The human brain contains about billion neurons. A neuron has a cell body, which includes the cell nucleus, and special extensions called axons pronounced AK-sonz and dendrites pronounced DEN-drahytz. Bundles of axons, called nerves, are found throughout the body. Axons and dendrites allow neurons to communicate, even across long distances.
Different types of neurons control or perform different activities. For instance, motor neurons transmit messages from the brain to the muscles to generate movement. The input to the nervous system derives from sensory cells of widely varying types, which transmute physical modalities such as light and sound into neural activity.
Internally, the nervous system contains complex webs of connections between nerve cells that allow it to generate patterns of activity that depend only partly on sensory input.
The nervous system is also capable of storing information over time, by dynamically modifying the strength of connections between neurons, as well as other mechanisms. The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans Finger, , chapter 1 , but their internal structure was not understood until it became possible to examine them using a microscope.
A microscopic examination shows that nerves consist primarily of the axons of neurons, along with a variety of membranes that wrap around them. The neurons that give rise to nerves do not generally lie within the nerves themselves — their cell bodies reside within the brain, central cord, or peripheral ganglia.
All animals more derived than sponges have nervous systems. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons Sakarya et al.
In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Cambrian period, over million years ago. The nervous system contains two main categories or types of cells: neurons and glial cells. The nervous system is defined by the presence of a special type of cell, the neuron sometimes called "neurone" or "nerve cell".
Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neuron possess an axon , a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts.
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. These include sensory neurons that transmute physical stimuli such as light and sound into neural signals, and motor neurons that transmute neural signals into activation of muscles or glands.
In many species, though, the majority of neurons receive all of their input from other neurons and send their output to other neurons. Glial cells named from the Greek word for "glue" are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system Allen, In the human brain, it is currently estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas Azevedo et al.
Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy pathogens and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets.
A very important set of glial cell oligodendrocytes in the vertebrate CNS, and Schwann cells in the PNS generate layers of a fatty substance called myelin that wrap around axons and provide electrical insulation that allows them to transmit signals much more rapidly and efficiently. The nervous system of vertebrate animals is divided into two parts called the central nervous system CNS and peripheral nervous system PNS.
The CNS is the largest part, and includes the brain and spinal cord. The CNS is enclosed and protected by meninges, a three-layered system of membranes, including a tough, leathery outer layer called the dura mater. The brain is also protected by the skull, and the spinal cord by the vertebral bones.
Blood vessels that enter the CNS are surrounded by cells that form a tight chemical seal called the blood-brain barrier , preventing many types of chemicals present in the body from gaining entry to the CNS. The peripheral nervous system PNS is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord.
The PNS is divided into "somatic" and "visceral" parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglion of the spinal cord. The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system.
Some authors also include sensory neurons whose cell bodies lie in the periphery for senses such as hearing as part of the PNS; others, however, omit them Hubbard, , p. The vertebrate nervous system can also be divided into areas called gray matter "grey matter" in British spelling and white matter. Gray matter which is only gray in preserved tissue, and is better described as pink or light brown in living tissue contains a high proportion of cell bodies of neurons.
White matter is composed mainly of myelin-coated axons, and takes its color from the myelin. White matter includes all of the body's nerves, and much of the interior of the brain and spinal cord.
Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There is an anatomical convention that a cluster of neurons in the brain is called a "nucleus", whereas a cluster of neurons in the periphery is called a "ganglion". There are, however, a few exceptions to this rule, notably the part of the brain called the basal ganglia. Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system.
They do, however, have homologs of many genes that play key roles in synaptic function in other animals. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density the signal-receiving part of a synapse Sakarya, However, the function of that structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction Jacobs et al.
Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system.
In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons.
In some cases groups of intermediate neurons are clustered into discrete ganglia Ruppert et al. The development of the nervous system in radiata is relatively unstructured.
Unlike bilaterians, radiata only have two primordial cell layers, the endoderm and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type Sanes et al.
The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared during the Cambrian period, — million years ago Balavoine, The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord or two parallel nerve cords , with an enlargement a "ganglion" for each body segment, with an especially large ganglion at the front, called the "brain".
It has not been definitively established whether the generic form of the bilaterian central nervous system is inherited from the so-called "Urbilaterian" — the last common ancestor of all existing bilaterians — or whether separate lines have evolved similar structures in parallel Northcutt, The CNS is separate from the peripheral nervous system, although the two systems are interconnected. The nerve axons of the CNS — the slender projections of nerve cells that carry impulses — are much shorter.
PNS nerve axons can be up to 1 meter long for instance, the nerve that activates the big toe whereas, within the CNS, they are rarely longer than a few millimeters. Much of the PNS has the ability to regenerate; if a nerve in your finger is severed, it can regrow. The CNS, however, does not have this ability. The components of the central nervous system are further split into a myriad of parts. Below, we will describe some of these sections in a little more detail.
Stroke blocks the blood supply to the brain and can be life threatening. Learn more about strokes, including the types, symptoms, and how treat and…. Peripheral neuropathy is most common among people with diabetes, causing impairment in the peripheral nervous system Find out here about other medical…. All about the central nervous system. Medically reviewed by Seunggu Han, M. What is the CNS? Brain Spinal cord White and gray matter Central glial cells Cranial nerves CNS diseases The central nervous system consists of the brain and spinal cord.
What is the central nervous system? The brain. Spinal cord. Share on Pinterest The spinal cord carries information from the brain to the rest of the body. White and gray matter.
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Skip to main content. Brain and nerves. Home Brain and nerves. Nervous system. Actions for this page Listen Print. Summary Read the full fact sheet. On this page. Neurones are the building blocks Central nervous system The peripheral nervous system Problems of the nervous system Where to get help Things to remember.
Neurones are the building blocks The basic building block of the nervous system is a nerve cell, or neurone. Central nervous system The brain and the spinal cord make up the central nervous system.
The spinal cord The spinal cord is connected to the brain and runs the length of the body. The peripheral nervous system Nerves connect the brain and spinal cord to the peripheral nervous system, which is what nerve tissue outside of the central nervous system is called.
The autonomic nervous system The autonomic nervous system is part of the peripheral nervous system. The somatic nervous system The somatic nervous system is also a part of the peripheral nervous system.
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