The nervous system derives its name from nerves—the cordlike bundles of axons that branch repeatedly to innervate tissues throughout the body. Nerves form remarkably coordinated networks that relay messages to and from the brain. These networks underlie the ability to sense stimuli and transmit signals to and from different parts of the body.
Nerve cells, or neurons, are the basic unit of the nervous system. The primary function of these cells is communication, which they do by sending electrical and chemical messages. Electric signaling is the fastest way to communicate, and nerve cells do this by sending electrical signals down their axons. Individual axons are microscopic in size and too small to see with the naked eye. Groups of axons that are bundled together outside of the brain and spinal cord form nerves. Nerves can also be too small to see. For example, the nerve that innervates taste buds on the tip of the tongue, called the chorda tympani, is about 1.7 × 10−5 inches in diameter—that is 150 times thinner than a single strand of hair. Other nerves are much larger. The widest nerve in the human body, which is also the longest nerve, is the sciatic nerve. This nerve is about three-quarters of an inch in diameter where it starts in the lower back. The sciatic nerve branches out to provide movement and feeling from the top of the leg all the way down to the foot.
Anatomy and Physiology
The function of a nerve depends on the individual axons, also called nerve fibers, that make up the nerve. Nerve fibers are broadly classified as afferent or efferent. Afferent fibers are sensory nerve fibers and constantly keep the central nervous system (CNS) informed of events that happen inside and outside of the body. This means that sensory fibers are continually sending information into the brain about what we sense in the environment, such as sights, sounds, tastes, and smells as well as details about the organs in the body. Efferent fibers are the motor nerve fibers and send information out of the brain to the many muscles and glands throughout the body. Motor fibers control both voluntary and involuntary movements. The autonomic nerve fibers are responsible for the involuntary actions of the body such as heart rate, blood pressure, muscle contractions of the digestive organs, and secretions from glands. Animals are usually unconscious of this kind of movement.
How fast electrical signals can travel down nerve fibers is determined by how thick the fiber is as well as the myelin that surrounds it. Some of the fastest nerve signals travel 400 feet in a second, which is around 144 miles per hour. Other fibers, such as those that convey pain, do not have any myelin; these nerve signals travel about three feet in a second, which is just over two miles per hour.
Nerve fibers are then bundled together to make up the peripheral nerves. Hence, nerves can also be broadly classified as sensory, motor, or mixed nerves, which contain both sensory and motor fibers. There are 12 pairs of cranial nerves that connect to the brain and 31 pairs of spinal nerves that connect to the spinal cord. Cranial nerves are numbered with Roman numerals in order of where they connect to the brain. These nerves primarily serve the sensory and motor functions of the head and neck region. Some cranial nerves are exclusively sensory nerves, such as the olfactory and optic nerves, while some are only motor nerves, such as the trochlear and abducens nerves that are involved in eye movement. The remaining cranial nerves are mixed.
Unlike the cranial nerves that serve one or a few specific functions to a broad region, the spinal nerves serve all functions to smaller segmental regions of the body. Therefore, all of the spinal nerves are mixed nerves. These nerves do not have names but are grouped and numbered according to the spinal cord level from which they stem—cervical, thoracic, lumbar, or sacral. The cervical nerves, which are numbered C1—C8, involve the shoulders, arms, neck, and hands. The thoracic nerves, T1—T12, run along the middle of the back and are involved with the chest, abdomen, and internal organs, including the heart, lungs, liver, and stomach. The lumbar nerves are L1—L5 and target muscles of the lower back, thighs, legs, and feet as well as some internal organs, such as the appendix, bladder, and reproductive organs. The sacral nerves are S1—S5 and target the hips, buttocks, thighs, and legs. The last pair of spinal nerves, simply called the coccygeal nerves, targets the skin over the back of the tailbone.
See also: Autonomic Nervous System; Central Nervous System; Cranial Nerves; Peripheral Nervous System
A history of the nervous system. (n.d.). Retrieved from http://www.stanford.edu/class/history13/earlysciencelab/body/nervespages/nerves.html
Neuron basics. (n.d.). Retrieved from http://www.animatlab.com/NeuralNetworkEditor/FiringRateNeuralNet/NeuronBasics.htm