Human Anatomy and Physiology: A Reading Material PDF

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material VII. NERVOUS SYSTEM The nervous system is the master control and communication system of the body. Every thought, action, and emotion reflects its activity. It communicates with body cells using electrical impulses, which are rapid and specific...

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material VII. NERVOUS SYSTEM The nervous system is the master control and communication system of the body. Every thought, action, and emotion reflects its activity. It communicates with body cells using electrical impulses, which are rapid and specific and cause almost immediate responses. The nervous system does not work alone to regulate and maintain body homeostasis; the endocrine system is a second important regulating system. Whereas the nervous system controls with rapid electrical nerve impulses, the endocrine system produces hormones that are released into the blood. Thus, the endocrine system acts in a more leisurely way. To carry out its normal role, the nervous system has three overlapping functions: (1) It uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. These changes are called stimuli, and the gathered information is called sensory input. (2) It processes and interprets the sensory input and decides what should be done at each moment—a process called integration. (3) It then causes a response, or effect, by activating muscles or glands (effectors) via motor output. An example will illustrate how these functions work together. When you are driving and see a red light just ahead (sensory input), your nervous system integrates this information (red light means “stop”) and sends motor output to the muscles of your right leg and foot. Your foot goes for the brake pedal (the response). A. Organization of the Nervous System Structural Classification The structural classification, which includes all nervous system organs, has two subdivisions—the central nervous system and the peripheral nervous system. The central nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal body cavity and act as the integrating and command centers of the nervous system. They interpret incoming sensory information and issue instructions based on past experience and current conditions. The peripheral nervous system (PNS) includes all parts of the nervous system outside the CNS. It consists mainly of the nerves that extend from the spinal cord and brain. Spinal nerves carry impulses to and from the spinal cord. Cranial nerves carry impulses to and from the brain. These nerves serve as communication lines. They link all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles. Functional Classification The functional classification scheme is concerned only with PNS structures. It divides them into two principal subdivisions. The sensory division, or afferent (literally “to go toward”) division, consists of nerves (composed of many individual nerve fibers) that convey impulses to the central nervous system from sensory receptors located in various parts of the body. The sensory division keeps the CNS constantly informed of events going on both inside and outside the body. Sensory fibers delivering impulses from the skin, skeletal muscles, and joints are called somatic sensory (afferent) fibers, whereas those transmitting impulses from the visceral organs are called visceral sensory (afferent) fibers. The motor division, or efferent division, carries impulses from the CNS to effector organs, the muscles and glands. These impulses activate muscles and glands; that is, they effect (bring about or cause) a motor response. The motor division in turn has two subdivisions: The somatic nervous system allows us to consciously, or voluntarily, control our skeletal muscles. Hence, we often refer to this subdivision as the voluntary nervous system. However, not all skeletal muscle activity controlled by this motor division is voluntary. Skeletal muscle reflexes, such as the stretch reflex, are initiated involuntarily by these same fibers.  The autonomic nervous system (ANS) regulates events that are automatic, or involuntary, such as the activity of smooth muscle, cardiac muscle, and glands. This subdivision, commonly called the involuntary nervous system, itself has two parts, the sympathetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. Although it is simpler to study the nervous system in terms of its subdivisions, remember that these subdivisions are made for the sake of convenience only. Remember that the nervous system acts as a coordinated unit, both structurally and functionally. Nerve Tissue: Structure and Function Even though it is complex, nervous tissue is made up of just two principal types of cells—supporting cells and neurons. Supporting Cells Supporting cells in the CNS are “lumped together” as neuroglia, literally, “nerve glue,” also called glial cells or glia. Neuroglia include many types of cells that support, insulate, and protect the delicate neurons. In addition, each of the different types of neuroglia has special functions. CNS neuroglia include the following: Astrocytes: abundant star-shaped cells that account for nearly half of neural tissue. Their numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient supply lines, the blood capillaries. Astrocytes form a living barrier between capillaries and neurons, help determine capillary permeability, and play a role in making exchanges between the two. In this way, they help protect the neurons from harmful substances that might be in the blood. Astrocytes also help control the chemical environment in the brain by “mopping up” leaked potassium ions, which are involved in generating a nerve impulse, and recapturing chemicals released for communication purposes. Microglia: spiderlike phagocytes that monitor the health of nearby neurons and dispose of debris, such as dead brain cells and bacteria. Ependymal cells: neuroglia that line the central cavities of the brain and the spinal cord. The beating of their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a protective watery cushion around the CNS. Oligodendrocytes: neuroglia that wrap their flat extensions (processes) tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths. Although neuroglia somewhat resemble neurons structurally (both cell types have cell extensions), they are not able to transmit nerve impulses, a function that is highly developed in neurons. Another important difference is that neuroglia never lose their ability to divide, whereas most neurons do. Consequently, most brain tumors are gliomas, or tumors formed by neuroglia. Supporting cells in the PNS come in two major varieties—Schwann cells and satellite cells. Schwann cells form the myelin sheaths around nerve fibers in the PNS. Satellite cells act as protective, cushioning cells for peripheral neuron cell bodies. Neurons Neurons, also called nerve cells, are highly specialized to transmit messages (nerve impulses) from one part of the body to another. Although neurons differ structurally from one another, they have many common features. All have a cell body, which contains the nucleus and one or more slender processes extending from the cell body. Cell Body The cell body is the metabolic center of the neuron. Its transparent nucleus contains a large nucleolus. The cytoplasm surrounding the nucleus contains the usual organelles, except that it lacks centrioles (which confirms the amitotic nature of most neurons). The rough ER, called Nissl bodies, and neurofibrils (intermediate filaments that are important in maintaining cell shape) are particularly abundant in the cell body.  Processes The arm-like processes, or fibers, vary in length from microscopic to about 7 feet in the tallest humans. The longest ones in humans reach from the lumbar region of the spine to the great toe. Neuron processes that convey incoming messages (electrical signals) toward the cell body are dendrites, whereas those that generate nerve impulses and typically conduct them away from the cell body are axons. Neurons may have hundreds of branching dendrites (dendr = tree), depending on the neuron type. However, each neuron has only one axon, which arises from a cone-like region of the cell body called the axon hillock. An occasional axon gives off a collateral branch along its length, but all axons branch profusely at their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hundreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters. As we said, axons transmit nerve impulses away from the cell body. When these impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular space between neurons, or between a neuron and its target cell. Each axon terminal is separated from the next neuron by a tiny gap called the synaptic cleft. Such a functional junction, where an impulse is transmitted from one neuron to another, is called a synapse (syn = to clasp or join). Although they are close, neurons never actually touch other neurons. Myelin Sheaths Most long nerve fibers are covered with a whitish, fatty material called myelin, which has a waxy appearance. Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses. Compare myelin sheaths to the many layers of insulation that cover the wires in an electrical cord; the layers keep the electricity flowing along the desired path just as myelin does for nerve fibers. Axons outside the CNS are myelinated by Schwann cells, as just noted. Many of these cells wrap themselves around the axon in a jelly-roll fashion. Initially, the membrane coil is loose, but the Schwann cell cytoplasm is gradually squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. This part of the Schwann cell, external to the myelin sheath, is called the neurilemma (“neuron husk”). Because the myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, called nodes of Ranvier, at regular intervals. As mentioned previously, myelinated fibers are also found in the central nervous system. Oligodendrocytes form CNS myelin sheaths. In the PNS, it takes many Schwann cells to make a single myelin sheath; but in the CNS, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. Thus, in the CNS, one oligodendrocyte can form many myelin sheaths. Although the myelin sheaths formed by oligodendrocytes and those formed by Schwann cells are similar, the CNS sheaths lack a neurilemma. Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the central nervous system. Homeostatic Imbalance: The importance of myelin insulation is best illustrated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, the electrical current is short-circuited and may “jump” to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The affected person may have visual and speech disturbances, lose the ability to control his or her muscles, and become increasingly disabled. Multiple sclerosis is an autoimmune disease in which the person’s own immune system attacks a protein component of the sheath. As yet there is no cure, but injections of interferon (a hormone-like substance released by some immune cells) appear to hold the symptoms at bay and provide some relief. Other drugs aimed at slowing the autoimmune response are also being used, though further research is needed to determine their long- term effects.  Terminology Clusters of neuron cell bodies and collections of nerve fibers are named differently in the CNS and in the PNS. For the most part, cell bodies are found in the CNS in clusters called nuclei. This well-protected location within the bony skull or vertebral column is essential to the well-being of the nervous system—remember that neurons do not routinely undergo cell division after birth. The cell body carries out most of the metabolic functions of a neuron, so if it is damaged, the cell dies and is not replaced. Small collections of cell bodies called ganglia (ganglion, singular) are found in a few sites outside the CNS in the PNS. Bundles of nerve fibers (neuron processes) running through the CNS are called tracts, whereas in the PNS they are called nerves. The terms white matter and gray matter refer respectively to myelinated versus unmyelinated regions of the CNS. As a general rule, the white matter consists of dense collections of myelinated fibers (tracts), and gray matter contains mostly unmyelinated fibers and cell bodies. Classification Neurons may be classified based on their function or their structure. Functional Classification Functionally, neurons are grouped according to the direction the nerve impulse travels relative to the CNS. On this basis, there are sensory, motor, and association neurons (interneurons). Neurons carrying impulses from sensory receptors (in the internal organs or the skin) to the CNS are sensory neurons, or afferent neurons. The cell bodies of sensory neurons are always found in a ganglion outside the CNS. Sensory neurons keep us informed about what is happening both inside and outside the body. The dendrite endings of the sensory neurons are usually associated with specialized receptors that are activated by specific changes occurring nearby. The simpler types of sensory receptors in the skin are cutaneous sense organs, and those in the muscles and tendons are proprioceptors. The pain receptors (actually bare nerve endings) are the least specialized of the cutaneous receptors. They are also the most numerous, because pain warns us that some type of body damage is occurring or is about to occur. However, strong stimulation of any of the cutaneous receptors (for example, by searing heat, extreme cold, or excessive pressure) is also interpreted as pain. The proprioceptors detect the amount of stretch, or tension, in skeletal muscles, their tendons, and joints. They send this information to the brain so that the proper adjustments can be made to maintain balance and normal posture. Propria comes from the Latin word meaning “one’s own,” and the proprioceptors constantly advise our brain of our own movements. Neurons carrying impulses from the CNS to the viscera and/or muscles and glands are motor neurons, or efferent neurons. The cell bodies of motor neurons are usually located in the CNS. The third category of neurons consists of the interneurons, or association neurons. They connect the motor and sensory neurons in neural pathways. Their cell bodies are typically located in the CNS. Structural Classification Structural classification is based on the number of processes, including both dendrites and axons, extending from the cell body. If there are several, the neuron is a multipolar neuron. Because all motor and association neurons are multipolar, this is the most common structural type. Neurons with two processes— one axon and one dendrite—are bipolar neurons. Bipolar neurons are rare in adults, found only in some special sense organs (eye, nose), where they act in sensory processing as receptor cells. Unipolar neurons have a single process emerging from the cell body as if the cell body were on a “cul-de-sac” off the “main road” that is the axon. However, the process is very short and divides almost immediately into proximal (central) and distal (peripheral) processes. Unipolar neurons are unique in that only the small branches at the end of the peripheral process are dendrites. The remainder of the peripheral process and the central process function as the axon; thus, in this case, the axon actually conducts nerve impulses both toward and away from the cell body. Sensory neurons found in PNS ganglia are unipolar. Physiology: Nerve Impulses Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neurons, muscles, or glands. Electrical Conditions of a Resting Neuron’s Membrane The plasma membrane of a resting, or inactive, neuron is polarized, which means that there are fewer positive ions sitting on the inner face of the neuron’s plasma membrane than there are on its outer face (1). The major positive ions inside the cell are potassium (K+), whereas the major positive ions outside the cell are sodium (Na+). As long as the inside remains more negative (fewer positive ions) than the outside, the neuron will stay inactive. Action Potential Initiation and Generation Many different types of stimuli excite neurons to become active and generate an impulse. For example, light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin. However, most neurons in the body are excited by neurotransmitter chemicals released by other neurons, as we will describe shortly. Regardless of the stimulus, the result is always the same—the permeability properties of the cell’s plasma membrane change for a very brief period. Normally, sodium ions cannot diffuse through the plasma membrane to any great extent, but when the neuron is adequately stimulated, the “gates” of sodium channels in the membrane open. Because sodium is in much higher concentration outside the cell, it then diffuses quickly into the neuron. (Remember the laws of diffusion?) This inward rush of sodium ions changes the polarity of the neuron’s membrane at that site, an event called depolarization (2). Locally, the inside is now more positive, and the outside is less positive, a local electrical situation called a graded potential. However, if the stimulus is strong enough and the sodium influx is great enough, the local depolarization (graded potential) activates the neuron to initiate and transmit a long-distance signal called an action potential, also called a nerve impulse in neurons. (3) The nerve impulse is an all-or- none response, like starting a car. It is either propagated (conducted, or sent) over the entire axon (4), or it doesn’t happen at all. The nerve impulse never goes partway along an axon’s length, nor does it die out with distance, as do graded potentials. Almost immediately after the sodium ions rush into the neuron, the membrane permeability changes again, becoming impermeable to sodium ions but permeable to potassium ions. So potassium ions are allowed to diffuse out of the neuron into the interstitial fluid, and they do so very rapidly. This outflow of positive ions from the cell restores the electrical conditions at the membrane to the polarized, or resting, state, an event called repolarization (5). After repolarization of the electrical conditions, the sodium-potassium pump restores the initial concentrations of the sodium and potassium ions inside and outside the neuron (6). This pump uses ATP (cellular energy) to pump excess sodium ions out of the cell and to bring potassium ions back into it. Until repolarization occurs, a neuron cannot conduct another impulse. Once begun, these sequential events spread along the entire neuronal membrane. The events just described explain propagation of a nerve impulse along unmyelinated fibers. Fibers that have myelin sheaths conduct impulses much faster because the nerve impulse literally jumps, or leaps, from node to node along the length of the fiber. This occurs because no electrical current can flow across the axon membrane where there is fatty myelin insulation. This faster type of electrical impulse propagation is called saltatory conduction (saltare = to dance or leap). Transmission of the Signal at Synapses So far we have explained only the irritability aspect of neuronal functioning. What about conductivity— how does the electrical impulse traveling along one neuron get across the synapse to the next neuron (or effector cell) to influence its activity? The answer is that the impulse doesn’t! Instead, a neurotransmitter chemical crosses the synapse to transmit the signal from one neuron to the next, or to the target cell. When the action potential reaches an axon terminal (1), the electrical change opens calcium channels. Calcium ions, in turn, cause the tiny vesicles containing neurotransmitter to fuse with the axonal membrane (2), and pore-like openings form, releasing the neurotransmitter into the synaptic cleft (3). The neurotransmitter molecules diffuse across the synaptic cleft* and bind to receptors on the membrane of the next neuron (4). If enough neurotransmitter is released, the whole series of events described above (sodium entry 5, depolarization, etc.) will occur, generating a graded potential and eventually a nerve impulse in the receiving neuron beyond the synapse. The electrical changes prompted by neurotransmitter binding are very brief because the neurotransmitter is quickly removed from the synaptic cleft (5), either by diffusing away, by reuptake into the axon terminal, or by enzymatic breakdown. This limits the effect of each nerve impulse to a period shorter than the blink of an eye. Notice that the transmission of an impulse is an electrochemical event. Transmission down the length of the neuron’s membrane is basically electrical, but the next neuron is stimulated by a neurotransmitter, which is a chemical. Because each neuron both receives signals from and sends signals to scores of other neurons, it carries on “conversations” with many different neurons at the same time. Physiology: Reflexes Although there are many types of communication between neurons, much of what the body must do every day is programmed as reflexes. Reflexes are rapid, predictable, and involuntary responses to stimuli. They are much like one-way streets—once a reflex begins, it always goes in the same direction. Reflexes occur over neural pathways called reflex arcs and involve both CNS and PNS structures. Think of a reflex as a preprogrammed response to a given stimulus. The types of reflexes that occur in the body are classed as either somatic or autonomic. Somatic reflexes include all reflexes that stimulate the skeletal muscles; these are still involuntary reflexes even though skeletal muscle normally is under voluntary control. When you quickly pull your hand away from a hot object, a somatic reflex is working. Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands. Secretion of saliva (salivary reflex) and changes in the size of the eye pupils (pupillary reflex) are two such reflexes. Autonomic reflexes regulate such body functions as digestion, elimination, blood pressure, and sweating. All reflex arcs have a minimum of five elements: a receptor (which reacts to a stimulus), an effector (the muscle or gland eventually stimulated), and sensory and motor neurons to connect the two. The synapse or interneurons between the sensory and motor neurons represents the fifth element—the CNS integration center. The simple patellar, or knee-jerk, reflex is an example of a two- neuron reflex arc, the simplest type in humans. The patellar reflex (in which the quadriceps muscle attached to the hit tendon is stretched) is familiar to most of us. It is usually tested during a physical exam to determine the general health of the motor portion of our nervous system.  Most reflexes are much more complex than the two-neuron reflex, involving synapses between one or more interneurons in the CNS (integration center). The flexor, or withdrawal, reflex is a three-neuron reflex arc in which the limb is withdrawn from a painful stimulus. A three- neuron reflex arc also consists of five elements— receptor, sensory neuron, interneuron, motor neuron, and effector. Because there is always a delay at synapses (it takes time for neurotransmitter to diffuse through the synaptic cleft), the more synapses there are in a reflex pathway, the longer the reflex takes to happen. Many spinal reflexes involve only spinal cord neurons and occur without brain involvement. As long as the spinal cord is functional, spinal reflexes, such as the flexor reflex, will work. By contrast, some reflexes require that the brain become involved because many different types of information have to be evaluated to arrive at the “right” response. The response of the pupils of the eyes to light is a reflex of this type. As noted earlier, reflex testing is an important tool in evaluating the condition of the nervous system. Reflexes that are exaggerated, distorted, or absent indicate damage or disease in the nervous system. Reflex changes often occur before a pathological condition becomes obvious in other ways. B. Central Nervous System Functional Anatomy of the Brain The adult brain’s unimpressive appearance gives few hints of its remarkable abilities. It is about two good fistfuls of pinkish gray tissue, wrinkled like a walnut and with the texture of cold oatmeal. It weighs a little over 3 pounds. Because the brain is the largest and most complex mass of nervous tissue in the body, we commonly discuss it in terms of its four major regions—cerebral hemispheres, diencephalon, brain stem, and cerebellum. Cerebral Hemispheres The paired cerebral hemispheres, collectively called the cerebrum, are the most superior part of the brain and together are a good deal larger than the other three brain regions combined. In fact, as the cerebral hemispheres develop and grow, they enclose and obscure most of the brain stem, so many brain stem structures cannot normally be seen unless a sagittal section is made. Picture how a mushroom cap covers the top of its stalk, and you have an idea of how the cerebral hemispheres cover the diencephalon and the superior part of the brain stem. The entire surface of the cerebrum exhibits elevated ridges of tissue called gyri (gyrus, singular; “twisters”), separated by shallow grooves called sulci (sulcus, singular; “furrows”). Less numerous are the deeper grooves called fissures, which separate large regions of the brain. Many of the fissures and gyri are important anatomical landmarks. The cerebral hemispheres are separated by a single deep fissure, the longitudinal fissure. Other fissures or sulci divide each cerebral hemisphere into a number of lobes, named for the cranial bones that lie over them. Each cerebral hemisphere has three basic regions: a superficial cortex of gray matter, which looks gray in fresh brain tissue; an internal area of white matter; and the basal nuclei, islands of gray matter situated deep within the white matter. 1. Cerebral Cortex Speech, memory, logical and emotional responses, consciousness, the interpretation of sensation, and voluntary movement are all functions of the cerebral cortex. Many of the functional areas of the cerebral hemispheres have been identified. The primary somatic  sensory area is located in the parietal lobe posterior to the central sulcus. Impulses traveling from the body’s sensory receptors (except for the special senses) are localized and interpreted in this area of the brain. The primary somatic sensory area allows you to recognize pain, differences in temperature, or a light touch. A spatial map, the sensory homunculus, has been developed to show how much tissue in the primary somatic sensory area is devoted to various sensory functions. Body regions with the most sensory receptors—the lips and fingertips—send impulses to neurons that make up a large part of the sensory area. Furthermore, the sensory pathways are crossed pathways— meaning that the left side of the primary somatic sensory area receives impulses from the right side of the body, and vice versa. Impulses from the special sense organs are interpreted in other cortical areas. For example, the visual area is located in the posterior part of the occipital lobe, the auditory area is in the temporal lobe bordering the lateral sulcus, and the olfactory area is deep inside the temporal lobe. The primary motor area, which allows us to consciously move our skeletal muscles, is anterior to the central sulcus in the frontal lobe. The axons of these motor neurons form the major voluntary motor tract—the pyramidal tract, or corticospinal tract, which descends to the cord. As in the primary somatic sensory cortex, the body is represented upside-down, and the pathways are crossed. Most of the neurons in the primary motor area control body areas having the finest motor control; that is, the face, mouth, and hands. The body map on the motor cortex, as you might guess, is called the motor homunculus. A specialized cortical area that is very involved in our ability to speak, Broca’s area, or motor speech area, is found at the base of the precentral gyrus (the gyrus anterior to the central sulcus). Damage to this area, which is located in only one cerebral hemisphere (usually the left), causes the inability to say words properly. You know what you want to say, but you can’t vocalize the words. Areas involved in higher intellectual reasoning and socially acceptable behavior are believed to be in the anterior part of the frontal lobes, the anterior association area. The frontal lobes also house areas involved with language comprehension. Complex memories appear to be stored in the temporal and frontal lobes. The posterior association area encompasses part of the posterior cortex. This area plays a role in recognizing patterns and faces, and blending several different inputs into an understanding of the whole situation. Within this area is the speech area, located at the junction of the temporal, parietal, and occipital lobes. The speech area allows you to sound out words. This area (like Broca’s area) is usually in only one cerebral hemisphere. 2. Cerebral White Matter Most of the remaining cerebral hemisphere tissue—the deeper cerebral white matter—is composed of fiber tracts carrying impulses to, from, or within the cortex. One very large fiber tract, the corpus callosum, connects the cerebral hemispheres. Such fiber tracts are called commissures. The corpus callosum arches above the structures of the brain stem and allows the cerebral hemispheres to communicate with one another. This is important because, as already noted, some of the cortical functional areas are in only one hemisphere. Association fiber tracts connect areas within a hemisphere, and projection fiber tracts connect the cerebrum with lower CNS centers, such as the brain stem.  3. Basal Nuclei Although most of the gray matter is in the cerebral cortex, there are several “islands” of gray matter, called the basal nuclei, buried deep within the white matter of the cerebral hemispheres. The basal nuclei help regulate voluntary motor activities by modifying instructions (particularly in relation to starting or stopping movement) sent to the skeletal muscles by the primary motor cortex. A tight band of projection fibers, called the internal capsule, passes between the thalamus and the basal nuclei. Homeostatic Imbalance Individuals who have problems with their basal nuclei are often unable to walk normally or carry out other voluntary movements in a normal way. Huntington’s disease and Parkinson’s disease are two examples of such syndromes. Diencephalon The diencephalon, or interbrain, sits atop the brain stem and is enclosed by the cerebral hemispheres. The major structures of the diencephalon are the thalamus, hypothalamus, and epithalamus. The thalamus, which encloses the shallow third ventricle of the brain, is a relay station for sensory impulses passing upward to the sensory cortex. As impulses surge through the thalamus, we have a crude recognition of whether the sensation we are about to have is pleasant or unpleasant. It is the neurons of the sensory cortex that actually localize and interpret the sensation. The hypothalamus (literally, “under the thalamus”) makes up the floor of the diencephalon. It is an important autonomic center because it plays a role in regulating body temperature, water balance, and metabolism. The hypothalamus is also the center for many drives and emotions, and as such it is an important part of the so-called limbic system, or “emotional-visceral brain.” For example, thirst, appetite, sex, pain, and pleasure centers are in the hypothalamus. Additionally, the hypothalamus regulates the pituitary gland (an endocrine organ) and produces two hormones of its own. The pituitary gland hangs from the anterior floor of the hypothalamus by a slender stalk. The mammillary bodies, reflex centers involved in olfaction (the sense of smell), bulge from the floor of the hypothalamus posterior to the pituitary gland. The epithalamus forms the roof of the third ventricle. Important parts of the epithalamus are the pineal gland (part of the endocrine system) and the choroid plexus of the third ventricle. The choroid plexuses, which are knots of capillaries within each of the four ventricles, form the cerebrospinal fluid. Brain Stem The brain stem is about the size of a thumb in diameter and approximately 3 inches (approximately 7.5 cm) long. Its structures are the midbrain, pons, and medulla oblongata. In addition to providing a pathway for ascending and descending tracts, the brain stem has many small gray matter areas. These nuclei produce the rigidly programmed autonomic behaviors necessary for survival. In addition, some are associated with the cranial nerves and control vital activities such as breathing and blood pressure. 1. Midbrain A relatively small part of the brain stem, the midbrain extends from the mammillary bodies to the pons inferiorly. The cerebral aqueduct, a tiny canal that travels through the midbrain, connects the third ventricle of the diencephalon to the fourth ventricle below. Anteriorly, the midbrain is composed primarily of two bulging fiber tracts, the cerebral peduncles (literally, “little feet of the cerebrum”), which convey ascending and descending impulses. Dorsally located are four rounded protrusions called the corpora quadrigemina because they reminded some anatomist of two pairs of twins (gemini). These bulging nuclei are reflex centers involved with vision and hearing.  2. Pons The pons is the rounded structure that protrudes just below the midbrain. Pons means “bridge,” and this area of the brain stem is mostly fiber tracts (bundles of nerve fibers in the CNS). However, it does have important nuclei involved in the control of breathing. 3. Medulla Oblongata The medulla oblongata is the most inferior part of the brain stem. It merges into the spinal cord below without any obvious change in structure. Like the pons, the medulla is an important fiber tract area. Additionally, the medulla is the area where the important pyramidal tracts (motor fibers) cross over to the opposite side. The medulla also contains many nuclei that regulate vital visceral activities. It contains centers that control heart rate, blood pressure, breathing, swallowing, and vomiting, among others. The fourth ventricle lies posterior to the pons and medulla and anterior to the cerebellum. 4. Reticular Formation Extending the entire length of the brain stem is a diffuse mass of gray matter, the reticular formation. The neurons of the reticular formation are involved in motor control of the visceral organs—for example, controlling smooth muscle in the digestive tract. A special group of reticular formation neurons, the reticular activating system (RAS), plays a role in consciousness and the awake/sleep cycle. The RAS also acts as a filter for the flood of sensory inputs that streams up the spinal cord and brain stem daily. Weak or repetitive signals are filtered out, but unusual or strong impulses do reach consciousness. Damage to this area can result in prolonged unconsciousness (coma). Cerebellum The large, cauliflower-like cerebellum projects dorsally from under the occipital lobe of the cerebrum. Like the cerebrum, the cerebellum has two hemispheres and a convoluted surface. The cerebellum also has an outer cortex made up of gray matter and an inner region of white matter. The cerebellum provides the precise timing for skeletal muscle activity and controls our balance. Thanks to its activity, body movements are smooth and coordinated. It plays its role less well when itis sedated by alcohol. Fibers reach the cerebellum from the equilibrium apparatus of the inner ear, the eye, the proprioceptors of the skeletal muscles and tendons, and many other areas. The cerebellum can be compared to an automatic pilot, continuously comparing the brain’s “intentions” with actual body performance by monitoring body position and the amount of tension in various body parts. When needed, the cerebellum sends messages to initiate the appropriate corrective measures. Homeostatic Imbalance If the cerebellum is damaged (for example, by a blow to the head, a tumor, or a stroke), movements become clumsy and disorganized—a condition called ataxia. Victims cannot keep their balance and may appear drunk because of the loss of muscle coordination. They are no longer able to touch their finger to their nose with eyes closed—a feat that healthy individuals accomplish easily.  Protection of the Central Nervous System Nervous tissue is soft and delicate, and even slight pressure can injure the irreplaceable neurons. As we saw in Chapter 5, nature tries to protect the brain and spinal cord by enclosing them within bone (the skull and vertebral column). Now, let’s focus on three additional protections for the CNS: the meninges, cerebrospinal fluid, and blood-brain barrier. 1. Meninges The three connective tissue membranes covering and protecting the CNS structures are meninges. The outermost layer, the leathery dura mater, meaning “tough or hard mother,” is a double-layered membrane where it surrounds the brain. One of its layers is attached to the inner surface of the skull, forming the periosteum (periosteal layer). The other, called the meningeal layer, forms the outermost covering of the brain and continues as the dura mater of the spinal cord. The dural layers are fused together except in three areas where they separate to enclose dural venous sinuses that collect venous blood, such as the superior sagittal sinus. In several places, the inner dural membrane extends inward to form a fold that attaches the brain to the cranial cavity. Two of these folds, the falx cerebri and the tentorium cerebelli, separate the cerebellum from the cerebrum. The middle meningeal layer is the web-like arachnoid mater. Arachnida means “spider,” and some think the arachnoid membrane looks like a cobweb. Its thread-like extensions span the subarachnoid space to attach it to the innermost membrane, the pia mater (“gentle mother”). The delicate pia mater clings tightly to the surface of the brain and spinal cord, following every fold. The subarachnoid space is filled with cerebrospinal fluid. (Remember that the choroid plexuses produce CSF). Specialized projections of the arachnoid membrane, arachnoid granulations, protrude through the dura mater. The cerebrospinal fluid is absorbed into the venous blood in the dural sinuses through the arachnoid granulations. Homeostatic Imbalance Meningitis, an inflammation of the meninges, is a serious threat to the brain because bacterial or viral meningitis may spread into the nervous tissue of the CNS. This condition of brain inflammation is called encephalitis. Meningitis is usually diagnosed by taking a sample of cerebrospinal fluid from the subarachnoid space surrounding the spinal cord. 2. Cerebrospinal Fluid Cerebrospinal fluid (CSF) is a watery “broth” with components similar to blood plasma, from which it forms. However, it contains less protein and more vitamin C, and its ion composition is different. The choroid plexuses—clusters of capillaries hanging from the “roof” in each of the brain’s ventricles, or enlarged chambers—continually form CSF from blood. The CSF in and around the brain and cord forms a watery cushion that protects the fragile nervous tissue from blows and other trauma, and helps the brain “float” so it is not damaged by the pressure of its own weight. Inside the brain, CSF is continually moving. It circulates from the two lateral ventricles (in the cerebral hemispheres) into the third ventricle (in the diencephalon) and then through the cerebral aqueduct of the midbrain into the fourth ventricle dorsal to the pons and medulla oblongata. Some of the fluid reaching the fourth ventricle simply continues down into the spinal cord, but most of it circulates into the subarachnoid space through three openings, the paired lateral apertures and the median aperture, in the walls of the fourth ventricle. The CSF returns to the blood in the dural venous sinuses through the arachnoid granulations. In this way, CSF is continually replaced. Ordinarily, CSF forms and drains at a constant rate so that its normal pressure and volume (150 ml—about half a cup) are maintained. Any significant changes in CSF composition (or the appearance of blood cells in it) could indicate meningitis or certain other brain pathologies (such as tumors or multiple sclerosis). A procedure called a lumbar (spinal) puncture can obtain a sample of CSF for testing. Because the withdrawal of fluid decreases CSF fluid pressure, the patient must remain horizontal (lying down) for 6 to 12 hours after the procedure to prevent an agonizingly painful “spinal headache.” Homeostatic Imbalance If something obstructs its drainage (for example, a tumor), CSF begins to accumulate and exert pressure on the brain. This condition is hydrocephalus, literally, “water on the brain.” Hydrocephalus in a newborn baby causes the head to enlarge as the brain increases in size. This is possible in an infant because the skull bones have not yet fused. However, in an adult this condition is likely to result in brain damage because the skull is hard, and the accumulating fluid creates pressure that crushes soft nervous tissue and could restrict blood flow into the brain. Today hydrocephalus is treated surgically by inserting a shunt (a plastic tube) to drain the excess fluid into a vein in the neck or abdomen. 3. The Blood-Brain Barrier No other body organ is so absolutely dependent on a constant internal environment as is the brain. Other body tissues can withstand the rather small fluctuations in the concentrations of hormones, ions, and nutrients that continually occur, particularly after eating or exercising. If the brain were exposed to such chemical changes, uncontrolled neural activity might result—remember that certain ions (sodium and potassium) are involved in initiating nerve impulses and that some amino acids serve as neurotransmitters. Consequently, neurons are kept separated from blood-borne substances by the blood-brain barrier, composed of the least permeable capillaries in the whole body. These capillaries are almost seamlessly bound together by tight junctions all around. Of water-soluble substances, only water, glucose, and essential amino acids pass easily through the walls of these capillaries. Metabolic wastes, such as urea, toxins, proteins, and most drugs, are prevented from entering brain tissue. Nonessential amino acids and potassium ions not only are prevented from entering the brain, but also are actively pumped from the brain into the blood across capillary walls. Although the bulbous “feet” of the astrocytes that cling to the capillaries may contribute to the barrier, the relative impermeability of the brain capillaries is most responsible for providing this protection. The blood-brain barrier is virtually useless against fats, respiratory gases, and other fat-soluble molecules that diffuse easily through all plasma membranes. This explains why blood-borne alcohol, nicotine, and anesthetics can affect the brain. Brain Dysfunctions Brain dysfunctions are unbelievably varied. Here, we will focus on traumatic brain injuries and cerebrovascular accidents. Traumatic Brain Injuries Head injuries are a leading cause of accidental death in the United States. Consider, for example, what happens if you crash into the rear end of another car. If you’re not wearing a seatbelt, your head will jerk forward and then violently stop as it hits the windshield. Brain trauma is caused not only by injury at the site of the blow, but also by the effect of the ricocheting brain hitting the opposite end of the skull. A concussion occurs when brain injury is slight. The victim may be dizzy, “see stars,” or lose consciousness briefly, but typically little permanent brain damage occurs. A brain contusion results from marked tissue destruction. If the cerebral cortex is injured, the individual may remain conscious, but severe brain stem contusions always result in a coma lasting from hours to a lifetime due to injury to the reticular activating system. After head blows, death may result from intracranial hemorrhage (bleeding from ruptured vessels) or cerebral edema (swelling of the brain due to inflammatory response to injury). Individuals who are initially alert and lucid following head trauma and then begin to deteriorate neurologically are most likely hemorrhaging or suffering the delayed consequences of edema, both of which compress vital brain tissue. Cerebrovascular Accidents Commonly called strokes, cerebrovascular accidents (CVAs) are the fifth leading cause of death in the United States. CVAs occur when blood circulation to a brain area is blocked, as by a blood clot or a ruptured blood vessel, and vital brain tissue dies. After a CVA, it is often possible to determine the area of brain damage by observing the patient’s symptoms. For example, if the patient has left-sided paralysis (a one-sided paralysis is called hemiplegia), the right motor cortex of the frontal lobe is most likely involved. Aphasias are a common result of damage to the left cerebral hemisphere, where the language areas are located. There are many types of aphasias, but the most common are motor aphasia, which involves damage to Broca’s area and a loss of ability to speak, and sensory aphasia, in which a person loses the ability to understand written or spoken language. Aphasias are maddening to victims because, as a rule, their intellect is unimpaired. Brain lesions can also cause marked changes in a person’s disposition (for example, a change from a sunny to a foul personality). In such cases, a tumor as well as a CVA might be suspected. CVA mortality rates have declined over the past 15 years. More than 75 percent of patients who have a CVA survive 1 year, and more than 50 percent survive for 5 years. Some patients recover at least part of their lost faculties because undamaged neurons spread into areas where neurons have died and take over some lost functions. Indeed, most of the recovery seen after brain injury is due to this phenomenon. Not all strokes are “completed.” Temporary brain ischemia, or restricted blood flow, is called a transient ischemic attack (TIA). TIAs last from 5 to 50 minutes and are characterized by symptoms such as numbness, temporary paralysis, and impaired speech. Although these defects are not permanent, they do constitute “red flags” that warn of impending, more serious CVAs. Spinal Cord The cylindrical spinal cord, which is approximately 17 inches (42 cm) long, is a glistening white continuation of the brain stem. The spinal cord provides a two-way conduction pathway to and from the brain, and it is a major reflex center (spinal reflexes are completed at this level). Enclosed within the vertebral column, the spinal cord extends from the foramen magnum of the skull to the first or second lumbar vertebra, where it ends just below the ribs. Like the brain, the spinal cord is cushioned and protected by meninges. Meningeal coverings do not end at the second lumbar vertebra (L2) but instead extend well beyond the end of the spinal cord in the vertebral canal. Because there is no possibility of damaging the cord beyond L3, the meningeal sac inferior to that point provides a nearly ideal spot for a lumbar puncture to remove CSF for testing. In humans, 31 pairs of spinal nerves arise from the cord and exit from the vertebral column to serve the body area close by. The spinal cord is about the size of a thumb for most of its length, but it is enlarged in the cervical and lumbar regions where the nerves serving the upper and lower limbs arise and leave the cord. Because the vertebral column grows faster than the spinal cord, the spinal cord does not reach the end of the vertebral column, and the spinal nerves leaving its inferior end must travel through the vertebral canal for some distance before exiting. This collection of spinal nerves at the inferior end of the vertebral canal is called the cauda equina because it looks so much like a horse’s tail (the literal translation of cauda equina). Gray Matter of the Spinal Cord and Spinal Roots The gray matter of the spinal cord looks like a butterfly or the letter H in cross section. The two posterior projections are the dorsal horns, or posterior horns; the two anterior projections are the ventral horns, or anterior horns. The gray matter surrounds the central canal of the cord, which contains CSF. Neurons with specific functions can be located in the gray matter. The dorsal horns contain interneurons. The cell bodies of the sensory neurons, whose fibers enter the cord by the dorsal root, are found in an enlarged area called the dorsal root ganglion. If the dorsal root or its ganglion is damaged, sensation from the body area served will be lost. The ventral horns of the gray matter contain cell bodies of motor neurons of the somatic (voluntary) nervous system, which send their axons out the ventral root of the cord. The dorsal and ventral roots fuse to form the spinal nerves. Homeostatic Imbalance Damage to the ventral root results in flaccid paralysis of the muscles served. In flaccid paralysis, nerve impulses do not reach the muscles affected; thus, no voluntary movement of those muscles is possible. The muscles begin to atrophy because they are no longer stimulated. White Matter of the Spinal Cord White matter of the spinal cord is composed of myelinated fiber tracts— some running to higher centers, some traveling from the brain to the cord, and some conducting impulses from one side of the spinal cord to the other. Because of the irregular shape of gray matter, the white matter on each side of the cord is divided into three regions—the dorsal column, lateral column, and ventral column. Each of the columns contains a number of fiber tracts made up of axons with the same destination and function. Tracts conducting sensory impulses to the brain are sensory, or afferent, tracts. Those carrying impulses from the brain to skeletal muscles are motor, or efferent, tracts. All tracts in the dorsal columns are ascending tracts that carry sensory input to the brain. The lateral and ventral columns contain both ascending and descending (motor) tracts. Homeostatic Imbalance If the spinal cord is transected (cut crosswise) or crushed, spastic paralysis results. The affected muscles stay healthy because they are still stimulated by spinal reflex arcs, and movement of those muscles does occur. However, movements are involuntary and not controllable. This can be as much of a problem as complete lack of mobility. In addition, because the spinal cord carries both sensory and motor impulses, a loss of feeling or sensory input occurs in the body areas below the point of cord destruction. Physicians often use a pin to see whether a person can feel pain after spinal cord injury—to find out whether regeneration is occurring. Pain is a hopeful sign in such cases. If the spinal cord injury occurs high in the cord, so that all four limbs are affected, the individual is a quadriplegic. If only the legs are paralyzed, the individual is a paraplegic. C. Peripheral Nervous System The peripheral nervous system (PNS) consists of nerves and scattered ganglia (groups of neuronal cell bodies found outside the CNS). We have already considered one type of ganglion—the dorsal root ganglion of the spinal cord. We will cover others in the discussion of the autonomic nervous system. Here, we will concern ourselves only with nerves. Structure of Nerve As noted earlier in this chapter, a nerve is a bundle of neuron fibers found outside the CNS. Within a nerve, neuron fibers, or processes, are wrapped in protective connective tissue coverings. Each fiber is surrounded by a delicate connective tissue sheath, an endoneurium. Groups of fibers are bound by a coarser connective tissue wrapping, the perineurium, to form fiber bundles, or fascicles. Finally, all the fascicles are bound together by a tough fibrous sheath, the epineurium, to form the cordlike nerve. Like neurons, nerves are classified according to the direction in which they transmit impulses. Nerves that carry impulses only toward the CNS are called sensory (afferent) nerves, whereas those that carry only motor fibers are motor (efferent) nerves. Nerves carrying both sensory and motor fibers are called mixed nerves; all spinal nerves are mixed nerves. Cranial Nerves The 12 pairs of cranial nerves primarily serve the head and neck. Only one pair (the vagus nerves) extends to the thoracic and abdominal cavities. The cranial nerves are numbered in sequence, and in most cases their names reveal the most important structures they control. Table 7.2 describes cranial nerves by name, number, course, and major function. The last column of the table describes how cranial nerves are tested, which is an important part of any neurological examination. You do not need to memorize these tests, but this information may help you understand cranial nerve function. As you read through the table, also look at Figure 7.23, which shows the location of the cranial nerves on the brain’s anterior surface. Most cranial nerves are mixed nerves; however, three pairs—the optic, olfactory, and vestibulocochlear nerves—are purely sensory in function. (The older name for the vestibulocochlear nerve is acoustic nerve, a name that reveals its role in hearing but not in equilibrium.) This study tool may help you remember the cranial nerves in order. The first letter of each word in the saying is the first letter of the cranial nerve to be remembered: “Oh, oh, oh, to touch and feel very good velvet at home.”  Spinal Nerves and Nerve Plexuses The 31 pairs of human spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord. Although each of the cranial nerves issuing from the brain is named specifically, the spinal nerves are named for the region of the cord from which they arise. Almost immediately after being formed, each spinal nerve divides into the dorsal ramus and ventral ramus (plural rami), making each spinal nerve only about ½ inch long. The rami, like the spinal nerves, contain both motor and sensory fibers. Thus, damage to a spinal nerve or either of its rami results both in loss of sensation and in flaccid paralysis of the area of the body served. The smaller dorsal rami serve the skin and muscles of the posterior body trunk. The ventral rami of spinal nerves T1 through T12 form the x, which supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk. The ventral rami of all other spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the limbs.  Autonomic Nervous System The autonomic nervous system (ANS) is the motor subdivision of the PNS that controls body activities automatically. It is composed of a specialized group of neurons that regulate cardiac muscle (the heart), smooth muscles (found in the walls of the visceral organs and blood vessels), and glands. Although all body systems contribute to homeostasis, the relative stability of our internal environment depends largely on the workings of the ANS. At every moment, signals flood from the visceral organs into the CNS, and the autonomic nervous system makes adjustments as necessary to best support body activities. For example, blood flow may be shunted to more “needy” areas, heart and breathing rate may be sped up or slowed down, blood pressure may be adjusted, and stomach secretions may be increased or decreased. Most of this fine-tuning occurs without our awareness or attention—few of us realize when our pupils dilate or our arteries constrict— hence the ANS is also called the involuntary nervous system. Somatic and Autonomic Nervous Systems Compared Our previous discussions of nerves in the efferent (motor) division of the PNS have focused on the somatic nervous system, the PNS subdivision that controls our skeletal muscles. So, before plunging into a description of autonomic nervous system anatomy, let’s note some important differences between the somatic and autonomic subdivisions of the PNS. Besides differences in their effector organs and in the neurotransmitters they release, the patterns of their motor pathways differ. In the somatic division, the cell bodies of the motor neurons are inside the CNS, and their axons (in spinal nerves) extend all the way to the skeletal muscles they serve. The autonomic nervous system, however, has a chain of two motor neurons. The first motor neuron of each pair, the preganglionic neuron, is in the brain or spinal cord. Its axon, the preganglionic axon (literally, the “axon before the ganglion”), leaves the CNS to form a synapse with the second motor neuron in a ganglion outside the CNS. The axon of this ganglionic neuron, the postganglionic axon, then extends to the organ it serves. The autonomic nervous system has two arms, the sympathetic and the parasympathetic. Both serve the same organs but cause essentially opposite effects, counterbalancing each other’s activities to keep body systems running smoothly. The sympathetic division mobilizes the body during extreme situations (such as fear, exercise, or rage), whereas the parasympathetic division allows us to “unwind” and conserve energy. We examine these differences in more detail shortly, but first let’s consider the structural characteristics of the two arms of the ANS. Anatomy of the Parasympathetic Division The preganglionic neurons of the parasympathetic division are located in brain nuclei of several cranial nerves—III, VII, IX, and X (the vagus being the most important of these) and in the S2 through S4 levels of the spinal cord, For this reason, the parasympathetic division is also called the craniosacral division. The neurons of the cranial region send their axons out in cranial nerves to serve the head and neck organs. There they synapse with the ganglionic motor neuron in a terminal ganglion. From the terminal ganglion, the postganglionic axon extends a short distance to the organ it serves. In the sacral region, the preganglionic axons leave the spinal cord and form the pelvic splanchnic nerves, also called the pelvic nerves, which travel to the pelvic cavity. In the pelvic cavity, the preganglionic axons synapse with the second motor neurons in terminal ganglia on, or close to, the organs they serve. Anatomy of the Sympathetic Division The sympathetic division is also called the thoracolumbar division because its preganglionic neurons are in the gray matter of the spinal cord from T1 through L2 (see Figure 7.27). The preganglionic axons leave the cord in the ventral root, enter the spinal nerve, and then pass through a ramus communicans, or small communicating branch, to enter a sympathetic trunk ganglion. The sympathetic trunk, or sympathetic chain lies alongside the vertebral column on each side. After it reaches the ganglion, the axon may synapse with the second (ganglionic) neuron in the sympathetic chain at the same level or a different level, and the postganglionic axon then reenters the spinal nerve to travel to the skin. Or, the preganglionic axon may pass through the ganglion without synapsing and form part of the splanchnic nerves. The splanchnic nerves travel to the viscera to synapse with the ganglionic neuron, found in a collateral ganglion anterior to the vertebral column. The major collateral ganglia— the celiac and the superior and inferior mesenteric ganglia— supply the abdominal and pelvic organs. The postganglionic axon then leaves the collateral ganglion and travels to serve a nearby visceral organ. Now that we have described the anatomical details, we are ready to examine ANS functions in a little more detail. Autonomic Functioning Body organs served by the autonomic nervous system receive fibers from both divisions. Exceptions are most blood vessels and most structure of the skin, some glands, and the adrenal medulla, all of which receive only sympathetic fibers. When both divisions serve the same organ, they cause antagonistic effects, mainly because their postganglionic axons release different neurotransmitters. The parasympathetic fibers, called cholinergic fibers, release acetylcholine. The sympathetic postganglionic fibers, called adrenergic fibers, release norepinephrine. The preganglionic axons of both divisions release acetylcholine. To emphasize the relative roles of the two arms of the ANS, we will focus briefly on situations in which each division is “in control.” Sympathetic Division The sympathetic division is often referred to as the “fight-or-flight” system. Its activity is evident when we are excited or find ourselves in emergency or threatening situations, such as being frightened by a stranger late at night. A pounding heart; rapid, deep breathing; cold, sweaty skin; a prickly scalp; and dilated eye pupils are sure signs of sympathetic nervous system activity. Under such conditions, the sympathetic nervous system increases heart rate, blood pressure, and blood glucose levels; dilates the bronchioles of the lungs; and brings about many other effects that help the individual cope with the stressor. Other examples are dilation of blood vessels in skeletal muscles (so that we can run faster or fight better) and withdrawal of blood from the digestive organs (so that the bulk of the blood can be used to serve the heart, brain, and skeletal muscles). The sympathetic nervous system is working at full speed not only when you are emotionally upset but also when you are physically stressed. For example, if you have just had surgery or run a marathon, your adrenal glands (activated by the sympathetic nervous system) will be pumping out epinephrine and norepinephrine. The effects of sympathetic nervous system activation continue for several minutes until its hormones are destroyed by the liver. Thus, although sympathetic nerve impulses themselves may act only briefly, the hormonal effects they provoke linger. The widespread and prolonged effects of sympathetic activation help explain why we need time to calm down after an extremely stressful situation. The sympathetic division generates a head of steam that enables the body to cope rapidly and vigorously with situations that threaten homeostasis. Its function is to provide the best conditions for responding to some threat, whether the best response is to run, to see better, or to think more clearly. Parasympathetic Division The parasympathetic division is most active when the body is at rest and not threatened in any way. This division, sometimes called the “rest-and-digest” system, is chiefly concerned with promoting normal digestion, with elimination of feces and urine, and with conserving body energy, particularly by decreasing demands on the cardiovascular system. Its activity is best illustrated by a person who relaxes after a meal and reads the newspaper. Blood pressure and heart and respiratory rates are being regulated at low-normal levels, the digestive tract is actively digesting food, and the skin is warm (indicating that there is no need to divert blood to skeletal muscles or vital organs). The eye pupils are constricted to protect the retinas from excessive damaging light, and the lenses of the eyes are “set” for close vision. We might also consider the parasympathetic division as the “housekeeping” system of the body. An easy way to remember the most important roles of the two ANS divisions is to think of the parasympathetic division as the D (digestion, defecation, and diuresis [urination]) division and the sympathetic division as the E (exercise, excitement, emergency, and embarrassment) division. Remember, however, that although it is easiest to think of the sympathetic and parasympathetic divisions as working in an all-or-none fashion, this is rarely the case. A dynamic balance exists between the two divisions, and both are continuously making fine adjustments. Also, although we have described the parasympathetic division as the “at rest” system, most blood vessels are controlled only by the sympathetic fibers regardless of whether the body is “on alert” or relaxing. D. Developmental Aspects of the Nervous System Because the nervous system forms during the first month of embryonic development, maternal infection early in pregnancy can have extremely harmful effects on the fetal nervous system. For example, German measles (rubella) in the mother often causes deafness and other types of CNS damage. Also, because nervous tissue has the highest metabolic rate in the body, lack of oxygen for even a few minutes kills neurons. Because smoking decreases the amount of oxygen that can be carried in the blood, a smoking mother may be sentencing her infant to possible brain damage. Radiation and various drugs (alcohol, opiates, cocaine, and others) can also be very damaging if taken during early fetal development. Homeostatic Imbalance In difficult deliveries, temporary lack of oxygen may lead to cerebral palsy, but this is only one of the suspected causes. Cerebral palsy is a neuromuscular disability in which the voluntary muscles are poorly controlled and spastic because of brain damage. About half of its victims have seizures, are intellectually disabled, and/or have impaired hearing or vision. Cerebral palsy is the largest single cause of physical disabilities in children. A number of other congenital malformations—triggered by genetic or environmental factors—also plague the CNS. Most serious are hydrocephalus, anencephaly, and spina bifida. Anencephaly is a birth defect in which the cerebrum fails to develop. Children with anencephaly cannot see, hear, or process sensory information; these babies typically die soon after birth. Spina bifida (“forked spine”) results when the vertebrae form incompletely (typically in the lumbosacral region). There are several varieties of spina bifida. In the least serious, a dimple, and perhaps a tuft of hair, appears over the site of malformation, but no neurological problems occur. In the most serious, meninges, nerve roots, and even parts of the spinal cord protrude from the spine, rendering the lower part of the spinal cord functionless. The child is unable to control the bowels or bladder, and the lower limbs are paralyzed. One of the last areas of the CNS to mature is the hypothalamus, which contains centers for regulating body temperature. For this reason, premature babies usually have problems controlling their loss of body heat and must be carefully monitored. The nervous system grows and matures all through childhood, largely as a result of myelination that goes on during this period. A good indication of the degree of myelination of particular neural pathways is the level of neuromuscular control in that body area. Neuromuscular coordination progresses in a superior to inferior direction and in a proximal to distal direction, and myelination occurs in the same sequence. The brain reaches its maximum weight in the young adult. Over the next 60 years or so, neurons are damaged and die; and brain weight and volume steadily decline. However, a nearly unlimited number of neural pathways is always available and ready to be developed, allowing us to continue to learn throughout life. As we grow older, the sympathetic nervous system gradually becomes less and less efficient, particularly in its ability to constrict blood vessels. When older people stand up quickly, they often become lightheaded or faint. The reason is that the sympathetic nervous system is not able to react quickly enough to counteract the pull of gravity by activating the vasoconstrictor fibers, and blood pools in the feet. This condition, orthostatic hypotension, is a type of low blood pressure resulting from changes in body position as described. Orthostatic hypotension can be prevented to some degree if the person changes position slowly, giving the sympathetic nervous system time to adjust. The usual cause of nervous system deterioration is circulatory system problems. For example, arteriosclerosis (decreased elasticity of the arteries) and high blood pressure reduce the supply of oxygen to brain neurons. A gradual decline of oxygen due to the aging process can lead to senility, characterized by forgetfulness, irritability, difficulty in concentrating and thinking clearly, and confusion. A sudden loss of blood and oxygen delivery to the brain results in a CVA (stroke), as described earlier. However, many people continue to enjoy intellectual lives and mentally demanding tasks their entire lives. In fact, fewer than 5 percent of people over age 65 demonstrate true senility. Sadly, many cases of “reversible senility,” caused by certain drugs, low blood pressure, constipation, poor nutrition, depression, dehydration, and hormone imbalances, go undiagnosed. The best way to maintain your mental abilities in old age may be to seek regular medical checkups throughout life. Although eventual shrinking of the brain is normal, some individuals (professional boxers and chronic alcoholics, for example) hasten the process. Whether a boxer wins the match or not, the likelihood of brain damage and atrophy increases with every blow. The expression “punch drunk” reflects the symptoms of slurred speech, tremors, abnormal gait, and dementia (mental illness) seen in many retired boxers. Everyone recognizes that alcohol has a profound effect on the mind as well as the body. CT scans of chronic alcoholics reveal reduced brain size at a fairly early age. Like boxers, chronic alcoholics tend to exhibit signs of mental deterioration unrelated to the aging process. The human cerebral hemispheres— our “thinking caps”—are awesome in their complexity. No less amazing are the brain regions that oversee all our subconscious, autonomic body functions—the diencephalon and brain stem—particularly when you consider their relatively insignificant size. The spinal cord, which acts as a reflex center, and the peripheral nerves, which provide communication links between the CNS and body periphery, are equally important to body homeostasis. Reference: Marieb, Elaine Nicpon; Keller, Suzanne M. (2018). Essentials of Human Anatomy and Physiology 12th Edition Pearson Education Inc., 300 Hudson Street, NY NY 10013