Nervous system

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For other uses, see Nervous system (disambiguation). Nervous system_sentence_0

Nervous system_table_infobox_0

Nervous systemNervous system_header_cell_0_0_0
DetailsNervous system_header_cell_0_1_0
IdentifiersNervous system_header_cell_0_2_0
LatinNervous system_header_cell_0_3_0 systema nervosumNervous system_cell_0_3_1
MeSHNervous system_header_cell_0_4_0 Nervous system_cell_0_4_1
FMANervous system_header_cell_0_5_0 Nervous system_cell_0_5_1

In biology, the nervous system is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. Nervous system_sentence_1

The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous system_sentence_2

Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. Nervous system_sentence_3

In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). Nervous system_sentence_4

The CNS consists of the brain and spinal cord. Nervous system_sentence_5

The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nervous system_sentence_6

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Nervous system_sentence_7

Spinal nerves serve both functions and are called mixed nerves. Nervous system_sentence_8

The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Nervous system_sentence_9

Somatic nerves mediate voluntary movement. Nervous system_sentence_10

The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. Nervous system_sentence_11

The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. Nervous system_sentence_12

The enteric nervous system functions to control the gastrointestinal system. Nervous system_sentence_13

Both autonomic and enteric nervous systems function involuntarily. Nervous system_sentence_14

Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. Nervous system_sentence_15

At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". Nervous system_sentence_16

Neurons have special structures that allow them to send signals rapidly and precisely to other cells. Nervous system_sentence_17

They send these signals in the form of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. Nervous system_sentence_18

A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. Nervous system_sentence_19

The connections between neurons can form neural pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. Nervous system_sentence_20

Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Nervous system_sentence_21

Nervous systems are found in most multicellular animals, but vary greatly in complexity. Nervous system_sentence_22

The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. Nervous system_sentence_23

The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. Nervous system_sentence_24

All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. Nervous system_sentence_25

The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants. Nervous system_sentence_26

The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Nervous system_sentence_27

Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply senesence. Nervous system_sentence_28

The medical specialty of neurology studies disorders of the nervous system and looks for interventions that can prevent or treat them. Nervous system_sentence_29

In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including diabetic neuropathy and demyelinating disorders such as multiple sclerosis and amyotrophic lateral sclerosis. Nervous system_sentence_30

Neuroscience is the field of science that focuses on the study of the nervous system. Nervous system_sentence_31

Structure Nervous system_section_0

The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body. Nervous system_sentence_32

Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope. Nervous system_sentence_33

The author Michael Nikoletseas wrote: Nervous system_sentence_34

A microscopic examination shows that nerves consist primarily of axons, along with different membranes that wrap around them and segregate them into fascicles. Nervous system_sentence_35

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. Nervous system_sentence_36

All animals more advanced than sponges have nervous systems. Nervous system_sentence_37

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. Nervous system_sentence_38

In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a nerve net, a diffuse network of isolated cells. Nervous system_sentence_39

In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Ediacaran period, over 550 million years ago. Nervous system_sentence_40

Cells Nervous system_section_1

The nervous system contains two main categories or types of cells: neurons and glial cells. Nervous system_sentence_41

Neurons Nervous system_section_2

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Structure of a typical neuronNervous system_table_caption_1
NeuronNervous system_header_cell_1_0_0

The nervous system is defined by the presence of a special type of cell—the neuron (sometimes called "neurone" or "nerve cell"). Nervous system_sentence_42

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 membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Nervous system_sentence_43

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; axons typically extend throughout the body in bundles called nerves. Nervous system_sentence_44

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. Nervous system_sentence_45

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; however in many species the great majority of neurons participate in the formation of centralized structures (the brain and ganglia) and they receive all of their input from other neurons and send their output to other neurons. Nervous system_sentence_46

Glial cells Nervous system_section_3

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. Nervous system_sentence_47

In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. Nervous system_sentence_48

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. Nervous system_sentence_49

A very important type of glial cell (oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system) generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Nervous system_sentence_50

Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system. Nervous system_sentence_51

Anatomy in vertebrates Nervous system_section_4

See also: List of nerves of the human body and List of regions in the human brain Nervous system_sentence_52

The nervous system of vertebrates (including humans) is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). Nervous system_sentence_53

The (CNS) is the major division, and consists of the brain and the spinal cord. Nervous system_sentence_54

The spinal canal contains the spinal cord, while the cranial cavity contains the brain. Nervous system_sentence_55

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. Nervous system_sentence_56

The brain is also protected by the skull, and the spinal cord by the vertebrae. Nervous system_sentence_57

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. Nervous system_sentence_58

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. Nervous system_sentence_59

The PNS is divided into somatic and visceral parts. Nervous system_sentence_60

The somatic part consists of the nerves that innervate the skin, joints, and muscles. Nervous system_sentence_61

The cell bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal cord. Nervous system_sentence_62

The visceral part, also known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. Nervous system_sentence_63

The autonomic nervous system itself consists of two parts: the sympathetic nervous system and the parasympathetic nervous system. Nervous system_sentence_64

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. Nervous system_sentence_65

The vertebrate nervous system can also be divided into areas called gray matter and white matter. Nervous system_sentence_66

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. Nervous system_sentence_67

White matter is composed mainly of myelinated axons, and takes its color from the myelin. Nervous system_sentence_68

White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Nervous system_sentence_69

Gray matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. Nervous system_sentence_70

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. Nervous system_sentence_71

There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia. Nervous system_sentence_72

Comparative anatomy and evolution Nervous system_section_5

Main article: Evolution of nervous systems Nervous system_sentence_73

Neural precursors in sponges Nervous system_section_6

Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. Nervous system_sentence_74

They do, however, have homologs of many genes that play key roles in synaptic function. Nervous system_sentence_75

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). Nervous system_sentence_76

However, the function of this structure is currently unclear. Nervous system_sentence_77

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. Nervous system_sentence_78

Radiata Nervous system_section_7

Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. Nervous system_sentence_79

In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. Nervous system_sentence_80

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. Nervous system_sentence_81

In some cases groups of intermediate neurons are clustered into discrete ganglia. Nervous system_sentence_82

The development of the nervous system in radiata is relatively unstructured. Nervous system_sentence_83

Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm. Nervous system_sentence_84

Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type. Nervous system_sentence_85

Bilateria Nervous system_section_8

The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. Nervous system_sentence_86

All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Ediacaran period, 550–600 million years ago. Nervous system_sentence_87

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". Nervous system_sentence_88

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. Nervous system_sentence_89

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. Nervous system_sentence_90

On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. Nervous system_sentence_91

The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain. Nervous system_sentence_92

Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups (superphyla) called protostomes and deuterostomes. Nervous system_sentence_93

Deuterostomes include vertebrates as well as echinoderms, hemichordates (mainly acorn worms), and Xenoturbellidans. Nervous system_sentence_94

Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of worms. Nervous system_sentence_95

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. Nervous system_sentence_96

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. Nervous system_sentence_97

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. Nervous system_sentence_98

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. Nervous system_sentence_99

Worms Nervous system_section_9

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. Nervous system_sentence_100

As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. Nervous system_sentence_101

These nerve cords are connected by transverse nerves like the rungs of a ladder. Nervous system_sentence_102

These transverse nerves help coordinate the two sides of the animal. Nervous system_sentence_103

Two ganglia at the head (the "nerve ring") end function similar to a simple brain. Nervous system_sentence_104

Photoreceptors on the animal's eyespots provide sensory information on light and dark. Nervous system_sentence_105

The nervous system of one very small roundworm, the nematode Caenorhabditis elegans, has been completely mapped out in a connectome including its synapses. Nervous system_sentence_106

Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. Nervous system_sentence_107

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. Nervous system_sentence_108

In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons. Nervous system_sentence_109

Arthropods Nervous system_section_10

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. Nervous system_sentence_110

Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. Nervous system_sentence_111

The head segment contains the brain, also known as the supraesophageal ganglion. Nervous system_sentence_112

In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Nervous system_sentence_113

Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. Nervous system_sentence_114

It controls the mouthparts, the salivary glands and certain muscles. Nervous system_sentence_115

Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. Nervous system_sentence_116

The sensory information from these organs is processed by the brain. Nervous system_sentence_117

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. Nervous system_sentence_118

A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Nervous system_sentence_119

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. Nervous system_sentence_120

"Identified" neurons Nervous system_section_11

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. Nervous system_sentence_121

In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique. Nervous system_sentence_122

In the roundworm C. Nervous system_sentence_123 elegans, whose nervous system is the most thoroughly described of any animal's, every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. Nervous system_sentence_124

One notable consequence of this fact is that the form of the C. elegans nervous system is completely specified by the genome, with no experience-dependent plasticity. Nervous system_sentence_125

The brains of many molluscs and insects also contain substantial numbers of identified neurons. Nervous system_sentence_126

In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. Nervous system_sentence_127

Every fish has two Mauthner cells, in the bottom part of the brainstem, one on the left side and one on the right. Nervous system_sentence_128

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. Nervous system_sentence_129

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. Nervous system_sentence_130

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. Nervous system_sentence_131

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. Nervous system_sentence_132

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. Nervous system_sentence_133

Mauthner cells have been described as command neurons. Nervous system_sentence_134

A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually. Nervous system_sentence_135

Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. Nervous system_sentence_136

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. Nervous system_sentence_137

Function Nervous system_section_12

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. Nervous system_sentence_138

There are multiple ways that a cell can send signals to other cells. Nervous system_sentence_139

One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. Nervous system_sentence_140

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. Nervous system_sentence_141

Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. Nervous system_sentence_142

It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second. Nervous system_sentence_143

At a more integrative level, the primary function of the nervous system is to control the body. Nervous system_sentence_144

It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. Nervous system_sentence_145

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. Nervous system_sentence_146

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. Nervous system_sentence_147

Neurons and synapses Nervous system_section_13

Most neurons send signals via their axons, although some types are capable of dendrite-to-dendrite communication. Nervous system_sentence_148

(In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Nervous system_sentence_149

Neural signals propagate along an axon in the form of electrochemical waves called action potentials, which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells. Nervous system_sentence_150

Synapses may be electrical or chemical. Nervous system_sentence_151

Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function. Nervous system_sentence_152

At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Nervous system_sentence_153

Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. Nervous system_sentence_154

The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals. Nervous system_sentence_155

When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft. Nervous system_sentence_156

The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Nervous system_sentence_157

Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. Nervous system_sentence_158

For example, release of the neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell. Nervous system_sentence_159

The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace). Nervous system_sentence_160

Nervous system_table_infobox_2

Structure of a typical chemical synapseNervous system_header_cell_2_0_0

There are literally hundreds of different types of synapses. Nervous system_sentence_161

In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors. Nervous system_sentence_162

Many synapses use more than one neurotransmitter—a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamate or GABA, along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Nervous system_sentence_163

Molecular neuroscientists generally divide receptors into two broad groups: chemically gated ion channels and second messenger systems. Nervous system_sentence_164

When a chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane. Nervous system_sentence_165

Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. Nervous system_sentence_166

When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription. Nervous system_sentence_167

According to a rule called Dale's principle, which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses. Nervous system_sentence_168

This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates. Nervous system_sentence_169

Because different targets can (and frequently do) use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still. Nervous system_sentence_170

Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and GABA, each have largely consistent effects. Nervous system_sentence_171

Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Nervous system_sentence_172

Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Nervous system_sentence_173

Because of this consistency, glutamatergic cells are frequently referred to as "excitatory neurons", and GABAergic cells as "inhibitory neurons". Nervous system_sentence_174

Strictly speaking, this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications. Nervous system_sentence_175

One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. Nervous system_sentence_176

The best-known form of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that use the neurotransmitter glutamate acting on a special type of receptor known as the NMDA receptor. Nervous system_sentence_177

The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a channel opens that permits calcium to flow into the target cell. Nervous system_sentence_178

The calcium entry initiates a second messenger cascade that ultimately leads to an increase in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. Nervous system_sentence_179

This change in strength can last for weeks or longer. Nervous system_sentence_180

Since the discovery of LTP in 1973, many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time. Nervous system_sentence_181

The reward system, that reinforces desired behaviour for example, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine as neurotransmitter. Nervous system_sentence_182

All these forms of synaptic modifiability, taken collectively, give rise to neural plasticity, that is, to a capability for the nervous system to adapt itself to variations in the environment. Nervous system_sentence_183

Neural circuits and systems Nervous system_section_14

The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. Nervous system_sentence_184

Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing, and there are seen to be countless types of information processing possible. Nervous system_sentence_185

Warren McCulloch and Walter Pitts showed in 1943 that even artificial neural networks formed from a greatly simplified mathematical abstraction of a neuron are capable of universal computation. Nervous system_sentence_186

Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. Nervous system_sentence_187

In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Nervous system_sentence_188

Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. Nervous system_sentence_189

Charles Sherrington, in his influential 1906 book The Integrative Action of the Nervous System, developed the concept of stimulus-response mechanisms in much more detail, and Behaviorism, the school of thought that dominated Psychology through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms. Nervous system_sentence_190

However, experimental studies of electrophysiology, beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining cell excitability and generating patterns of activity intrinsically, without requiring an external stimulus. Nervous system_sentence_191

Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. Nervous system_sentence_192

When intrinsically active neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive. Nervous system_sentence_193

A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior. Nervous system_sentence_194

Reflexes and other stimulus-response circuits Nervous system_section_15

The simplest type of neural circuit is a reflex arc, which begins with a sensory input and ends with a motor output, passing through a sequence of neurons connected in series. Nervous system_sentence_195

This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. Nervous system_sentence_196

The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane. Nervous system_sentence_197

If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. Nervous system_sentence_198

There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. Nervous system_sentence_199

One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. Nervous system_sentence_200

The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. Nervous system_sentence_201

The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away. Nervous system_sentence_202

In reality, this straightforward schema is subject to numerous complications. Nervous system_sentence_203

Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Nervous system_sentence_204

Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex. Nervous system_sentence_205

Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain. Nervous system_sentence_206

For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. Nervous system_sentence_207

The initial sensory response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain stem, are not all that different from those in a simple reflex, but the intermediate stages are completely different. Nervous system_sentence_208

Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei. Nervous system_sentence_209

These areas perform signal-processing functions that include feature detection, perceptual analysis, memory recall, decision-making, and motor planning. Nervous system_sentence_210

Feature detection is the ability to extract biologically relevant information from combinations of sensory signals. Nervous system_sentence_211

In the visual system, for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world. Nervous system_sentence_212

Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming a hierarchy of processing stages. Nervous system_sentence_213

At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. Nervous system_sentence_214

By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. Nervous system_sentence_215

The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina. Nervous system_sentence_216

Intrinsic pattern generation Nervous system_section_16

Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Nervous system_sentence_217

Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. Nervous system_sentence_218

When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos. Nervous system_sentence_219

A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator. Nervous system_sentence_220

Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. Nervous system_sentence_221

One of the most important types of temporal pattern is circadian rhythmicity—that is, rhythmicity with a period of approximately 24 hours. Nervous system_sentence_222

All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Nervous system_sentence_223

Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Nervous system_sentence_224

Animals as diverse as insects and vertebrates share a similar genetic clock system. Nervous system_sentence_225

The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. Nervous system_sentence_226

The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus. Nervous system_sentence_227

Mirror neurons Nervous system_section_17

Main article: Mirror neuron Nervous system_sentence_228

A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Nervous system_sentence_229

Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Nervous system_sentence_230

Such neurons have been directly observed in primate species. Nervous system_sentence_231

Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. Nervous system_sentence_232

In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex and the inferior parietal cortex. Nervous system_sentence_233

The function of the mirror system is a subject of much speculation. Nervous system_sentence_234

Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling (see the common coding theory). Nervous system_sentence_235

They argue that mirror neurons may be important for understanding the actions of other people, and for learning new skills by imitation. Nervous system_sentence_236

Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. Nervous system_sentence_237

However, to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation. Nervous system_sentence_238

There are neuroscientists who caution that the claims being made for the role of mirror neurons are not supported by adequate research. Nervous system_sentence_239

Development Nervous system_section_18

Main articles: Development of the nervous system and Development of the nervous system in humans Nervous system_sentence_240

In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory. Nervous system_sentence_241

All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole. Nervous system_sentence_242

The gastrula has the shape of 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. Nervous system_sentence_243

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. Nervous system_sentence_244

The inner portion of the neural plate (along the midline) is destined to become the central nervous system (CNS), the outer portion the peripheral nervous system (PNS). Nervous system_sentence_245

As development proceeds, a fold called the neural groove appears along the midline. Nervous system_sentence_246

This fold deepens, and then closes up at the top. Nervous system_sentence_247

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. Nervous system_sentence_248

The sequence of stages from neural plate to neural tube and neural crest is known as neurulation. Nervous system_sentence_249

In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the organizer region. Nervous system_sentence_250

For decades, though, the nature of neural induction defeated every attempt to figure it out, until finally it was resolved by genetic approaches in the 1990s. Nervous system_sentence_251

Induction of neural tissue requires inhibition of the gene for a so-called bone morphogenetic protein, or BMP. Nervous system_sentence_252

Specifically the protein BMP4 appears to be involved. Nervous system_sentence_253

Two proteins called Noggin and Chordin, both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. Nervous system_sentence_254

It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. Nervous system_sentence_255

In some animals, however, another type of molecule called Fibroblast Growth Factor or FGF may also play an important role in induction. Nervous system_sentence_256

Induction of neural tissues causes formation of neural precursor cells, called neuroblasts. Nervous system_sentence_257

In drosophila, neuroblasts divide asymmetrically, so that one product is a "ganglion mother cell" (GMC), and the other is a neuroblast. Nervous system_sentence_258

A GMC divides once, to give rise to either a pair of neurons or a pair of glial cells. Nervous system_sentence_259

In all, a neuroblast is capable of generating an indefinite number of neurons or glia. Nervous system_sentence_260

As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a family of secreted signaling molecules called neurotrophins which regulate the growth and survival of neurons. Nervous system_sentence_261

Zhu et al. Nervous system_sentence_262

identified DNT1, the first neurotrophin found in flies. Nervous system_sentence_263

DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila. Nervous system_sentence_264

Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation. Nervous system_sentence_265

Pathology Nervous system_section_19

Main article: Neurology Nervous system_sentence_266

See also: Psychiatry Nervous system_sentence_267

The central nervous system is protected by major physical and chemical barriers. Nervous system_sentence_268

Physically, the brain and spinal cord are surrounded by tough meningeal membranes, and enclosed in the bones of the skull and vertebral column, which combine to form a strong physical shield. Nervous system_sentence_269

Chemically, the brain and spinal cord are isolated by the blood–brain barrier, which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. Nervous system_sentence_270

These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences. Nervous system_sentence_271

Although nerves tend to lie deep under the skin except in a few places such as the ulnar nerve near the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Nervous system_sentence_272

Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. Nervous system_sentence_273

If a nerve is completely transected, it will often regenerate, but for long nerves this process may take months to complete. Nervous system_sentence_274

In addition to physical damage, peripheral neuropathy may be caused by many other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain–Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or shingles, or poisoning by toxins such as heavy metals. Nervous system_sentence_275

Many cases have no cause that can be identified, and are referred to as idiopathic. Nervous system_sentence_276

It is also possible for nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with local anesthetic drugs such as lidocaine. Nervous system_sentence_277

Physical damage to the spinal cord may result in loss of sensation or movement. Nervous system_sentence_278

If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Nervous system_sentence_279

Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves. Nervous system_sentence_280

See also Nervous system_section_20

Nervous system_unordered_list_0

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: system.