The nervous system relies on specialized cells called neurons to transmit information. Each neuron is composed of a cell body, dendrites that receive signals, and a single, elongated fiber that sends signals outward. This outbound fiber is the nerve axon, a structure that functions as the primary transmission line for the nervous system, enabling coordinated movement, sensory perception, and cognitive function.
Defining the Nerve Axon
A nerve axon is the long projection of a nerve cell that conducts electrical impulses away from the neuron’s cell body, or soma. Its purpose is to transmit information to other neurons, muscles, and glands. While a neuron has only one axon, its length varies greatly; some in the sciatic nerve stretch from the base of the spinal cord to the toes, while others are less than a millimeter long.
Axons are structurally distinct from dendrites, which are typically shorter, more branched, and function to receive signals. In contrast, the axon maintains a more constant diameter along its length and acts as the neuron’s sole output channel. This structure allows it to carry electrochemical signals to its intended target.
The axon is a metabolically active part of the cell that depends on a system known as axonal transport. This process moves essential molecules and organelles from the cell body to the distant axon terminals, supplying the synapse with the neurotransmitters and enzymes necessary for communication.
Anatomy of an Axon
The structure of an axon is adapted for its function of signal transmission. It originates from the neuron’s cell body at a specialized region called the axon hillock. This area sums up incoming signals, and if the total signal reaches a certain threshold, an electrical impulse is generated.
The axon is a tail-like structure filled with cytoplasm, referred to as axoplasm, and enclosed by a cell membrane known as the axolemma. Many axons are insulated by a fatty substance called myelin, which forms a protective layer called the myelin sheath. This sheath is not continuous but is interrupted by small gaps called the nodes of Ranvier.
Myelin is produced by different types of glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The axon terminates in several small branches, each ending in a swelling known as an axon terminal. It is at these terminals that the neuron communicates with its target cell by releasing chemical messengers called neurotransmitters across a synapse.
Signal Transmission Along the Axon
The signal transmitted along an axon is an electrical impulse called an action potential. Once initiated at the axon hillock, it travels down the entire length of the axon without diminishing in strength. The action potential is generated by the rapid movement of sodium (Na+) and potassium (K+) ions across the axolemma through voltage-gated ion channels.
When a neuron is at rest, there is a stable electrical difference across its membrane, known as the resting membrane potential. A sufficiently strong stimulus triggers the voltage-gated sodium channels at the axon hillock to open, allowing sodium ions into the cell. This influx of positive charge causes a rapid reversal of the membrane potential, a phase called depolarization, which propagates the action potential down the axon.
In myelinated axons, this process is accelerated through saltatory conduction. The myelin sheath acts as an insulator, preventing ion leakage, so the electrical current flows quickly underneath the myelinated segments until it reaches a node of Ranvier. At these nodes, which are rich in voltage-gated Na+ channels, the action potential is regenerated to its full strength before “jumping” to the next node. This method is faster and more energy-efficient than continuous conduction in unmyelinated axons.
Axonal Damage and Related Disorders
Disruption to the structure or function of nerve axons can have serious neurological consequences, as it interrupts the flow of information within the nervous system. Axonal damage can stem from traumatic injuries, compression of nerves, metabolic problems, or neurodegenerative diseases. The specific symptoms depend on the type and location of the affected neurons but can include loss of sensation, muscle weakness, and impaired coordination.
Multiple sclerosis (MS) is an example where the immune system attacks the myelin sheath in the central nervous system, which slows or blocks nerve signal conduction. Peripheral neuropathies involve damage to the axons of the peripheral nervous system and can be caused by conditions like diabetes or inherited disorders such as Charcot-Marie-Tooth disease.
Neurodegenerative diseases like Amyotrophic Lateral Sclerosis (ALS) and Parkinson’s Disease also feature significant axonal degeneration. In ALS, the deterioration of motor neuron axons leads to progressive muscle paralysis. The capacity for repair after injury differs between the central (CNS) and peripheral (PNS) nervous systems, as axon damage within the CNS is often permanent.