Axon

An axon is a long, slender projection extending from a neuron that conducts electrical impulses away from the cell body. Essential to the communication network of the vertebrate nervous system, axons serve as the primary transmission routes through which neurons relay information to other nerve cells, muscles, and glands. They display distinctive structural and functional features that set them apart from dendrites, the other main type of neuronal extension.

Structure and Morphology of Axons

Axons typically maintain a uniform diameter along their length, in contrast to the tapering structure of dendrites. They may arise from the cell body or from a dendrite, the latter forming what are known as axon-carrying dendrites. A neuron never possesses more than one axon, though invertebrate axons may contain multiple semi-independent functional regions.
The axon is enveloped by a specialised membrane termed the axolemma, and its internal cytoplasm, known as axoplasm, contains microtubules and neurofilaments that support structural integrity and enable transport. At their distal end, axons arborise into numerous branches known as telodendria, each terminating in an axon terminal or endfoot. These terminals participate in synapses, the specialised junctions through which signals are conveyed to other neurons, muscle fibres, or gland cells. Synapses may occur:

• At terminal endfeet.
• Along the axon shaft in the form of en passant boutons, which can number in the hundreds or thousands along a single axon.
• Occasionally onto the same neuron that produced the axon, forming an autapse.

A collection of axons constitutes a nerve tract in the central nervous system (CNS) and a nerve fascicle in the peripheral nervous system (PNS). The largest white-matter tract in placental mammals, including humans, is the corpus callosum, containing roughly 200 million axons and linking the two cerebral hemispheres.

Functional Role in Neural Transmission

Axons are essential for propagating action potentials, the rapid electrical impulses that serve as the communication currency of the nervous system. In sensory neurons such as pseudounipolar cells, axons function as afferent nerve fibres, carrying signals from peripheral receptors towards the CNS. In motor pathways, axons transmit impulses away from the CNS to muscles and glands.
Axonal length varies widely. In humans, some axons measure only a millimetre, whereas others exceed a metre. The longest belong to the sciatic nerve, which extends from the lumbar spinal cord to the big toe. Axon diameter also varies considerably: most mammalian axons measure about 1 micrometre, while some reach 20 micrometres. Certain species exhibit exceptionally large axons, such as the squid giant axon, nearly 1 millimetre thick, adapted for rapid conduction.

Myelinated and Unmyelinated Axons

Axons can be classified as myelinated or unmyelinated. Myelination involves the formation of a fatty insulating sheath composed of:

• Schwann cells in the PNS.
• Oligodendrocytes in the CNS.

This sheath increases conduction efficiency by enabling saltatory conduction, where action potentials jump between exposed gaps known as nodes of Ranvier. Myelinated axons form the bulk of white matter, while neuronal cell bodies and dendrites constitute grey matter. Bundles of myelinated fibres that cross the midline of the brain are called commissural fibres, with the corpus callosum being the largest example.

Classification of Nerve Fibres

Nerve fibres may be classified functionally and physiologically:

• Group A, B, and C fibres:
  • Group A and B fibres are myelinated.
  • Group C fibres are unmyelinated.
  • These groups include both sensory and motor fibres.
• Type I–IV sensory fibres:
  • This classification is based on sensory function and conduction velocity.

Both systems aid in understanding the behaviour of axons in health and disease.

Axonal Region and Initial Segment

The axonal compartment comprises the axon hillock, initial segment, the axon shaft, telodendria, terminals, and associated myelin. The axon hillock marks the transition from soma to axon and plays a crucial part in integrating incoming signals. The axon initial segment (AIS) is a highly specialised, unmyelinated region containing large numbers of voltage-gated sodium channels. It is the primary site for action potential initiation.
Key features of the AIS include:

• A length typically between 20 and 60 micrometres.
• A dense array of sodium channels anchored by cytoskeletal proteins such as Ankyrin-G.
• Structural plasticity, allowing adjustments in location and length to modulate neuronal excitability.

This plasticity ensures stable function within neural circuits and supports neuronal polarity, where dendrites receive input and the axon delivers output.

Axonal Branching and Connectivity

Axons frequently form elaborate networks. In the CNS, axons often branch extensively, creating numerous synaptic connections. For example, cerebellar granule cells generate two parallel fibres from a T-shaped branch point. Such branching enables a single axon to influence multiple neural targets and coordinate complex responses.

Axonal Transport Mechanisms

Efficient communication between the soma and distal axon terminals requires robust transport systems. In the axoplasm, microtubules and neurofilaments support axonal transport, which moves:

• Structural proteins for axon growth.
• Vesicles containing neurotransmitters.
• Organelles such as mitochondria.
• Waste materials for recycling or degradation.

This bidirectional transport is vital for maintaining axonal health and function.

Axonal Disorders and Clinical Relevance

Axonal dysfunction contributes to many neurological conditions affecting both the CNS and PNS. Disorders may arise from degeneration, impaired myelination, trauma, metabolic disturbances, or inherited defects. Examples include:

• Peripheral neuropathies.
• Demyelinating diseases such as multiple sclerosis.
• Axonal injury associated with trauma.
• Genetic conditions affecting ion channels or cytoskeletal proteins.