The Nervous System 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,...

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 influenced by the nervous system in some way. If the nervous systems stops functioning, the body can stay alive only with the Instructors may assign assistance of life-supporting a related “Building machines. Vocabulary” activity using Mastering A&P Y ou are driving down the freeway when a horn The nervous system does not work alone to reg- blares on your right, and you swerve to your left. ulate and maintain body homeostasis; the endocrine You are with a group of friends but are not pay- system is a second important regulating system. ing attention to any particular conversation, yet when Whereas the nervous system controls with rapid you hear your name, you immediately focus in. What ­electrical nerve impulses, the endocrine system pro- do these events have in common? They are everyday duces hormones that are released into the blood. examples of nervous system function, which has your Thus, the endocrine system acts in a more leisurely body cells humming with activity all the time. way. We will discuss the endocrine system in detail The nervous system is the master control and in Chapter 9. communication system of the body. Every thought, To carry out its normal role, the nervous system action, and emotion reflects its activity. It communi- has three overlapping functions (Figure 7.1): (1) It cates with body cells using electrical impulses, which uses its millions of sensory receptors to monitor are rapid and specific and cause almost immediate changes occurring both inside and outside the body. responses. These changes are called stimuli, and the gathered 242 M07_MARI1942_13_SE_C07.indd 242 01/02/2021 14:39  Chapter 7: The Nervous System 243 We have only one nervous system, but because of its complexity, it is difficult to consider all its parts at Sensory input the same time. So, to simplify its study, we divide it Integration in terms of its structures (structural classification) or Sensory receptor in terms of its activities (functional classification). We briefly describe each of these classification schemes next, and Figure 7.2 illustrates their rela- tionships. You do not need to memorize this whole Motor output scheme now, but as you are reading the descriptions, try to get a “feel” for the major parts and how they fit Brain and spinal cord together. This will make learning easier as you make Effector Figure 7.1 The nervous system’s functions. Central Nervous System (brain and spinal cord) 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 7 integration. (3) It then causes a response, or effect, by Peripheral Nervous System (cranial and spinal nerves) activating muscles or glands (effectors) via motor output. CONCEPT LINK These three overlapping nervous system functions are very similar to a feedback loop (Chapter 1, p. 41). Recall Sensory Motor that in a feedback loop, a receptor receives sensory (afferent) (efferent) input, which it sends to the brain (control center) for processing (integration); the brain then analyzes the information and determines the appropriate output, which leads to a motor response. Sense Somatic Autonomic organs (voluntary) (involuntary) An example will illustrate how these functions work together. When you are driving and see a red Skeletal Cardiac and light just ahead (sensory input), your nervous system muscles smooth muscle, glands 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). 7.1 Organization of the Nervous System Parasympathetic Sympathetic Learning Objectives Figure 7.2 Organization of the nervous system. ✓✓ List the general functions of the nervous system. As the flowchart shows, the central nervous system ✓✓ Explain the structural and functional classifications receives input via sensory fibers and issues commands of the nervous system. via motor fibers. The sensory and motor fibers together ✓✓ Define central nervous system and peripheral form the nerves that constitute the peripheral nervous nervous system, and list the major parts of each. system. M07_MARI1942_13_SE_C07.indd 243 01/02/2021 14:39  244 Essentials of Human Anatomy and Physiology your way through this chapter. Later you will see all this subdivision as the voluntary nervous system. these terms and concepts again in more detail. However, not all skeletal muscle activity con- trolled by this motor division is voluntary. 7.1a Structural Classification Skeletal muscle reflexes, such as the stretch reflex The structural classification, which includes all ner- (described later in the chapter), are involuntary. vous system organs, has two subdivisions—the cen- The autonomic (aw0to-nom9ik) nervous sys- tral nervous system and the peripheral nervous tem (ANS) regulates events that are involuntary system (see Figure 7.2). (no conscious control), such as the activity of The central nervous system (CNS) consists of smooth muscle, cardiac muscle, and glands. This the brain and spinal cord, which occupy the dorsal subdivision, commonly called the involuntary body cavity and act as the integrating and command nervous system, itself has two parts, the sympa- centers of the nervous system. They interpret incom- thetic and parasympathetic, which typically bring ing sensory information and issue instructions based about opposite effects. What one stimulates, the on past experience and current conditions. other inhibits. We will describe these later. The peripheral (pĕ-rif9er-al) nervous system Although it is simpler to study the nervous sys- (PNS) includes all parts of the nervous system out- tem in terms of its subdivisions, remember that these side the CNS. It consists mainly of the nerves that subdivisions are made for the sake of convenience extend from the spinal cord and brain. Spinal nerves only. Remember that the nervous system acts as a carry impulses to and from the spinal cord. Cranial coordinated unit, both structurally and functionally. (kra9ne-al) nerves carry impulses to and from the brain. These nerves serve as communication lines. They link all parts of the body by carrying impulses Did You Get It? from the sensory receptors to the CNS and from the 1. Name the structures that make up the CNS and CNS to the appropriate glands or muscles. those that make up the PNS. For the answer, see Appendix A. 7.1b Functional Classification The functional classification scheme is concerned only with PNS structures. It divides them into two 7.2 Nervous Tissue: Structure principal subdivisions (see Figure 7.2). The sensory division, or afferent (af9er-ent; lit- and Function erally “to go toward”) division, consists of nerves Learning Objective (composed of many individual nerve fibers) that ✓✓ Describe the structures and functions of neurons convey impulses to the central nervous system from and neuroglia. sensory receptors located in various parts of the body. The sensory division keeps the CNS constantly Even though it is complex, nervous tissue is made up informed of events going on both inside and outside of just two principal types of cells—supporting cells the body. Sensory fibers delivering impulses from and neurons. the skin, skeletal muscles, and joints are called somatic (soma 5 body) sensory (afferent) fibers, 7.2a Supporting Cells whereas those transmitting impulses from the vis- Supporting cells in the CNS are “lumped together” ceral organs are called visceral sensory (­afferent) fibers. as neuroglia (nu-rog9le-ah), literally, “nerve glue,” The motor division, or efferent (ef9er-rent) also called glial cells or glia. Neuroglia include many division (think: efferent exits the CNS), carries impulses types of cells that support, insulate, and protect the from the CNS to effector organs, the muscles and glands. delicate neurons (Figure 7.3). In addition, each of These impulses activate muscles and glands; that is, the different types of neuroglia has special functions. they effect (bring about or cause) a motor response. CNS neuroglia include the following: The motor division in turn has two subdivisions Astrocytes: abundant star-shaped cells (see Figure 7.2): that account for nearly half of neural tissue The somatic (so-mat9ik) nervous system (Figure 7.3a). Their numerous projections have allows us to consciously (voluntarily), control swollen ends that cling to neurons, bracing them our skeletal muscles. Hence, we often refer to and anchoring them to their nutrient supply M07_MARI1942_13_SE_C07.indd 244 01/02/2021 14:39  Chapter 7: The Nervous System 245 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 7 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 fluid–filled cavities. Figure 7.3 Supporting cells (neuroglia) of nervous tissue. lines, the blood capillaries. Astrocytes form a liv- Microglia (mi-kro9-gle-ah): spiderlike phago- ing barrier between capillaries and neurons, help cytes that monitor the health of nearby neurons determine capillary permeability, and play a role and dispose of debris, such as dead brain cells in making exchanges between the two. In this and bacteria (Figure 7.3b). way, they help protect the neurons from harmful Ependymal (ĕ-pen9dı̆-mal) cells: neuroglia that substances that might be in the blood. Astrocytes line the central cavities of the brain and the spi- also help control the chemical environment in nal cord (Figure 7.3c). They participate in the the brain by “mopping up” leaked potassium production of cerebrospinal fluid (CSF) and the ions, which are involved in generating a nerve beating of their cilia helps to circulate the cere- impulse, and recapturing chemicals released for brospinal fluid that fills those cavities and forms communication purposes. a protective watery cushion around the CNS. M07_MARI1942_13_SE_C07.indd 245 01/02/2021 14:39  246 Essentials of Human Anatomy and Physiology Oligodendrocytes (ol0ı̆-go-den9dro-sı̆tz): neu- bodies, and neurofibrils (intermediate filaments roglia that wrap their flat extensions (processes) that are important in maintaining cell shape) are tightly around CNS nerve fibers, producing fatty particularly abundant in the cell body. insulating coverings called myelin sheaths (Figure 7.3d). Processes The armlike processes, or fibers, vary in Although neuroglia somewhat resemble neurons length from microscopic to over 3 feet long, reaching structurally (both cell types have cell extensions), from the lumbar region of the spine to the great toe. they are not able to transmit nerve impulses, a func- Neuron processes that convey incoming messages tion that is highly developed in neurons. Another (electrical signals) toward the cell body are dendrites important difference is that neuroglia never lose (den9drıˉtz), whereas those that generate nerve their ability to divide, whereas most neurons do. impulses and typically conduct them away from the Consequently, most brain tumors are gliomas, or cell body are axons (ak9sonz). Neurons may have tumors formed by neuroglia. hundreds of branching dendrites (dendr 5 tree), Supporting cells in the PNS come in two major depending on the structural type. However, each neu- varieties—Schwann cells and satellite cells ron has only one axon, which arises from a conelike (Figure 7.3e). Schwann cells form the myelin region of the cell body called the axon hillock. sheaths around nerve fibers in the PNS. Satellite An occasional axon gives off a collateral branch cells act as protective, cushioning cells for peripheral along its length, but all axons branch profusely at neuron cell bodies. their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hun- dreds of tiny vesicles, or membranous sacs, that con- Did You Get It? tain chemicals called neurotransmitters (review our discussion of events at the neuromuscular junction in 2. Which neuroglia are most abundant in the body? Which produce the insulating material called myelin? Chapter 6). As we said, axons transmit nerve impulses 3. Why is a brain tumor more likely to be formed from away from the cell body. When these impulses reach neuroglia than from neurons? the axon terminals, they stimulate the release of neu- For answers, see Appendix A. rotransmitters into the extracellular space between neurons, or between a neuron and its target cell. 7.2b Neurons Each axon terminal is separated from the next neuron or its target by a tiny gap called the synaptic Anatomy (sı̆-nap9tik) cleft. Such a functional junction, where Learning Objectives an impulse is transmitted from one neuron to another, ✓✓ Describe the general structure of a neuron, and is called a synapse (syn 5 to clasp or join). Although name its important anatomical regions. they are close, neurons never actually touch other neu- ✓✓ Describe the composition of gray matter and rons or target cells. You will learn more about syn- white matter. apses and the events that occur there a bit later. Neurons, also called nerve cells, are highly special- Myelin Sheaths Most long nerve fibers are covered ized to transmit messages (nerve impulses) from one with a whitish, fatty material called myelin (mi9ĕ- part of the body to another. Although neurons differ lin), which has a waxy appearance. Myelin protects structurally from one another, they have many com- and insulates the fibers and increases the speed of mon features (Figure 7.4). All have a cell body, which nerve impulse transmission. Compare myelin contains the nucleus and one or more slender pro- sheaths to the many layers of insulation that cover cesses extending from the cell body. the wires in an electrical cord; the layers keep the electricity flowing along the desired path just as Cell Body The cell body is the metabolic center of myelin does for nerve fibers. Axons outside the CNS the neuron. Its transparent nucleus contains a large are myelinated by Schwann cells, as previously nucleolus. The cytoplasm surrounding the nucleus noted. Many of these cells wrap themselves around contains the usual organelles, except that it lacks the axon in a jelly-roll fashion (Figure 7.5, p. 248). centrioles (which confirms the amitotic nature of Initially, the membrane coil is loose, but the most neurons). The rough ER, called Nissl (nis9l) Schwann cell cytoplasm is gradually squeezed from M07_MARI1942_13_SE_C07.indd 246 01/02/2021 14:39  Chapter 7: The Nervous System 247 Dendrite Cell Mitochondrion body Nissl substance Axon hillock Axon Neurofibrils Collateral Nucleus branch Nucleolus One 7 Schwann cell Node of Axon Ranvier terminal Schwann cells, forming the myelin sheath on axon (a) Neuron cell body Dendrite Figure 7.4 Structure of a typical motor neuron. (a) Diagrammatic view. (b) Scanning electron micro- graph showing the cell body and (b) dendrites (6153). M07_MARI1942_13_SE_C07.indd 247 01/02/2021 14:39  248 Essentials of Human Anatomy and Physiology husk”). Because the myelin sheath is formed by Q  hy does the myelin sheath that is produced by W Schwann cells have gaps in it? many individual Schwann cells, it has gaps, or inden- tations, called nodes of Ranvier (rahn-vĕr), at regu- lar intervals (see Figure 7.4). Schwann cell As mentioned previously, myelinated fibers are cytoplasm also found in the central nervous system. Oli­ Schwann cell Axon plasma membrane godendrocytes form CNS myelin sheaths (see Fig­ ure 7.3d). In the PNS, it takes many Schwann cells Schwann cell to make a single myelin sheath; but in the CNS, the nucleus oligodendrocytes with their many flat extensions can (a) 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 (b) central nervous system. Homeostatic Neurilemma Imbalance 7.1 Myelin The importance of myelin insulation is best illus- sheath trated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) grad- ually destroys the myelin sheaths around CNS fibers (c) by converting them to hardened sheaths called ­scleroses. As this happens, the electrical current is Figure 7.5 Relationship of Schwann cells to axons short-­circuited and may “jump” to another demye- in the peripheral nervous system. (a–c) As illustrated linated neuron. In other words, nerve signals do not (top to bottom), a Schwann cell envelops part of an axon always reach the intended target. The affected person in a trough and then rotates around the axon. Most of may have visual and speech disturbances, lose the the Schwann cell cytoplasm comes to lie just beneath the exposed part of its plasma membrane. The tight coil ability to control his or her muscles, and become of plasma membrane material surrounding the axon is increasingly disabled. Multiple sclerosis is an auto- the myelin sheath. The Schwann cell cytoplasm and immune disease in which the person’s own immune exposed membrane are referred to as the neurilemma. system attacks a protein component of the sheath. As yet there is no cure, but injections of beta interferon between the membrane layers. When the wrapping (a hormonelike substance released by some immune process is done, a tight coil of wrapped membranes, cells) appear to hold the symptoms at bay and pro- the myelin sheath, encloses the axon. Most of the vide some relief. Other drugs aimed at slowing the Schwann cell cytoplasm ends up just beneath the autoimmune response are also being used, though outermost part of its plasma membrane. This part of further research is needed to determine their long- the Schwann cell, external to the myelin sheath, is term effects. called the neurilemma (nu0rı̆-lem9mah, “neuron Terminology Clusters of neuron cell bodies and the sheath. collections of nerve fibers are named differently in each Schwann cell forming only one tiny segment of the CNS and in the PNS. For the most part, cell bod- A ies are found in the CNS in clusters called nuclei. arrange themselves end to end along the nerve fiber, The sheath is produced by many Schwann cells that This well-protected location within the bony skull or M07_MARI1942_13_SE_C07.indd 248 01/02/2021 14:39  Chapter 7: The Nervous System 249 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 7 To effectors (muscles and glands) Figure 7.6 Neurons classified by system; most cell bodies are in Interneurons (association neurons) function. Sensory (afferent) neurons ganglia in the PNS. Motor (efferent) complete the communication conduct impulses from sensory neurons transmit impulses from pathway between sensory and motor receptors (in the skin, viscera, the CNS (brain or spinal cord) to neurons; their cell bodies reside in muscles) to the central nervous effectors in the body periphery. the CNS. vertebral column is essential to the well-being of the Neurons may be classified based on their function or nervous system—remember that neurons do not their structure. routinely undergo cell division after birth. The cell body carries out most of the metabolic functions of Functional Classification Functionally, neurons are a neuron, so if it is damaged, the cell dies and is not grouped according to the direction the nerve impulse replaced. Small collections of cell bodies called travels relative to the CNS. On this basis, there are ­ganglia (gang9le-ah; ganglion, singular) are found in sensory, motor, and association neurons (interneu- a few sites outside the CNS in the PNS. rons) (Figure 7.6). Neurons carrying impulses from Bundles of nerve fibers (neuron processes) run- sensory receptors (in the internal organs or the skin) ning through the CNS are called tracts, whereas in to the CNS are sensory neurons, or afferent the PNS they are called nerves. The terms white mat- neurons. (Recall that afferent means “to go toward.”) ter and gray matter refer respectively to myelinated The cell bodies of sensory neurons are always found versus unmyelinated regions of the CNS. As a general in a ganglion outside the CNS. Sensory neurons keep rule, the white matter consists of dense collections us informed about what is happening both inside of myelinated fibers (tracts), and gray matter con- and outside the body. tains mostly unmyelinated fibers and cell bodies. The dendrite endings of the sensory neurons are usually associated with specialized receptors that Classification are activated by specific changes occurring nearby. Learning Objectives (We cover the very complex receptors of the special ✓✓ Classify neurons according to structure and senses—vision, hearing, equilibrium, taste, and function. smell—separately in Chapter 8). The simpler types of ✓✓ List the types of general sensory receptors and sensory receptors in the skin are cutaneous sense describe their functions. organs, and those in the muscles and tendons are M07_MARI1942_13_SE_C07.indd 249 01/02/2021 14:39  250 Essentials of Human Anatomy and Physiology (a) Free nerve endings (pain (b) Meissner’s corpuscle and temperature receptors) (touch receptor) (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. proprioceptors (pro0pre-o-sep9torz) (shown in proprioceptors constantly advise our brain of our Figure 4.3 and Figure 7.7). The pain receptors (actu- own movements. ally bare nerve endings) are the least specialized of Neurons carrying impulses from the CNS to the the cutaneous receptors. They are also the most viscera and/or muscles and glands are motor numerous, because pain warns us that some type of neurons, or efferent neurons (see Figure 7.6). The body damage is occurring or is about to occur. cell bodies of motor neurons are usually located in However, strong stimulation of any of the cutaneous the CNS. receptors (for example, by searing heat, extreme cold, The third category of neurons consists of the or excessive pressure) is also interpreted as pain. interneurons, or association neurons. They The proprioceptors detect the amount of stretch, ­connect the motor and sensory neurons in neural or tension, in skeletal muscles, their tendons, and pathways. Their cell bodies are typically located in joints. They send this information to the brain so the CNS. that the proper adjustments can be made to main- tain balance and normal posture. Propria comes Structural Classification The structural classification from the Latin word meaning “one’s own,” and the of neurons is based on the number of processes, M07_MARI1942_13_SE_C07.indd 250 01/02/2021 14:39  Chapter 7: The Nervous System 251 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 Axon the cell body in multipolar and bipolar neurons? Dendrites Which part releases neurotransmitters? 7. Your professor tells you that one neuron transmits a (a) Multipolar neuron nerve impulse at the rate of 1 meter per second and another neuron conducts at the rate of 40 meters per second. Which neuron has the myelinated axon? For answers, see Appendix A. Cell body 7.2c Physiology: Nerve Impulses Learning Objectives Dendrite Axon ✓✓ Describe the events that lead to the generation of (b) Bipolar neuron a nerve impulse and its conduction from one neuron to another. ✓✓ Define reflex arc, and list its elements. 7 Dendrites Neurons have two major functional properties: Cell body Short single ­irritability, the ability to respond to a stimulus by pro- process ducing a nerve impulse, and conductivity, the a­ bility to transmit the impulse to other neurons, muscles, or Axon glands. Peripheral Central process process Electrical Conditions of a Resting Neuron’s Mem­ (c) Unipolar neuron brane The plasma membrane of a resting, or inactive, neuron is polarized, which means that Figure 7.8 Classification of neurons on the basis of there are fewer positive ions sitting on the inner face structure. of the neuron’s plasma membrane than there are on its outer face (Figure 7.9 1 , p. 252). The major pos- including both dendrites and axons, extending from itive ions inside the cell are potassium (K1), whereas the cell body (Figure 7.8). If there are several, the neu- the major positive ions outside the cell are sodium ron is a multipolar neuron. Because all motor and (Na1). The polarized membrane is more permeable association neurons are multipolar, this is the most to K1 than to Na1 at rest, maintaining a more nega- common structural type. Neurons with two pro- tive inside (fewer positive ions) compared to outside, cesses—one axon and one dendrite—are bipolar neu- as K1 ions exit the cell. This maintains the inactive, rons. Bipolar neurons are rare in adults, found only in resting state of the neuron. some special sense organs (eye, nose), where they act in sensory processing as receptor cells. Unipolar neu- Action Potential Initiation and Generation Many rons have a single process emerging from the cell body different types of stimuli excite neurons to become as if the cell body were on a “cul-de-sac” off the “main active and generate an impulse. For example, light road” that is the axon. However, the process is very excites the eye receptors, sound excites some of the short and divides almost immediately into proximal ear receptors, and pressure excites some cutaneous (central) and distal (peripheral) processes. Unipolar receptors of the skin. However, most neurons in the neurons are unique in that only the small branches at body are excited by neurotransmitter chemicals re­ the end of the peripheral process are dendrites. The leased by other neurons, as we will describe shortly. remainder of the peripheral process and the central Regardless of the stimulus, the result is always process function as the axon; thus, in this case, the the same—the permeability properties of the cell’s axon actually conducts nerve impulses both toward plasma membrane change for a very brief period. and away from the cell body. Sensory neurons found in Normally, sodium ions cannot diffuse through the PNS ganglia are unipolar. (Refer back to Figure 7.6.) M07_MARI1942_13_SE_C07.indd 251 01/02/2021 14:39  252 Essentials of Human Anatomy and Physiology [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 more permeable to K+ ions (which exit the + + + + + + + cell resulting in fewer positive ions inside) at rest. 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. – – + If the stimulus is strong enough, depolarization causes +Na+ membrane polarity to be completely reversed, and an action + + – – – – – potential is initiated. (If the stimulus is not strong enough, the cell returns to a resting membrane state and no action – – + + + + + potential occurs.) + + + – – – – – 4 Propagation of the action potential. Depolarization of – – – + + + + + the first membrane patch causes permeability changes in the adjacent membrane, and the events described in step 2 are + + + + – – – repeated. Thus, the action potential propagates rapidly along the entire length of the membrane like dominoes falling one – – – – + + + after another, or a crowd doing the "wave". 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+ exterior Na+ Na+ Na+– K+ pump 6 Initial ionic conditions restored. The ionic conditions K+ Na+ Diffusion K+ Diffusion of the resting state are restored later by the activity of the K+ Plasma sodium-potassium pump. Three sodium ions are ejected for membrane every two potassium ions carried back into the cell. The neuron is now ready to "fire" again. K+ Cell K+ eText interior Mastering A&P > Study Area > Interactive Physiology (IP) Figure 7.9 The nerve impulse. Video M07_MARI1942_13_SE_C07.indd 252 01/02/2021 14:39  Chapter 7: The Nervous System 253 plasma membrane to any great extent, but when Homeostatic the neuron is adequately stimulated, the “gates” of Imbalance 7.2 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 “floods” (by diffusion) into the neuron. block nerve impulses by altering membrane perme- This inward rush of sodium ions changes the polar- ability to ions, mainly sodium ions. As we have seen, ity of the neuron’s membrane at that site, an event no sodium entry 5 no action potential. called depolarization (Figure 7.9 2 ). Locally, the Cold and continuous pressure hinder impulse inside is now more positive, and the outside is less conduction because they interrupt blood circulation positive, a local electrical situation called a graded (and hence the delivery of oxygen and nutrients) to potential. However, if the stimulus is strong enough the neurons. For example, your fingers get numb and the sodium influx is great enough, the local depo- when you hold an ice cube for more than a few sec- larization (graded potential) activates the neuron to onds. Likewise, when you sit on your foot, it “goes to initiate and transmit a long-distance signal called an sleep.” When you warm your fingers or remove the action potential, or a nerve impulse. 3 The nerve pressure from your foot, the impulses begin to be impulse is an all-or-nothing response, like starting a transmitted again, leading to an unpleasant prickly car. It is either propagated (conducted, or sent) over feeling. the entire axon 4 , or it doesn’t happen at all. The 7 nerve impulse never goes partway along an axon’s length, nor does it die out with distance, as do Transmission of the Signal at Synapses So far we graded potentials. have explained only the irritability aspect of neuro- Almost immediately after the sodium ions rush nal function. What about conductivity—how does into the neuron, the membrane permeability the electrical impulse traveling along one neuron get changes again, becoming impermeable to sodium across the synapse to the next neuron (or effector ions but permeable to potassium ions. So potas- cell) to influence its activity? sium ions are allowed to rapidly diffuse out of the The answer is that the impulse doesn’t! Instead, a neuron into the interstitial fluid. This outflow of neurotransmitter chemical crosses the synapse to positive ions from the cell restores the electrical transmit the signal from one neuron to the next, or to ­conditions at the membrane to the polarized, or the target cell (as occurred at the neuromuscular junc- resting, state, an event called repolarization tion; see Chapter 6). When the action potential (Figure 7.9 5 ). After repolarization of the electrical reaches an axon terminal (Figure 7.10 1 , p. 254), the conditions, the sodium-potassium pump restores electrical change opens calcium channels. Calcium ionic conditions, the initial concentrations of the ions, in turn, cause the tiny vesicles containing neu- sodium and potassium ions inside and outside the rotransmitter to fuse with the axonal membrane 2 , neuron 6. This pump uses ATP (cellular energy) to releasing the neurotransmitter into the synaptic cleft pump excess sodium ions back out of the cell and to by exocytosis 3. The neurotransmitter molecules dif- bring potassium ions back into it. Until repolariza- fuse across the synaptic cleft* and bind to receptors on tion occurs, and resting ionic conditions are restored, a the membrane of the next neuron 4. If enough neu- neuron cannot conduct another impulse. Once begun, rotransmitter is released, the whole series of events these sequential events spread along the entire neu- described above (sodium entry 5 , depolarization, ronal membrane. etc.) will occur, generating a graded potential and The events just described explain propagation of eventually a nerve impulse in the receiving neuron a nerve impulse along unmyelinated fibers. Fibers beyond the ­synapse. The electrical changes prompted that have myelin sheaths conduct impulses much by neurotransmitter binding are very brief because faster because the nerve impulse literally jumps, or the neurotransmitter is quickly removed from the leaps, from node to node along the length of the fiber. This occurs because no electrical current can *Although most neurons communicate via the chemical flow across the axon membrane where there is fatty type of synapse described above, there are some examples of myelin insulation. This faster type of electrical ­electrical synapses, in which the neurons are physically joined impulse propagation is called saltatory (sal9tah-to0re) by gap junctions and electrical currents actually flow from conduction (saltare 5 to dance or leap). one neuron to the next. M07_MARI1942_13_SE_C07.indd 253 01/02/2021 14:39  254 Essentials of Human Anatomy and Physiology Axon of synaptic cleft 6 , either by diffusing away, by reuptake transmitting into the axon terminal, or by enzymatic breakdown. neuron This limits the effect of each nerve impulse to a period Receiving shorter than the blink of an eye. neuron CONCEPT LINK 1 Action Recall what you learned about the events at the neuro- Dendrite potential muscular junction between a neuron and a muscle arrives. fiber (Figure 6.5, p. 207). Events at synapses are very Ca2+ Ca2+ similar, except that the "target" is another neuron, and Vesicles in some locations of the body, neurotransmitters other Axon terminal Synaptic than acetylcholine are released. cleft Notice that the transmission of an impulse is an electrochemical event. Transmission down the length of the neuron’s axon 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 2 Vesicle Transmitting neuron many different neurons at the same time. Also, dif- fuses with 4 Neurotrans- ferent neurotransmitters are released in different plasma 3 Neurotrans- mitter binds parts of the body, depending on “who” is talking membrane. mitter is to receptor (which neurons) and “what” they are trying to released into on receiving express (usually stimulation or inhibition). synaptic cleft. neuron's membrane. 7.2d Physiology: Reflexes Although there are many types of communication between neurons, much of what the body must do Synaptic every day is programmed as reflexes. Reflexes are cleft Ion Neurotransmitter rapid, predictable, and involuntary responses to stimuli. channels molecules 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 Receiving neuron involve both CNS and PNS structures. Think of a reflex as a preprogrammed response to a given stimulus. Neurotransmitter is The types of reflexes that occur in the body are Neurotransmitter broken down and classed as either somatic or autonomic. Somatic released. reflexes stimulate the skeletal muscles; these are still Receptor Na+ involuntary reflexes even though skeletal muscle Na+ normally is under voluntary control. When you quickly pull your hand away from a hot object, a somatic reflex is working. Autonomic reflexes regu- late the activity of smooth muscles, the heart, and glands. Secretion of saliva (salivary reflex) and changes in the size of the eye pupils (pupillary 5 Ion channel opens. 6 Ion channel closes. reflex) are two such reflexes. Autonomic reflexes reg- ulate such body functions as digestion, elimination, Figure 7.10 How neurons communicate at blood pressure, and sweating. chemical synapses. The events occurring at the All reflex arcs have a minimum of five elements synapse are numbered in order. (Figure 7.11a): a receptor (which reacts to a stimulus), M07_MARI1942_13_SE_C07.indd 254 01/02/2021 14:39  Chapter 7: The Nervous System 255 Stimulus at distal Skin Spinal cord end of neuron (in cross section) 2 Sensory neuron 3 Integration 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 7 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. M07_MARI1942_13_SE_C07.indd 255 01/02/2021 14:39  256 Essentials of Human Anatomy and Physiology an effector (the muscle or gland eventually stimu- 7.3 Central Nervous System lated), and sensory and motor neurons to connect the two. The synapse or interneurons between the 7.3a Functional Anatomy sensory and motor neurons represents the fifth of the Brain element—the CNS integration center. Learning Objective The simple patellar (pah-tel9ar), or knee-jerk, reflex is an example of a two-neuron reflex arc, the ✓✓ Identify and indicate the functions of the major regions of the cerebral hemispheres, diencephalon, simplest type in humans (Figure 7.11b). The patellar brain stem, and cerebellum on a human brain reflex (in which the quadriceps muscle attached to model or diagram. the hit tendon is stretched) is familiar to most of us. It is usually tested during a physical exam to deter- The adult brain’s unimpressive appearance gives mine the general health of the motor portion of our few hints of its remarkable abilities. It is about two nervous system. good fistfuls of pinkish gray tissue, wrinkled like a Most reflexes are much more complex than the walnut and with the texture of cold oatmeal. It two-neuron reflex, involving synapses between one weighs a little over 3 pounds. Because the brain is or more interneurons in the CNS (integration cen- the largest and most complex mass of nervous ter). The flexor, or withdrawal, reflex is a three-neuron ­tissue in the body, we commonly discuss it in terms reflex arc in which the limb is withdrawn from a of its four major regions—cerebral hemispheres, painful stimulus (see Figure 7.11c). A three-neuron ­diencephalon (di0en-sef9ah-lon), brain stem, and cere­ reflex arc also consists of five elements—receptor, bellum (Figure 7.12, and Table 7.1, p. 258). sensory neuron, interneuron, motor neuron, and effector. Because there is always a delay at synapses Cerebral Hemispheres (it takes time for neurotransmitter to diffuse through The paired cerebral (suh re9bral) hemispheres, col- the synaptic cleft), the more synapses there are in a lectively called the cerebrum, are the most superior reflex pathway, the longer the reflex takes to happen. part of the brain and together are a good deal larger Many spinal reflexes involve only spinal cord neu- than the other three brain regions combined. In fact, rons and occur without brain involvement. As long as as the cerebral hemispheres develop and grow, they the spinal cord is functional, spinal reflexes, such as enclose and obscure most of the brain stem, so many the flexor reflex, will work. By contrast, some reflexes brain stem structures cannot normally be seen unless require that the brain become involved because many a sagittal section is made. Picture how a mushroom different types of information have to be evaluated to cap covers the top of its stalk, and you have an idea arrive at the “right” response. The response of the of how the cerebral hemispheres cover the dienceph- pupils of the eyes to light is a reflex of this type. alon and the superior part of the brain stem (see As noted earlier, reflex testing is an important Figure 7.12). tool in evaluating the condition of the nervous sys- The entire surface of the cerebrum exhibits ele- tem. Reflexes that are exaggerated, distorted, or vated ridges of tissue called gyri (ji9re; gyrus, singular; absent indicate damage or disease in the nervous sys- “twisters”), separated by shallow grooves called sulci tem. Reflex changes often occur before a pathologi- (sul9ki; sulcus, singular; “furrows”). Less numerous cal condition becomes obvious in other ways. are the deeper grooves called fissures (Fig­ure 7.13a, p. 259), which separate large regions of the brain. Many of the fissures and gyri are important anatom- Did You Get It? ical landmarks. The cerebral hemispheres are sepa- 8. What is the difference between a graded potential rated by a single deep fissure, the longitudinal fissure. and an action potential? Other fissures or sulci divide each cerebral hemi- 9. Explain the difference between a synaptic cleft and a synapse. How is a stimulus transmitted across a sphere into a number of lobes, named for the cranial synapse? bones that lie over them (see Figure 7.13a and b). 10. Which portion(s) of a neuron is (are) likely to be Each cerebral hemisphere has three basic regions: associated with a sensory receptor or a sensory a superficial cortex of gray matter, which looks gray organ? in fresh brain tissue; an internal area of white matter; 11. What is a reflex? and the basal nuclei, islands of gray matter situated For answers, see Appendix A. deep within the white matter. We consider these regions next. M07_MARI1942_13_SE_C07.indd 256 01/02/2021 14:39  Chapter 7: The Nervous System 257 Cerebral Cerebral hemisphere 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, superior part of the brain stem. The regions of the human brain. are forced to grow posteriorly left cerebral ­hemisphere is drawn so The brain can be considered in and laterally over the other brain that it looks transparent, to reveal terms of four main parts: cerebral regions by the bones of the skull. the location of the deeply situated ­hemispheres, diencephalon, brain (b) In the adult brain, the cerebral diencephalon and superior part of 7 stem, and cerebellum. (a) In the ­hemispheres, now highly ­convoluted, the brain stem. ­developing brain, the cerebral enclose the ­diencephalon and the Cerebral Cortex Speech, memory, logical and emo­ For example, the visual area is located in the posterior tional responses, consciousness, the interpretation part of the occipital lobe, the auditory area is in the of sensation, and voluntary movement are all func- temporal lobe bordering the lateral sulcus, and the tions of the cerebral cortex. Many of the functional olfactory area is deep inside the temporal lobe. areas of the cerebral hemispheres have been The primary motor area, which allows us to identified (Figure 7.13c). The primary somatic consciously move our skeletal muscles, is anterior to sensory area is located in the parietal lobe posteri- the central sulcus in the frontal lobe. The axons of or to the central sulcus. Impulses traveling from the these motor neurons form the major voluntary body’s sensory receptors (except for the special motor tract—the pyramidal tract, or corticospinal senses) are localized and interpreted in this area of (kor0tı̆-ko-spi9nal) tract, which descends to the spi- the brain. The primary somatic sensory area allows nal cord. As in the primary somatic sensory cortex, the you to recognize pain, differences in temperature, or body is represented upside-down, and the pathways a light touch. are crossed. Most of the neurons in the primary motor A spatial map, the sensory homunculus area control body areas having the finest motor ­(ho-mung9ku-lus; “little man”), has been devel- control; that is, the face, mouth, and hands (see ­ oped to show how much tissue in the primary Figure 7.14). The body map on the motor ­cortex, as somatic sensory area is devoted to various sensory you might guess, is called the motor homunculus. functions. (Figure 7.14, p. 260; note that the body is A specialized cortical area that is very involved in represented in an upside-down manner). Body our ability to speak is Broca’s (bro9kahz) area. Also regions with the most sensory receptors—the lips called the motor speech area (see Figure 7.13c), it helps and fingertips—send impulses to neurons that us speak by sending the motor signals that allow us to make up a large part of the sensory area. form words with our mouths. It is found at the base of Furthermore, the sensory pathways are crossed path- the precentral gyrus (the gyrus anterior to the central ways—meaning that the left side of the primary sulcus). Damage to this area, which is located in only somatic sensory area receives impulses from the one cerebral hemisphere (usually the left), causes the right side of the body, and vice versa. inability to say words properly. You know what you Impulses from the special sense organs are inter- want to say, but you can’t vocalize the words. preted in other cortical areas (see Figure 7.13b and c). (Text continues on page 260.) M07_MARI1942_13_SE_C07.indd 257 01/02/2021 14:39  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 7.15) 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 ­(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 258 M07_MARI1942_13_SE_C07.indd 258 01/02/2021 14:39  Chapter 7: The Nervous System 259 Precentral Central sulcus Figure 7.13 Left lateral view of the gyrus Postcentral gyrus brain. (a) Diagrammatic view of major Frontal lobe Parietal lobe structural areas. (b) Photograph of a brain. (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 Lateral sulcus areas). Pastel colors (pink and pale blue) 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 Frontal matter (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) M07_MARI1942_13_SE_C07.indd 259 01/02/2021 14:39  260 Essentials of Human Anatomy and Physiology Posterior Motor Sensory Motor map in Anterior Sensory map in precentral gyrus postcentral gyrus Should d rm Trunk Neck Hea Trunk Arm Hip Leg rea Knee Fo w Elbo t Arm Wri Hip Ha o nd s Elb er er Fi nd Ha s ng ng w er Fi Knee Th s um b um Foot b Ne Th e c Bro k Ey w se No e Eye Toes c Fa s Face Genitals Lip Lips Teeths Gum Jaw Jaw Tongue Tongue Primary motor Primary somatic Pharynx cortex sensory cortex Intra- Swallowing (precentral gyrus) (postcentral gyrus) abdominal Figure 7.14 Sensory and motor by the amount of the gyrus occu- the diagram, and the somatic sensory areas of the cerebral cortex. The pied 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 cere- and socially acceptable behavior are believed to be bral hemisphere tissue—the deeper cerebral white in the anterior part of the frontal lobes, the anterior matter (see Figures 7.13a and 7.15)—is composed of association area. The frontal lobes also house areas fiber tracts carrying impulses to, from, or within the involved with language comprehension. Complex cortex. One very large fiber tract, the c­ orpus callosum memories appear to be stored in the temporal and (kah-lo9sum), connects the cerebral hemispheres frontal lobes. (Figure 7.15). Such fiber tracts are called commissures. The posterior association area encompasses The corpus callosum arches above the structures of part of the posterior cortex. This area plays a role in the brain stem and allows the cerebral hemispheres to recognizing patterns and faces, and blending several communicate with one another. This is important different inputs into an understanding of the whole because, as already noted, some of the cortical func- situation. Within this area is the speech area, located tional areas are in only one hemisphere. Association at the junction of the temporal, parietal, and occipi- fiber tracts connect areas within a hemisphere, and tal lobes. The speech area allows you to sound out projection fiber tracts connect the cerebrum with lower words. This area (like Broca’s area) is usually in only CNS centers, such as the brain stem. one cerebral hem