The nervous system is a network of specialized cells that coordinate the actions of an animal and send signals from one part of its body to another. These cells send signals either as electrochemical waves traveling along thin fibers called axons, or as chemicals released onto other cells. The nervous system is composed of neurons and other specialized cells called glial cells (plural form glia).
In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain and spinal cord. The neurons of the central nervous system are interconnected in complex arrangements and transmit electrochemical signals from one to another. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. Sensory neurons are activated by inputs impinging on them from outside or inside the body, and send signals that inform the central nervous system of ongoing events. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect neurons to muscles or other effector organs. The interaction of the different neurons form neural circuits that regulate an organism's perception of the world and its body and behavior.
Nervous systems are found in most multicellular animals, but vary greatly in complexity. Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion. Radiata, including jellyfish, have a nervous system consisting of a simple nerve net. Bilaterian animals, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, spinal cord, and peripheral nerves.
Structure
The nervous system is defined by the presence of a special type of cell, the neuron. Neurons are distinctive in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical.
All animals more advanced than sponges have a nervous system. 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. 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 500 million years ago.
Vertebrates
The nervous system of vertebrate animals is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
Central Nervous System
Main article: Central nervous systemThe central nervous system (CNS) is the largest part of the nervous system, and includes the brain and spinal cord. The spinal cavity holds and protects the spinal cord, while the head contains and protects the brain. The CNS is covered by the meninges, a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.
Peripheral nervous system
Main article: Peripheral nervous systemThe PNS is a regional term for the collective nervous structures that do not lie in the CNS. The bodies of the nerve cells lie in the CNS, either in the brain or the spinal cord, and the longer of the cellular processes of these cells, known as axons, extend through the limbs and the flesh of the torso. The large majority of the axons, which are commonly called nerves, are considered to be PNS.
The autonomic nervous system, which mediates involuntary behaviors such as heartbeat and breathing, consists of two parts: the sympathetic nervous system and the parasympathetic nervous system.
The cell bodies of afferent PNS nerves lie in the dorsal root ganglia.
Invertebrates
Neural precursors in sponges
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons. They do, however, have homologs of many genes that play key roles in synaptic function. In particular, recent studies have shown that they possess a group of proteins that cluster together to form a structure resembling a postsynaptic density—the signal-receiving part of a synapse. The function of this structure is currrently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves, which mediate some simple actions such as whole-body contraction.
Radiata
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 that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.
The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, 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.
Bilateria
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 in the Cambrian period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".
The basic architecture of the bilaterian nervous system is seen most clearly during embryonic development. All bilaterian animals at an early stage of development form a gastrula, which is a disk with three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal organs; a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the ectoderm, which gives rise to the skin and nervous system. In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the CNS, the outer portion the PNS. As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation.
Worms
Planaria, a type of flatworm, have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large ganglia at the head end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark.
The nervous system of the roundworm Caenorhabditis elegans has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans , males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons
Arthropods
Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly . Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia .
The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Imme
Nervous system - Wikipedia, the free encyclopedia
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