Central Nervous System

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The central nervous system (CNS) refers to the portion of the nervous system that lies within the skull and spinal column and receives nervous impulses from sense receptors throughout the organism, regulates bodily processes, and organizes and directs behavior. .Anatomically, the CNS comprises the brain and spinal cord, which float within the cranial cavity of the skull and the vertebral canal of the spinal column in a liquid matrix called cerebrospinal fluid, which also fills their hollows and serves as a protective cushion against damage. CNS tissue is further protected by three enfolding membranes called the meninges. The outer and toughest, the dura mater, attaches to skull and spine, encasing the spongy arachnoid membrane within which the cerebrospinal fluid circulates. The soft pia mater is contiguous with the outer layer of brain and cord.

The basic structural unit of nervous tissue is the nerve cell or neuron, a specialized body cell of elongated shape (from a few microns to several feet in length), whose enhanced reactivity and conductivity permit it to propagate or conduct an electrical impulse along its length and to chemically stimulate adjacent neurons to do likewise at specialized junctions called synapses. The nervous system is made up of billions of neurons, which interconnect every part of the organism to monitor and regulate it. Receptor neurons lead like the twigs of a tree inward to branches and thence to great trunks, called nerves, which enter the CNS and ascend into the brain. There, effector neurons originate and descend to exit the CNS as nerves branching repeatedly out to regulate all muscle tissue and therefore all bodily activity. Twelve bilateral pairs of cranial nerves enter the brain directly. The cord is the origin of 31 bilateral pairs of spinal nerves, which exit the CNS through openings between adjacent vertebrae. Each spinal nerve contains both entering receptor fibers and departing effector fibers. The nerve divides on reaching the cord, with sensory fibers entering on the back and motor fibers exiting on the front.

The spinal cord is thus a great pathway for ascending and descending nerve tracts, but connectedness is a property of the CNS, within which a third type of neuron, the interneuron, is found. Interneurons connect effector and receptor neurons, and by repeated branchings of their tips may synapse at either end with many hundreds of other neurons. The functional unit of the nervous system is the reflex arc, which so links receptor and effector neurons that a stimulus at a sense receptor capable of causing its nerve to conduct will automatically trigger an effector neuron to produce a response in a muscle or gland. Some reflexes are extremely simple, but most are not. The CNS is hierarchically organized, with higher centers being stimulated by and acting upon lower centers, so that progressively more complex reflexes are organized progressively higher in the CNS. Certain muscle stretch reflexes operate spinally for the most part. Respiratory reflexes are largely centered in the brain stem, the part of the brain that is contiguous to the spinal cord. Homeostatic reactions depend upon reflexes organized higher yet, in the hypothalamus, which may give rise to motivational states such as hunger and thirst. It is thought that by means of progressively more complex reflexes (some inborn, but most acquired through learning) all functions of the CNS are conducted, including the higher mental functions, the seat of which is the brain. The CNS is also symmetrically organized. Midline structures like the cord have two symmetrical halves. Other structures are duplicated, such as the two cerebral hemispheres. Most fibers cross the midline (e.g., the left brain controls the right hand).

The brain is an organ of unparalleled complexity of parts and function, a reality that may be obscured by summary description. A great deal has nevertheless been learned about the pathways that are followed by ascending and descending nerve tracts. Much of the CNS is white matter, the encased processes or extensions of nerve cells, bundles of which indicate pathways called tracts. The nerve bodies are not encased and are present as gray matter, clusters of which indicate centers of activity called nuclei. Evolutionary influences have given characteristic shapes to the complex arrangements of neurons in the CNS, permitting to be named and located on charts or in living tissue.

The gross anatomy of the brain, in greatly oversimplified summary, may be divided into three regions: (1) the brain stem, the parts of which (medulla, pons, mesencephalon) contain the nuclei of the brain stem reticular formation, which is vital in consciousness and the level of arousal of the brain above; (2) the cerebellum, a center for the smooth regulation of motor behavior; and (3) the cerebrum, which is of greatest interest to psychology for its organizing role in the higher mental functions and emotion. Between brain stem and cerebrum are the thalamus and hypothalamus, which some authorities class with the brain stem, some with the cerebrum. Thalamic nuclei largely integrate and relay sensory impulses upward to the cerebrum. Hypothal-amic nuclei, however, are vital in the regulation of homeo-static reactions and in integrating the reflexes of the nuclei of the limbic system, structures embedded deep within the cerebrum that give rise to emotional experience and expression.

The cerebrum's deeply fissured gray outer surface, the hemispheres of its cerebral cortex, is the terminus of sensory processes and the origin of motor processes. Much of this area is given over to association areas of interneurons, whose complex interconnections give rise to memory, speech, purposive behavior, and, generally, the higher mental functions.

The pathways, relays, and sensory and motor areas of the brain have been mapped by largely physical and physiological methods. But the nature of the higher mental processes of humans remains elusive because they cannot be charted thus. As J. Minckler observed, the structure and function of nervous tissue are so intertwined that they must be studied together. At some levels of the CNS, the appropriate units of function are physiological. Other levels are best studied through discrete behaviors. Still more complex functions of brain, however, require the scrutiny of complex patterns or styles of behavior, and the highest levels of brain function shade into issues of intelligence, logic, purpose, and consciousness, themselves as little understood as the brain.

The study of the CNS in humans is thus the study of brain-behavior and brain-mind relationships, a field in which psychology is heavily involved. That there is a relationship between brain and mind is well established and has been observed for a very long time. C. J. Golden noted that Pythagoras, in 500 b.c., linked brain and human reasoning. In the second century a.d., Galen of Pergamum observed the effects on consciousness of brain injury in gladiators and described animals rendered senseless by pressure on their brains.

Galen was incorrect in attributing mental processes to the fluid-filled hollows of the brain, a view which nevertheless endured until the Renaissance. Modern concepts regarding brain functions did not begin to develop until the 1800s. This delay resulted from vitalistic and imprecise views of both brain and mind, and it endured until a more scientific and reductionistic view of both brain and behavior emerged. Rarnon y Cajal forwarded neuron theory in the late 1800s and received a Nobel Prize in physiology in 1906, the same year that C. S. Sherrington, who developed the concept of the reflex arc, published on integrative mechanisms of the nervous system. Galton's work with the behavioral measurement of individual differences contributed greatly to the emerging science of psychometrics or mental measurement. In the early 1900s, J. B. Watson moved psychology toward the study of behavior rather than mental states. He and B. F. Skinner both contributed to a science and technology of behavior that has meshed well with biology in permitting brain behavior studies. But the complexities of mind or behavior and brain are such that the more we learn, the more there remains to be learned. In 1974, G. Sommerhoff put it thus: "The peculiar fascination of the brain lies in the fact that there is probably no other object of scientific enquiry about which we know so much and yet understand so little."

At the heart of the problem lies the fact that the nervous system, so simple in basic elements, is so complex in arrangements. As Hubbard observed in 1975, it is easy to imagine neuronal arrangements capable of causing muscles to contract or glands to empty, but it is difficult to imagine such arrangements permitting the aging Beethoven to compose works he could no longer hear. The sheer complexity of interconnections, which could well permit such complex behaviors, virtually defies understanding. Some five million neurons, for example, may lie beneath a single square centimeter of brain surface, each of which synapses with perhaps 600 other neurons. Virtually the entire depth and surface of the brain may be involved in any given behavior.

To be understood, the brain (and possibly all the CNS) must be understood as a whole. Yet owing to limits in theory, knowledge, and perhaps capacity, we must approach the whole through study of the parts, viewed at many levels and from many perspectives. Full understanding of the CNS thus lies beyond any one discipline. Psychology, however, contributes in many ways to the expanding interdisciplinary study of the CNS, called neuroscience. Psychologists have put forward or contributed to models of mind compatible with known facts of brain function and have helped develop new models of neural function drawing on and contributing to computer modeling. They have also used neuroscientific findings to develop broad models of human behavior. Psychologists also commonly contribute directly to knowledge of brain-behavior relationships through experimental and clinical neuropsychology.

Experimental neuropsychologists have long studied such things as the behavioral derangements caused by known lesions and other disturbances of CNS tissue in animals. Clinical neuropsychologists have increasingly used qualitative and quantitative aspects of behavior on special tasks to deduce or infer the probable locus and nature of brain tissue impairment in individuals. The accuracy of such assessments reached very substantial levels by the 1970s, and behavioral mappings of the strengths and deficits of brain-injured individuals contribute significantly to current treatment and rehabilitation efforts.

Ongoing developments in knowledge and methodology continue to require new connections among the disciplines comprising the evolving field of neuroscience.

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