nerve terminal

A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.

A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon, within the nerve, is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.

Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium. Nerve cells (often called neurons) are further classified as sensory, motor, or mixed nerves.

In the central nervous system, the analogous structures are known as nerve tracts.

Structure

Each nerve is covered on the outside by a dense sheath of connective tissue, the epineurium. Beneath this is a layer of fat cells, the perineurium, which forms a complete sleeve around a bundle of axons. Perineurial septae extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium. This forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres. Nerves are bundled and often travel along with blood vessels, since the neurons of a nerve have fairly high energy requirements.

Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid. This acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood-nerve barrier similar to the blood–brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized using magnetic resonance neurography, and thus MR neurography can identify nerve irritation and/or injury.

Categories

Nerves are categorized into three groups based on the direction that signals are conducted:

* Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin.
* Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands.
* Mixed nerves contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. All spinal nerves are mixed nerves, and some of the cranial nerves are also mixed nerves.
Nerves can be categorized into two groups based on where they connect to the central nervous system:
* Spinal nerves innervate (distribute to/stimulate) much of the body, and connect through the vertebral column to the spinal cord and thus to the central nervous system. They are given letter-number designations according to the vertebra through which they connect to the spinal column.
* Cranial nerves innervate parts of the head, and connect directly to the brain (especially to the brainstem). They are typically assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included. In addition, cranial nerves have descriptive names.

Terminology

Specific terms are used to describe nerves and their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to "innervate" that section of the body or organ. Other terms relate to whether the nerve affects the same side ("ipsilateral") or opposite side ("contralateral") of the body, to the part of the brain that supplies it.

Development

Nerve growth normally ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling".

Regeneration

If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons can regenerate and remake the synaptic connections with neurons with the help of guidepost cells. This is also referred to as neuroregeneration.

The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud. When one of the growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is very slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect.

Function

A nerve conveys information in the form of electrochemical impulses (as nerve impulses known as action potentials) carried by the individual neurons that make up the nerve. These impulses are extremely fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, where the message is converted from electrical to chemical and then back to electrical.

Nerves can be categorized into two groups based on function:
* An afferent nerve fiber conducts sensory information from a sensory neuron to the central nervous system, where the information is then processed. Bundles of fibres or axons, in the peripheral nervous system are called nerves, and bundles of afferent fibers are known as ''sensory nerves''.
* An efferent nerve fiber conducts signals from a motor neuron in the central nervous system to muscles. Bundles of these fibres are known as ''efferent nerves''.

Nervous system

The nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain, brainstem and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to all remaining body parts.

Nerves that transmit signals from the CNS are called ''motor'' or '' efferent'' nerves, while those nerves that transmit information from the body to the CNS are called ''sensory'' or ''afferent''. Spinal nerves serve both functions and are called ''mixed'' nerves. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement.

The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

Clinical significance

Cancer can spread by invading the spaces around nerves. This is particularly common in head and neck cancer, prostate cancer and colorectal cancer.

Nerves can be damaged by physical injury as well as conditions like carpal tunnel syndrome and repetitive strain injury. Autoimmune diseases such as Guillain–Barré syndrome, neurodegenerative diseases, polyneuropathy, infection, neuritis, diabetes, or failure of the blood vessels surrounding the nerve all cause nerve damage, which can vary in severity.

Multiple sclerosis is a disease associated with extensive nerve damage. It occurs when the macrophages of an individual's own immune system damage the myelin sheaths that insulate the axon of the nerve.

A pinched nerve occurs when pressure is placed on a nerve, usually from swelling due to an injury, or pregnancy and can result in pain, weakness, numbness or paralysis, an example being