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THE NERVOUS SYSTEM+
General Function None of the body system is capable of functioning alone. All are interdependent and work together as one unit so that normal conditions within the body may prevail. Control of the body’s billions of cells is accomplished mainly by two communication systems: the nervous system and the endocrine system. Both systems transmit information from one part of the body to another, but they do it in different ways. The nervous system transmits information very rapidly by nerve impulses conducted from one body area to another. The endocrine system transmits information more slowly by chemicals secreted by ductless glands into blood steam and circulated from glands to other parts of the body. The nervous system serves as the chief coordinating agency. Conditions both within and outside the body are constantly changing; the purpose of the nervous system is to respond to these internal and external changes (known as stimuli) and so cause the body to adapt to new conditions. It is through the nerve impulse sent to the various organs by the nervous system that a person's internal harmony and the balance between the person and the environment are maintained. The nervous system has been compared to a telephone exchange, in that the brain and the spinal cord act as switching centers and the nerve trunks act as cables for carrying messages to and from these centers. Cells of nervous system and their functions The two types of cells found in the nervous system are called neurons or nerve cells and neuroglia , which are specialized connective tissue cells. Neurons conduct impulses, whereas Neuroglia supports neurons. Neurons Each neuron consists of three parts: a main part called the neuron cell body, one or more branching projections called dendrites, and one elongated projection known as an axon.
Dendrites are the processes or projections that transmit impulses to the neuron cell bodies, and axons are the processes that transmit impulses away from the neuron cell bodies. Myelin sheath is a white, fatty substance formed by Schwann cells that wrap around some axons outside the central nervous system. Such fibers are called myelinated fibers. While Nodes of Ranvier are indentions between adjacent Schwann cells. Neurons can be classified structurally and functionally. The three types of functional classification of neurons are according to the direction in which they transmit impulses. These are: sensory neurons, motor neurons, and interneuron. Sensory neurons transmit impulses to the spinal cord and brain from all parts of the body. Motor neurons transmit impulses in the opposite direction-away from the brain and spinal cord. They do not conduct impulses to all parts of the body but only to two kinds of tissue-muscle and glandular epithelial tissue. Interneurons conduct impulses from sensory neurons to motor neurons. Sensory neurons are also called afferent neurons; motor neurons are called efferent neurons, and interneurons are called central or connecting neurons. Diagram of a typical neuron showing dendrites, a cell Body, and an axon
Instead, they are special types of connective tissue cells. Their name is appropriate because it is derived from Greek word glia meaning "glue." One function of neuroglia cells is to hold the functioning neurons together and protect them. Examples: Astrocytes: star-shaped cells, and are the main supporting tissue of the CNS. Epidymal cells : these cells form the epithelial lining of the ventricles and central canal of the spinal cord. Those cells form choroid plexuses that secrete cerebral spinal fluid. Microglia cells : these are derived from monocytes that migrate from blood into the nervous system before birth. They are found in the area of blood vessels. They enlarge and become phargocytic, removing microbes and damaged tissue in areas of inflammation and cell destruction. Oligodendrocytes : these are found in clusters around nerve cell bodies where they play a supportive function; form and maintain myelin, having the function as Schwann cells in peripheral nerves. Impulse Generation and Conduction The Nerve Impulse The cell membrane of an unstimulated (resting) neuron carries an electric charge. Because of positive and negative ions concentrated on either side of the membrane, the inside of the membrane at rest is negative as compared with the outside. A nerve impulse is a local reversal in the charge on the nerve cell membrane that then spreads along the membrane like an electric current. This sudden electrical change in the membrane is called an action potential. A stimulus, then, is any force that can start an action potential. This electric change results from rapid shifts in sodium and potassium ions across the cell membrane: Na+ (usually extracellular, and K+ intracellular). The reversal occurs very rapidly (in less than one thousandth of a second) and is followed by a rapid return of the membrane to its original state so that it can be stimulated again.
A myelinated nerve fiber conducts impulses more rapidly than an unmyelinated fiber of the same size because the electrical impulse "jumps" from node (space) to node in the myelin sheath instead of traveling continuously along the fiber. The Synapse Each neuron is a separate unit, and there is no anatomic connection between neurons. How then is it possible for neurons to communicate? In other words, how does the axon of one neuron make functional contact with the membrane of another neuron? This is accomplished by the
Central neuron-a cell or cells within the CNS. These neurons may carry impulses to and from the brain, may function within the brain, or may distribute impulses to different regions of the spinal cord. Motor neuron, or efferent neuron-a cell that carries impulses away from the CNS. Effectors- a muscle or a gland outside the CNS that carries out a response. At its simplest, a reflex arc can involve just two neurons, one sensory and one motor, with a synapse in the CNS. There are very few reflex arcs that require only this minimal number of neurons. The knee jerk reflex is one of the few examples in humans. Most reflex arcs involve many more, even hundreds, of connecting neurons within the central nervous system. Division of the Nervous System The nervous system as a whole consists of two principal divisions called the central nervous system and peripheral nervous system. Because the brain and spinal cord occupy a midline or central location in the body, they are together called the central nervous system or CNS. Similarly, the usual designation for the nerves of the body is the peripheral nervous system or PNS. Use of the term peripheral is appropriate because nerves extend to outlying or peripheral parts of the body. A subdivision of the peripheral nervous system called the autonomic nervous system (ANS) structures that regulate the body's autonomic or involuntary functions (for example, the heart rate, the contractions of the stomach, and intestines, and the secretion of chemical compounds by glands). Central Nervous System
The CNS as its name implies, is centrally located. Its two major structures, the brain and spinal cord, are found along the midsagittal plane of the body. The brain is protected in the cranial
And the arachnoid, which is the membrane between the dura and pia mater. The term arachnoid means "cobweblike." It comes from arachne, which is the Greek word for spider. The meninges that form the protective covering around the spinal cord also extend up and around the brain to enclose it completely. Flow of cerebral spinal fluid (diagram) Fluid fills the subarachnoid spaces between the pia mater and arachnoid in the brain and spinal cord. This fluid is called cerebrospinal fluid (CSF). Cerebrospinal fluid also fills spaces in the brain called cerebral ventricles. There are two lateral ventricles: One lies inside the right half of the cerebrum (the largest part of the human brain), and the other lies inside the left half of the cerebrum. The 3rd, and the 4th ventricles. CSF is one of the body's circulating fluids. It forms continually from fluid filtering out of the blood in a network of brain capillaries known as the choroid plexus and into the ventricles.
CSF seeps from the lateral ventricles into the third ventricle and flows down through the cerebral aqueduct into the fourth ventricle. It moves from the fourth ventricle into the small, tube like central canal of the cord and out into the subarachnoid spaces. Then it moves leisurely down and around the cord and up and around the brain (in the subarachnoid spaces of their meninges) and returns to the blood (in the veins of the brain) Divisions of the Nervous system CENTRAL NERVOUS SYSTEM Autonomic (involuntary) Nerves Brain, and Spinal cord Divisions of the Brain
The diencephalon is a small but important part of the brain located between the midbrain inferiorly and the cerebrum superiorly. It consists of two major structures: the hypothalamus and the thalamus. The ventricle of the diencephalons is the 3rd ventricle. Hypothalamus : The hypothalamus, as its name suggests, is located inferior to the thalamus. The posterior pituitary gland, the stalk that attaches it to the undersurface of the brain, and areas of gray matter located in the sidewalls of a fluid-filled space called the third ventricle are extensions of the hypothalamus. Measured by size, it is one of the least significant parts of the brain, but measured its contribution to healthy survival; it is one of the most important brain structures. Impulses from neurons whose dendrites and cell bodies lie in the hypothalamus are conducted by their axons to neurons located in the spinal cord, and many of these impulses are then relayed to muscles and glands all over the body. Thus the hypothalamus exerts a major control over virtually all-internal organs. Among the vital functions that it helps control are the heartbeat, constriction and dilation of blood vessels, and contractions of the stomach and intestines. Some neurons in the hypothalamus function in a surprising way; they make the hormones that the posterior pituitary gland secretes into the blood. Because of one of these hormones (called antidiuretic hormone or ADH) affects the volume of urine excreted, the hypothalamus plays an essential role in maintaining the body's water balance. Some of the neurons in the hypothalamus function as endocrine glands. Their axons secrete chemicals called releasing hormones into the blood, which then carries them to the anterior pituitary gland. Releasing hormones, as their name suggests, control the release of certain anterior pituitary hormones. These in turn influence the hormone secretion of other endocrine glands. Thus the hypothalamus indirectly helps control the functioning of every cell in the body. The hypothalamus is a crucial part of the mechanism for maintaining body temperature. Therefore a marked elevation in body temperature in the absence of disease frequently characterizes injuries or other abnormalities of the hypothalamus.
In addition, this important center is involved in functions such as the regulation of water balance; sleep cycles, and the control of appetite and many emotions involved in pleasure, fear, anger, sexual arousal, and pain. Thalamus Just superior to the hypothalamus is a dumb-bell shaped section or largely gray matter called the thalamus. Each enlarged end of the dumbbell lies in a lateral wall of the third ventricle. The thin center section of the thalamus passes from left to right through the third ventricle. The thalamus is composed chiefly of dendrites and cell bodies of neurons that have axons extending up to the sensory areas of the cerebrum. It performs the following functions: It helps produce sensations. Its neurons relay impulses to the cerebral cortex from the sense organ of the body. It associates sensations with emotions. Almost all sensations are accompanied by a feeling of some degree of pleasantness or unpleasantness. The way that these pleasant and unpleasant feelings are produced is unknown except that they seem to be associated with the arrival of sensory impulses in thalamus. It plays a part in the so -called arousal or alerting mechanism. It contains important nuclei such as medial geniculate which is responsible for auditory sense and lateral geniculate which is responsible for vision. Cerebellum The cerebellum is the second largest part of the human brain. It lies under the occipital lobe of the cerebrum. In the cerebellum, gray matter composes the outer layer, and white matter composes the bulk of the interior. Function. Most of our knowledge about cerebellar functions has come from observing patients who have some sort of disease of the cerebellum and from animals that have had the cerebellum removed. From such observations, we know that the cerebellum plays an essential part in the production of normal movements.
The corpus callosum is a broad band of fibres passing between corresponding cortical areas of the two hemispheres. Association fibers - unite adjacent or widely separated gyri of the same hemisphere. Projection fibers - ascend from or descend to lower lying parts of the central nervous system. BRAIN AREA FUNCTION Brain stem Medulla oblongata Two-way conduction pathway between the spinal cord and higher brain centers; cardiac, respiratory, and vasomotor control center Pons Two-way conduction pathway between areas of the brain and other regions of the body; Influences respiration Midbrain Two-way conduction pathway; relay for visual and auditory Impulses Diencephalon Hypothalamus Regulation of body temperature, water balance, sleeps cycle control appetite, and sexual arousal Thalamus Sensory relay station from various body areas to cerebral cortex; emotions and alerting or arousal mechanisms Cerebellum Muscle coordination; maintenance of equilibrium and posture Cerebrum Sensory perception, emotions willed movements, consciousness, and memory Functions of the cerebral cortex It’s within the cerebral cortex that nerve impulses are received and analyzed. These activities form the basis of knowledge. The brain “stores” information, much of which can be recalled on demand by means of a phenomenon called memory. It’s in the cortex that thought processes such as association, judgment, and discrimination takes place. Conscious deliberations and voluntary actions also arise from there. Although the various brain areas act in coordination to produce behavior, particular functions are localized in the cortex of each lobe:
Draw the diagram of functional areas
control of the skeletal muscles. A larger region of the lobe is involved in planning and conscious thought. It also contains two areas important in speech. The parietal lobe : occupies the superior part of each hemisphere and lies posterior to the central sulcus. A large region of the parietal lobe integrates information from all sensory areas. The estimation of distances, sizes, and shapes also take place here. The gyrus just behind the central sulcus contains the primary sensory area, where impulses from the skin, such as touch, pain, and temperature are interpreted. As with the motor cortex, the greater the intensity of sensation from a particular area, the tongue or fingers, for example, the more area of the cortex is involved. The temporal lobe : lies inferior to the lateral sulcus and folds under the hemisphere on each side. This lobe contains the auditory areas for receiving and interpreting impulses from the ear. The olfactory area, concerned with the sense of smell, is located in the medial part of the temporal lobe and is not visible from the surface; it’s stimulated by impulses arising from receptors in the nose. The occipital lobe : lies posterior to the parietal lobe and extend over the cerebellum. This lobe contains for interpreting impulses arising from the retina of the eye. Communication areas The ability to communicate by written and verbal means is an interesting example of how areas of the cerebral cortex are interrelated. The intergration of these areas is closely connected with the learning process. The auditory receiving area: detects sound impulses transmitted from the environment, where as the surrounding area; the association area, interprets the sounds such as; integrating the pitch, loudness, and direction of sound. Another area is located in the left side in most people; the comprehension area, or wernick area. This area functions in speech recognition and the meaning of words. Someone who suffers damage in this area of the brain, as by stroke, will have difficulty in understanding the meaning of speech. The beginnings of language are learned by hearing; thus, the auditory area for understanding sounds is near the auditory receiving areas of the cortex. Babies often appear to understand what
is being said long before they do any talking themselves. It is several years before the children learn to read or write words. The visual areas of the occipital cortex: visual receiving area; visual images of language are collected. The visual association area that lies anterior interprets these visual impulses as words. The ability to read with understanding also develops in this area. The motor areas for spoken and written communication: lie anterior to the most inferior part of the frontal lobe’s motor cortex. The specialized cortical region; motor speech area, or Broca area, plans the sequence of muscle contractions in the tongue, larynx, and soft palate required to form meaningful sentences. There is a functional relation among areas of the brain. Many neurons must work together to enable to enable a person to receive, interpret, and respond to verbal and written messages as well as to touch and other sensory stimuli. Memory and the learning process Memory is the mental faculty for recalling ideas. Short-term memory refers to the retention of bits of information for a few seconds or perhaps a few minutes, after which the information for a few minutes after which it’s lost unless reinforced. Long-term memory refers o the storage of information that can be recalled at a later time. There is a tendency for a memory to become more fixed the more often the person repeats the remembered experience; thus short term memory signals can lead to long term memories. Physiological studies show that rehearsal of the same information again and again accelerates the degree of short term memory transfer into long term memory. It’s also noted that the brain is able to organize information so that new ideas are stored in the same areas in which similar ones have been stored before. Along the border between the cerebrum and the diencephalon is a region known as the limbic system. This system is involved in emotional states and behavior. It’s also important for short term memories and the consolidation of long term memories, and thus is essential for learning. Spinal Cord Location of the Spinal Cord
