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The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, spinal cord , or peripheral ganglia. All animals more advanced 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. 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".

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. Glial cells named from the Greek for "glue" are non-neuronal cells that provide support and nutrition , maintain homeostasis , form myelin , and participate in signal transmission in the nervous system. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.

The nervous system of vertebrates including humans is divided into the central nervous system CNS and the peripheral nervous system PNS. The CNS is the major division, and consists of the brain and the spinal cord. The CNS is enclosed and protected by the 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 vertebrae. The peripheral nervous system PNS is a collective term for the nervous system structures that do not lie within the CNS. 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 ganglia 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.

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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. The vertebrate nervous system can also be divided into areas called grey matter "gray matter" in American spelling and white matter. White matter is composed mainly of myelinated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Grey 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 or spinal cord is called a nucleus , whereas a cluster of neurons in the periphery is called a ganglion. 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. 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.

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. 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. 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.

The vast majority of existing animals are bilaterians , meaning animals with left and right sides that are approximate mirror images of each other.

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All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Ediacaran period, — million years ago. Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature.

On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain. Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups superphyla called protostomes and deuterostomes. There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral usually bottom side of the body, whereas in deuterostomes the nerve cord is on the dorsal usually top side.

In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates.

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Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline. Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms 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.

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These transverse nerves help coordinate the two sides of the animal. Two ganglia at the head the " nerve ring " end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark. The nervous system of one very small roundworm, the nematode Caenorhabditis elegans , has been completely mapped out in a connectome including its synapses. 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 female hermaphrodites , have different numbers of neurons and groups of neurons that perform sex-specific functions.

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. The head segment contains the brain, also known as the supraesophageal ganglion.

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In the insect nervous system , the brain is anatomically divided into the protocerebrum , deutocerebrum , and tritocerebrum. Immediately behind the brain is the subesophageal ganglion , which is composed of three pairs of fused ganglia. It controls the mouthparts , the salivary glands and certain muscles. Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation.

The sensory information from these organs is processed by the brain. In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling.

A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called neuropil , in the interior. A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties.

In the roundworm C. One notable consequence of this fact is that the form of the C. The brains of many molluscs and insects also contain substantial numbers of identified neurons. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes.

The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape , then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish.

Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response. Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells.

One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. It is also much faster: the fastest nerve signals travel at speeds that exceed meters per second. At a more integrative level, the primary function of the nervous system is to control the body.

The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.