- An axon is a long, slender projection of a neuron (nerve cell) that conducts electrical impulses away from the neuron’s cell body (soma) toward other neurons, muscle cells, or glands.
- It is one of the key structures in the nervous system responsible for the rapid transmission of action potentials, which are electrical signals that enable communication within and between different parts of the body. The axon plays an essential role in integrating sensory input and motor output, coordinating both voluntary and involuntary functions.
- Typically, a neuron has a single axon, which can vary greatly in length—from a few micrometers in the brain to over a meter in the human sciatic nerve. The axon originates at a specialized region of the cell body known as the axon hillock, where incoming signals from the dendrites and soma are integrated. If the combined signals exceed a certain threshold, an action potential is initiated and travels down the axon to its terminal branches.
- Many axons are surrounded by a myelin sheath, a fatty insulating layer formed by Schwann cells in the peripheral nervous system (PNS) and by oligodendrocytes in the central nervous system (CNS). Myelin dramatically increases the speed and efficiency of electrical conduction through saltatory conduction, where the action potential “jumps” between exposed regions called nodes of Ranvier. This mechanism allows for faster signal transmission compared to unmyelinated axons, which conduct impulses more slowly and continuously along their membrane.
- At the distal end of the axon are the axon terminals or synaptic boutons, which form synapses with target cells. Here, the electrical signal is converted into a chemical signal: neurotransmitters are released into the synaptic cleft and bind to receptors on the next cell, triggering a new electrical response or a biochemical change. This process is central to how the brain processes information, how muscles contract, and how glands secrete substances.
- Axons are supported internally by a cytoskeletal framework made up of microtubules, neurofilaments, and actin filaments, which maintain their shape and facilitate the transport of cellular materials. This axonal transport system is essential for moving proteins, organelles, and other substances between the soma and the axon terminal. Disruptions in axonal transport are associated with several neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS).
- Damage to axons, whether from trauma, ischemia, toxins, or disease, can lead to impaired nerve function or even permanent loss of sensation and movement. In the PNS, axons have some ability to regenerate, guided by Schwann cells and their extracellular matrix. However, in the CNS, axonal regeneration is limited, which is why spinal cord injuries and brain damage often result in lasting deficits.
