Essentials of Human Anatomy & Physiology Global Edition PDF

7 The Nervous System WHAT As the primary control system of the body, the nervous system provides for higher mental HOW function and emotional expression, maintains homeostasis, and...

7 The Nervous System WHAT As the primary control system of the body, the nervous system provides for higher mental HOW function and emotional expression, maintains homeostasis, and Communication by regulates the activities of the nervous system involves muscles and glands. a combination of electrcial and chemical signals. WHY All body systems are under the control or regulation INSTrucTorS of the nervous system. If the nervous New Building systems stops functioning, the body Vocabulary coaching can stay alive only with the Activities for this assistance of life-supporting chapter are assignable machines. in which are rapid and specific and cause almost immediate responses. You are driving down the freeway when a The nervous system does not work alone to but are not paying attention to any particular con- regulate and maintain body homeostasis; the versation, yet when you hear your name, you endocrine system is a second important regulating system. Whereas the nervous system controls with immediately focus in. What do these events have rapid electrical nerve impulses, the endocrine sys- in common? They are everyday examples of ner- tem produces hormones that are released into the vous system function, which has your body cells blood. Thus, the endocrine system acts in a more humming with activity nearly all the time. leisurely way. We will discuss the endocrine sys- The nervous system is the master control and communication system of the body. Every thought, tem in detail in Chapter 9. action, and emotion reflects its activity. It commu- To carry out its normal role, the nervous system nicates with body cells using electrical impulses, has three overlapping functions (Figure 7.1, p. 252): 251 252 Essentials of Human Anatomy and Physiology Central Nervous System Sensory input (brain and spinal cord) Integration Sensory receptor Peripheral Nervous System (cranial and spinal nerves) Motor output Brain and spinal cord Effector Figure 7.1 The nervous system’s functions. Sensory Motor (afferent) (efferent) (1) It uses its millions of sensory receptors to monitor changes occurring both inside and out- side the body. These changes are called stimuli, and the gathered information is called sensory input. (2) It processes and interprets the sensory Sense Somatic Autonomic input and decides what should be done at each organs (voluntary) (involuntary) moment—a process called integration. (3) It then Skeletal Cardiac and causes a response, or effect, by activating muscles muscles smooth muscle, ➔ or glands (effectors) via motor output. ConCeptLink These three overlapping nervous system functions glands are very similar to a feedback loop (Chapter 1, p. 45). Recall that in a feedback loop, a receptor receives sensory input, which it sends to the brain (control center) for processing (integration); the brain then analyzes the information and determines the appro- Parasympathetic Sympathetic priate output, which leads to a motor response. ➔ Figure 7.2 organization of the nervous system. An example will illustrate how these functions As the flowchart shows, the central nervous system work together. When you are driving and see a receives input via sensory fibers and issues commands red light just ahead (sensory input), your nervous via motor fibers. The sensory and motor fibers together form the nerves that constitute the peripheral nervous system integrates this information (red light means system. “stop”) and sends motor output to the muscles of your right leg and foot. Your foot goes for the brake pedal (the response). We have only one nervous system, but because of its organization of the Nervous complexity, it is difficult to consider all its parts at the same time. So, to simplify its study, we divide it in System terms of its structures (structural classi- fication) or in terms of its activities (functional classification). We ➔ Learning Objectives briefly describe each of these classification schemes List the general functions of the nervous system. next, and Figure 7.2 illus- trates their relationships. □□ You do not need to memorize this whole scheme now, Explain the structural and functional classifications □ □of the nervous system. but as you are reading the descriptions, try to get a “feel” for the major parts and how they fit together. □ □Define central nervous system and peripheral nervous system, and list the major parts of each. This will  Chapter 7: The Nervous System 253 make learning easier as you make your way through The somatic (so-mat′ik) nervous system this chapter. Later you will see all these terms and allows us to consciously, or voluntarily, control concepts again in more detail. our skeletal muscles. Hence, we often refer to Structural classification this subdivision as the voluntary nervous system. However, not all skeletal muscle activ- The structural classification, which includes all ity controlled by this motor division is volun- nervous system organs, has two subdivisions—the tary. Skeletal muscle reflexes, such as the central nervous system and the peripheral nervous stretch reflex (described later in the chapter), system (see Figure 7.2). are initiated involuntarily by these same fibers. The central nervous system (CNS) consists of The autonomic (aw″to-nom′ik) nervous sys- the brain and spinal cord, which occupy the dorsal tem (ANS) regulates events that are automatic, body cavity and act as the integrating and com- or involuntary, such as the activity of smooth mand centers of the nervous system. They interpret muscle, cardiac muscle, and glands. This subdivi- incoming sensory information and issue instructions sion, commonly called the involuntary ner- based on past experience and current conditions. vous system, itself has two parts, the sympa- The peripheral (pĕ˘-rif′er-al) nervous system thetic and parasympathetic, which typically bring (PNS) includes all parts of the nervous system out- about opposite effects. What one stimulates, the side the CNS. It consists mainly of the nerves that other inhibits. We will describe these later. extend from the spinal cord and brain. Spinal nerves 7 Although it is simpler to study the nervous sys- carry impulses to and from the spinal cord. Cranial tem in terms of its subdivisions, remember that these (kra′ne-al) nerves carry impulses to and from the subdivisions are made for the sake of convenience brain. These nerves serve as communication lines. only. Remember that the nervous system acts as a They link all parts of the body by carrying impulses coordinated unit, both structurally and functionally. from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles. Did You Get It? Functional classification 1. Name the structures that make up the CNS and those The functional classification scheme is concerned that make up the PNS. only with PNS structures. It divides them into two For the answer, see Appendix A. principal subdivisions (see Figure 7.2). The sensory division, or afferent (af′er-ent; literally “to go toward”) division, consists of Nervous Tissue: Structure nerves (composed of many individual nerve fibers) and Function that convey impulses to the central nervous system ➔ Learning Objective from sensory receptors located in various parts of □ Describe the structures and functions of neurons the body. The sensory division keeps the CNS con- and neuroglia. stantly informed of events going on both inside and outside the body. Sensory fibers delivering Even though it is complex, nervous tissue is made up impulses from the skin, skeletal muscles, and of just two principal types of cells—supporting cells joints are called somatic (soma = body) sensory and neurons. (afferent) fibers, whereas those transmitting Supporting cells impulses from the visceral organs are called vis- ceral sensory (afferent) fibers. Supporting cells in the CNS are “lumped together” as neuroglia (nu-rog′le-ah), literally, “nerve glue,” also The motor division, or efferent (ef′er-rent) called glial cells or glia. Neuroglia include many types division, carries impulses from the CNS to effec- tor of cells that support, insulate, and pro- tect the organs, the muscles and glands. These impulses delicate neurons (Figure 7.3). In addition, each of the activate muscles and glands; that is, they effect different types of neuroglia has special functions. (bring about or cause) a motor response. CNS neuroglia include the following: The motor division in turn has two subdivi- Astrocytes: abundant star-shaped cells that sions (see Figure 7.2): account for nearly half of neural tissue 254 Essentials of Human Anatomy and Physiology Capillary Myelin sheath Neuron Process of oligodendrocyte Astrocyte Nerve fibers (a) Astrocytes are the most abundant (d) Oligodendrocytes have processes that form and versatile neuroglia. myelin sheaths around CNS nerve fibers. Satellite cells Cell body of neuron Neuron Schwann cells Microglial (forming myelin sheath) cell Nerve fiber (b) Microglial cells are phagocytes that (e) Satellite cells and Schwann cells (which form defend CNS cells. myelin) surround neurons in the PNS. Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal Figure 7.3 Supporting cells (neuroglia) of nervous tissue. (Figure 7.3a). Their numerous projections have recapturing chemicals released for communica- swollen ends that cling to neurons, bracing them tion purposes. and anchoring them to their nutrient sup- ply Microglia (mi-krog′le-ah): spiderlike phago- lines, the blood capillaries. Astrocytes form a cytes that monitor the health of nearby neu- living barrier between capillaries and neu- rons, rons and dispose of debris, such as dead brain help determine capillary permeability, and play a cells and bacteria (Figure 7.3b). role in making exchanges between the two. In this Ependymal (ĕ˘-pen′d˘ı̆ -mal) cells: neuroglia way, they help protect the neu- rons from harmful substances that might be in the blood. Astrocytes that line the central cavities of the brain and also help control the chemical environment in the the spinal cord (Figure 7.3c). The beating of brain by “mop- ping up” leaked potassium ions, their cilia helps to circulate the cerebrospinal which are involved in generating a nerve impulse, fluid that fills those cavities and forms a pro- and tective watery cushion around the CNS.  Chapter 7: The Nervous System 255 Oligodendrocytes (ol″˘ı̆ -go-den′dro-sˉı̄ tz): filaments that are important in maintaining cell neuroglia that wrap their flat extensions (pro- shape) are particularly abundant in the cell body. cesses) tightly around the nerve fibers, produc- ing fatty insulating coverings called myelin Processes The armlike processes, or fibers, sheaths (Figure 7.3d). vary in length from microscopic to about 7 feet in Although neuroglia somewhat resemble neu- the tallest humans. The longest ones in humans rons structurally (both cell types have cell exten- reach from the lumbar region of the spine to the sions), they are not able to transmit nerve impulses, great toe. Neuron processes that convey incoming a function that is highly developed in neurons. messages (electrical signals) toward the cell body Another important difference is that neuroglia are dendrites (den′drˉı̄ tz), whereas those that gen- never lose their ability to divide, whereas most erate nerve impulses and typically conduct them neurons do. Consequently, most brain tumors are away from the cell body are axons (ak′sonz). gliomas, or tumors formed by neuroglia. Neurons may have hundreds of branching den- Supporting cells in the PNS come in two major drites (dendr = tree), depending on the neuron varieties—Schwann cells and satellite cells (Figure type. However, each neuron has only one axon, 7.3e). Schwann cells form the myelin sheaths which arises from a conelike region of the cell around nerve fibers in the PNS. Satellite cells act body called the axon hillock. as protective, cushioning cells for peripheral neu- An occasional axon gives off a collateral branch along its length, but all axons branch profusely at 7 ron cell bodies. Did You Get It? their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hun- 2. Which neuroglia are most abundant in the body? dreds of tiny vesicles, or membranous sacs, that Which produce the insulating material called myelin? contain chemicals called neurotransmitters 3. Why is a brain tumor more likely to be formed from (review our discussion of events at the neuromus- neuroglia than from neurons? cular junction in Chapter 6). As we said, axons For answers, see Appendix A. transmit nerve impulses away from the cell body. When these impulses reach the axon terminals, Neurons they stimulate the release of neurotransmitters into the extracellular space between neurons, or Anatomy between a neuron and its target cell. ➔ Learning Objectives Each axon terminal is separated from the next □ □Describe the general structure of a neuron, and neuron by a tiny gap called the synaptic (s˘ı̆ - name its important anatomical regions. nap′tik) cleft. Such a functional junction, where an □ □Describe the composition of gray matter and white impulse is transmitted from one neuron to another, matter. is called a synapse (syn = to clasp or join). Neurons, also called nerve cells, are highly spe- Although they are close, neurons never actually cialized to transmit messages (nerve impulses) from touch other neurons. You will learn more about one part of the body to another. Although neurons synapses and the events that occur there a bit later. differ structurally from one another, they have many common features (Figure 7.4, p. 256). All have a cell Myelin Sheaths Most long nerve fibers are cov- body, which contains the nucleus and one or more ered with a whitish, fatty material called myelin slender processes extending from the cell body. (mi′ĕ˘-lin), which has a waxy appearance. Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses. Compare Cell Body The cell body is the metabolic center of the myelin sheaths to the many layers of insulation that neuron. Its transparent nucleus contains a large cover the wires in an electrical cord; the layers keep nucleolus. The cytoplasm surrounding the nucleus the electricity flowing along the desired path just as contains the usual organelles, except that it lacks myelin does for nerve fibers. Axons outside the CNS centrioles (which confirms the amitotic nature of are myelinated by Schwann cells, as just noted. most neurons). The rough ER, called Nissl (nis′l) Many of these cells wrap themselves around the bodies, and neurofibrils (intermediate axon in a jelly-roll fashion (Figure 7.5, p. 257). Initially, the membrane coil is loose, but the 256 Essentials of Human Anatomy and Physiology Dendrite Cell Mitochondrion body Nissl substance Axon hillock Axon Neurofibrils Collateral branch Nucleus Nucleolus One Schwann cell Node of Axon terminal Ranvier Schwann cells, forming the myelin sheath on axon (a) Neuron cell body Dendr ite Figure 7.4 Structure of a typical motor neuron. (a) Diagrammatic view. (b) Scanning electron micrograph showing the cell body and (b) dendrites (615*).  Chapter 7: The Nervous System 257 Schwann cell cytoplasm is gradually squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes, Q: Why does the myelin sheath that is produced by Schwann cells have gaps in it? the myelin sheath, encloses the axon. Most of the Schwann cell Schwann cell cytoplasm ends up just beneath the cytoplasm outermost part of its plasma membrane. This part of Schwann cell the Schwann cell, external to the myelin sheath, is Axon plasma membrane called the neurilemma (nu″r˘ı̆ -lem′mah, “neuron Schwann cell husk”). Because the myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, nucleus called nodes of Ranvier (rahn-vēˉr), at regular (a) intervals (see Figure 7.4). As mentioned previously, myelinated fibers are also found in the central nervous system. Oligodendrocytes form CNS myelin sheaths (see Figure 7.3d). 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 7 fibers at the same time. Thus, in the CNS, one oligo- (b) dendrocyte can form many myelin sheaths. Although the myelin sheaths formed by oligoden- drocytes and those formed by Schwann cells are Neurilemma similar, the CNS sheaths lack a neurilemma. Because Myelin the neurilemma remains intact (for the most part) sheath 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. (c) Homeostatic Imbalance 7.1 Figure 7.5 relationship of Schwann cells to axons in the peripheral nervous system. (a–c) As illustrated The importance of myelin insulation is best illus- (top to bottom), a Schwann cell envelops part of an axon trated by observing what happens when myelin is in a trough and then rotates around the axon. Most of not there. The disease multiple sclerosis (MS) the Schwann cell cytoplasm comes to lie just beneath the exposed part of its plasma membrane. The tight coil of gradually destroys the myelin sheaths around CNS plasma membrane material surrounding the axon is the fibers by converting them to hardened sheaths myelin sheath. The Schwann cell cytoplasm and exposed called scleroses. As this happens, the electrical cur- membrane are referred to as the neurilemma. rent is short-circuited and may “jump” to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The Terminology Clusters of neuron cell bodies and affected person may have visual and speech distur- collections of nerve fibers are named differently in the bances, lose the ability to control his or her CNS and in the PNS. For the most part, cell bodies are muscles, and become increasingly disabled. found in the CNS in clusters called nuclei. This well- Multiple sclero- sis is an autoimmune disease in protected location within the bony skull or vertebral which the person’s own immune system attacks a column is essential to the well-being of the nervous protein component of the sheath. As yet there is no system—remember that neurons do not routinely cure, but injections of interferon (a hormonelike undergo cell division substance released by some immune cells) appear to hold the symptoms at bay and provide some relief. Other drugs aimed at slowing the each Schwann cell forming only one tiny segment of A: arrange themselves end to end along the nerve fiber, autoimmune response are also being used, though The sheath is produced by several Schwann cells that further research is needed to deter- the sheath. mine their long-term effects. --------------------------- --✚ 258 Essentials of Human Anatomy and Physiology Central process (axon) Sensory Cell neuron Spinal cord body (central nervous system) Ganglion Dendrites Peripheral process (axon) Afferent transmission Interneuron (association neuron) Receptors Peripheral nervous system Efferent transmission Motor neuron To effectors (muscles and glands) Figure 7.6 Neurons classified by system; most cell bodies are in ganglia (association neurons) complete the function. Sensory (afferent) neurons in the PNS. Motor (efferent) neurons communication pathway between conduct impulses from sensory transmit impulses from the CNS (brain sensory and motor neurons; their cell receptors (in the skin, viscera, or spinal cord) to effectors in the body bodies reside in the CNS. muscles) to the central nervous periphery. Interneurons after birth. The cell body carries out most of the Functional Classification Functionally, neurons are metabolic functions of a neuron, so if it is dam- grouped according to the direction the nerve aged, the cell dies and is not replaced. Small col- impulse travels relative to the CNS. On this basis, lections of cell bodies called ganglia (gang′le-ah; there are sensory, motor, and association neurons ganglion, singular) are found in a few sites outside (interneurons) (Figure 7.6). Neurons carrying the CNS in the PNS. impulses from sensory receptors (in the internal Bundles of nerve fibers (neuron processes) organs or the skin) to the CNS are sensory neu- running through the CNS are called tracts, rons, or afferent neurons. (Recall that afferent whereas in the PNS they are called nerves. The means “to go toward.”) The cell bodies of sensory terms white matter and gray matter refer respec- neurons are always found in a ganglion outside the tively to myelinated versus unmyelinated regions CNS. Sensory neurons keep us informed about what of the CNS. As a general rule, the white matter is happening both inside and outside the body. consists of dense collections of myelinated fibers The dendrite endings of the sensory neurons (tracts), and gray matter contains mostly unmy- are usually associated with specialized receptors elinated fibers and cell bodies. that are activated by specific changes occurring nearby. (We cover the very complex receptors of classification the special senses—vision, hearing, equilibrium, ➔ Learning Objectives taste, and smell—separately in Chapter 8). The □ □classify neurons according to structure and function. simpler types of sensory receptors in the skin are cutaneous sense organs, and those in the □ □List the types of general sensory receptors and describe their functions. muscles and t endons are proprioceptors (pro″pre-o-sep′torz) (shown in Figure 4.3 and Neurons may be classified based on their function Figure 7.7). The pain receptors (actually bare nerve or their structure.  Chapter 7: The Nervous System 259 (a) Free nerve endings (pain (b) Meissner’s corpuscle and temperature receptors) (touch receptor) 7 (d) Golgi tendon organ (proprioceptor) (c) Lamellar corpuscle (deep pressure receptor) (e) Muscle spindle (proprioceptor) Figure 7.7 Types of sensory receptors. Parts (d) and (e) represent two types of proprioceptors. endings) are the least specialized of the cutaneous comes from the Latin word meaning “one’s own,” receptors. They are also the most numerous, because and the proprioceptors constantly advise our brain of pain warns us that some type of body damage is our own movements. occurring or is about to occur. However, strong Neurons carrying impulses from the CNS to the stimulation of any of the cutaneous recep- tors (for viscera and/or muscles and glands are motor example, by searing heat, extreme cold, or excessive neurons, or efferent neurons (see Figure 7.6). The pressure) is also interpreted as pain. cell bodies of motor neurons are usually located in The proprioceptors detect the amount of the CNS. stretch, or tension, in skeletal muscles, their ten- The third category of neurons consists of the dons, and joints. They send this information to the interneurons, or association neurons. They brain so that the proper adjustments can be made connect the motor and sensory neurons in neural to maintain balance and normal posture. Propria 260 Essentials of Human Anatomy and Physiology and distal (peripheral) processes. Unipolar neu- rons are unique in that only the small branches at the end of the peripheral process are dendrites. The Cell body remainder of the peripheral process and the central Axon process function as the axon; thus, in this case, the Dendrites axon actually conducts nerve impulses both toward and away from the cell body. Sensory neurons found (a) Multipolar neuron in PNS ganglia are unipolar. Did You Get It? 4. How does a tract differ from a nerve? Cell body 5. How does a ganglion differ from a nucleus? 6. Which part of a neuron conducts impulses toward the cell body in multipolar and bipolar neurons? Which part releases neurotransmitters? Dendrite Axon 7. Your professor tells you that one neuron transmits a nerve impulse at the rate of 1 meter per second and (b) Bipolar neuron another neuron conducts at the rate of 40 meters per second. Which neuron has the myelinated axon? For answers, see Appendix A. Dendrites Cell body Short single Physiology: Nerve Impulses process ➔ Learning Objectives Axon □ □Describe the events that lead to the generation of a nerve impulse and its conduction from one Peripheral Central neuron to another. process process □ □Define reflex arc, and list its elements. (c) Unipolar neuron Neurons have two major functional properties: Figure 7.8 classification of neurons on the basis irritability, the ability to respond to a stimulus and of structure. convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neu- rons, muscles, or glands. pathways. Their cell bodies are typically located in the CNS. Electrical Conditions of a Resting Neuron’s Membrane The plasma membrane of a resting, or inactive, Structuralclassification Structural Classificationis neuron is polarized, which means that there are based on the number of processes, including both fewer positive ions sitting on the inner face of the dendrites and axons, extending from the cell body neuron’s plasma membrane than there are on its (Figure 7.8). If there are several, the neuron is a outer face (Figure 7.9 1). The major positive ions multipolar neuron. Because all motor and asso- inside the cell are potassium (K+), whereas the ciation neurons are multipolar, this is the most major positive ions outside the cell are sodium common structural type. Neurons with two pro- (Na+). As long as the inside remains more negative cesses—one axon and one dendrite—are bipolar (fewer positive ions) than the outside, the neuron neurons. Bipolar neurons are rare in adults, found will stay inactive. only in some special sense organs (eye, nose), where they act in sensory processing as receptor Action Potential Initiation and Generation cells. Unipolar neurons have a single process Many emerging from the cell body as if the cell body different types of stimuli excite neurons to become were on a “cul-de-sac” off the “main road” that is active and generate an impulse. For example, light the axon. However, the process is very short and excites the eye receptors, sound excites some of the divides almost immediately into proximal (central) ear receptors, and pressure excites some cutaneous  Chapter 7: The Nervous System 261 [Na+] +– + +– + – + + + – – + – – – 1 Resting membrane is polarized. In the resting state, the external face of the membrane is slightly positive; its internal [K+] face is slightly negative. The chief extracellular ion is sodium (Na+), whereas the chief intracellular ion is potassium (K+). – – –– – – – The membrane is relatively impermeable to both ions. + + + + + + + Na+ +– + + + – + + 2 Stimulus initiates local depolarization. A stimulus – – – –+ + – – changes the permeability of a local "patch" of the membrane, + Na+ and sodium ions diffuse rapidly into the cell. This changes the polarity of the membrane (the inside becomes more positive; – – –– – – – the outside becomes more negative) at that site. + + + + + + + Na+ – – – –+ + – – +– + + + + 3 Depolarization and generation of an action potential. 7 + – If the stimulus is strong enough, depolarization causes +Na+ membrane polarity to be completely reversed, and an action + + – – – – – potential is initiated. – – + + + + + +– + – – –+ – – + 4 Propagation of the action potential. Depolarization of – + + + + – the first membrane patch causes permeability changes in the adjacent membrane, are repeated. Thus, and the the events action described potential in step 2 rapidly propagates + + + + – – – along the entire length of the membrane. – – –– + + + K+ + + + – + – –+ –+ +– – + – + 5 Repolarization. Potassium ions diffuse out of the cell as the membrane permeability changes again, restoring the – K+ negative charge on the inside of the membrane and the – – –+ + + + positive charge on the outside surface. Repolarization occurs in the same direction as depolarization. + + + – – – – Cell Na+ Na+– K+ exterior Na + Na + pump K+ 6 Initial ionic conditions restored. The ionic conditions of the resting state are restored later by the activity of the Na Diffusion K Diffusion K+ Plasma sodium-potassium pump. Three sodium ions are ejected for + + membrane every two potassium ions carried back into the cell. K+ Cell K+ inter ior Figure 7.9 The nerve impulse. 262 Essentials of Human Anatomy and Physiology receptors of the skin. However, most neurons in the faster because the nerve impulse literally jumps, or body are excited by neurotransmitter chemicals leaps, from node to node along the length of the fiber. released by other neurons, as we will describe This occurs because no electrical current can flow shortly. across the axon membrane where there is fatty Regardless of the stimulus, the result is always myelin insulation. This faster type of electrical the same—the permeability properties of the cell’s impulse propagation is called saltatory (sal′tah- to″re) plasma membrane change for a very brief period. conduction (saltare = to dance or leap). Normally, sodium ions cannot diffuse through the plasma membrane to any great extent, but when Homeostatic Imbalance 7.2 the neuron is adequately stimulated, the “gates” of sodium channels in the membrane open. Because A number of factors can impair the conduction of sodium is in much higher concentration outside the impulses. For example, sedatives and anesthetics cell, it then diffuses quickly into the neuron. block nerve impulses by altering membrane per- (Remember the laws of diffusion?) This inward rush meability to ions, mainly sodium ions. As we have of sodium ions changes the polarity of the neuron’s seen, no sodium entry = no action potential. membrane at that site, an event called depolariza- Cold and continuous pressure hinder impulse tion (Figure 7.9 2). Locally, the inside is now more conduction because they interrupt blood circula- positive, and the outside is less positive, a local tion (and hence the delivery of oxygen and nutri- electrical situation called a graded potential. ents) to the neurons. For example, your fingers get However, if the stimulus is strong enough and the numb when you hold an ice cube for more than a sodium influx is great enough, the local depolariza- few seconds. Likewise, when you sit on your foot, it tion (graded potential) activates the neuron to initi- “goes to sleep.” When you warm your fingers or ate and transmit a long-distance signal called an remove the pressure from your foot, the impulses action potential, also called a nerve impulse in begin to be transmitted again, leading to an neurons. 3 The nerve impulse is an all-or-none unpleasant prickly feeling. response, like starting a car. It is either propagated ____________________✚ (conducted, or sent) over the entire axon 4, or it Transmission of the Signal at Synapses So far doesn’t happen at all. The nerve impulse never we have explained only the irritability aspect of goes partway along an axon’s length, nor does it neuronal functioning. What about conductivity— die out with distance, as do graded potentials. how does the electrical impulse traveling along Almost immediately after the sodium ions rush one neuron get across the synapse to the next into the neuron, the membrane permeability neuron (or effector cell) to influence its activity? changes again, becoming impermeable to sodium The answer is that the impulse doesn’t! Instead, ions but permeable to potassium ions. So potas- a neurotransmitter chemical crosses the synapse to sium ions are allowed to diffuse out of the neuron transmit the signal from one neuron to the next, or into the interstitial fluid, and they do so very rap- to the target cell (as occurred at the neuromuscular idly. This outflow of positive ions from the cell junction; see Chapter 6). When the action potential restores the electrical conditions at the membrane reaches an axon terminal (Figure 7.10 1), the elec- to the polarized, or resting, state, an event called trical change opens calcium channels. Calcium repolarization (Figure 7.9 5). After repolarization ions, in turn, cause the tiny vesicles containing of the electrical conditions, the sodium-potassium neurotransmitter to fuse with the axonal membrane pump restores the initial concentrations of the 2, and porelike openings form, releasing the neu- sodium and potassium ions inside and outside the rotransmitter into the synaptic cleft 3. The neu- neuron 6. This pump uses ATP (cellular energy) to rotransmitter molecules diffuse across the synaptic pump excess sodium ions out of the cell and to cleft* and bind to receptors on the membrane of bring potassium ions back into it. Until repolariza- the next neuron 4. If enough neurotransmitter is tion occurs, a neuron cannot conduct another impulse. Once begun, these sequential events spread along the entire neuronal membrane. *Although most neurons communicate via the chemical type of synapse described above, there are some examples of The events just described explain propagation electrical synapses, in which the neurons are physically of a nerve impulse along unmyelinated fibers. Fibers joined by gap junctions and electrical currents actually flow that have myelin sheaths conduct impulses much from one neuron to the next.  Chapter 7: The Nervous System 263 released, the whole series of events described above Axon of (sodium entry 1, depolarization, etc.) will occur, transmitting generating a graded potential and eventually a nerve neuron impulse in the receiving neuron beyond the synapse. Receiving The electrical changes prompted by neurotransmitter neuron binding are very brief because the neurotransmitter is quickly removed from the syn- aptic cleft 6, either by diffusing away, by reuptake into the axon terminal, or by enzymatic break- 1 Action down. This limits the effect of each nerve impulse Dendr ite potential to a period shorter than the blink of an eye. arrives. Notice that the transmission of an impulse is an Vesicles electrochemical event. Transmission down the length Axon terminal Synaptic of the neuron’s membrane is basically electrical, but cleft 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. 7 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 2Vesicle Transmitting neuron are rapid, predictable, and involuntary responses fuses with 4 Neurotrans- plasma 3 Neurotrans- mitter binds to stimuli. They are much like one-way streets— membrane. mitter is to receptor once a reflex begins, it always goes in the same released into on receiving direction. Reflexes occur over neural pathways synaptic cleft. neuron's called reflex arcs and involve both CNS and PNS membrane. structures. Think of a reflex as a preprogrammed response to a given stimulus. Synaptic The types of reflexes that occur in the body are cleft Ion Neurotransmitter classed as either somatic or autonomic. Somatic channels molecules reflexes include all reflexes that stimulate the skel- etal muscles; these are still involuntary reflexes even though skeletal muscle normally is under voluntary control. When you quickly pull your hand away Receiving neuron from a hot object, a somatic reflex is working. Neurotransmitter is Autonomic reflexes regulate the activity of smooth Neurotransmitter broken down and muscles, the heart, and glands. Secretion of saliva released. (salivary reflex) and changes in the size of the eye Receptor Na+ pupils (pupillary reflex) are two such reflexes. Na+ Autonomic reflexes regulate such body functions as digestion, elimination, blood pressure, and sweating. All reflex arcs have a minimum of five ele- ments (Figure 7.11a, p. 264): a receptor (which reacts to a stimulus), an effector (the muscle or gland eventually stimulated), and sensory and motor 5 Ion channel opens. 6 Ion channel closes. neurons to connect the two. The synapse or interneurons between the sensory and motor neu- Figure 7.10 How neurons communicate at rons represents the fifth element—the CNS inte- chemical synapses. The events occurring at the gration center. synapse are numbered in order. 264 Essentials of Human Anatomy and Physiology Stimulus at distal Skin Spinal cord end of neuron (in cross section) 2 Sensory neuron 3Integration 1 Receptor center 4 Motor neuron 5 Effector Interneuron (a) Five basic elements of reflex arc 1 Sensory (stretch) receptor 2 Sensory (afferent) neuron 3 4 Motor (efferent) neuron 5 Effector organ (b) Two-neuron reflex arc 1 Sensory receptor 2 Sensory (afferent) neuron 3 Interneuron 4 Motor (efferent) neuron 5 Effector organ (c) Three-neuron reflex arc Figure 7.11 Simple reflex arcs.  Chapter 7: The Nervous System 265 The simple patellar (pah-tel′ar), or knee-jerk, reflex is an example of a two-neuron reflex arc, the central Nervous System simplest type in humans (Figure 7.11b). The patellar Functional Anatomy of the Brain reflex (in which the quadriceps muscle attached to ➔ Learning Objective the hit tendon is stretched) is familiar to most of us. It is usually tested during a physical exam to □ Identify and indicate the functions of the major regions of the cerebral hemispheres, diencephalon, determine the general health of the motor portion of brain stem, and cerebellum on a human brain our nervous system. model or diagram. Most reflexes are much more complex than the two-neuron reflex, involving synapses between The adult brain’s unimpressive appearance gives few one or more interneurons in the CNS (integration hints of its remarkable abilities. It is about two good center). The flexor, or withdrawal, reflex is a fistfuls of pinkish gray tissue, wrinkled like a walnut three-neuron reflex arc in which the limb is with- and with the texture of cold oatmeal. It weighs a little drawn from a painful stimulus (see Figure 7.11c). over 3 pounds. Because the brain is the largest and A three-neuron reflex arc also consists of five ele- most complex mass of nervous tis- sue in the body, ments—receptor, sensory neuron, interneuron, we commonly discuss it in terms of its four major motor neuron, and effector. Because there is regions—cerebral hemispheres, dien- cephalon always a delay at synapses (it takes time for neu- (di″en-sef′ah-lon), brain stem, and cere- bellum rotransmitter to diffuse through the synaptic cleft), (Figure 7.12, p. 266 and Table 7.1, p. 269). 7 the more synapses there are in a reflex pathway, cerebral Hemispheres the longer the reflex takes to happen. The paired cerebral (suh re′bral) hemispheres, collectively called the cerebrum, are the most Many spinal reflexes involve only spinal cord superior part of the brain and together are a good neurons and occur without brain involvement. As deal larger than the other three brain regions com- long as the spinal cord is functional, spinal reflexes, bined. In fact, as the cerebral hemispheres develop such as the flexor reflex, will work. By contrast, and grow, they enclose and obscure most of the some reflexes require that the brain become brain stem, so many brain stem structures cannot involved because many different types of normally be seen unless a sagittal section is made. information have to be evaluated to arrive at the Picture how a mushroom cap covers the top of its “right” response. The response of the pupils of the stalk, and you have an idea of how the cerebral eyes to light is a reflex of this type. hemispheres cover the diencephalon and the As noted earlier, reflex testing is an important superior part of the brain stem (see Figure 7.12). tool in evaluating the condition of the nervous system. Reflexes that are exaggerated, distorted, or absent indicate damage or disease in the ner- The entire surface of the cerebrum exhibits vous system. Reflex changes often occur before a elevated ridges of tissue called gyri (ji′re; gyrus, pathological condition becomes obvious in other singular; “twisters”), separated by shallow grooves ways. called sulci (sul′ki; sulcus, singular; “furrows”). Less numerous are the deeper grooves called Did You Get It? fissures (Figure 7.13a, p. 267), which separate large regions of the brain. Many of the fissures and 8. What is the difference between a graded potential and an action potential? gyri are important anatomical landmarks. The cere- 9. Explain the difference between a synaptic cleft and a bral hemispheres are separated by a single deep synapse. How is a stimulus transmitted across a fissure, the longitudinal fissure. Other fissures or synapse? sulci divide each cerebral hemisphere into a num- 10. Which portion(s) of a neuron is (are) likely to be ber of lobes, named for the cranial bones that lie associated with a sensory receptor or a sensory organ? over them (see Figure 7.13a and b). 11. What is a reflex? Each cerebral hemisphere has three basic For answers, see Appendix A. 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 266 Essentials of Human Anatomy and Physiology Cerebral hemisphere Cerebral hemisphere Outline of diencephalon Diencephalon Midbrain Cerebellum Cerebellum Brain stem Brain stem (a) 13 weeks (b) Adult brain Figure 7.12 Development and hemispheres, initially smooth, are superior part of the brain stem. The regions of the human brain. The forced to grow posteriorly and left cerebral hemisphere is drawn so brain can be considered in terms laterally over the other brain regions that it looks transparent, to reveal of four main parts: cerebral by the bones of the skull. (b) In the location of the deeply situated hemispheres, diencephalon, brain the adult brain, the cerebral diencephalon and superior part of stem, and cerebellum. (a) In the hemispheres, now highly convoluted, the brain stem. developing brain, the cerebral enclose the diencephalon and the matter situated deep within the white matter. We Impulses from the special sense organs are consider these regions next. interpreted in other cortical areas (see Figure 7.13b and c). For example, the visual area is Cerebral Cortex Speech, memory, logical and located in the posterior part of the occipital lobe, emotional responses, consciousness, the interpre- the auditory area is in the temporal lobe border- tation of sensation, and voluntary movement are all ing the lateral sulcus, and the olfactory area is functions of the cerebral cortex. Many of the deep inside the temporal lobe. functional areas of the cerebral hemispheres have The primary motor area, which allows us to been identified (Figure 7.13c). The primary somatic consciously move our skeletal muscles, is anterior sensory area is located in the parietal lobe posterior to the central sulcus in the frontal lobe. The axons to the central sulcus. Impulses traveling from the of these motor neurons form the major voluntary body’s sensory receptors (except for the special motor tract—the pyramidal tract, or corticospi- senses) are localized and inter- preted in this area of nal (kor″t˘ı̆ -ko-spi′nal) tract, which descends to the brain. The primary somatic sensory area allows the cord. As in the primary somatic sensory cortex, you to recognize pain, differences in temperature, or a the body is represented upside-down, and the light touch. pathways are crossed. Most of the neurons in A spatial map, the sensory homunculus the primary motor area control body areas having (ho-mung′ku-lus; “little man”), has been developed the finest motor control; that is, the face, mouth, to show how much tissue in the primary somatic and hands (see Figure 7.14). The body map on the sensory area is devoted to various sensory func- motor cortex, as you might guess, is called the tions. (Figure 7.14, p. 268; note that the body is motor homunculus. represented in an upside-down manner). Body A specialized cortical area that is very involved in regions with the most sensory receptors—the lips our ability to speak, Broca’s (bro′kahz) area, or and fingertips—send impulses to neurons that motor speech area (see Figure 7.13c), is found at make up a large part of the sensory area. the base of the precentral gyrus (the gyrus anterior Furthermore, the sensory pathways are crossed to the central sulcus). Damage to this area, which is pathways—meaning that the left side of the pri- located in only one cerebral hemisphere (usually mary somatic sensory area receives impulses from the left), causes the inability to say words properly. the right side of the body, and vice versa. You know what you want to say, but you can’t vocalize the words.  Chapter 7: The Nervous System 267 Precentral Central sulcus Figure 7.13 Left lateral view of the gyrus Postcentral gyrus brain. (a) Diagrammatic view of major structural areas. (b) Photograph of a brain. Frontal lobe Parietal lobe (c) Functional areas of the cerebral Parieto-occipital hemisphere, diagrammatic view. More sulcus (deep) intense colors (red and blue) indicate primary cortical areas (motor and sensory areas). Pastel colors (pink and pale blue) Lateral sulcus represent association areas of the cerebral Occipital lobe cortex. Temporal lobe Cerebellum Pons Medulla Parietal lobe Cerebral cortex oblongata (gray matter) Spinal Gyrus cord Left cerebral hemisphere 7 Sulcus Cerebral white Fissure matter Frontal (a deep sulcus) lobe (a) Occipital Temporal lobe lobe Superior Cerebellum Inferior Brain (b) stem Central sulcus Primary motor area Primary somatic sensory area Premotor area Anterior Gustatory area (taste) association area Speech/language Working memory (outlined by dashes) and judgment Problem Posterior association solving area Language comprehension Broca's area Visual area (motor speech) Olfactory area Auditory area (c) 268 Essentials of Human Anatomy and Physiology Posterior Motor Sensor y Motor map in Anter ior Sensory map in precentral gyrus postcentral gyrus Shoul Hip d Ha arm Neck Hea Trunk Trunk Hip Arm Knee Leg Elbo t For w Arm Wri Ha der e o nd Knee rs Fin Elb nd s ge w ge Fin rs Th um b um Foot b Nec e Th k Ey Bro se w Eye No e Toes c Fa s Face Genitals Lip Lips Teeth Gums Jaw Jaw Tongue Tongue Primary motor Primary somatic Phar ynx cor tex sensory cortex Intra- Swallowing (precentral gyrus) (postcentral gyrus) abdominal Figure 7.14 Sensory and motor by the amount of the gyrus occupied the diagram, and the somatic sensory areas of the cerebral cortex. The by the body area diagrams cortex is on the right. relative amount of cortical tissue (homunculi). The primary motor devoted to each function is indicated cortex is shown on the left side of Areas involved in higher intellectual reasoning Cerebral White Matter Most of the remaining and socially acceptable behavior are believed to cerebral hemisphere tissue—the deeper cerebral be in the anterior part of the frontal lobes, the white matter (see Figures 7.13a and 7.15)—is anterior association area. The frontal lobes also composed of fiber tracts carrying impulses to, from, house areas involved with language comprehen- or within the cortex. One very large fiber tract, the sion. Complex memories appear to be stored in corpus callosum (kah-lo′sum), connects the the temporal and frontal lobes. cerebral hemispheres (Figure 7.15). Such fiber The posterior association area encompasses tracts are called commissures. The corpus callosum part of the posterior cortex. This area plays a role arches above the structures of the brain stem and in recognizing patterns and faces, and blending allows the cerebral hemispheres to communicate several different inputs into an understanding of with one another. This is important because, as the whole situation. Within this area is the speech already noted, some of the cortical functional areas area, located at the junction of the temporal, pari- are in only one hemisphere. Association fiber tracts etal, and occipital lobes. The speech area allows connect areas within a hemisphere, and projection you to sound out words. This area (like Broca’s fiber tracts connect the cerebrum with lower CNS area) is usually in only one cerebral hemisphere. centers, such as the brain stem. Table 7.1 Functions of Major Brain regions region Function Cerebral hemispheres cortex: Gray matter: Localizes and interprets sensory inputs Controls voluntary and skilled skeletal muscle activity Acts in intellectual and emotional processing Basal nuclei: Subcortical motor centers help control skeletal muscle movements (see Figure 7.14) Diencephalon Thalamus: Relays sensory impulses to cerebral cortex Relays impulses between cerebral motor cortex and lower motor centers Involved in memory Hypothalamus: Chief integration center of autonomic (involuntary) nervous system Regulates body temperature, food intake, water balance, and thirst Regulates hormonal output of anterior pituitary gland and acts as an endocrine organ 7 (producing ADH and oxytocin) Limbic system—A functional system: Includes cerebral and diencephalon structures (e.g., hypothalamus and anterior thalamic nuclei) Mediates emotional response; involved in memory processing Brain stem Midbrain: Contains visual and auditory reflex centers Contains subcortical motor centers Contains nuclei for cranial nerves III and IV; contains projection fibers (e.g., fibers of the pyramidal tracts) Pons: Relays information from the cerebrum to the cerebellum Cooperates with the medullary centers to control respiratory rate and depth Contains nuclei of cranial nerves V–VII; contains projection fibers Medulla oblongata: Relays ascending sensory pathway impulses from skin and proprioceptors Contains nuclei controlling heart rate, blood vessel diameter, respiratory rate, vomiting, etc. Relays sensory information to the cerebellum Contains nuclei of cranial nerves VIII–XII; contains projection fibers Site of crossover of pyramids reticular formation—A functional system: Maintains cerebral cortical alertness; filters out repetitive stimuli Helps regulate skeletal and visceral muscle activity Cerebellum cerebellum: Processes information from cerebral motor cortex, proprioceptors, and visual and equilibrium pathways Provides “instructions” to cerebral motor cortex and subcortical motor centers, resulting in smooth, coordinated skeletal muscle movements Responsible for proper balance and posture 270 Essentials of Human Anatomy and Physiology Longitudinal fissure Association fibers Super ior Lateral Commissural fibers (corpus callosum) ventricle Corona Basal nuclei radiata Fornix Internal Thalamus capsule Third ventricle Pons Projection fibers Medulla oblongata Figure 7.15 Frontal section (facing posteriorly) of the brain showing commissural, association, and projection fibers running through the cerebrum and the lower cNS. Notice the internal capsule that passes between the thalamus and the basal nuclei. Basal Nuclei Although most of the gray matter is in Did You Get It? the cerebral cortex, there are several “islands” of 12. What are the three major regions of the cerebrum? gray matter, called the basal nuclei, buried deep 13. What is the composition of white matter of the brain? within the white matter of the cerebral hemi- spheres For answers, see Appendix A. (see Figure 7.15). The basal nuclei help regulate voluntary motor activities by modifying instructions Diencephalon (particularly in relation to starting or stopping The diencephalon, or interbrain, sits atop the brain movement) sent to the skeletal muscles by the stem and is enclosed by the cerebral hemi- spheres primary motor cortex. A tight band of pro- jection (see Figure 7.12). The major structures of the fibers, called the internal capsule, passes between diencephalon are the thalamus, hypothala- mus, and the thalamus and the basal nuclei. epithalamus (Figure 7.16). The thala- mus, which encloses the shallow third ventricle of the brain, is a Homeostatic Imbalance 7.3 relay station for sensory impulses passing upward to the sensory cortex. As impulses surge through the Individuals who have problems with their basal nuclei thalamus, we have a crude recognition of whether the are often unable to walk normally or carry out other sensation we are about to have is pleasant or voluntary movements in a normal way. Huntington’s unpleasant. It is the neurons of the sensory cortex disease and Parkinson’s disease are two examples of that actually localize and interpret the sensation. such syndromes. (See “A Closer Look” on pp. 278–279. ---------------------------------- -------------✚  Cerebral hemisphere Third ventricle Corpus callosum Choroid plexus of third ventricle Occipital lobe of cerebral hemisphere Thalamus Anterior (encloses third ventricle) commissure Pineal gland (part of epithalamus) Hypothalamus Corpora quadrigemina Optic chiasma Cerebral Midbrain aqueduct Pituitary gland Cerebral peduncle Mammillary body Fourth ventricle Pons Choroid plexus (part of epithalamus) Medulla oblongata Cerebellum 7 Spinal cord (a) Figure 7.16 Diencephalon and brain stem structures. (a) A midsagittal section of the brain illustrating the diencephalon (purple) and brain stem (green). (Figure continues on page 272.) The hypothalamus (literally, “under the thala- Brain Stem mus”) makes up the floor of the diencephalon. It is The brain stem is about the size of a thumb in an important autonomic center because it plays a diameter and approximately 3 inches (approxi- role in regulating body temperature, water bal- mately 7.5 cm) long. Its structures are the mid- ance, and metabolism. The hypothalamus is also the brain, pons, and medulla oblongata. In addition to center for many drives and emotions, and as such it providing a pathway for ascending and descend- is an important part of the so-called limbic system, ing tracts, the brain stem has many small gray or “emotional-visceral brain.” For example, thirst, mat- ter areas. These nuclei produce the rigidly appetite, sex, pain, and pleasure centers are in the programmed autonomic behaviors necessary for hypothalamus. Additionally, the hypothalamus survival. In addition, some are associated with the regulates the pituitary gland (an endocrine organ) cranial nerves and control vital activities such as and produces two hormones of its own. The breathing and blood pressure. Identify the brain pituitary gland hangs from the ante- rior floor of stem areas (see Figure 7.16) as you read the the hypothalamus by a slender stalk. (We discuss its descriptions that follow. function in Chapter 9.) The mam- millary bodies, reflex centers involved in olfac- tion (the sense of Midbrain A relatively small part of the brain smell), bulge from the floor of the hypothalamus posterior to the pituitary gland. stem, the midbrain extends from the mammil- lary bodies to the pons inferiorly. The cerebral The epithalamus (ep″˘ı̆ -thal′ah-mus) forms aqueduct, a tiny canal that travels through the the roof of the third ventricle. Important parts of midbrain, connects the third ventricle of the dien- the epithalamus are the pineal gland (part of the cephalon to the fourth ventricle below. Anteriorly, endocrine system) and the choroid (ko′roid) the midbrain is composed primarily of two bulging plexus of the third ventricle. The choroid plex- fiber tracts, the cerebral peduncles (pe′dun klz) uses, which are knots of capillaries within each of (literally, “little feet of the cerebrum”), which con- the four ventricles, form the cerebrospinal fluid. vey ascending and descending impulses. Dorsally 272 Essentials of Human Anatomy and Physiology Figure 7.16 (continued) (b) The reticular formation, which extends the length of Radiations to cerebral the brain stem. Ascending arrows indicate cortex sensory input to the cerebrum. Descending arrows indicate efferent output of reticular neurons. Auditory Visual impulses impulses Reticular formation Descending motor projections Ascending general sensory to spinal cord tracts (touch, pain, temperature) (b) located are four rounded protrusions called the muscle in the digestive tract. A special group of corpora quadrigemina (kor′por-ah kwah″dr˘ı̆ - reticular formation neurons, the reticular activat- jem′˘ı̆ -nah) because they reminded some anatomist ing system (RAS), plays a role in consciousness of two pairs of twins (gemini). These bulging nuclei and the awake/sleep cycle (Figure 7.16b). The RAS are reflex centers involved with vision and hearing. 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, The pons (ponz) is the rounded structure Pons but unusual or strong impulses do reach con- that protrudes just below the midbrain. Pons sciousness. Damage to this area can result in pro- means “bridge,” and this area of the brain stem is longed unconsciousness (coma). mostly fiber tracts (bundles of nerve fibers in the Cerebellum CNS). However, it does have important nuclei The large, cauliflower-like cerebellum (ser″e- involved in the control of breathing. bel′um) projects dorsally from under the occipital lobe of the cerebrum. Like the cerebrum, the cer- Medulla Oblongata The medulla oblongata ebellum has two hemispheres and a convoluted (mĕ˘-dul′ah ob″long-gă˘′tah) is the most inferior surface. The cerebellum also has an outer cortex part of the brain stem. It merges into the spinal made up of gray matter and an inner region of white cord below without any obvious change in struc- matter. ture. Like the pons, the medulla is an important fiber tract area. Additionally, the medulla is the The cerebellum provides the precise timing for area where the important pyramidal tracts (motor skeletal muscle activity and controls our balance. fibers) cross over to the opposite side. The medulla Thanks to its activity, body movements are smooth also contains many nuclei that regulate vital vis- and coordinated. It plays its role less well when it ceral activities. It contains centers that control is sedated by alcohol. Fibers reach the cerebellum heart rate, blood pressure, breathing, swallowing, from the equilibrium apparatus of the inner ear, the and vomiting, among others. The fourth ventri- eye, the proprioceptors of the skeletal muscles and cle lies posterior to the pons and medulla and tendons, and many other areas. The cerebellum anterior to the cerebellum. can be compared to an automatic pilot, continu- ously comparing the brain’s “intentions” with actual Reticular Formation Extending the entire length body performance by monitoring body position of the brain stem is a diffuse mass of gray matter, and the amount of tension in various body parts. the reticular formation. The neurons of the retic- When needed, the cerebellum sends messages to ular formation are involved in motor control of the initiate the appropriate corrective measures. visceral organs—for example, controlling smooth  Chapter 7: The Nervous System 273 collect venous blood, such as the superior sagittal Homeostatic Imbalance 7.4 sinus. If the cerebellum is damaged (for example, by a In several places, the inner dural membrane extends blow to the head, a tumor, or a stroke), move- inward to form a fold that attaches the brain to the ments become clumsy and disorganized—a con- cranial cavity. Two of these folds, the falx (falks) dition called ataxia (uh tax′e uh). Victims cannot cerebri and the tentorium cerebelli, separate the keep their balance and may appear drunk because cerebellum from the cerebrum (shown in Figure 7.17). of the loss of muscle coordination. They are no longer able to touch their finger to their nose with The middle meningeal layer is the weblike arachnoid eyes closed—a feat that healthy individuals (ah-rak′noid) mater (see Figure 7.17). Arachnida accomplish easily. --------------------------------------- means “spider,” and some think the arachnoid membrane looks like a cobweb. Its threadlike ----------------✚ extensions span the subarachnoid space to attach it Did You Get It? to the innermost membrane, the pia (pi′ah) mater 14. Which brain region controls such vital activities as breathing and blood pressure—cerebrum, brain stem, (“gentle mother”). The delicate pia mater clings or cerebellum? tightly to the surface of the brain and spinal cord, 15. What is the function of the cerebellum? following every fold. 16. In what major brain region are the thalamus, The subarachnoid space is filled with cerebro- hypothalamus, and pineal gland found? 7 spinal fluid. (Remember that the chor