<text>Bannister, Roger, ed., Brain's Clinical Neurology, 6th ed.(1985); Bullock, T. H., Introduction to Nervous Systems (1977); Chalmers, N.,et al., eds., The Biological Bases of Behaviour (1971); Chapouthier, G., andMatras, J. J., The Nervous System and How It Functions (1986); Eccles, J. C.,The Understanding of the Brain, 2d ed. (1977); Fentress, J. C., SimplerNetworks and Behavior (1976); Gilroy, J., Basic Neurology (1982); Kee, LeongS., An Introduction to the Human Nervous System (1987); Kuffler, S. W., andNichols, J. G., From Neuron to Brain, 2d ed. (1984); Nathan, Peter, The NervousSystem, 3d ed. (1988); Noback, Charles R., and Demarest, Robert J., The HumanNervous Systtem (1985); Parnavelas, John G., et al., eds., The Making of theNervous System (1988); Steklis, Horst D., and Erwin, J., eds., Neurosciences(1987); Toates, Frederick, Biological Foundations of Behavior (1986).</text>
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<text>The precision and intricacy of nervous system architecture is well illustrated in the mammalian cerebellum, a large structure at the back of the head that plays an important role in various forms of sensory-motor coordination. Five types of neuron--Purkinje, Golgi, stellate, basket, and granule cells--are located here and are distributed in precise relationship to one another, permitting excitation and inhibition for the control of a variety of skilled tasks. A similar ordered complexity is evident among the five cell types of the vertebrate retina--photoreceptor, horizontal, bipolar, amacrine, and retinal ganglion cells. The highly ordered interactions among these cells process the rich details of the visual environment. Similarly impressive specializations are found in other receptors and central nervous system structures throughout all higher forms of animal life.</text>
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<text><span class="style1">yelencephalonThe </span><span class="style2"><a href="#" class="group">BRAIN</a></span><span class="style1"> connects with the spinal cord at the medulla, or myelencephalon, the site of the more primitive regulatory functions. Great fiber tracts between the brain and spinal cord cross in the medulla. The entrance and exit points of 12 cranial nerves, which serve a variety of somatic and visceral functions, are located in the medulla as well as in the next higher level of the brain, the pons.Metencephalon, Cerebellum, and MesencephalonThe pons (the metencephalon) and the cerebellum are responsible for many basic tasks of sensory and motor coordination. The next level of the brain, the mesencephalon (midbrain), is involved with still more complex functions of sensory-motor processing. The brain stem (medulla, pons, and midbrain) houses the reticular formation, a complex structure that combines many otherwise separate sensory and motor functions. The reticular formation also influences generalized levels of consciousness, including cycles of waking and sleeping.DiencephalonThe diencephalon contains complex integrative structures, including the thalamus, which coordinates multiple sensory signals, and the hypothalamus, which plays a critical role in motivated behaviors, such as feeding, drinking, mating, and fighting. The hypothalamus is a major site of neurosecretion, connecting the nervous system to the endocrine (hormone) system by means of the pituitary.TelencephalonThe telencephalon, largely the cerebrum, is the apex of sophisticated brain structure and undoubtedly plays a major role in the more subtle emotional, coordinative, and intellectual functions. The cerebral cortex, the highly complex outer portion, is divided into three major zones: the archipallium, the paleopallium, and the neopallium. It is at the cortical level that the most-striking features of precise nervous-system organization are represented. For example, in the motor cortex as well as the somatosensory ("body" sensory) cortex of mammals, various body segments are anatomically localized in proportion to their relative importance. These regions are even further specialized; for example, in the somatosensory cortex, specific columns of cells provide awareness of joint movement, light touch, or deflection of body hair. Similarly, cortical regions are present for the processing of auditory, visual, olfactory, and other such stimuli. In the visual cortex, columns of cells respond specifically to visual inputs that have certain orientations. </span></text>
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<text>BRAIN</text>
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<text>At least four distinguishable classes of operation exist for the vertebrate nervous system. First, sensory inputs can generate the specific details of motor response, a direct transfer process. More commonly, the details of sensory input depend upon the integration of afferent nerve stimulation as well as the current state of responsiveness of interneurons and motor neurons. Afferent signals can also serve to trigger responses, the characteristics of which are basically inherent to the structure stimulated. Finally, integrative and motor cells can fire spontaneously, even in the absence of sensory input. In each case, the vertebrate spinal cord normally acts in cooperation with the brain. </text>
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<text><span class="style1">erhaps the most interesting feature derived from studies of nervous system reflexes is the highly intimate and intricate connection between sensory inputs and motor outputs. The general principle of </span><span class="style2"><a href="#" class="group">FEEDBACK</a></span><span class="style1"> control appears to be important for complex nervous-system functions. Mammalian muscle fibers, for example, contain specialized sensory cells (spindle organs) that convey to the spinal cord information concerning their current state of contraction. The output to these muscles from both the spinal cord and the brain depends, in part, upon the signals from spindle fibers. Further, the brain can send special signals by way of the efferent system, which selectively changes the bias of these muscle spindles and affects the responsiveness of the muscle to signals of the motor neurons. Thus, sensory signals affect motor responses, which in turn affect sensory signals. </span></text>
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<text>FEEDBACK</text>
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<text><span class="style1">ne of the more interesting and important features of the spinal cord is that even when it is separated from the brain, it can control fundamental integrative functions known as reflexes. A </span><span class="style2"><a href="#" class="group">REFLEX</a></span><span class="style1"> can be considered schematically in terms of a sensory (receptor or afferent) cell that excites an interneuron, which in turn excites a motor (efferent) neuron. A variable lapse between the stimulus and the response reflects the time necessary to process the signal carried by the reflex afferent nerves. The strength of the response corresponds to a summation through time as well as over the different pathways. The response to a standard signal may take some time to develop fully. After the signal is removed the response may continue for a certain period of time. Different reflexes within the system interact with each other in complex patterns of mutual excitation and inhibition. </span></text>
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<text>REFLEX</text>
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<text>Lower OrganismsPrimitive nervous systems, such as those found in the nerve nets of coelenterates, do not have nerve-cell bodies that are grouped into ganglia or true brains. Individual neurons are poorly differentiated and not insulated by myelin sheaths, and the signals can travel in any direction across a synapse. The flatworms (Platyhelminthes) have a more highly ordered nervous system that is bilaterally symmetrical. The soma are collected into ganglia, and axons are grouped together, forming nerves. Flatworms receive environmental information by means of well-differentiated sense organs. The segmented roundworms (Annelida) show an even greater sophistication of nerves, controlled by a true brain. Highly complex and sophisticated patterns of nervous system organization are apparent in insects and crustaceans (Arthropoda) and mollusks (Mollusca).VertebratesIn the various vertebrate species sensory and motor nerve fibers are separated within the various spinal cord segments. Clearly demarcated synaptic regions contain cell bodies (gray matter), and equally well-defined tracts of myelinated axons (white matter) exist. A large, well-organized brain coordinates all nervous system functions.Spinal CordA vertebrate spinal cord in cross-section resembles a gray butterfly--the gray matter with cell bodies and associated synapses is arranged within a white oval, which contains the incoming (afferent) and outgoing (efferent) axons. In each segment sensory afferent fibers are concentrated into dorsal roots that are connected to associated ganglia outside of the spinal cord. Efferent fibers are concentrated in the ventral roots. The spinal cord's cervical, thoracic, lumbar, and sacral regions, control incoming, integrative, and outgoing nervous functions for successive body parts.Autonomic SystemThe spinal cord also houses and protects the two main branches of the Autonomic Nervous System, which controls involuntary, unconscious actions of smooth muscle and glands. These two branches are the sympathetic and parasympathetic systems. The thoracic and lumbar segments of the spinal cord contain nerves of the sympathetic system. The sympathetic system serves an adrenergic function, mobilizing the organism in a "fight or flight" reaction in emergencies. The parasympathetic system, located in cranial and sacral segments, is primarily cholinergic in function, serving to relax the organism. </text>
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<text><span class="style1">Synaptic CleftThe points of closest affinity and functional connection between neurons are known as synapses. Actually, most neurons do not signal each other by direct anatomical contact; they are normally separated by a small synaptic cleft of approximately 200 angstroms (one angstrom equals one hundred-millionth of a centimeter). </span><span class="style2"><a href="#" class="group">NEUROTRANSMITTERS</a></span><span class="style1">--chemical substances that effect impulses--are released into the synaptic cleft from the presynaptic terminals. They are received by the postsynaptic membrane of the receiving neuron. Occasionally, tight junctions exist, in which the transmission of information from one neuron to the next is primarily electrical, not chemical. Most common synapses occur between the axon of the sender cell and the dendrites or soma of the receiver cell. Synapses between two axons, dendrites, or soma are also known to occur. The particular synaptic location between adjacent cells can affect the types of signals that are transmitted. Also, in higher animals the synapses carry information primarily in one direction only, greatly enhancing the sophistication of neural codes. </span></text>
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<text>NEUROTRANSMITTERS</text>
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<text> Structure and FunctionThe axons of many vertebrate neurons are covered with a myelin sheath, which acts like insulation on a wire and greatly extends the efficiency of the conduction of spike discharges. This insulating sheath is made of glial cells. In the vertebrate central nervous system, glial cells are distinguished as oligodendroglia, and in the peripheral nervous system they are called Schwann cells. True myelin sheaths are not found in invertebrates, although similar coverings do exist in fast-conducting giant fiber systems of earthworms and shrimps. Myelin sheaths promote speed and reliability of nerve-impulse conduction. The 12 micron myelinated axon of a frog can conduct a signal as rapidly as the 350 micron axon of a squid. The one million myelinated axons in a mammalian optic nerve, which is 2 mm in diameter, would need an estimated diameter of nearly 50 mm to function at the same efficiency if they did not have a myelin sheath. Nodes of RanvierMyelinated fibers have small gaps, called nodes of Ranvier, between successive glial cells; the action potentials between them jump in a process called saltatory conduction. Humans with myelin deficiency, such as those afflicted with multiple sclerosis, can experience serious difficulties in nervous system functioning because of decreased speeds and reliability of conduction. </text>
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<text><span class="style1">Graded PotentialIn spite of their physical diversity, most nerve cells operate in a similar way--they generate and carry two basic types of electrochemical signals known as graded potentials and spike discharges (see </span><span class="style2"><a href="#" class="group">NEUROPHYSIOLOGY</a></span><span class="style1">). Because of the unequal distribution of some ions across the nerve-cell membrane, an inactive nerve cell has what is known as a resting potential, with the inside of the cell having a negative charge with respect to the outside of the cell. When the neuron is depolarized, this potential difference between inside and outside is reduced, and the nerve cell propagates an impulse to the end of the fiber. The graded-potential change, caused by depolarization of the dendrites, moves along the branches toward the soma. Spike DischargeIf the total depolarization is great enough, one or more spike discharges can be generated at the base of the axon and carried toward the synaptic terminal. In spikes, the frequency of discharges codes for the intensity of the signal. When these spikes reach synaptic bulbs, they induce the release of special chemical transmitters that have been stored in vesicles and can give rise to graded, postsynaptic potentials in the dendrites or soma of the next neuron. HyperpolarizationA neuron becomes hyperpolarized when its resting potential increases, causing the neuron to be less likely to carry messages. The neuron, as a result, becomes inhibited rather than excited. Certain transmitters are excitatory, stimulating nerve-impulse propagation, and others are inhibitory, causing hyperpolarization to occur. </span></text>
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<text>NEUROPHYSIOLOGY</text>
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<text>Receptors are neurons that bring information into the nervous system. Classically, sensory reception has been divided into the five senses of hearing, vision, touch, taste, and smell. Today receptors are more commonly classified in terms of the physical forms of stimulation that excite them: chemoreception, electroreception, mechanoreception, photoreception, and thermoreception. Nocireceptors are stimulated by damage to tissues. </text>
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<text><span class="style1"> neuron usually is considered multipolar, referring to the many processes emanating from the soma, and heteropolar, because these processes are anatomically distinct (axons and dendrites). Multipolar-heteropolar neurons are the dominant class of neural cells in the vertebrate central nervous system. Most sensory neurons are bipolar and are found in the peripheral nervous system. In higher invertebrates, such as insects, neurons with a single process connected to the soma most commonly function both as interneurons and motor neurons. These nerve cells are classified as unipolar. When the processes of a neuron cannot be readily distinguished on anatomical grounds, the neuron is called isopolar. Isopolar neurons appear to be the most primitive nerve cells; they commonly occur, for example, in the nerve nets of coelenterates (jellyfish and hydra). Higher vertebrates also contain some isopolar nerve cells, such as the amacrine cells of the human retina. It is likely that all of these types of neurons developed from </span><span class="style2"><a href="#" class="group">EPITHELIUM</a></span><span class="style1"> cells (cells of skin and organ membranes), which exhibit a limited capacity for excitation and conduction. </span></text>
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<text>EPITHELIUM</text>
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<text><span class="style1"> neuron of the human brain has three main regions: the cell body (soma), the dendrites, and the axon. The cell body, which ranges in size from 2 to 500 micra (or microns) in diameter. It contains the basic constituents of most animal </span><span class="style2"><a href="#" class="group">CELLS</a></span><span class="style1">--a nucleus, which has a nucleolus and chromosomes; and such cytoplasmic bodies as </span><span class="style2"><a href="#" class="group">RIBOSOMES</a></span><span class="style1">, </span><span class="style2"><a href="#" class="group">MITOCHONDRIA</a></span><span class="style1">, and </span><span class="style2"><a href="#" class="group">ENDOPLASMIC RETICULUM</a></span><span class="style1">. The dendrites are branched structures that have cytoplasmic continuity with the cell body. They function to receive signals from other nerve cells. The axon is a long fiber--up to 9 m (30 ft) in some whales--that is relatively uniform in diameter and often is covered with a myelin sheath. It normally serves to transmit information from one neuron to adjacent neurons. Neurons of many animals also contain specialized structures that serve diverse functions. Nissl bodies contain ribonucleoproteins and are especially involved in protein synthesis. Fibrillar structures have neurofilaments and microtubules and appear to participate in the transport of various substances throughout the neuron. Synaptic bulbs, often located at the ends of axons, are clear swellings that contain round, flattened vesicles and seem to be involved with the transmission of impulses. </span></text>
<text>The basic building block of a nervous system is the neuron, or nerve cell, a cell that is specialized for the transmission of information into, within, and out of the animal. The human brain alone has about one trillion individual nerve cells, any one of which may have several thousand direct connections to other nerve cells in the system. </text>
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<text><span class="style1"> highly organized collection of cells known as nerve cells, or neurons, constitutes the nervous system, which is found in all higher forms of animal life. These nerve cells collect information from the environment by means of receptors. They coordinate the information with the internal activities of the organism in a process known as integration. They also store information in terms of </span><span class="style2"><a href="#" class="group">MEMORY</a></span><span class="style1"> and generate adaptive patterns of behavior. Nervous systems vary greatly in form and complexity. They range from the simple nerve nets of such coelenterates as jellyfish to the elaborate, segmentally organized, and bilaterally symmetrical structures of higher invertebrates and most vertebrate species. Unicellular animals, such as those of the genus Protozoa, do not have nervous systems. Although these animals can produce coordinated responses to their environments, the fine gradations and various combinations of responses that characterize the nervous systems of higher animals are absent. Simple multicelled animals, such as sponges (genus Porifera), can give localized responses to various types of stimulation, but because they lack nervous systems they do not have the ability to integrate signals and responses in such a way as to achieve flexibility and unity of action. Nevertheless, these simple animals have cells surrounded by membranes that selectively permit the passage of electrically charged chemicals (ions). These in turn stimulate some degree of behavioral response (see </span><span class="style2"><a href="#" class="group">BIOPOTENTIAL</a></span><span class="style1">). In animals with nervous systems the membrane properties of individual nerve cells are extremely specialized. As a result, electrochemical signals are able to carry information from one neuron to the next, often in complex patterns and combinations. Further, in more highly evolved forms of animal life many types of signals arise that involve both excitation and inhibition; these animal forms also have developed specialized regions within the nervous system for particular functions such as seeing, hearing, feeding, and mating. The nervous system of higher organisms such as human beings is divided into a central system that comprises the </span><span class="style2"><a href="#" class="group">BRAIN</a></span><span class="style1"> and </span><span class="style2"><a href="#" class="group">SPINAL CORD</a></span><span class="style1">, and a peripheral system that comprises the remaining nervous tissue. The central nervous system coordinates the activity of the system as a whole. The motor nervous system innervates skeletal muscle, skin, and joints and controls voluntary actions, whereas the autonomic nervous system coordinates mainly involuntary actions such as heartbeat. </span></text>
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<text>BIOPOTENTIALBRAINMEMORYSPINAL CORD</text>
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<text>Nervous System HyperTextBook 2.0 Level 1</text>
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<text>Nervous SystemTHE NEURONStructure and FunctionPolarityReceptorsNEURAL FUNCTIONMYELINSYNAPSETYPES OF NERVOUS SYSTEMSREFLEXFeedbackOperation of NeuronsTHE BRAININTEGRATIONBibliography</text>
 (January 1993).iso/Files/Hyper/N-O/Nervous System HTB 2L1.cpt/Nervous System HTB 2.0 L1/BMAP_8942.png){kind=link}
 (January 1993).iso/Files/Hyper/N-O/Nervous System HTB 2L1.cpt/Nervous System HTB 2.0 L1/PATs.png){kind=link}