo away from the cell body toward the axon tip (anterograde transport). Dynein carries cargo in the opposite direction, back toward the cell body (retrograde transport), returning used materials and signaling molecules. Fast transport moves membrane-bound organelles, proteins, and genetic material at rates of 50 to 400 millimeters per day. Slow transport carries structural components like cytoskeletal fragments at less than 8 millimeters per day. Both speeds are essential: fast transport keeps the synapse supplied with neurotransmitter-filled vesicles, while slow transport maintains the axon’s structural framework.
What Happens at the Axon Terminal
When an action potential arrives at the axon terminal, it opens calcium channels in the membrane. Calcium floods in from outside the cell, where its concentration is much higher. This calcium influx triggers tiny vesicles filled with neurotransmitters to fuse with the terminal’s membrane and spill their contents into the synaptic gap. The released neurotransmitters drift across and bind to receptors on the next cell, either exciting it toward firing its own action potential or inhibiting it from doing so. This conversion from electrical to chemical signaling is how neurons communicate across the gaps between them.
What Happens When Axons Are Damaged
Axon damage is a central feature of several neurological conditions. In multiple sclerosis, the immune system attacks the myelin sheath, but the axons themselves also sustain significant injury. Axonal loss is actually the main driver of permanent disability in MS, not just the loss of myelin. When an axon is cut or severely damaged, the segment separated from the cell body degenerates in a process called Wallerian degeneration. The disconnected portion breaks down progressively from the injury site outward because it’s been cut off from the cell body’s supply of proteins and energy.
Even before outright severing occurs, axons show signs of distress. Transport disruptions cause proteins to pile up at injury sites, and structural proteins in the axon’s skeleton lose their normal chemical modifications, weakening the fiber. Research in MS tissue has found that these transport disturbances and structural changes appear not only within active disease lesions but also in surrounding tissue that looks normal on imaging, suggesting axonal damage extends beyond the visible areas of inflammation. Because mature neurons in the central nervous system have very limited ability to regenerate axons, this damage tends to be permanent, which is why preserving axonal health is a major focus in neurology.