A nerve is a closed, rope-like bundle of nerve fibers with a sheath of connective tissue in the peripheral nervous system. This interacts with other nerves to form the pathway for electrochemical impulses to be transmitted along the nerve fibers to peripheral organs.
What are nerves?
Schematic diagram showing the anatomy and structure of a nerve cell. Click to enlarge. The totality of all nerve cells (neurons) and glial cells (isolation and connection of neurons) is called the nervous system. The brain and spinal cord make up the Central Nervous System (CNS), and all nerves outside the CNS belong to the Peripheral Nervous System (PNS). The simplest nerves are unipolar nerves – simple sensory cells such as those of the eyes; biopolar nerve cells, on the other hand, transmit sensory impulses. The most common nerve cells are so-called multipolar cells, which occur primarily in the spinal cord. Afferent nerves conduct signals from sensory neurons, for example the mechanoreceptors in the skin, to the central nervous system. Efferent nerves, in turn, conduct signals from the central nervous system to muscles and glands. The PNS includes both the spinal nerves (connecting to the spinal cord through the spine), the cranial nerves (connecting to the brainstem), and the nerves of the visceral nervous system (enteric nervous system).
Anatomy and structure
Neurons have specialized structures to quickly and accurately send signals to other cells. Each nerve cell forms a cord-like structure consisting of a cell body with projections called axons (nerve fibers). Within a nerve, each axon is surrounded by a layer of connective tissue (endoneurium). This endoneurium consists of an inner sheath of glycocalix and an outer delicate network of collagen fibers. Within the endoneurium, the individual nerve fibers are surrounded by a protein liquid similar to the cerebrospinal fluid in the central nervous system. The nerve fibers are bundled into groups (fascicles) and are again encased by connective tissue (perineurium). The connective tissue enveloping the entire nerve is called the epineurium. Nerve growth normally stops at puberty but can be stimulated again by a molecular mechanism.
Functions and tasks
The primary function of the nerves and nervous system is to provide sensorimotor control of the body. This is done by extracting information from the environment using sensory receptors, sending signals encoding that information into the central nervous system, processing the information for an appropriate response, and sending output signals to muscles or glands to activate the response in question. Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions. The exchanges between them take the form of electrochemical nerve impulses traveling at speeds of up to 120 m/s. They travel from one neuron to the next; when a synapse crosses, the message in question is converted from an electrical impulse to a chemical message (neurotransmitter) and then back to an electrical impulse. The connections between neurons form neural circuits that create perception and determine the behavior of the organism.
Diseases
The nervous system, like the rest of the organism, is susceptible to disruption, whether as a result of genetic defects, physical damage due to trauma or poison, infection, or simple aging processes. The medical specialty of neurology deals with the causes of nervous system dysfunction and its treatment or prevention. In the peripheral nervous system, the most common type of problem is a failure of nerve conduction velocity, which can have a variety of causes. These include diabetic neuropathy and demyelinating diseases such as multiple sclerosis and amyotrophic lateral sclerosis. A pinched nerve occurs when pressure is placed on a nerve, such as due to swelling from an injury or as a result of pregnancy. Nerve damage or pinched nerves are usually accompanied by pain, numbness, weakness or paralysis, some of which may be felt far from the actual site of damage. If a nerve is damaged, radiating pain occurs in all areas from which the nerve cell receives input.Disorders of the nerves are usually diagnosed by physical examination, including testing of reflexes, walking and other directed movements, muscle responses, and sensory input supplemented by an EMG.
Typical and common disorders
- Nerve pain
- Nerve inflammation
- Polyneuropathy
- Epilepsy
