Principles of Anatomy and Physiology 15th Edition PDF

CHAPTER 12 Nervous Tissue Nervous Tissue and Homeostasis The excitable characteristic of nervous tissue allows for the generation of nerve impulses (action potentials) that provide communication with and regulation of most body organs. Both the nervous and endocrine systems have the same obje...

CHAPTER 12 Nervous Tissue Nervous Tissue and Homeostasis The excitable characteristic of nervous tissue allows for the generation of nerve impulses (action potentials) that provide communication with and regulation of most body organs. Both the nervous and endocrine systems have the same objective: nerves (Chapter 13), and of the brain and cranial nerves (Chapter 14). to keep controlled conditions within limits that maintain life. The The autonomic nervous system, which operates without voluntary nervous system regulates body activities by responding rapidly control, will be covered in Chapter 15. Chapter 16 will discuss the using nerve impulses; the endocrine system responds by releasing somatic senses—touch, pressure, warmth, cold, pain, and others— hormones. Chapter 18 compares the roles of both systems in and their sensory and motor pathways to show how nerve impulses maintaining homeostasis. pass into the spinal cord and brain or from the spinal cord and brain The nervous system is also responsible for our perceptions, to muscles and glands. Exploration of the nervous system concludes behaviors, and memories, and it initiates all voluntary movements. with a discussion of the special senses: smell, taste, vision, hearing, Because this system is quite complex, we discuss its structure and and equilibrium (Chapter 17). function in several chapters. This chapter focuses on the organization of the nervous system and the properties of neurons (nerve cells) Q Did you ever wonder how the human nervous system and neuroglia (cells that support the activities of neurons). We then coordinates and integrates all body systems so rapidly and examine the structure and functions of the spinal cord and spinal efficiently? 403 404 CHAPTER 1 2 Nervous Tissue The motor or efferent division of the PNS conveys output from 12.1 Overview of the Nervous the CNS to effectors (muscles and glands). This division is further sub- System divided into a somatic nervous system and an autonomic nervous system (Figure 12.1b). The somatic nervous system (SNS) (soˉ -MAT- ik; soma = body) conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the OBJECTIVES action of this part of the PNS is voluntary. The autonomic nervous system (ANS) (aw′-toˉ -NOM-ik; auto- = self; -nomic = law) conveys Describe the organization of the nervous system. output from the CNS to smooth muscle, cardiac muscle, and glands. Describe the three basic functions of the nervous system. Because its motor responses are not normally under conscious con- trol, the action of the ANS is involuntary. The ANS is comprised of two main branches, the sympathetic nervous system and the parasym- Organization of the Nervous System pathetic nervous system. With a few exceptions, effectors receive innervation from both of these branches, and usually the two branch- With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the es have opposing actions. For example, neurons of the sympathetic nervous system is one of the smallest and yet the most complex of nervous system increase heart rate, and neurons of the parasympa- the 11 body systems. This intricate network of billions of neurons and thetic nervous system slow it down. In general, the parasympathetic even more neuroglia is organized into two main subdivisions: the cen- nervous system takes care of “rest-and-digest” activities, and the tral nervous system and the peripheral nervous system. Neurology sympathetic nervous system helps support exercise or emergency deals with normal functioning and disorders of the nervous system. A actions—the so-called “fight-or-flight” responses. A third branch of neurologist (noo-ROL-oˉ-jist) is a physician who diagnoses and treats the autonomic nervous system is the enteric nervous system (ENS) disorders of the nervous system. (en-TER-ik; enteron = intestines), an extensive network of over 100 million neurons confined to the wall of the gastrointestinal (GI) tract. The ENS helps regulate the activity of the smooth muscle and glands of the GI Central Nervous System The central nervous system tract. Although the ENS can function independently, it communicates (CNS) consists of the brain and spinal cord (Figure 12.1a). The with and is regulated by the other branches of the ANS. brain is the part of the CNS that is located in the skull and contains about 85 billion neurons. The spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled Functions of the Nervous System by the bones of the vertebral column. The spinal cord contains about 100 million neurons. The CNS processes many different kinds of The nervous system carries out a complex array of tasks. It allows us incoming sensory information. It is also the source of thoughts, to sense various smells, produce speech, and remember past events; emotions, and memories. Most signals that stimulate muscles to in addition, it provides signals that control body movements and reg- contract and glands to secrete originate in the CNS. ulates the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory (input), integrative (pro- cess), and motor (output). Peripheral Nervous System The peripheral nervous Sensory function. Sensory receptors detect internal stimuli, such system (PNS) (pe-RIF-e-ral) consists of all nervous tissue outside as an increase in blood pressure, or external stimuli (for example, a the CNS (Figure 12.1a). Components of the PNS include nerves and raindrop landing on your arm). This sensory information is then car- sensory receptors. A nerve is a bundle of hundreds to thousands of ried into the brain and spinal cord through cranial and spinal nerves. axons plus associated connective tissue and blood vessels that lies Integrative function. The nervous system processes sensory in- outside the brain and spinal cord. Twelve pairs of cranial nerves formation by analyzing it and making decisions for appropriate emerge from the brain and thirty-one pairs of spinal nerves emerge responses—an activity known as integration. from the spinal cord. Each nerve follows a defined path and serves a specific region of the body. The term sensory receptor refers to a Motor function. Once sensory information is integrated, the ner- structure of the nervous system that monitors changes in the external vous system may elicit an appropriate motor response by activating or internal environment. Examples of sensory receptors include touch effectors (muscles and glands) through cranial and spinal nerves. receptors in the skin, photoreceptors in the eye, and olfactory (smell) Stimulation of the effectors causes muscles to contract and glands receptors in the nose. to secrete. The PNS is divided into sensory and motor divisions (Figure The three basic functions of the nervous system occur, for exam- 12.1b). The sensory or afferent division of the PNS conveys input into ple, when you answer your cell phone after hearing it ring. The sound the CNS from sensory receptors in the body. This division provides the of the ringing cell phone stimulates sensory receptors in your ears CNS with sensory information about the somatic senses (tactile, ther- (sensory function). This auditory information is subsequently relayed mal, pain, and proprioceptive sensations) and special senses (smell, into your brain where it is processed and the decision to answer the taste, vision, hearing, and equilibrium). phone is made (integrative function). The brain then stimulates the  12.1 Overview of the Nervous System 405 FIGURE 12.1 Organization of the nervous system. (a) Subdivisions of the nervous system. (b) Nervous system organizational chart; blue boxes represent sensory components of the peripheral nervous system, red boxes represent motor components of the PNS, and green boxes represent effectors (muscles and glands). The two main subdivisions of the nervous system are (1) the central nervous system (CNS), which consists CNS: of the brain and spinal cord, and (2) the peripheral Brain PNS: nervous system (PNS), which consists of all nervous Cranial tissue outside the CNS. nerves Spinal cord Spinal nerves Enteric plexuses in small intestine Sensory receptors in skin (a) CENTRAL NERVOUS SYSTEM (CNS) Sensory input Motor output PERIPHERAL NERVOUS SYSTEM (PNS) Sensory Motor Division Division Somatic nervous system Autonomic nervous system Sympathetic Parasympathetic Enteric nervous Somatic senses Special senses nervous system nervous system system Smooth muscle, cardiac muscle, Smooth muscle and Skeletal muscle and glands glands of GI tract (b) Q What are some of the functions of the CNS? 406 CHAPTER 1 2 Nervous Tissue contraction of specific muscles that will allow you to grab the phone a neuron through specific ion channels in its plasma membrane. and press the appropriate button to answer it (motor function). Once begun, a nerve impulse travels rapidly and at a constant strength. Some neurons are tiny and propagate impulses over a short dis- Checkpoint tance (less than 1 mm) within the CNS. Others are the longest cells in 1. What is the purpose of a sensory receptor? the body. The neurons that enable you to wiggle your toes, for exam- 2. What are the components and functions of the SNS and ANS? ple, extend from the lumbar region of your spinal cord (just above waist level) to the muscles in your foot. Some neurons are even longer. 3. Which subdivisions of the PNS control voluntary actions? Those that allow you to feel a feather tickling your toes stretch all the Involuntary actions? way from your foot to the lower portion of your brain. Nerve impulses 4. Explain the concept of integration and provide an example. travel these great distances at speeds ranging from 0.5 to 130 meters per second (1 to 290 mi/hr). Parts of a Neuron 12.2 Histology of Nervous Tissue Most neurons have three parts: (1) a cell body, (2) dendrites, and (3) an axon (Figure 12.2). The cell body, also known as the perikaryon (per′-i-KAR-ē-on) or soma, contains a nucleus surrounded by cytoplasm that includes typical cellular organelles OBJECTIVES such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell bodies also contain free ribosomes and prominent clusters of rough Contrast the histological characteristics and the functions of neu- endoplasmic reticulum, termed Nissl bodies (NIS-el). The ribosomes rons and neuroglia. are the sites of protein synthesis. Newly synthesized proteins Distinguish between gray matter and white matter. produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS. The cytoskeleton includes both neurofibrils (noo-rō-FĪ- Nervous tissue comprises two types of cells—neurons and neuroglia. brils), composed of bundles of intermediate filaments that provide These cells combine in a variety of ways in different regions of the the cell shape and support, and microtubules (mī-krō-TOO-būls′), nervous system. In addition to forming the complex processing net- which assist in moving materials between the cell body and axon. works within the brain and spinal cord, neurons also connect all Aging neurons also contain lipofuscin (līp′-o-FYŪS-īn), a pigment regions of the body to the brain and spinal cord. As highly specialized that occurs as clumps of yellowish brown granules in the cytoplasm. cells capable of reaching great lengths and making extremely intri- Lipofuscin is a product of neuronal lysosomes that accumulates as cate connections with other cells, neurons provide most of the the neuron ages, but does not seem to harm the neuron. A collection unique functions of the nervous system, such as sensing, thinking, of neuron cell bodies outside the CNS is called a ganglion (GANG-lē- remembering, controlling muscle activity, and regulating glandular on = sculling or knot; ganglia is plural). secretions. As a result of their specialization, most neurons have lost A nerve fiber is a general term for any neuronal process (exten- the ability to undergo mitotic divisions. Neuroglia are smaller cells, sion) that emerges from the cell body of a neuron. Most neurons have but they greatly outnumber neurons—perhaps by as much as 25 two kinds of processes: multiple dendrites and a single axon. Den- times. Neuroglia support, nourish, and protect neurons, and main- drites (DEN-drīts = little trees) are the receiving or input portions of a tain the interstitial fluid that bathes them. Unlike neurons, neuroglia neuron. The plasma membranes of dendrites (and cell bodies) con- continue to divide throughout an individual’s lifetime. Both neurons tain numerous receptor sites for binding chemical messengers from and neuroglia differ structurally depending on whether they are other cells. Dendrites usually are short, tapering, and highly branched. located in the central nervous system or the peripheral nervous In many neurons the dendrites form a tree-shaped array of processes system. These differences in structure correlate with the differences extending from the cell body. Their cytoplasm contains Nissl bodies, in function of the central nervous system and the peripheral nervous mitochondria, and other organelles. system. The single axon (= axis) of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a Neurons long, thin, cylindrical projection that often joins to the cell body at a cone-shaped elevation called the axon hillock (HIL-lok = small hill). Like muscle cells, neurons (nerve cells) (NOO-rons) possess electri- The part of the axon closest to the axon hillock is the initial seg- cal excitability (ek-sīt′-a-BIL-i-tē), the ability to respond to a stimu- ment. In most neurons, nerve impulses arise at the junction of the lus and convert it into an action potential. A stimulus is any change axon hillock and the initial segment, an area called the trigger zone, in the environment that is strong enough to initiate an action from which they travel along the axon to their destination. An axon potential. An action potential (nerve impulse) is an electrical signal contains mitochondria, microtubules, and neurofibrils. Because that propagates (travels) along the surface of the membrane of a rough endoplasmic reticulum is not present, protein synthesis does neuron. It begins and travels due to the movement of ions (such as not occur in the axon. The cytoplasm of an axon, called axoplasm, is sodium and potassium) between interstitial fluid and the inside of surrounded by a plasma membrane known as the axolemma  12.2 Histology of Nervous Tissue 407 FIGURE 12.2 Structure of a multipolar neuron. A multipolar neuron has a cell body, several short dendrites, and a single long axon. Arrows indicate the direction of information flow: dendrites → cell body → axon → axon terminals. The basic parts of a neuron are dendrites, a cell body, and an axon. Dendritic spines Dendrites Cell body Axon collateral Initial segment Axon hillock Mitochondrion Neurofibril Axon Nucleus Cytoplasm Nucleus of Schwann cell Nissl bodies (a) Parts of a neuron Schwann cell: Cytoplasm Dendrite Myelin sheath Nerve impulse Neurolemma Cell body Node of Ranvier Axon Neurofibril Axon: Axoplasm Steve Gschmeissner/SPL/GettyImages SEM 1500x Axolemma (b) Motor neuron Dendrite Neuroglial cell Cell body Axon terminal Nucleus Synaptic end bulb Axon Mark Nielsen LM 400x (c) Motor neuron Q What roles do the dendrites, cell body, and axon play in communication of signals? 408 CHAPTER 1 2 Nervous Tissue (lemma = sheath or husk). Along the length of an axon, side branches transport that occurs in an anterograde (forward) direction moves called axon collaterals may branch off, typically at a right angle to the organelles and synaptic vesicles from the cell body to the axon termi- axon. The axon and its collaterals end by dividing into many fine proc- nals. Fast axonal transport that occurs in a retrograde (backward) esses called axon terminals or axon telodendria (tēl′-ō-DEN-drē-a). direction moves membrane vesicles and other cellular materials The site of communication between two neurons or between a from the axon terminals to the cell body to be degraded or recycled. neuron and an effector cell is called a synapse (SIN-aps). The tips of Substances that enter the neuron at the axon terminals are also some axon terminals swell into bulb-shaped structures called synap- moved to the cell body by fast retrograde transport. These sub- tic end bulbs; others exhibit a string of swollen bumps called vari- stances include trophic chemicals such as nerve growth factor and cosities (var′-i-KOS-i-tēz). Both synaptic end bulbs and varicosities harmful agents such as tetanus toxin and the viruses that cause rabies, contain many tiny membrane-enclosed sacs called synaptic vesicles herpes simplex, and polio. that store a chemical called a neurotransmitter (noo′-rō-trans′-MIT- ter). A neurotransmitter is a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell. Structural Diversity in Neurons Neurons display great diversity in size and shape. For example, their cell bodies range in Many neurons contain two or even three types of neurotransmitters, diameter from 5 micrometers ( μm) (slightly smaller than a red blood each with different effects on the postsynaptic cell. cell) up to 135 μm (barely large enough to see with the unaided Because some substances synthesized or recycled in the neuron eye). The pattern of dendritic branching is varied and distinctive for cell body are needed in the axon or at the axon terminals, two types of neurons in different parts of the nervous system. A few small neurons transport systems carry materials from the cell body to the axon termi- lack an axon, and many others have very short axons. As we have nals and back. The slower system, which moves materials about already discussed, the longest axons are almost as long as a person is 1–5 mm per day, is called slow axonal transport. It conveys axoplasm tall, extending from the toes to the lowest part of the brain. in one direction only—from the cell body toward the axon terminals. Slow axonal transport supplies new axoplasm to developing or regen- erating axons and replenishes axoplasm in growing and mature axons. Classification of Neurons Both structural and functional Fast axonal transport, which is capable of moving materials a features are used to classify the various neurons in the body. distance of 200–400 mm per day, uses proteins that function as “mo- tors” to move materials along the surfaces of microtubules of the STRUCTURAL CLASSIFICATION Structurally, neurons are classified neuron’s cytoskeleton. Fast axonal transport moves materials in according to the number of processes extending from the cell body both directions—away from and toward the cell body. Fast axonal (Figure 12.3): FIGURE 12.3 Structural classification of neurons. Breaks indicate that axons are longer than shown. A multipolar neuron has many processes extending from the cell body, a bipolar neuron has two, and a unipolar neuron has one. Dendrites Cell body Trigger zone Dendrites Dendrite Peripheral process Cell body Myelin sheath Trigger zone Cell body Trigger zone Axon Axon Central process Myelin sheath Myelin sheath Axon Axon Axon terminal terminal terminal (a) Multipolar neuron (b) Bipolar neuron (c) Unipolar neuron Q Which type of neuron shown in this figure is the most abundant type of neuron in the CNS?  12.2 Histology of Nervous Tissue 409 1. Multipolar neurons usually have several dendrites and one axon FUNCTIONAL CLASSIFICATION Functionally, neurons are classified (Figure 12.3a). Most neurons in the brain and spinal cord are of this according to the direction in which the nerve impulse (action poten- type, as well as all motor neurons (described shortly). tial) is conveyed with respect to the CNS (Figure 12.5). 2. Bipolar neurons have one main dendrite and one axon (Fig- 1. Sensory neurons or afferent neurons (AF-er-ent NOO-ronz; af- = ure 12.3b). They are found in the retina of the eye, the inner ear, toward; -ferrent = carried) either contain sensory receptors at their and the olfactory area (olfact = to smell) of the brain. distal ends (dendrites) (see also Figure 12.10) or are located just 3. Unipolar neurons have dendrites and one axon that are fused after sensory receptors that are separate cells. Once an appropriate together to form a continuous process that emerges from the cell stimulus activates a sensory receptor, the sensory neuron forms an body (Figure 12.3c). These neurons are more appropriately called action potential in its axon and the action potential is conveyed pseudounipolar neurons (soo′-dō-ū′-ni-PŌ-lar) because they into the CNS through cranial or spinal nerves. Most sensory neu- begin in the embryo as bipolar neurons. During development, the rons are unipolar in structure. dendrites and axon fuse together and become a single process. The 2. Motor neurons or efferent neurons (EF-e-rent; ef- = away from) dendrites of most unipolar neurons function as sensory receptors convey action potentials away from the CNS to effectors (muscles that detect a sensory stimulus such as touch, pressure, pain, or and glands) in the periphery (PNS) through cranial or spinal nerves thermal stimuli. The trigger zone for nerve impulses in a unipolar (see also Figure 12.10). Motor neurons are multipolar in structure. neuron is at the junction of the dendrites and axon (Figure 12.3c). The impulses then propagate toward the synaptic end bulbs. The 3. Interneurons or association neurons are mainly located within the cell bodies of most unipolar neurons are located in the ganglia of CNS between sensory and motor neurons (see also Figure 12.10). spinal and cranial nerves. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activat- In addition to the structural classification scheme just described, ing the appropriate motor neurons. Most interneurons are multipolar some neurons are named for the histologist who first described them in structure. or for an aspect of their shape or appearance; examples include Purkinje cells (pur-KIN-jē) in the cerebellum and pyramidal cells (pi-RAM-i-dal), found in the cerebral cortex of the brain, which have Neuroglia pyramid-shaped cell bodies (Figure 12.4). Neuroglia (noo-RŌG-lē-a; -glia = glue) or glia (GLĒ-a) make up about half the volume of the CNS. Their name derives from the idea of early histologists that they were the “glue” that held nervous tissue together. We now know that neuroglia are not merely passive bystand- FIGURE 12.4 Two examples of CNS neurons. Arrows indicate the ers but rather actively participate in the activities of nervous tissue. direction of information flow. Generally, neuroglia are smaller than neurons, and they are 5 to 25 times more numerous. In contrast to neurons, glia do not generate The dendritic branching pattern often is distinctive for a particular type or propagate action potentials, and they can multiply and divide in of neuron. the mature nervous system. In cases of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons. Brain tumors derived from glia, called gliomas (glē-Ō-mas), tend to be highly malignant and to grow rapidly. Of the six types of neuroglia, four—astrocytes, oligodendrocytes, microglia, and ependymal cells— are found only in the CNS. The remaining two types—Schwann cells Dendrites and satellite cells—are present in the PNS. Cell body Neuroglia of the CNS Neuroglia of the CNS can be classified on the basis of size, cytoplasmic processes, and intracellular organization into four types: astrocytes, oligodendrocytes, microglial cells, and ependymal cells (Figure 12.6). ASTROCYTES These star-shaped cells have many processes and are Axon the largest and most numerous of the neuroglia. There are two types of astrocytes (AS-trō-sīts; astro- = star; -cyte = cell). Protoplasmic astrocytes have many short branching processes and are found in Axon gray matter (described shortly). Fibrous astrocytes have many long terminal unbranched processes and are located mainly in white matter (also described shortly). The processes of astrocytes make contact with (a) Purkinje cell (b) Pyramidal cell blood capillaries, neurons, and the pia mater (a thin membrane Q How did the pyramidal cells get their name? around the brain and spinal cord). 410 CHAPTER 1 2 Nervous Tissue FIGURE 12.5 Functional classification of neurons. Neurons are divided into three functional classes: sensory neurons, interneurons, and motor neurons. PERIPHERAL NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM Cell body Sensory receptor (dendrites) Axon Nerve impulse Sensory neuron (usually unipolar) Dendrites Cell body Interneuron (usually multipolar) Nerve Axon impulse Motor neuron (usually multipolar) Nerve impulse Effectors: Dendrites muscles or glands Cell body Axon Q Which functional class of neurons is responsible for integration? The functions of astrocytes include the following: 3. In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the 1. Astrocytes contain microfilaments that give them considerable brain. strength, which enables them to support neurons. 4. Astrocytes help to maintain the appropriate chemical environ- 2. Processes of astrocytes wrapped around blood capillaries isolate ment for the generation of nerve impulses. For example, they neurons of the CNS from various potentially harmful substances regulate the concentration of important ions such as K+; take up in blood by secreting chemicals that maintain the unique selective excess neurotransmitters; and serve as a conduit for the passage permeability characteristics of the endothelial cells of the capil- of nutrients and other substances between blood capillaries and laries. In effect, the endothelial cells create a blood–brain barrier, neurons. which restricts the movement of substances between the blood and interstitial fluid of the CNS. Details of the blood–brain barrier 5. Astrocytes may also play a role in learning and memory by influenc- are discussed in Chapter 14. ing the formation of neural synapses (see Section 16.5).  12.2 Histology of Nervous Tissue 411 FIGURE 12.6 Neuroglia of the central nervous system. Neuroglia of the CNS are distinguished on the basis of size, cytoplasmic processes, and intracellular organization. Cells of pia mater Protoplasmic Oligodendrocyte astrocyte Node of Ranvier Microglial cell Myelin sheath Axon Neuron Oligodendrocyte Neuroglial cell Blood capillary Fibrous astrocytes Protoplasmic astrocyte Neurons Microglial cell Ependymal cell Microvillus Cilia Ventricle Types of neuroglial cells Thomas Deerinck Deerinck/Science Source Images SEM Q Which CNS neuroglia function as phagocytes? OLIGODENDROCYTES These resemble astrocytes but are smaller cord). Functionally, ependymal cells produce, possibly monitor, and and contain fewer processes. Processes of oligodendrocytes (OL-i- assist in the circulation of cerebrospinal fluid. They also form the gō-den′-drō-sīts; oligo- = few; -dendro- = tree) are responsible for blood–cerebrospinal fluid barrier, which is discussed in Chapter 14. forming and maintaining the myelin sheath around CNS axons. As you will see shortly, the myelin sheath is a multilayered lipid and protein Neuroglia of the PNS Neuroglia of the PNS completely covering around some axons that insulates them and increases the surround axons and cell bodies. The two types of glial cells in the PNS speed of nerve impulse conduction. Such axons are said to be myeli- are Schwann cells and satellite cells (Figure 12.7). nated (MĪ-e-li-nā-ted). SCHWANN CELLS These cells encircle PNS axons. Like oligodendro- MICROGLIAL CELLS OR MICROGLIA These neuroglia are small cells cytes, they form the myelin sheath around axons. A single oligoden- with slender processes that give off numerous spinelike projections. drocyte myelinates several axons, but each Schwann cell (SCHVON or Microglial cells or microglia (mī-KROG-lē-a; micro- = small) function SCHWON) myelinates a single axon (Figure 12.7a; see also Fig- as phagocytes. Like tissue macrophages, they remove cellular debris ure 12.8a, c). A single Schwann cell can also enclose as many as 20 or formed during normal development of the nervous system and more unmyelinated axons (axons that lack a myelin sheath) (Fig- phagocytize microbes and damaged nervous tissue. ure 12.7b). Schwann cells participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS. EPENDYMAL CELLS Ependymal cells (ep-EN-de-mal; epen- = above; -dym- = garment) are cuboidal to columnar cells arranged in a single SATELLITE CELLS These flat cells surround the cell bodies of neurons layer that possess microvilli and cilia. These cells line the ventricles of of PNS ganglia (Figure 12.7c). Besides providing structural support, the brain and central canal of the spinal cord (spaces filled with cere- satellite cells (SAT-i-līt) regulate the exchanges of materials between brospinal fluid, which protects and nourishes the brain and spinal neuronal cell bodies and interstitial fluid. 412 CHAPTER 1 2 Nervous Tissue FIGURE 12.7 Neuroglia of the peripheral nervous system. Neuroglia of the PNS completely surround axons and cell bodies of neurons. Neuron cell body in a ganglion Satellite cell Node of Ranvier Schwann cell Schwann cell Myelin sheath Schwann cell Myelinated axon Unmyelinated axons Axon (a) (b) (c) Q How do Schwann cells and oligodendrocytes differ with respect to the number of axons they myelinate? Myelination spiral around CNS axons, forming a myelin sheath. A neurolemma is not present, however, because the oligodendrocyte cell body and nu- As you have already learned, axons surrounded by a multilayered lipid cleus do not envelop the axon. Nodes of Ranvier are present, but they and protein covering, called the myelin sheath, are said to be myeli- are fewer in number. Axons in the CNS display little regrowth after nated (Figure 12.8a). The sheath electrically insulates the axon of a injury. This is thought to be due, in part, to the absence of a neuro- neuron and increases the speed of nerve impulse conduction. Axons lemma, and in part to an inhibitory influence exerted by the oligoden- without such a covering are said to be unmyelinated (Figure 12.8b). drocytes on axon regrowth. Two types of neuroglia produce myelin sheaths: Schwann cells The amount of myelin increases from birth to maturity, and its (in the PNS) and oligodendrocytes (in the CNS). Schwann cells begin presence greatly increases the speed of nerve impulse conduction. An to form myelin sheaths around axons during fetal development. Each infant’s responses to stimuli are neither as rapid nor as coordinated as Schwann cell wraps about 1 millimeter (1 mm = 0.04 in.) of a single those of an older child or an adult, in part because myelination is still axon’s length by spiraling many times around the axon (Figure 12.8a). in progress during infancy. Eventually, multiple layers of glial plasma membrane surround the axon, with the Schwann cell’s cytoplasm and nucleus forming the out- Collections of Nervous Tissue ermost layer. The inner portion, consisting of up to 100 layers of Schwann cell membrane, is the myelin sheath. The outer nucleated The components of nervous tissue are grouped together in a variety of cytoplasmic layer of the Schwann cell, which encloses the myelin ways. Neuronal cell bodies are often grouped together in clusters. The sheath, is the neurolemma (sheath of Schwann) (noo′-rō-LEM-ma). A axons of neurons are usually grouped together in bundles. In addi- neurolemma is found only around axons in the PNS. When an axon is tion, widespread regions of nervous tissue are grouped together as injured, the neurolemma aids regeneration by forming a regeneration either gray matter or white matter. tube that guides and stimulates regrowth of the axon. Gaps in the myelin sheath, called nodes of Ranvier (RON-vē-ā), appear at inter- Clusters of Neuronal Cell Bodies Recall that a ganglion vals along the axon (Figure 12.8; see also Figure 12.2). Each Schwann (plural is ganglia) refers to a cluster of neuronal cell bodies located cell wraps one axon segment between two nodes. in the PNS. As mentioned earlier, ganglia are closely associated In the CNS, an oligodendrocyte myelinates parts of several axons. with cranial and spinal nerves. By contrast, a nucleus is a cluster of Each oligodendrocyte puts forth about 15 broad, flat processes that neuronal cell bodies located in the CNS.  12.2 Histology of Nervous Tissue 413 FIGURE 12.8 Myelinated and unmyelinated axons. Notice that one layer of Schwann cell plasma membrane surrounds unmyelinated axons. Axons surrounded by a myelin sheath produced either by Schwann cells in the PNS or by oligoden- drocytes in the CNS are said to be myelinated. Schwann cell: Nucleus Cytoplasm Schwann cell: Cytoplasm Nucleus Node of Ranvier Axon Unmyelinated axons (b) Transverse section of unmyelinated axons Neurolemma Myelinated axon Myelin sheath (a) Transverse sections of stages in the formation of a myelin sheath Axon: Unmyelinated Myelinated Schwann cell Myelin sheath Neurolemma David M. Phillips/Science TEM 25,000x David M. Phillips/Science TEM 25,000x Source Source (c) Transverse section of (d) Transverse section of myelinated axon unmyelinated axons Q What is the functional advantage of myelination? Bundles of Axons Recall that a nerve is a bundle of axons Gray and White Matter In a freshly dissected section of the that is located in the PNS. Cranial nerves connect the brain to the brain or spinal cord, some regions look white and glistening, and others periphery, whereas spinal nerves connect the spinal cord to the appear gray (Figure 12.9). White matter is composed primarily of periphery. A tract is a bundle of axons that is located in the CNS. myelinated axons. The whitish color of myelin gives white matter its Tracts interconnect neurons in the spinal cord and brain. name. The gray matter of the nervous system contains neuronal cell 414 CHAPTER 1 2 Nervous Tissue FIGURE 12.9 Distribution of gray matter and white matter in the spinal cord and brain. White matter consists primarily of myelinated axons of many neurons. Gray matter consists of neuron cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. Frontal plane through brain Transverse plane through spinal cord Gray matter White matter (a) Transverse section of spinal cord (b) Frontal section of brain White matter Gray matter Mark Nielsen Mark Nielsen (c) Transverse section of spinal cord (d) Frontal section of brain Q What is responsible for the white appearance of white matter? bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a 12.3 Electrical Signals in gray color and there is little or no myelin in these areas. Blood vessels are present in both white and gray matter. In the spinal cord, the white Neurons: An Overview matter surrounds an inner core of gray matter that, depending on how imaginative you are, is shaped like a butterfly or the letter H in transverse OBJECTIVES section; in the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, the cerebrum and cerebellum (Figure 12.9). Describe the cellular properties that permit communication among The arrangement of gray matter and white matter in the spinal cord and neurons and effectors. brain is discussed more extensively in Chapters 13 and 14, respectively. Compare the basic types of ion channels, and explain how they relate to graded potentials and action potentials. Checkpoint 5. Describe the parts of a neuron and the functions of each. Like muscle fibers, neurons are electrically excitable. They communi- 6. Give several examples of the structural and functional cate with one another using two types of electrical signals: (1) Graded classifications of neurons. potentials (described shortly) are used for short-distance communi- 7. What is a neurolemma, and why is it important? cation only. (2) Action potentials (also described shortly) allow com- 8. With reference to the nervous system, what is a nucleus? munication over long distances within the body. Recall that an action potential in a muscle fiber is called a muscle action potential. When  12.3 Electrical Signals in Neurons: An Overview 415 an action potential occurs in a neuron (nerve cell), it is called a nerve 2 The graded potential triggers the axon of the sensory neuron to action potential (nerve impulse). To understand the functions of form a nerve action potential, which travels along the axon into graded potentials and action potentials, consider how the nervous the CNS and ultimately causes the release of neurotransmitter at system allows you to feel the smooth surface of a pen that you have a synapse with an interneuron. picked up from a table (Figure 12.10): 3 The neurotransmitter stimulates the interneuron to form a 1 As you touch the pen, a graded potential develops in a sensory graded potential in its dendrites and cell body. receptor in the skin of the fingers. FIGURE 12.10 Overview of nervous system functions. Graded potentials and nerve and muscle action potentials are in- Right side of brain Left side of brain volved in the relay of sensory stimuli, integrative functions such as perception, and motor activities. 5 Cerebral cortex Brain Interneuron Upper motor neuron Thalamus 6 4 3 Interneuron Sensory neuron 7 Spinal cord 2 Lower motor neuron Key: Graded potential Nerve action potential Muscle action potential 1 8 Sensory receptor Neuromuscular junction Skeletal muscles Q In which region of the brain does perception primarily occur? 416 CHAPTER 1 2 Nervous Tissue 4 In response to the graded potential, the axon of the interneuron Ion Channels forms a nerve action potential. The nerve action potential travels along the axon, which results in neurotransmitter release at the When ion channels are open, they allow specific ions to move across next synapse with another interneuron. the plasma membrane, down their electrochemical gradient—a 5 This process of neurotransmitter release at a synapse followed concentration (chemical) difference plus an electrical difference. by the formation of a graded potential and then a nerve action Recall that ions move from areas of higher concentration to areas of potential occurs over and over as interneurons in higher parts lower concentration (the chemical part of the gradient). Also, posi- of the brain (such as the thalamus and cerebral cortex) are tively charged cations move toward a negatively charged area, and activated. Once interneurons in the cerebral cortex, the outer negatively charged anions move toward a positively charged area (the part of the brain, are activated, perception occurs and you are electrical aspect of the gradient). As ions move, they create a flow of able to feel the smooth surface of the pen touch your fingers. electrical current that can change the membrane potential. As you will learn in Chapter 14, perception, the conscious Ion channels open and close due to the presence of “gates.” The awareness of a sensation, is primarily a function of the cerebral gate is a part of the channel protein that can seal the channel pore cortex. shut or move aside to open the pore (see Figure 3.6). The electrical signals produced by neurons and muscle fibers rely on four types of Suppose that you want to use the pen to write a letter. The ner- ion channels: leak channels, ligand-gated channels, mechanically- vous system would respond in the following way (Figure 12.10): gated channels, and voltage-gated channels: 6 A stimulus in the brain causes a graded potential to form in the 1. The gates of leak channels randomly alternate between open and dendrites and cell body of an upper motor neuron, a type of closed positions (Figure 12.11a). Typically, plasma membranes motor neuron that synapses with a lower motor neuron farther have many more potassium ion (K+) leak channels than sodium ion down in the CNS in order to contract a skeletal muscle. The (Na+) leak channels, and the potassium ion leak channels are leakier graded potential subsequently causes a nerve action potential than the sodium ion leak channels. Thus, the membrane’s perme- to occur in the axon of the upper motor neuron, followed by ability to K+ is much higher than its permeability to Na+. Leak neurotransmitter release. channels are found in nearly all cells, including the dendrites, cell 7 The neurotransmitter generates a graded potential in a lower bodies, and axons of all types of neurons. motor neuron, a type of motor neuron that directly supplies 2. A ligand-gated channel opens and closes in response to the bind- skeletal muscle fibers. The graded potential triggers the ing of a ligand (chemical) stimulus. A wide variety of chemical formation of a nerve action potential and then release of the ligands—including neurotransmitters, hormones, and particular neurotransmitter at neuromuscular junctions formed with ions—can open or close ligand-gated channels. The neurotrans- skeletal muscle fibers that control movements of the fingers. mitter acetylcholine, for example, opens cation channels that allow 8 The neurotransmitter stimulates the muscle fibers that control Na+ and Ca2+ to diffuse inward and K+ to diffuse outward (Figure finger movements to form muscle action potentials. The muscle 12.11b). Ligand-gated channels are located in the dendrites of action potentials cause these muscle fibers to contract, which some sensory neurons, such as pain receptors, and in dendrites allows you to write with the pen. and cell bodies of interneurons and motor neurons. The production of graded potentials and action potentials de- 3. A mechanically-gated channel opens or closes in response to me- pends on two basic features of the plasma membrane of excitable chanical stimulation in the form of vibration (such as sound waves), cells: the existence of a resting membrane potential and the pres- touch, pressure, or tissue stretching (Figure 12.11c). The force dis- ence of specific types of ion channels. Like most other cells in the torts the channel from its resting position, opening the gate. Exam- body, the plasma membrane of excitable cells exhibits a membrane ples of mechanically-gated channels are those found in auditory potential, an electrical potential difference (voltage) across the receptors in the ears, in receptors that monitor stretching of internal membrane. In excitable cells, this voltage is termed the resting organs, and in touch receptors and pressure receptors in the skin. membrane potential. The membrane potential is like voltage 4. A voltage-gated channel opens in response to a change in mem- stored in a battery. If you connect the positive and negative termi- brane potential (voltage) (Figure 12.11d). Voltage-gated channels nals of a battery with a piece of wire, electrons will flow along the participate in the generation and conduction of action potentials in wire. This flow of charged particles is called current. In living cells, the axons of all types of neurons. the flow of ions (rather than electrons) constitutes the electrical Table 12.1 presents a summary of the four major types of ion channels current. in neurons. Graded potentials and action potentials occur because the membranes of neurons contain many different kinds of ion channels that open or close in response to specific stimuli. Because the lipid Checkpoint bilayer of the plasma membrane is a good electrical insulator, the 9. What types of electrical signals occur in neurons? main paths for current to flow across the membrane are through the 10. Why are voltage-gated channels important? ion channels.  12.3 Electrical Signals in Neurons: An Overview 417 FIGURE 12.11 Ion channels in the plasma membrane. (a) Leak channels randomly open and close. (b) A chemical stimulus—here, the neurotransmitter acetylcholine—opens a ligand-gated channel. (c) A mechanical stimulus opens a mechanically-gated channel. (d) A change in membrane potential opens voltage-gated K+ channels during an action potential. The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels. Extracellular fluid Plasma membrane Cytosol K+ leak channel K+ leak channel closed + open K+ K Channel randomly opens and closes (a) Leak channel Ligand-gated Ca2+ Ligand-gated channel closed Acetylcholine channel open Na+ Chemical stimulus opens the channel K+ (b) Ligand-gated channel Mechanically-gated channel Ca2+ Mechanically- closed gated channel open Na+ Mechanical stimulus opens the channel (c) Mechanically-gated channel Voltage-gated Voltage-gated K+ channel K+ K+ channel open K+ closed Change in membrane potential opens the channel Voltage = –70 mV Voltage = –50 mV (d) Voltage-gated channel Q What type of gated channel is activated by a touch on the arm? 418 CHAPTER 1 2 Nervous Tissue TAB L E 1 2.1 Ion Channels in Neurons TYPE OF ION CHANNEL DESCRIPTION LOCATION Leak channels Gated channels that randomly open and close. Found in nearly all cells, including dendrites, cell bodies, and axons of all types of neurons. Ligand-gated channels Gated channels that open in response to binding of Dendrites of some sensory neurons such as pain ligand (chemical) stimulus. receptors and dendrites and cell bodies of interneurons and motor neurons. Mechanically-gated channels Gated channels that open in response to mechanical Dendrites of some sensory neurons such as touch stimulus (such as touch, pressure, vibration, or tissue receptors, pressure receptors, and some pain receptors. stretching). Voltage-gated channels Gated channels that open in response to voltage Axons of all types of neurons. stimulus (change in membrane potential). equal buildup of positive ions in the extracellular fluid (ECF) along the 12.4 Resting Membrane outside surface of the membrane (Figure 12.12a). Such a separation of positive and negative electrical charges is a form of potential Potential energy, which is measured in volts or millivolts (1 mV = 0.001 V). The greater the difference in charge across the membrane, the larger the membrane potential (voltage). Notice in Figure 12.12a that the OBJECTIVE buildup of charge occurs only very close to the membrane. The cyto- sol or extracellular fluid elsewhere in the cell contains equal numbers Describe the factors that maintain a resting membrane potential. of positive and negative charges and is electrically neutral. The resting membrane potential of a cell can be measured in the The resting membrane potential exists because of a small buildup of following way: The tip of a recording microelectrode is inserted inside negative ions in the cytosol along the inside of the membrane, and an the cell, and a reference electrode is placed outside the cell in the FIGURE 12.12 Resting membrane potential. To measure resting membrane potential, the tip of the recording microelectrode is inserted inside the neuron, and the reference electrode is placed in the extracellular fluid. The electrodes are connected to a voltmeter that measures the difference in charge across the plasma membrane (in this case −70 mV, indicating that the inside of the cell is negative relative to the outside). The resting membrane potential is an electrical potential difference (voltage) that exists across the plasma membrane of an excitable cell under resting conditions. 0 mV Extracellular fluid Plasma membrane Cytosol –70 mV +70 mV Extracellular fluid + + Equal numbers of – Voltmeter – + – + and – charges – + in most of ECF + + + + Extracellular Reference Recording fluid electrode microelectrode Resting membrane potential + + + + + + + + + + + (an electrical potential difference across the plasma membrane) – – – – Cytosol Equal numbers of + – + – + and – charges – – – – – – – – – – – – + – + in most of cytosol Cytosol (a) Distribution of charges that produce the resting membrane potential of a neuron (b) Measurement of the resting membrane potential of a neuron Q The resting membrane potential of a neuron typically is −70 mV. What does this mean?  12.4 Resting Membrane Potential 419 extracellular fluid. Electrodes are devices that conduct electrical cytosol, however, the main cation is K+, and the two dominant charges. The recording microelectrode and the reference electrode anions are phosphates attached to molecules, such as the three are connected to an instrument known as a voltmeter, which detects phosphates in ATP, and amino acids in proteins. Because the plasma the electrical difference (voltage) across the plasma membrane (Fig- membrane typically has more K+ leak channels than Na+ leak chan- ure 12.12b). In neurons, the resting membrane potential ranges from nels, the number of potassium ions that diffuse down their concen- −40 to −90 mV. A typical value is −70 mV. The minus sign indicates tration gradient out of the cell into the ECF is greater than the num- that the inside of the cell is negative relative to the outside. A cell that ber of sodium ions that diffuse down their concentration gradient exhibits a membrane potential is said to be polarized. Most body from the ECF into the cell. As more and more positive potassium ions cells are polarized; the membrane potential varies from +5 mV to exit, the inside of the membrane becomes increasingly negative, and −100 mV in different types of cells. the outside of the membrane becomes increasingly positive. The resting membrane potential arises from three major factors: 2. Inability of most anions to leave the cell. Another factor contrib- 1. Unequal distribution of ions in the ECF and cytosol. A major factor utes to the negative resting membrane potential: Most anions in- that contributes to the resting membrane potential is the unequal side the cell are not free to leave (Figure 12.13). They cannot follow distribution of various ions in extracellular fluid and cytosol (Figure the K+ out of the cell because they are attached to nondiffusible 12.13). Extracellular fluid is rich in Na+ and chloride ions (Cl−). In molecules such as ATP and large proteins. FIGURE 12.13 Three factors that contribute to the resting membrane potential. (1) Because the plasma membrane has more K+ leak channels (blue) than Na+ leak channels (rust), the number of K+ ions that leave the cell is greater than the number of Na+ ions that enter the cell. As more and more K+ ions leave the cell, the inside of the membrane becomes increasingly negative and the outside of the membrane becomes increasingly positive. (2) Trapped anions (turquoise and red) cannot follow K+ out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins. (3) The electrogenic Na+–K+ ATPase (purple) expels 3 Na+ ions for every 2 K+ ions imported. The resting membrane potential is determined by three major factors: (1) unequal distribution of ions in the ECF and cytosol, (2) inability of most anions to leave the cell, and (3) the electrogenic nature of the Na+–K+ ATPases. Extracellular fluid Plasma membrane Cytosol Extracellular + fluid Chloride + – + + – – + – ion – – + – + + – – + + + – Sodium + – + + + – ion – + – – – – + + Na –K ATPase K+ leak channel Na+ leak channel 3 Na+ + + + + + + + + + + + + Resting + + membrane potential – – – – – – – – – – – Cytosol Phosphate + – ATP 2 K+ – –– ion – + ADP –– –– – Protein – + + – + –– + + + – Potassium + + + –– + + + – – ion + + – – + Q Suppose that the plasma membrane of a neuron has more Na+ leak channels than K+ leak channels. What effect would this have on the resting membrane potential? 420 CHAPTER 1 2 Nervous Tissue 3. Electrogenic nature of the Na+–K+ ATPases. Membrane perme- FIGURE 12.14 Graded potentials. Most graded potentials occur in the ability to Na+ is very low because there are only a few sodium leak dendrites and cell body (areas colored blue). channels. Nevertheless, sodium ions do slowly diffuse inward, down their concentration gradient. Left unchecked, such inward During a hyperpolarizing graded potential, the membrane potential is leakage of Na+ would eventually destroy the resting membrane inside more negative than the resting level; during a depolarizing graded potential. The small inward Na+ leak and outward K+ leak are off- potential, the membrane potential is inside less negative than the rest- set by the Na+–K+ ATPases (sodium–potassium pumps) (Figure ing level. 12.13). These pumps help maintain the resting membrane poten- tial by pumping out Na+ as fast as it leaks in. At the same time, the Na+–K+ ATPases bring in K+. However, the potassium ions even- tually leak back out of the cell as they move down their concen- tration gradient. Recall that the Na+–K+ ATPases expel three Na+ for each two K+ imported (see Figure 3.10). Since these pumps remove more positive charges from the cell than they bring into the cell, they are electrogenic, which means they contribute to the negativity of the resting membrane potential. Their total contribu- tion, however, is very small: only −3 mV of the total −70 mV resting membrane potential in a typical neuron. Membrane potential –60 in millivolts (mV) Resting membrane potential Checkpoint –70 11. What is the typical resting membrane potential of a neuron? –80 12. How do leak channels contribute to resting membrane potential? 0 10 Time in milliseconds (msec) (a) Hyperpolarizing graded potential 12.5 Graded Potentials Membrane potential –60 in millivolts (mV) OBJECTIVE –70 Describe how a graded potential is generated. Resting membrane potential –80 A graded potential is a small deviation from the resting membrane 0 10 Time in milliseconds (msec) potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). When the (b) Depolarizing graded potential response makes the membrane more polarized (inside more nega- Q What kind of graded potential describes a change in tive), it is termed a hyperpolarizing graded potential (hī-per-PŌ-lar- membrane potential from −70 to −60 mV? From −70 to ī′-zing) (Figure 12.14a). When the response makes the membrane −80 mV? less polarized (inside less negative), it is termed a depolarizing graded potential (Figure 12.14b). A graded potential occurs when a sti