Nerve

Nerves are enclosed, cable-like bundles of axons that form the fundamental conduits of the peripheral nervous system. They provide pathways through which electrochemical impulses travel between the central nervous system and the body’s organs, muscles and sensory receptors. Each nerve contains many axons—extensions of individual neurons—supported by specialised connective tissues and glial cells that protect and insulate the fibres.

Structural Organisation

A nerve is composed of several layers of connective tissue that form a robust, protective framework:

• Endoneurium surrounds each individual axon. This delicate sheath includes an inner glycocalyx layer and an outer mesh of fine collagen fibres.
• Perineurium encloses bundles of axons known as fascicles. It consists of flattened cells arranged to create a protective barrier, helping to regulate the nerve’s internal environment.
• Epineurium forms the outermost layer, a dense sheath that binds all fascicles together and offers mechanical strength.

Axons within nerves may be myelinated by Schwann cells, which increase the speed of impulse conduction. Within the endoneurium, fibres are bathed in endoneurial fluid, which functions analogously to cerebrospinal fluid and contributes to a blood–nerve barrier that restricts the passage of molecules.
Nerves frequently travel beside blood vessels because neuronal tissue has high metabolic demands. Injury or irritation can lead to increased endoneurial fluid, detectable on imaging techniques such as magnetic resonance neurography.

Categories of Nerves

Nerves are classified according to the direction of impulse transmission:

• Afferent nerves carry sensory information from the periphery to the central nervous system.
• Efferent nerves transmit motor commands from the central nervous system to muscles and glands.
• Mixed nerves contain both sensory and motor fibres. All spinal nerves and several cranial nerves fall into this category.

They are further divided based on their connections to the central nervous system:

• Spinal nerves, which emerge from the spinal cord and innervate most of the body.
• Cranial nerves, which connect directly to the brain and supply structures of the head and neck. These are designated by Roman numerals and descriptive names.

The term innervation refers to the supply of nerve fibres to a particular organ or region. Nerves may act on the same side of the body as their origin (ipsilateral action) or cross to affect the opposite side (contralateral action.

Development and Regeneration

Nerve growth is most active during childhood and adolescence, although certain molecular pathways—such as notch signalling—can reactivate growth under specific conditions. When a peripheral axon is damaged but the neuron’s cell body remains intact, regeneration is possible.
Regeneration follows a defined process:

1. The axon distal to the injury site degenerates.
2. Schwann cells and the neurilemma form a regeneration tube.
3. Nerve growth factors stimulate the sprouting of new axonal processes.
4. A sprout that enters the regeneration tube grows toward the original target.

Although regeneration can restore function, it is slow and seldom perfect, often leaving some degree of impairment.

Function and Impulse Transmission

Nerves conduct action potentials, which are rapid electrochemical signals transmitted along axons. Myelinated fibres can conduct impulses at speeds up to around 120 m/s. At synapses, electrical signals convert into chemical signals before being reconverted to electrical form within the next neuron. This process underlies communication throughout the nervous system, allowing organisms to respond to internal and external stimuli.
The nervous system comprises the central nervous system, including the brain and spinal cord, and the peripheral nervous system, which contains the nerves connecting the CNS to the rest of the body.

Clinical Significance

Neurological examination provides the first step in diagnosing nerve disorders, assessing reflexes, coordination, sensation and muscle strength. Additional tests such as electromyography, nerve conduction studies and computed tomography may follow.
Nerve damage can result from:

• Mechanical injury or compression, including pinched nerves
• Repetitive strain disorders such as carpal tunnel syndrome
• Autoimmune diseases like Guillain–Barré syndrome
• Neurodegenerative conditions and polyneuropathies
• Infections, diabetes or vascular impairment

Symptoms may include pain, numbness, weakness or paralysis. Referred pain occurs when altered signalling causes symptoms in locations distant from the injury. Some cancers invade nerve sheaths, a phenomenon seen in prostate, colorectal and head and neck cancers. Diseases such as multiple sclerosis affect the myelin sheath, disrupting nerve conduction.

Nerves in Other Animals

In vertebrates, identified neurons—neurons with unique properties across all individuals of a species—are rare. In simpler nervous systems, however, many neurons are uniquely identifiable. Among vertebrates, the best-known examples are the Mauthner cells in fish. Each fish possesses two of these giant brainstem neurons, whose powerful synapses initiate a rapid escape reflex known as the C-start response. A single action potential in a Mauthner cell can trigger an immediate, large-scale movement, illustrating the efficiency and precision of specialised neural circuits.
Nerves and neurons thus play critical roles across species, forming the basis for sensation, movement and coordinated behaviour.