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 the motor neurons that run from your spinal cord down to your feet. It is the primary output line of the neuron, and without it, your brain couldn’t communicate with the rest of your body.
Basic Structure of an Axon
A neuron has three main parts: the cell body (which houses the nucleus), branching extensions called dendrites that receive incoming signals, and the axon that sends signals outward. The axon begins at a region called the axon hillock, a small mound where the cell body tapers into the fiber. Just beyond the hillock sits the axon initial segment, a specialized stretch packed with ion channels that serves as the trigger zone for electrical signals. This is where the neuron “decides” whether an incoming signal is strong enough to fire.
From there, the axon extends outward, sometimes branching along its length, until it reaches its endpoints: small bulb-shaped structures called axon terminals. These terminals sit close to the next cell in line, separated by a tiny gap called the synapse. The whole structure, from hillock to terminal, can be as short as a fraction of a millimeter in the brain or as long as a meter in the peripheral nerves of your legs.
How Axons Differ From Dendrites
Dendrites and axons look and behave differently. Dendrites are typically short, heavily branched, and designed to receive signals from other neurons. Axons are longer, thinner, and built to transmit signals over distance. The classic model of information flow is straightforward: dendrites collect, the cell body processes, and the axon sends. Reality is more complex. Some dendrites can fire their own electrical signals and even release chemical messengers, and some axons form direct connections with other axons. But the general division of labor holds for most neurons in your body.
How Signals Travel Down the Axon
The electrical signal that moves along an axon is called an action potential. It works through a rapid chain reaction involving charged particles (ions) flowing in and out of the axon membrane. The process has three stages.
First, sodium ions rush into the axon through channels that snap open when the membrane reaches a certain voltage threshold. This influx of positive charge causes the inside of the axon to spike from its resting negative voltage to a positive one, a phase called depolarization that lasts about one millisecond. Second, slower potassium channels open and potassium ions flow out, pulling the voltage back down in a phase called repolarization. Third, because those potassium channels are slow to close, the voltage briefly dips below its normal resting level before settling back. This whole cycle then triggers the adjacent stretch of membrane to fire, sending the signal forward like a wave.
Why Myelin Makes Signals Faster
Many axons are wrapped in myelin, a fatty insulating layer produced by specialized support cells. Myelin doesn’t cover the axon continuously. Instead, it forms segments with small exposed gaps between them called nodes of Ranvier. Rather than crawling along every point on the membrane, the electrical signal jumps from node to node, a process called saltatory conduction. This dramatically increases speed: unmyelinated axons conduct signals at roughly 0.5 to 10 meters per second, while myelinated axons can reach up to 150 meters per second.
Myelination begins before birth, with early insulation appearing around 28 weeks of gestation in structures critical for reflexes and survival, like the brainstem and sensory pathways. It continues after birth and progresses from the brainstem forward into the higher brain regions. This prolonged development is one reason that motor coordination, decision-making, and other complex abilities take years to fully mature.
The Axon’s Internal Supply Chain
Because axons can be extraordinarily long, the cell body can’t simply rely on diffusion to supply the far end with proteins, energy molecules, and other essentials. Instead, axons run an active transport system along internal tracks made of protein filaments called microtubules.
Outbound transport (from the cell body toward the terminals) is driven by motor proteins from the kinesin family, which walk along microtubules carrying vesicles, organelles, and building materials at speeds up to about 1 micrometer per second, or roughly 400 millimeters per day. Inbound transport (from the terminals back to the cell body) is handled by a different motor protein called dynein, which hauls aging proteins and organelles back for recycling at similar speeds. There’s also a slower transport system, moving structural proteins like cytoskeletal components at less than 8 millimeters per day. If this supply chain breaks down, the axon degenerates, which is a feature of several neurodegenerative diseases.
What Happens at the Axon Terminal
When an action potential reaches the end of the axon, it triggers a cascade that converts the electrical signal into a chemical one. Voltage-sensitive calcium channels at the terminal open, allowing calcium ions to flood in. This calcium surge happens fast, and within a few hundred microseconds it activates specialized sensor proteins that cause tiny vesicles filled with chemical messengers (neurotransmitters) to fuse with the terminal membrane and spill their contents into the synaptic gap. Those neurotransmitters then bind to receptors on the next cell, either exciting it toward firing its own signal or inhibiting it from doing so.
This conversion from electrical to chemical signaling is the fundamental mechanism behind everything your nervous system does: moving a muscle, feeling pain, forming a memory, regulating your heartbeat.
Axon Damage and Regeneration
Axons in the peripheral nervous system, the nerves outside your brain and spinal cord, can regenerate after injury. The regrowth rate is relatively slow, typically 1 to 4 millimeters per day, which is governed by the speed of the axon’s internal transport system. This means recovery from a nerve injury in the arm or leg can take weeks to months depending on how far the new axon tip needs to grow to reach its target. Injuries closer to the hand or foot tend to recover better than those near the shoulder or hip simply because the distance is shorter.
Axons in the brain and spinal cord are a different story. The central nervous system contains molecular signals that actively inhibit regrowth, which is a major reason why spinal cord injuries and strokes cause lasting damage. Researchers have found that certain conditioning techniques can partially overcome these inhibitory signals, but reliable central nervous system axon repair remains one of the biggest unsolved problems in neuroscience.