What Are Two General Ways In Which Nervous Systems Differ Among Animal Groups?

This article has non yet been published; it may contain inaccuracies, unapproved changes, or be unfinished.

The nervous organisation is the part of an creature'south body that coordinates its beliefs and transmits signals between dissimilar body areas. In vertebrates it consists of 2 master parts, called the central nervous system (CNS) and the peripheral nervous arrangement (PNS). The CNS contains the brain and spinal string. The PNS consists mainly of nerves, which are long fibers that connect the CNS to every other part of the body, simply too includes other components such as peripheral ganglia, sympathetic and parasympathetic ganglia, and the enteric nervous organization, a semi-independent part of the nervous arrangement whose function is to control the gastrointestinal organization.

At the cellular level, the nervous system is defined by the presence of a special blazon of cell, called the neuron, likewise known as a "nerve cell". Neurons have special properties that permit them to transport signals rapidly and precisely to other cells. 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 to other neurons, called synapses. A cell that receives a synaptic signal from a neuron (a postsynaptic neuron) may be excited, inhibited, or otherwise modulated. The connections between neurons form neural circuits that can generate very complex patterns of dynamical activity. Along with neurons, the nervous arrangement as well contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Contempo testify suggests that glia may also take a substantial signaling part.

Nervous systems are found in almost all multicellular animals, merely vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges and microscopic bloblike organisms called placozoans and mesozoans. The nervous systems of ctenophores (rummage jellies) and cnidarians (e.g., anemones, hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types of animals, with the exception of echinoderms and a few types of worms, take a nervous system containing a brain, a central string (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.

At the well-nigh basic level, the function of the nervous organisation is to control motion of the organism and to impact the environment (due east.g., through pheromones). This is achieved by sending signals from i cell to others, or from ane part of the trunk to others. The output from the nervous system derives from signals that travel to muscle cells, causing muscles to exist activated, and from signals that travel to endocrine cells, causing hormones to be released into the bloodstream or other internal fluids. The input to the nervous organisation derives from sensory cells of widely varying types, which transmute physical modalities such as light and sound into neural activeness. 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 arrangement is also capable of storing data over fourth dimension, past dynamically modifying the strength of connections between neurons, every bit well equally other mechanisms.

Contents

• 1 Structure
  • one.1 Cells
    • ane.1.i Neurons
    • i.1.ii Glial cells
• two Beefcake in vertebrates
• 3 Comparative anatomy and evolution
  • 3.i Neural precursors in sponges
  • three.2 Radiata
  • 3.3 Bilateria
    • iii.iii.one Annelids
    • iii.iii.2 Ecdysozoa
      • 3.3.2.1 Nematodes
      • 3.3.2.ii Arthropods
  • iii.4 "Identified" neurons
• iv Function
  • 4.one Neurons and synapses
  • 4.2 Neural circuits and systems
  • iv.iii Reflexes and other stimulus-response circuits
  • iv.4 Intrinsic blueprint generation
• 5 References

Structure

The nervous system derives its name from fretfulness, which are cylindrical bundles of fibers that emanate from the brain and central cord, and co-operative repeatedly to innervate every part of the torso. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans (Finger, 2001, chapter 1), merely their internal structure was not understood until information technology became possible to examine them using a microscope. A microscopic exam shows that nerves consist primarily of the axons of neurons, forth with a multifariousness of membranes that wrap around them. The neurons that give rise to fretfulness do not generally lie within the nerves themselves — their cell bodies reside within the brain, central string, 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-prison cell signalling mechanisms that are precursors to those of neurons (Sakarya et al., 2007). 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 upward the peachy majority of existing species, the nervous organisation has a common structure that originated early in the Cambrian menstruum, over 500 million years ago.

Cells

The nervous system contains 2 master categories or types of cells: neurons and glial cells.

Neurons

The nervous arrangement is divers past the presence of a special type of cell, the neuron (sometimes chosen "neurone" or "nerve jail cell"). Neurons tin can exist distinguished from other cells in a number of ways, but their almost primal 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. Axons frequently travel through the body in bundles called fretfulness (in the PNS) or tracts (in the CNS).

Even in the nervous organisation of a single species such as humans, hundreds of different types of neurons exist, with a wide multifariousness of morphologies and functions. These include sensory neurons that transmute concrete 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

Glial cells (named from the Greek word for "glue") are non-neuronal cells that provide support and nutrition, maintain homeostasis, course myelin, and participate in signal transmission in the nervous system (Allen, 2009). 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 unlike brain areas (Azevedo et al., 2009). Among the well-nigh 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 of import set up of glial jail cell (oligodendrocytes in the vertebrate CNS, and Schwann cells in the PNS) generate layers of a fat substance called myelin that wrap around axons and provide electrical insulation that allows them to transmit signals much more chop-chop and efficiently.

Anatomy in vertebrates

Figure 1: Major divisions of the vertebrate nervous system.

The nervous system of vertebrate animals is divided into two parts called the central nervous organization (CNS) and peripheral nervous arrangement (PNS).

The CNS is the largest part, and includes the encephalon and spinal string. The CNS is enclosed and protected past meninges, a three-layered organization of membranes, including a tough, leathery outer layer chosen the dura mater. The brain is also protected by the skull, and the spinal cord past the vertebral bones. Claret 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 nowadays in the body from gaining entry to the CNS.

The peripheral nervous arrangement (PNS) is a commonage term for the nervous system structures that do not prevarication inside the CNS. The large majority of the axon bundles chosen 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 function consists of the nerves that innervate the peel, joints, and muscles. The cell bodies of somatic sensory neurons lie in dorsal root ganglion of the spinal cord. The visceral part, too known as the autonomic nervous system, contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous organization itself consists of two parts: the sympathetic nervous organization and the parasympathetic nervous organization. Some authors as well include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) equally function of the PNS; others, however, omit them (Hubbard, 1974, p. seven).

The vertebrate nervous organisation can too exist divided into areas chosen gray matter ("grey thing" in British spelling) and white matter. Gray matter (which is just gray in preserved tissue, and is ameliorate described equally pink or light brown in living tissue) contains a loftier proportion of cell bodies of neurons. White matter is composed mainly of myelin-coated axons, and takes its color from the myelin. White affair includes all of the body'southward fretfulness, and much of the interior of the brain and spinal string. Gray matter is found in clusters of neurons in the encephalon and spinal cord, and in cortical layers that line their surfaces. At that place 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, yet, a few exceptions to this rule, notably the part of the brain called the basal ganglia.

Comparative beefcake and evolution

Neural precursors in sponges

Sponges have no cells continued to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, take homologs of many genes that play key roles in synaptic function in other animals. Recent studies accept shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving function of a synapse) (Sakarya, 2007). Even so, the role 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 elementary actions such every bit whole-trunk contraction (Jacobs et al., 2007).

Radiata

Jellyfish, rummage jellies, and related animals have lengthened nerve nets rather than a central nervous arrangement. In most jellyfish the nerve cyberspace is spread more or less evenly beyond the body; in comb jellies information technology is concentrated near the mouth. The nerve nets consist of sensory neurons, which choice up chemic, tactile, and visual signals; motor neurons, which can activate contractions of the torso 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 detached ganglia (Ruppert et al., 2004).

The evolution of the nervous system in radiata is relatively unstructured. Different bilaterians, radiata but have two primordial prison 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., 2006).

Bilateria

Figure 2: Nervous system of a generic bilaterian fauna, in the form of a nerve cord with segmental enlargements, and a "brain" at the front end. (Note: this cartoon shows the nerve cord on the dorsal side of the body, but as the commodity explains, in protostomes it generally lies on the ventral side.)

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 idea to have descended from a mutual wormlike ancestor that appeared during the Cambrian period, 550–600 meg years ago (Balavoine, 2003). The fundamental bilaterian body grade is a tube with a hollow gut crenel running from mouth to anus, and a nerve cord (or 2 parallel nerve cords), with an enlargement (a "ganglion") for each torso segment, with an peculiarly large ganglion at the forepart, chosen the "encephalon". Information technology has not been definitively established whether the generic course of the bilaterian key nervous system is inherited from the and then-called "Urbilaterian" — the last mutual ancestor of all existing bilaterians — or whether divide lines accept evolved similar structures in parallel (Northcutt, 2012). On one mitt, the presence of a shared set of genetic markers, besides as a tripartite brain construction shared past widely separated species (Hirth, 2010), suggest common derivation; on the other mitt, the fact that some modern types of bilaterians (such as echinoderms) lack a central nerve cord, while many lack recognizably tripartite brains, suggest that this might have been the primitive state (Northcutt, 2012).

Vertebrates, annelids, crustaceans, and insects all show the segmented bilaterian body plan at the level of the nervous system. In mammals, the spinal cord contains a serial of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the trunk surface and underlying musculature. On the limbs, the layout of the innervation design is complex, but on the trunk it gives ascent to a series of narrow bands. The top three segments belong to the brain, giving ascension to the forebrain, midbrain, and hindbrain (Ghysen, 2003).

Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups (superphyla) called protostomes and deuterostomes (Erwin et al., 2002). Deuterostomes include vertebrates as well as echinoderms, hemichordates (mainly acorn worms), and Xenoturbellidans (Bourlat et al., 2006). Protostomes, the more than diverse group, include arthropods, molluscs, and numerous types of worms. At that place is a basic difference between the two groups in the placement of the nervous organization within the torso: protostomes possess a nerve cord on the ventral (unremarkably bottom) side of the trunk, whereas in deuterostomes the nerve cord is on the dorsal (usually meridian) side. In fact, numerous aspects of the trunk are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Nearly anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was showtime proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for instance, have nerve cords that run along the ventral midline of the trunk, while all vertebrates have spinal cords that run along the dorsal midline (Lichtneckert and Reichert, 2005).

Annelids

Effigy three: Earthworm nervous organization. Pinnacle: side view of the front of the worm. Bottom: nervous system in isolation, viewed from above

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. Equally an example, earthworms have dual nerve cords running forth the length of the body and merging at the tail and the oral fissure. These nerve cords are connected to each other by transverse nerves resembling the rungs of a ladder. These transverse nerves help coordinate movement of the 2 sides of the creature. Two ganglia at the head end part equally a simple brain. Photoreceptors in the animal'south eyespots provide sensory data on light and dark (Adey, WR).

Ecdysozoa

Ecdysozoa are animals that shed their cuticle. These include nematodoes and arthropods.

Nematodes

The nervous system of i particular type of nematode, the tiny roundworm Caenorhabditis elegans, has been mapped out downwardly to the synaptic level. This has been possible considering in this species, every individual worm (ignoring mutations and sex differences) has an identical set of neurons, with the same locations and chemical features, and the same connections to other cells. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are mapped. The nervous system of C. elegans is sexually dimorphic; the nervous systems of the ii sexes, males and hermaphrodites, have dissimilar numbers of neurons and groups of neurons that perform sex-specific functions. Males accept exactly 383 neurons, while hermaphrodites accept exactly 302 neurons (Hobert, 2005), an unusual characteristic called eutely.

Arthropods

Arthropods, such as insects and crustaceans, accept a nervous system fabricated upward of a series of ganglia, continued by a pair of ventral nerve cords running along the length of the abdomen (Chapman, 1998). Nearly trunk segments take one ganglion on each side, but some are fused to course the brain and other large ganglia. The head segment contains the brain, as well known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately backside the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. Information technology controls the mouthparts, the salivary glands and certain muscles. Many arthropods have well-developed sensory organs, including compound optics for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the encephalon.

In arthropods, about 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 non participate in signalling. A protoplasmic fiber, called the main neurite, runs from the cell torso and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, about parts of the insect encephalon accept passive jail cell bodies arranged effectually the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called "neuropil", in the interior (Chapman, 1998). At that place are, however, important exceptions to this rule, including the mushroom bodies, which play a key part in learning and retentivity.

"Identified" neurons

A neuron is called identified if it has backdrop that distinguish it from every other neuron in the same animal — such as location, neurotransmitter, gene expression pattern, and connectivity — and if every individual organism belonging to the same species has 1 and merely one neuron with the same prepare of properties (Hoyle and Wiersma, 1977). In vertebrate nervous systems very few neurons are "identified" in this sense — in humans, there are believed to be none — only in simpler nervous systems, some or all neurons may exist thus unique. As mentioned above, in the roundworm Caenorhabditis Elegans every neuron in the body is uniquely identifiable, with the aforementioned location and the same connections in every individual worm.

The brains of many molluscs and insects also comprise substantial numbers of identified neurons (Hoyle and Wiersma, 1977). In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish (Stein, 1999). Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and 1 on the right. Each Mauthner jail cell has an axon that crosses over, innervating neurons at the same brain level and and then traveling down through the spinal cord, making numerous connections equally information technology goes. The synapses generated by a Mauthner prison cell are so powerful that a single action potential gives rising to a major behavioral response: within milliseconds the fish curves its trunk into a C-shape, then straightens, thereby propelling itself rapidly forward. Functionally this is a fast escape response, triggered about hands past a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the simply identified neurons in fish — there are about 20 more than types, including pairs of "Mauthner prison cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells normally 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 (Stein, 1999, p. 112). Such neurons announced most unremarkably in the fast escape systems of various species — the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology considering of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, yet, 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 (Simmons and Young, 1999).

Role

The ultimate function of the nervous system is to control the body, especially its movement in the surround. It does this by extracting information from the surroundings using sensory receptors, sending signals that encode this information into the central nervous arrangement, processing the data to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The development of a complex nervous organisation has made it possible for various animal species to have advanced perceptual capabilities 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 information technology possible to have language, abstruse representation of concepts, transmission of culture, and many other features of human being society that would non exist without the human brain.

At the most basic level, the nervous system sends signals from i cell to others, or from one part of the body to others. There are multiple ways that a cell can transport signals to other cells. One is by releasing chemicals chosen hormones into the internal apportionment, so that they can diffuse to distant sites. In dissimilarity to this "broadcast" mode of signaling, the nervous system provides "betoken-to-point" signals — neurons projection their axons to specific target areas and brand synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is as well much faster: the fastest nerve signals travel at speeds that exceed 100 meters per 2nd.

Neurons and synapses

Effigy 4: Major elements in synaptic transmission. An electrochemical moving ridge called an action potential travels forth the axon of a neuron. When the wave reaches a synapse, it provokes release of neurotransmitter molecules, which bind to chemical receptor molecules located in the membrane of the target cell.

Most neurons ship signals via their axons, although some types are capable of emitting signals from their dendrites. In fact, some types of neurons such as the amacrine cells of the retina have no axon, and communicate just via their dendrites. Neural signals propagate forth an axon in the form of electrochemical waves chosen action potentials, which emit jail cell-to-cell signals at points of contact chosen "synapses".

Synapses may be electrical or chemical. Electric synapses pass ions directly between neurons (Hormuzdi et al., 2004), but chemic synapses are much more than mutual, and much more diverse in function. At a chemical synapse, the cell that sends signals is called presynaptic, and the jail cell that receives signals is chosen postsynaptic. Both the presynaptic and postsynaptic regions of contact are full of molecular mechanism that carries out the signalling process. The presynaptic surface area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals. When calcium enters the presynaptic terminal through voltage-gated calcium channels, an arrays of molecules embedded in the membrane are activated, and cause the contents of some vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft. The neurotransmitter and then binds to chemical receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the upshot on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more than circuitous means. For case, release of the neurotransmitter acetylcholine at a synaptic contact betwixt a motor neuron and a muscle prison cell depolarizes the musculus cell and starts a series of events, which results in a contraction of the muscle prison cell. The unabridged synaptic manual process takes merely a fraction of a millisecond, although the furnishings on the postsynaptic cell may last much longer (fifty-fifty indefinitely, in cases where the synaptic betoken leads to the formation of a retention trace).

At that place are literally hundreds of different types of synapses, even within a single species. In fact, there are over a hundred known neurotransmitter chemicals, and many of them activate multiple types of receptors. Many synapses apply more than i neurotransmitter — a common arrangement is for a synapse to utilise one fast-acting minor-molecule neurotransmitter such equally glutamate or GABA, along with 1 or more than peptide neurotransmitters that play slower-acting modulatory roles. Neuroscientists by and large divide receptors into ii broad groups: ligand-gated ion channels and 1000-protein coupled receptors (GPCRs) that rely on 2d messenger signaling. When a ligand-gated ion channel is activated, it opens a aqueduct that allow specific types of ions to menstruation beyond the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory by bringing the membrane potential closer or farther from threshold for triggering an action potential. When a GPCR is activated, it starts a cascade of molecular interactions inside the target jail cell, which may ultimately produce a wide diversity of complex furnishings, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.

According to Dale's principle, which has merely a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses (Strata and Harvey, 1999). This does non mean, though, that a neuron exerts the same issue on all of its targets, because the upshot of a synapse depends not on the neurotransmitter, simply on the receptors that information technology activates. Because different targets can (and oft do) use dissimilar types of receptors, it is possible for a neuron to have excitatory furnishings on one set of target cells, inhibitory effects on others, and complex modulatory effects on others withal. Nevertheless, information technology happens that the 2 most widely used neurotransmitters, glutamate and gamma-Aminobutyric acid (GABA), each have largely consequent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. (There are a few exceptional situations in which GABA has been found to take excitatory furnishings, mainly during early evolution. For a review encounter Marty and Llano, 2005.) Because of this consistency, glutamatergic cells are often referred to equally "excitatory neurons", and GABAergic cells as "inhibitory neurons". 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 fifty-fifty in scholarly publications.

One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic forcefulness. The best-understood course of neural memory is a process called long-term potentiation (abbreviated LTP), which operates at synapses that utilize the neurotransmitter glutamate acting on a special blazon of receptor known every bit the NMDA receptor (Cooke and Bliss, 2006). The NMDA receptor has an "associative" property: if the two cells involved in the synapse are both activated at approximately the same time, a aqueduct opens that permits calcium to flow into the target jail cell (Bliss and Collingridge, 1993). The calcium entry initiates a second messenger cascade that ultimately leads to an increment in the number of glutamate receptors in the target cell, thereby increasing the effective strength of the synapse. This change in strength can last for weeks or longer. Since the discovery of LTP in 1973, many other types of synaptic retentiveness traces have been found, involving increases or decreases in synaptic strength that are induced by varying weather condition, and last for variable periods of time (Cooke and Bliss, 2006). Reward learning, for case, depends on a variant form of LTP that is conditioned on an extra input coming from a reward-signalling pathway that uses dopamine equally neurotransmitter (Kauer and Malenka, 2007). All these forms of synaptic modifiability, taken collectively, give ascent to neural plasticity, that is, to a capability for the nervous organization to adapt itself to variations in the environment.

Neural circuits and systems

The basic neuronal function of sending signals to other cells includes a adequacy for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide diverseness of functions, including characteristic detection, pattern generation, and timing (Dayan and Abbott, 2005). In fact, it is difficult to assign limits to the types of information processing that tin be carried out by neural networks: Warren McCulloch and Walter Pitts proved in 1943 that even artificial neural networks formed from a greatly simplified mathematical brainchild of a neuron are capable of universal computation. Given that individual neurons can generate complex temporal patterns of action independently, the range of capabilities possible for even small groups of neurons are beyond current understanding.

Figure v: Illustration of pain pathway, from René Descartes's Treatise of Human being

Historically, for many years the predominant view of the role of the nervous system was as a stimulus-response associator (Sherrington, 1906). In this formulation, neural processing begins with stimuli that actuate 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 musculus contraction, i.e., to overt responses. Descartes believed that all of the behaviors of animals, and well-nigh of the behaviors of humans, could exist explained in terms of stimulus-response circuits, although he besides believed that higher cerebral functions such as language were not capable of being explained mechanistically. Charles Sherrington, in his influential 1906 book The Integrative Activeness of the Nervous System, developed the concept of stimulus-response mechanisms in much more particular, and Behaviorism, the school of thought that dominated Psychology through the middle of the 20th century, attempted to explicate every attribute of homo behavior in stimulus-response terms (Baum, 2005).

Nevertheless, experimental studies of electrophysiology, offset in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for generating patterns of activeness intrinsically, without requiring an external stimulus (Piccolino, 2002). Neurons were constitute to exist capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation. When intrinsically agile neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more than extensive. A modern formulation views the office of the nervous organisation partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns — both types of activeness interact with each other to generate the full repertoire of behavior.

Reflexes and other stimulus-response circuits

Effigy 6: Simplified schema of basic nervous system part: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal string and then out to motor neurons

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 in between. For case, consider the "withdrawal reflex" causing the hand to wiggle back after a hot stove is touched. The circuit begins with sensory receptors in the peel that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes rut to change the electrical field across the membrane. If the change in electrical potential is big enough, information technology evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which projection (send axonal output) to the aforementioned region of the spinal cord, others projecting into the encephalon. Ane target is a fix of spinal interneurons that project to motor neurons decision-making the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong plenty, 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. The excitatory signals induce contraction of the muscle cells, which causes the articulation angles in the arm to alter, pulling the arm away.

In reality, this straightforward schema is subject to numerous complications. 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 attune the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.

Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the encephalon. Consider, for example, what happens when an object in the periphery of the visual field moves, and a person looks toward it. The initial sensory response, in the retina of the centre, and the final motor response, in the oculomotor nuclei of the encephalon stem, are non all that different from those in a simple reflex, but the intermediate stages are completely dissimilar. Instead of a i or two stride chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cognitive cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei. These areas perform signal-processing functions that include feature detection, perceptual analysis, memory recall, decision-making, and motor planning.

Characteristic detection is the ability to excerpt biologically relevant information from combinations of sensory signals. In the visual system, for case, sensory receptors in the retina of the eye are simply individually capable of detecting "dots of light" in the outside world. Second-level visual neurons receive input from groups of primary receptors, college-level neurons receive input from groups of 2d-level neurons, and so on, forming a hierarchy of processing stages. At each phase, important information is extracted from the signal ensemble and unimportant information is discarded. Past the stop of the process, input signals representing "dots of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The well-nigh sophisticated sensory processing occurs inside the brain, merely complex feature extraction likewise takes place in the spinal cord and in peripheral sensory organs such as the retina.

Intrinsic pattern generation

Although stimulus-response mechanisms are the easiest to understand, the nervous organisation is also capable of controlling the trunk in means that practise not require an external stimulus, past means of internally generated patterns of activity. Because of the variety of voltage-sensitive ion channels that can exist 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 betwixt high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other past excitatory or inhibitory synapses, the resulting networks are capable of a wide diversity of dynamical behaviors, including attractor dynamics, periodicity, and fifty-fifty anarchy. A network of neurons that uses its internal construction to generate spatiotemporally structured output, without requiring a correspondingly structured stimulus, is called a central design generator.

Internal pattern generation operates on a wide range of fourth dimension scales, from milliseconds to hours or longer. 1 of the most of import types of temporal pattern is circadian rhythmicity — that is, rhythmicity with a period of approximately 24 hours. All animals that accept been studied show circadian fluctuations in neural activity, which control cyclic alternations in behavior such as the sleep-wake bicycle. Experimental studies dating from the 1990s have shown that cyclic 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. Animals equally diverse every bit insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by lite only continues to operate even when light levels are held constant and no other external time-of-mean solar day cues are available. The clock genes are expressed in many parts of the nervous arrangement besides as many peripheral organs, just in mammals all of these "tissue clocks" are kept in synchrony past signals that emanate from a master timekeeper in a tiny part of the encephalon called the suprachiasmatic nucleus.

References

• Adey, WR (1951). The nervous system of the earthworm Megascolex. J. Comp. Neurol. 94 (1): 57–103. doi:10.1002/cne.900940104. PMID: 14814220.

• Allen NJ, Barres BA (2009). Neuroscience: Glia - more than but brain glue. Nature 457 (7230): 675–7. doi:10.1038/457675a. PMID: 19194443.

• Azevedo FA, Carvalho LR, Grinberg LT, et al. (2009). Equal numbers of neuronal and nonneuronal cells make the human being encephalon an isometrically scaled-up primate brain. J. Comp. Neurol. 513 (v): 532–41. doi:10.1002/cne.21974. PMID: 19226510.

• Balavoine Chiliad (2003). The segmented Urbilateria: A testable scenario. Int. Comp. Biol. 43 (one): 137–47. doi:x.1093/icb/43.ane.137. Archived from the original on Dec. 2003. http://icb.oxfordjournals.org/cgi/content/full/43/one/137.

• Baum WM (2005). Understanding behaviorism: Behavior, Culture and Evolution . Blackwell. ISBN 978-i-4051-1262-8.

• Elation Boob tube, Collingridge GL (Jan 1993). A synaptic model of retentivity: long-term potentiation in the hippocampus. Nature 361 (6407): 31–ix. doi:x.1038/361031a0. PMID: 8421494.

• Bourlat SJ, Juliusdottir T, Lowe CJ, et al. (2006). Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature 444 (7115): 85–8. doi:10.1038/nature05241. PMID: 17051155.

• Chapman RF (1998). "Ch. 20: Nervous system". The insects: structure and function . Cambridge University Press. pp. 533–568. ISBN 978-0-521-57890-5.

• Cooke SF, Elation TV (2006). Plasticity in the human primal nervous system. Brain 129 (Pt 7): 1659–73. doi:10.1093/encephalon/awl082. PMID: 16672292.

• Dayan P, Abbott LF (2005). Theoretical Neuroscience: Computational and Mathematical Modeling of Neural Systems . MIT Printing. ISBN 978-0-262-54185-5.

• Erwin DH, Davidson EH (2002). The concluding common bilaterian ancestor. Development 129 (13): 3021–32. PMID: 12070079.

• Finger Due south (2001). "Ch. 1: The brain in artifact". Origins of neuroscience: a history of explorations into brain function . Oxford Univ. Press. ISBN 978-0-19-514694-3.

• Ghysen A (2003). The origin and evolution of the nervous system. Int. J. Dev. Biol. 47 (7–8): 555–62. PMID: 14756331. Archived from the original on Dec. 2003. http://www.ijdb.ehu.es/spider web/paper.php?doi=14756331.

• Hirth F (2010). On the origin and development of the tripartite brain. Brain Behav. Evol. 76 (i): 3–10. doi:10.1159/000320218. PMID: 20926853.

• Hobert, O (2005). Specification of the nervous organisation. WormBook, ed. The C. elegans Research Community, doi:10.1895/wormbook.1.12.i, http://www.wormbook.org.

• Hormuzdi SG, Filippov MA, Mitropoulou G, et al. (2004). Electric synapses: a dynamic signaling system that shapes the action of neuronal networks. Biochim. Biophys. Acta 1662 (1–2): 113–37. doi:ten.1016/j.bbamem.2003.10.023. PMID: 15033583.

• Hoyle Thousand, Wiersma CAG (1977). Identified neurons and behavior of arthropods . Plenum Press. ISBN 978-0-306-31001-0.

• Hubbard JI (1974). The peripheral nervous arrangement . Plenum Press. p. vii. ISBN 978-0-306-30764-5.

• Jacobs DK, Nakanishi Due north, Yuan D, et al. (2007). Evolution of sensory structures in basal metazoa. Integr Comp Biol 47 (5): 712–723. doi:10.1093/icb/icm094. PMID: 21669752. Archived from the original on Nov. 2007. http://icb.oxfordjournals.org/cgi/content/full/47/5/712.

• Kauer JA, Malenka RC (2007). Synaptic plasticity and habit. Nat. Rev. Neurosci. 8 (eleven): 844–58. doi:x.1038/nrn2234. PMID: 17948030.

• Lichtneckert R, Reichert H (2005). Insights into the urbilaterian brain: conserved genetic patterning mechanisms in insect and vertebrate encephalon development. Heredity 94 (five): 465–77. doi:10.1038/sj.hdy.6800664. PMID: 15770230.

• Marty A, Llano I (2005). Excitatory effects of GABA in established encephalon networks. Trends Neurosci. 28 (6): 284–9. doi:10.1016/j.tins.2005.04.003. PMID: 15927683.

• McCulloch WS, Pitts W (1943). A logical calculus of the ideas immanent in nervous activity. Balderdash. Math. Biophys. 5 (4): 115–133. doi:10.1007/BF02478259.

• Northcutt RG (2012). Evolution of centralized nervous systems: two schools of evolutionary idea. Proc. Natl. Acad. Sci. U.S.A. 109 Suppl 1: 10626–33. doi:x.1073/pnas.1201889109. PMID: 22723354.

• Piccolino Yard (November 2002). Fifty years of the Hodgkin-Huxley era. Trends Neurosci. 25 (11): 552–3. doi:ten.1016/S0166-2236(02)02276-2. PMID: 12392928.

• Ruppert EE, Fox RS, Barnes RD (2004). Invertebrate Zoology (7 ed.). Brooks/Cole. pp. 111–124. ISBN 0-03-025982-seven.

• Sakarya O, Armstrong KA, Adamska One thousand, et al. (2007). Vosshall, Leslie. ed. A mail service-synaptic scaffold at the origin of the creature kingdom. PLoS ONE two (vi): e506. doi:10.1371/journal.pone.0000506. PMID: 17551586. }

• Sanes DH, Reh TA, Harris WA (2006). Development of the nervous system . Bookish Press. pp. 3–iv. ISBN 978-0-12-618621-v.

• Sherrington CS (1906). [Integrative Activeness of the Nervous System]. Scribner. Archived from the original on Dec. 2010. http://books.google.com/?id=6KwRAAAAYAAJ.

• Simmons PJ, Young D (1999). Nervus cells and animal behaviour . Cambridge University Press. p. 43. ISBN 978-0-521-62726-9.

• Strata P, Harvey R (1999). Dale's principle. Brain Res. Bull. 50 (5–vi): 349–50. doi:10.1016/S0361-9230(99)00100-eight. PMID: 10643431.

• Stein PSG (1999). Neurons, Networks, and Motor Behavior . MIT Press. pp. 38–44. ISBN 978-0-262-69227-4.

Source: http://www.scholarpedia.org/article/Nervous_system

Posted by: martinhambsood.blogspot.com

Share this post