Nervous system (redirected from Human Nervous System)

nervous system, network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system. Invertebrate animals show varying degrees of complexity in their nervous systems, but it is in the vertebrate animals (phylum Chordata, subphylum Vertebrata) that the system reaches its greatest complexity.

Anatomy and Function

In vertebrates the system has two main divisions, the central and the peripheral nervous systems. The central nervous system consists of the brain

Anatomy and Function

.....  and spinal cord. Linked to these are the cranial, spinal, and autonomic nerves, which, with their branches, constitute the peripheral nervous system. The brain might be compared to a computer and its memory banks, the spinal cord to the conducting cable for the computer's input and output, and the nerves to a circuit supplying input information to the cable and transmitting the output to muscles and organs.

The nervous system is built up of nerve cells, called neurons, which are supported and protected by other cells. Of the 200 billion or so neurons making up the human nervous system, approximately half are found in the brain. From the cell body of a typical neuron extend one or more outgrowths (dendrites), threadlike structures that divide and subdivide into ever smaller branches. Another, usually longer structure called the axon also stretches from the cell body. It sometimes branches along its length but always branches at its microscopic tip. When the cell body of a neuron is chemically stimulated, it generates an impulse that passes from the axon of one neuron to the dendrite of another; the junction between axon and dendrite is called a synapse. Such impulses carry information throughout the nervous system. Electrical impulses may pass directly from axon to axon, from axon to dendrite, or from dendrite to dendrite.

So-called white matter in the central nervous system consists primarily of axons coated with light-colored myelin produced by certain neuroglial cells. Nerve cell bodies that are not coated with white matter are known as gray matter. Nonmyelinated axons that are outside the central nervous system are enclosed only in a tubelike neurilemma sheath composed of Schwann cells, which are necessary for nerve regeneration. There are regular intervals along peripheral axons where the myelin sheath is interrupted. These areas, called nodes of Ranvier, are the points between which nerve impulses, in myelinated fibers, jump, rather than pass, continuously along the fiber (as is the case in unmyelinated fibers). Transmission of impulses is faster in myelinated nerves, varying from about 3 to 300 ft (1–91 m) per sec.

Both myelinated and unmyelinated dendrites and axons are termed nerve fibers; a nerve is a bundle of nerve fibers; a cluster of nerve cell bodies (neurons) on a peripheral nerve is called a ganglion. Neurons are located either in the brain, in the spinal cord, or in peripheral ganglia. Grouped and interconnected ganglia form a plexus, or nerve center. Sensory (afferent) nerve fibers deliver impulses from receptor terminals in the skin and organs to the central nervous system via the peripheral nervous system. Motor (efferent) fibers carry impulses from the central nervous system to effector terminals in muscles and glands via the peripheral system.

The peripheral system has 12 pairs of cranial nerves: olfactory, optic, oculomotor, trochlear, trigeminal, abducent, facial, vestibulo-cochlear (formerly known as acoustic), glossopharyngeal, vagus, spinal accessory, and hypoglossal. These have their origin in the brain and primarily control the activities of structures in the head and neck. The spinal nerves arise in the spinal cord, 31 pairs radiating to either side of the body: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Autonomic Nervous System

The autonomic nerve fibers form a subsidiary system that regulates the iris of the eye and the smooth-muscle action of the heart, blood vessels, glands, lungs, stomach, colon, bladder, and other visceral organs not subject to willful control. Although the autonomic nervous system's impulses originate in the central nervous system, it performs the most basic human functions more or less automatically, without conscious intervention of higher brain centers. Because it is linked to those centers, however, the autonomic system is influenced by the emotions; for example, anger can increase the rate of heartbeat. All of the fibers of the autonomic nervous system are motor channels, and their impulses arise from the nerve tissue itself, so that the organs they innervate perform more or less involuntarily and do not require stimulation to function.

Autonomic nerve fibers exit from the central nervous system as part of other peripheral nerves but branch from them to form two more subsystems: the sympathetic and parasympathetic nervous systems, the actions of which usually oppose each other. For example, sympathetic nerves cause arteries to contract while parasympathetic nerves cause them to dilate. Sympathetic impulses are conducted to the organs by two or more neurons. The cell body of the first lies within the central nervous system and that of the second in an external ganglion. Eighteen pairs of such ganglia interconnect by nerve fibers to form a double chain just outside the spine and running parallel to it. Parasympathetic impulses are also relayed by at least two neurons, but the cell body of the second generally lies near or within the target organ.

The Nervous System and Reflexes

In general, nerve function is dependent on both sensory and motor fibers, sensory stimulation evoking motor response. Even the autonomic system is activated by sensory impulses from receptors in the organ or muscle. Where especially sensitive areas or powerful stimuli are concerned, it is not always necessary for a sensory impulse to reach the brain in order to trigger motor response. A sensory neuron may link directly to a motor neuron at a synapse in the spinal cord, forming a reflex arc that performs automatically. Thus, tapping the tendon below the kneecap causes the leg to jerk involuntarily because the impulse provoked by the tap, after traveling to the spinal cord, travels directly back to the leg muscle. Such a response is called an involuntary reflex action.

Commonly, the reflex arc includes one or more connector neurons that exert a modulating effect, allowing varying degrees of response, e.g., according to whether the stimulation is strong, weak, or prolonged. Reflex arcs are often linked with other arcs by nerve fibers in the spinal cord. Consequently, a number of reflex muscle responses may be triggered simultaneously, as when a person shudders and jerks away from the touch of an insect. Links between the reflex arcs and higher centers enable the brain to identify a sensory stimulus, such as pain; to note the reflex response, such as withdrawal; and to inhibit that response, as when the arm is held steady against the prick of a hypodermic needle.

Reflex patterns are inherited rather than learned, having evolved as involuntary survival mechanisms. But voluntary actions initiated in the brain may become reflex actions through continued association of a particular stimulus with a certain result. In such cases, an alteration of impulse routes occurs that permits responses without mediation by higher nerve centers. Such responses are called conditioned reflexes, the most famous example being one of the experiments Ivan Pavlov performed with dogs. After the dogs had learned to associate the provision of food with the sound of a bell, they salivated at the sound of the bell even when food was not offered. Habit formation and much of learning are dependent on conditioned reflexes. To illustrate, the brain of a student typist must coordinate sensory impulses from both the eyes and the muscles in order to direct the fingers to particular keys. After enough repetition the fingers automatically find and strike the proper keys even if the eyes are closed. The student has "learned" to type; that is, typing has become a conditioned reflex.

Disorders of the Nervous System

A number of diseases can significantly affect the proper functioning of the nervous system. Parkinson's disease, Huntington's disease, myasthenia gravis, and amyotrophic lateral sclerosis (commonly known as Lou Gehrig's disease) are some of the more severe diseases affecting the nervous system. Strokes, which are related to circulatory disorders, also may have permanent effects on the nervous system. Certain plant derivatives, such as belladonna, cocaine, and caffeine, have a variety of stimulatory, inhibitory, and hallucinatory effects on the nervous system.

Bibliography

See D. Ottoson, Physiology of the Nervous System (1982); G. Chapouthier and J. J. Matras, The Nervous System and How It Functions (1986); L. S. Kee, Introduction to the Human Nervous System (1987); P. Nathan, The Nervous System (3d ed. 1988); J. G. Panavelas et al., ed. The Making of the Nervous System (1988).

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nervous system

System of specialized cells (neurons, or nerve cells) that conduct stimuli from a sensory receptor through a neuron network to the site (e.g., a gland or muscle) where the response occurs. In humans, it consists of the central and peripheral nervous systems, the former consisting of the brain and spinal cord and the latter of the nerves, which carry impulses to and from the central nervous system. The cranial nerves handle head and neck sensory and motor activities, except the vagus nerve, which conducts signals to visceral organs. Each spinal nerve is attached to the spinal cord by a sensory and a motor root. These exit between the vertebrae and merge to form a large mixed nerve, which branches to supply a defined area of the body. Disorders include amyotrophic lateral sclerosis, chorea, epilepsy, myasthenia gravis, neural tube defect, parkinsonism, and poliomyelitis. Effects of disorders range from transient tics and minor personality changes to major personality disruptions, seizures, paralysis, and death.

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Nervous system (invertebrate)

All multicellular organisms have a nervous system, which may be defined as assemblages of cells specialized by their shape and function to act as the major coordinating organ of the body. Nervous tissue underlies the ability to sense the environment, to move and react to stimuli, and to generate and control all behavior of the organism. Compared to vertebrate nervous systems, invertebrate systems are somewhat simpler and can be more easily analyzed. Invertebrate nerve cells tend to be much larger and fewer in number than those of vertebrates. They are also easily accessible and less complexly organized; and they are hardy and amenable to revealing experimental manipulations. However, the rules governing the structure, chemistry, organization, and function of nervous tissue have been strongly conserved phylogenetically. Therefore, although humans and the higher vertebrates have unique behavioral and intellectual capabilities, the underlying physical-chemical principles of nerve cell activity and the strategies for organizing higher nervous systems are already present in the lower forms. Thus neuroscientists have taken advantage of the simpler nervous systems of invertebrates to acquire further understanding of those processes by which all brains function. See Nervous system (vertebrate)

Invertebrate and vertebrate nerve cells differ more in quantity, or degree, than in qualitative features. Aside from differences in size and numbers, the most striking difference is that invertebrate neurons have a unipolar shape, whereas most vertebrate neurons are multipolar. An additional general contrast between invertebrate and vertebrate nervous systems is that invertebrates tend to have more neurons displaced to the periphery (outside the central nervous system) and to perform more integrative and processing functions in the periphery. Vertebrates perform almost all their integration within the central nervous system, using interneurons. Invertebrate nervous systems also seem to have a greater potential for regrowth, regeneration, or repair after damage than do vertebrate nerve cells. Many invertebrates continue to add new nerve cells to their ganglia with age; vertebrates, in general, do not. Only vertebrate neurons have myelin sheaths, a specialized wrapping of glial membrane around axons, increasing their conduction speed. Invertebrates tend to enhance conduction velocity by using giant axons, particularly for certain escape responses.

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Nervous system (vertebrate)

A coordinating and integrating system which functions in the adaptation of an organism to its environment. An environmental stimulus causes a response in an organism when specialized structures, receptors, are excited. Excitations are conducted by nerves to effectors which act to adapt the organism to the changed conditions of the environment.

Comparative morphology

The brain of all vertebrates, including humans, consists of three basic divisions: prosencephalon, mesencephalon, and rhombencephalon (Fig. 1). The individual divisions or patterns of the brain do not function separately to bring about a final response; rather, each pattern acts on a common set of connections in the spinal cord.

Lateral views of several vertebrate brains showing evolutionary relationships

Spinal patterns are the final common patterns used by all higher brain pathways to influence all organs of the body. These reflexes are divided into two basic patterns: the monosynaptic arc and the multisynaptic arc. The monosynaptic arc, or myotatic reflex, maintains tonus and posture in vertebrates and consists of two neurons, a sensory and a motor neuron.

The multisynaptic arc, or flexor reflex, is the pattern by which an animal withdraws a part of its body from a noxious stimulus. Both sensory neurons and internuncial neurons send information to brain centers. Coordinated limb movement is based on a connective pattern of neurons at the spinal level.

The structure of the spinal cord and its connections are basically similar among all vertebrates. The major evolutionary changes in the spinal cord have been the increased segregation of cells and fibers of a common function from cells and fibers of other functions and the increase in the length of fibers which connect brain centers with spinal centers. See Postural equilibrium

The rhombencephalon of the brain is subdivided into a roof, or cerebellum, and a floor, or medulla oblongata. The medulla is similar to the spinal cord and is divided into a dorsal sensory region and a ventral motor region. It is an integrating and relay area between higher brain centers and the spinal cord. In addition to these nuclei and their connections, the medulla consists of both ascending and descending pathways to and from higher brain centers. The same basic connections occur throughout vertebrates.

In mammals, the cerebellum does not initiate movement; it only times the length of muscle contractions and orders the sequence in which muscles should contract to bring about a movement. The command to initiate a movement is received from the cerebral cortex (Fig. 2). Similarly, the cerebral cortex receives information regarding limb position and state of muscular contraction to ensure that its commands can be carried out by the cerebellum.

Mammalian brain in sagittal section

The mesencephalon is divided into a roof or optic tectum and a floor or tegmentum. The tegmentum contains the nuclei of the oculomotor and trochlear cranial nerves and a rostral continuation of the sensory nucleus of the trigeminal cranial nerve.

In the evolution of vertebrates, the prosencephalon develops as two major divisions, the diencephalon and the telencephalon. The diencephalon retains the tubular form and serves as a relay and integrating center for information passing to and from the telencephalon and lower centers. The telencephalon is divided into a pair of cerebral hemispheres and an unpaired tel­en­cephalon medium.

There are three divisions of the diencephalon in all vertebrates: an epithalamus which forms the roof of the neural tube, a thalamus which forms the walls of the neural tube, and a hypothalamus which forms the floor of the neural tube. The epithalamus and hypothalamus are primarily concerned with autonomic functions such as homeostasis. The thalamus is subdivided into dorsal and ventral regions. The dorsal region relays and integrates sensory information, and the ventral thalamus relays and integrates motor information. See Homeostasis, Instinctive behavior

The telencephalon is the most complex brain division in vertebrates. It is divided into a roof, or pallium, and a floor, or basal region. The pallium is divided into three primary divisions: a medial PI or hippocampal division, a dorsal PII or general pallial division, and a lateral PII division, often called the pyriform pallium.

The most striking change in the telencephalon of land vertebrates involves the PIIIa component. In mammals, it has proliferated with the PIIb component of the dorsal pallium to produce the mammalian neocortex. In all land vertebrates except amphibians, the PIIb and the PIIIa components, along with the corpus striatum (BI and BII), are the highest centers for the analysis of sensory information and motor coordination. The PI, PIIa, PIIIb, BIII, and posterior parts of BI and BII form part of the limbic system which is concerned with behavioral regulation.

Comparative histology

The nervous system is composed of several basic cell types, including nerve cells called neurons, interstitial cells called neurolemma (cells of Schwann), satellite cells, oligodendroglia, and astroglia; and several connective-tissue cell types, including fibroblasts and microglia, blood vessels, and extracellular fluids.

Each neuron possesses three fundamental properties, involving specialized capacity to react to stimuli, to transmit the resulting excitation rapidly to other portions of the cell, and to influence other neurons, muscle, or glandular cells. Each neuron consists of a cell body (soma), one to several cytoplasmic processes called dendrites, and one process called an axon. Cell bodies vary from about 7 to more than 70 micrometers in diameter; each contains a nucleus and several cytoplasmic structures, including Nissl (chromophil) granules, mitochondria, and neurofibrils. The cell body is continuously synthesizing new cytoplasm, especially protein, which flows down the cell processes. The dendrites range from a fraction of a millimeter to a few millimeters in length. An axon may range from about a millimeter up to many feet in length. The site where two neurons come into contact with each other and where influences of one neuron are transmitted to the other neuron is called a synapse. Neurotransmitters are secreted across the presynaptic membrane into the synaptic cleft where they may excite (excitatory synapse) or inhibit (inhibitory synapse) the postsynaptic membrane. See Biopotentials and ionic currents, Sensation, Synaptic transmission

There are three layers of connective tissue membranes, the meninges, covering the brain and spinal cord: the inner, pia mater; the middle layer, the arachnoid; and the outermost, the dura mater. Between the pia mater and the arachnoid is the subarachnoid space; this space and the ventricular cavities within the brain are filled with an extracellular fluid, the cerebrospinal fluid.

Comparative embryology

The anlage of the nervous system is formed in the outer germ layer, the ectoderm, although some later contributions are also obtained from the middle germ layer, the mesoderm. In most vertebrates a neural plate is formed, which later folds into a neural groove, then closes to form a neural tube. The formation of neural tissue within the ectoderm is due to inductive influences from underlying chordomesodermal structures. See Developmental biology, Embryonic induction, Neural crest

When the neural tube is developing, a segmentation of the central nervous system occurs by the formation of transverse bulges, neuromeres. At the time of neuromeric segmentation, the brain is subdivided into the so-called brain vesicles by local widenings of its lumen. In the rostral end more or less well-developed hemispheres are formed; in the middle of the brain anlage the mesencephalic bulge develops; and behind the latter the walls of the tube thicken into cerebellar folds. In this way the brain anlage is divided into five sections: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon, and its cavity is divided into the rudiments of the adult ventricles.

In spite of the extraordinary variation in adult morphology of the vertebrate brain in different species, the early phases of development are essentially similar. The spinal cord remains as a comparatively slightly differentiated tube.

The cranial or cerebral nerves are the peripheral nerves of the head that are related to the brain. Twelve pairs of cranial nerves have been distinguished in human anatomy and these nerves have been numbered rostrally to caudally as follows:

•  I. Olfactory nerve, fila olfactoria
•  II. Optic nerve, fasciculus opticus
•  III. Oculomotor nerve
•  IV. Trochlear nerve
•  V. Trigeminal nerve, in most vertebrates divided into three branches: ophthalmic, maxillary, and mandibular
•  VI. Abducens nerve
•  VII. Facial nerve
•  VIII. Statoacoustic nerve
•  IX. Glossopharyngeal nerve
•  X. Vagus nerve
•  XI. Accessory nerve
•  XII. Hypoglossal nerve

The spinal ganglia are formed from the neural crest which grows out like a continuous sheet from the dorsal margin of the neural tube and is secondarily split up into cell groups, the ganglia, by a segmentating influence from the somites. Fibers grow out from the ganglionic cells and form the sensory fibers of the spinal nerves. Motor nerve fibers emerge from cells situated in the ventral horns of the spinal cord. The ventral motor fibers and the dorsal sensory fibers fuse to form a common stem, which is again laterally divided into branches, innervating the corresponding segment of the body.

The ganglia of the sympathetic nervous system develop ventrolateral to the spinal cord as neural crest derivatives. At first a continual column of sympathetic nerve cells is formed; it later subdivides into segmental ganglia.

The parasympathetic system is made up of preganglionic fibers emanating as general visceromotor fibers from the brain and from the sacral cord segments. Cells migrate to form the peripheral ganglia along them. See Autonomic nervous system