Axon

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Neuron

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Human brain right dissected lateral view description

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Neuron Cell Body

Myelinated neuron

Myelin sheath (1)

Axon is a long, slender projection of a neuron, or nerve cell, that typically conducts electrical impulses away from the neuron's cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In the nervous system, axons can vary greatly in length, from a fraction of an inch to several feet.

Structure

An axon is one of the several types of cytoplasmic protrusions from the cell body of the neuron; other types include dendrites and a single axon hillock. The axon hillock is the area of the cell body from which the axon originates. The structure of an axon is divided into four main parts: the axon hillock, the initial segment, the axon proper, and the axon terminal.

Axon Hillock

The Axon hillock is the cone-shaped area on the cell body from which the axon originates. It is considered the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon.

Initial Segment

The initial segment is the part of the axon closest to the axon hillock. It is the region of the axon that has the lowest threshold for electrical stimulation, thus serving as the axon's trigger zone for action potentials.

Axon Proper

The axon proper, or axon shaft, is the long, slender part of the axon that extends from the initial segment to the axon terminals. It is responsible for the conduction of the action potential along the neuron.

Axon Terminal

The axon terminal, or terminal bouton, is the end part of the axon that makes synaptic contacts with other neurons or effector cells (muscles, glands). This is where the transmission of signals to other cells occurs.

Function

The primary function of an axon is to conduct electrical impulses away from the neuron's cell body. These impulses, known as action potentials, are critical for the communication between neurons within the nervous system. Axons can also have additional roles, including the transport of materials between the cell body and the synapses at the axon terminals, a process known as axonal transport.

Types of Axons

Axons can be classified based on their myelination or their diameter.

Myelinated Axons

Myelinated axons are covered by a fatty substance called myelin, which acts as an insulator and increases the speed at which electrical impulses are conducted along the axon. Myelinated axons are typically found in the white matter of the brain and spinal cord.

Unmyelinated Axons

Unmyelinated axons lack a myelin sheath. These axons are typically slower at conducting electrical impulses than myelinated axons. Unmyelinated axons are usually found in the grey matter of the brain and spinal cord.

Clinical Significance

Damage to axons can lead to a variety of neurological disorders, depending on the location and extent of the injury. Diseases such as multiple sclerosis are characterized by the degeneration of myelin sheaths in the central nervous system, which affects the conduction of action potentials along the axons.