A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in myelin. Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves. Nerves are part of the peripheral nervous system. Afferent nerves convey sensory signals to the brain, for example from skin or organs, while efferent nerves conduct stimulatory signals from the brain to the muscles and glands.
These signals, sometimes called nerve impulses, are also known as action potentials: Rapidly traveling electrical waves, which begin typically in the cell body of a neuron and propagate rapidly down the axon to its tip or "terminus." The signals cross over from the terminus to the adjacent neuron through a gap called the synapse.
Nerves may contain fibers that all serve the same purpose; for example motor nerves (efferent), the axons of which all terminate on muscle fibers and stimulate contraction. On the other hand, sensory nerves (afferent) receive input from sensory organs. Nerves can also be mixed, containing both efferent and afferent fibers.
Most nerves synapse in the spinal cord. Efferent nerves enter the spinal cord through the dorsal (i.e. closest to the back) horn of the spinal cord, while afferent nerves leave the spinal cord through the ventral (i.e. closest to the belly) horn. The cranial nerves do not synapse in the spinal cord; they connect to the central nervous system directly to and from the brain.
Damage to nerves can be caused by physical injury, swelling (e.g. carpal tunnel syndrome), autoimmune diseases (e.g. Guillain-Barré syndrome), diabetes, or failure of the blood vessels surrounding the nerve. Pinched nerves occur when pressure is placed on a nerve, usually from swelling due to an injury or pregnancy. Nerve damage or pinched nerves are usually accompanied by pain, numbness, weakness, or paralysis. Patients may feel these symptoms in areas far from the actual site of damage, a phenonmenon called referred pain. Referred pain occurs because when a nerve is damaged, signalling is defective from all parts of the area which the nerve receives input, not just the site of the damage.