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403
CHAPTER 12
Both the nervous and endocrine systems have the same objective: to keep controlled conditions within limits that maintain life. The nervous system regulates body activities by responding rapidly using nerve impulses; the endocrine system responds by releasing hormones. Chapter 18 compares the roles of both systems in maintaining homeostasis. The nervous system is also responsible for our perceptions, behaviors, and memories, and it initiates all voluntary movements. Because this system is quite complex, we discuss its structure and function in several chapters. This chapter focuses on the organization of the nervous system and the properties of neurons (nerve cells) and neuroglia (cells that support the activities of neurons). We then examine the structure and functions of the spinal cord and spinal
nerves (Chapter 13), and of the brain and cranial nerves (Chapter 14). The autonomic nervous system, which operates without voluntary control, will be covered in Chapter 15. Chapter 16 will discuss the somatic senses—touch, pressure, warmth, cold, pain, and others— and their sensory and motor pathways to show how nerve impulses pass into the spinal cord and brain or from the spinal cord and brain to muscles and glands. Exploration of the nervous system concludes with a discussion of the special senses: smell, taste, vision, hearing, and equilibrium (Chapter 17).
Q Did you ever wonder how the human nervous system coordinates and integrates all body systems so rapidly and efficiently?
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.
404 CHAPTER 12 Nervous Tissue
The motor or efferent division of the PNS conveys output from the CNS to effectors (muscles and glands). This division is further sub- 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 action of this part of the PNS is voluntary. The autonomic nervous system (ANS) (aw′-toˉ -NOM-ik; auto- = self; -nomic = law) conveys output from the CNS to smooth muscle, cardiac muscle, and glands. 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- pathetic nervous system. With a few exceptions, effectors receive innervation from both of these branches, and usually the two branch- es have opposing actions. For example, neurons of the sympathetic nervous system increase heart rate, and neurons of the parasympa- thetic nervous system slow it down. In general, the parasympathetic nervous system takes care of “rest-and-digest” activities, and the sympathetic nervous system helps support exercise or emergency actions—the so-called “fight-or-flight” responses. A third branch of the autonomic nervous system is the enteric nervous system (ENS) (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 tract. Although the ENS can function independently, it communicates with and is regulated by the other branches of the ANS.
Functions of the Nervous System
The nervous system carries out a complex array of tasks. It allows us to sense various smells, produce speech, and remember past events; in addition, it provides signals that control body movements and reg- ulates the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory (input), integrative (pro- cess), and motor (output).
- Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then car- ried into the brain and spinal cord through cranial and spinal nerves.
- Integrative function. The nervous system processes sensory in- formation by analyzing it and making decisions for appropriate responses—an activity known as integration.
- Motor function. Once sensory information is integrated, the ner- vous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete. The three basic functions of the nervous system occur, for exam- ple, when you answer your cell phone after hearing it ring. The sound of the ringing cell phone stimulates sensory receptors in your ears (sensory function). This auditory information is subsequently relayed into your brain where it is processed and the decision to answer the phone is made (integrative function). The brain then stimulates the
12.1 (^) Overview of the Nervous
System
OBJECTIVES
- Describe the organization of the nervous system.
- Describe the three basic functions of the nervous system.
Organization of the Nervous System
With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the nervous system is one of the smallest and yet the most complex of the 11 body systems. This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: the cen- tral nervous system and the peripheral nervous system. Neurology deals with normal functioning and disorders of the nervous system. A neurologist (noo-ROL-oˉ-jist) is a physician who diagnoses and treats disorders of the nervous system.
Central Nervous System The central nervous system
(CNS) consists of the brain and spinal cord ( Figure 12.1a ). The 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 by the bones of the vertebral column. The spinal cord contains about 100 million neurons. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.
Peripheral Nervous System The peripheral nervous
system (PNS) (pe-RIF-e-ral) consists of all nervous tissue outside the CNS ( Figure 12.1a ). Components of the PNS include nerves and sensory receptors. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge 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 structure of the nervous system that monitors changes in the external or internal environment. Examples of sensory receptors include touch receptors in the skin, photoreceptors in the eye, and olfactory (smell) receptors in the nose. The PNS is divided into sensory and motor divisions ( Figure 12.1b ). The sensory or afferent division of the PNS conveys input into the CNS from sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses (tactile, ther- mal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium).
406 CHAPTER 12 Nervous Tissue
a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength. Some neurons are tiny and propagate impulses over a short dis- tance (less than 1 mm) within the CNS. Others are the longest cells in the body. The neurons that enable you to wiggle your toes, for exam- 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. Those that allow you to feel a feather tickling your toes stretch all the way from your foot to the lower portion of your brain. Nerve impulses travel these great distances at speeds ranging from 0.5 to 130 meters per second (1 to 290 mi/hr).
Parts of a Neuron 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 such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies (NIS-el). The ribosomes are the sites of protein synthesis. Newly synthesized proteins 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- brils), composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules (mī-krō-TOO-būls′), which assist in moving materials between the cell body and axon. Aging neurons also contain lipofuscin (līp′-o-FYŪS-īn), a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lipofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron. A collection of neuron cell bodies outside the CNS is called a ganglion (GANG-lē- on = sculling or knot; ganglia is plural). A nerve fiber is a general term for any neuronal process (exten- sion) that emerges from the cell body of a neuron. Most neurons have two kinds of processes: multiple dendrites and a single axon. Den- drites (DEN-drīts = little trees) are the receiving or input portions of a neuron. The plasma membranes of dendrites (and cell bodies) con- tain numerous receptor sites for binding chemical messengers from other cells. Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles. The single axon (= axis) of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a long, thin, cylindrical projection that often joins to the cell body at a cone-shaped elevation called the axon hillock (HIL-lok = small hill). The part of the axon closest to the axon hillock is the initial seg- ment. In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, an area called the trigger zone , from which they travel along the axon to their destination. An axon contains mitochondria, microtubules, and neurofibrils. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon. The cytoplasm of an axon, called axoplasm , is surrounded by a plasma membrane known as the axolemma
contraction of specific muscles that will allow you to grab the phone and press the appropriate button to answer it (motor function).
Checkpoint
1. What is the purpose of a sensory receptor? 2. What are the components and functions of the SNS and ANS? 3. Which subdivisions of the PNS control voluntary actions? Involuntary actions? 4. Explain the concept of integration and provide an example.
12.2 (^) Histology of Nervous Tissue
OBJECTIVES
- Contrast the histological characteristics and the functions of neu- rons and neuroglia.
- Distinguish between gray matter and white matter.
Nervous tissue comprises two types of cells—neurons and neuroglia. These cells combine in a variety of ways in different regions of the nervous system. In addition to forming the complex processing net- works within the brain and spinal cord, neurons also connect all regions of the body to the brain and spinal cord. As highly specialized cells capable of reaching great lengths and making extremely intri- cate connections with other cells, neurons provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. As a result of their specialization, most neurons have lost the ability to undergo mitotic divisions. Neuroglia are smaller cells, but they greatly outnumber neurons—perhaps by as much as 25 times. Neuroglia support, nourish, and protect neurons, and main- tain the interstitial fluid that bathes them. Unlike neurons, neuroglia continue to divide throughout an individual’s lifetime. Both neurons and neuroglia differ structurally depending on whether they are located in the central nervous system or the peripheral nervous system. These differences in structure correlate with the differences in function of the central nervous system and the peripheral nervous system.
Neurons
Like muscle cells, neurons (nerve cells) (NOO-rons) possess electri- cal excitability (ek-sīt′-a-BIL-i-tē), the ability to respond to a stimu- lus and convert it into an action potential. A stimulus is any change in the environment that is strong enough to initiate an action potential. An action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of
12.2 Histology of Nervous Tissue 407
The basic parts of a neuron are dendrites, a cell body, and an axon.
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.
Q What roles do the dendrites, cell body, and axon play in communication of signals?
Axon
Initial segment Axon hillock Mitochondrion
Cytoplasm
Neurofibril
Dendritic spines
Nucleus
Nissl bodies
Nucleus of Schwann cell
Schwann cell:
Node of Ranvier
Neurofibril
Axon terminal
Synaptic end bulb
Axon:
Cytoplasm Myelin sheath
Axoplasm
Nerve impulse
Axolemma
Neurolemma
Dendrites Cell body
Axon collateral
(a) Parts of a neuron
Cell body
Axon
Dendrite
SEM 1500x (b) Motor neuron
Steve Gschmeissner/SPL/GettyImages
Neuroglial cell
Cell body
Nucleus
Axon
Dendrite
LM 400x (c) Motor neuron
Mark Nielsen
12.2 Histology of Nervous Tissue 409
FUNCTIONAL CLASSIFICATION Functionally, neurons are classified according to the direction in which the nerve impulse (action poten- tial) is conveyed with respect to the CNS ( Figure 12.5 ).
1. Sensory neurons or afferent neurons (AF-er-ent NOO-ronz; af- = toward; -ferrent = carried) either contain sensory receptors at their distal ends (dendrites) (see also Figure 12.10 ) or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neu- rons are unipolar in structure. 2. Motor neurons or efferent neurons (EF-e-rent; ef- = away from) convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves (see also Figure 12.10 ). Motor neurons are multipolar in structure. 3. Interneurons or association neurons are mainly located within the CNS between sensory and motor neurons (see also Figure 12.10 ). Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activat- ing the appropriate motor neurons. Most interneurons are multipolar in structure.
Neuroglia
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- ers but rather actively participate in the activities of nervous tissue. Generally, neuroglia are smaller than neurons, and they are 5 to 25 times more numerous. In contrast to neurons, glia do not generate or propagate action potentials, and they can multiply and divide in 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 and satellite cells—are present in the PNS.
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 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 gray matter (described shortly). Fibrous astrocytes have many long unbranched processes and are located mainly in white matter (also described shortly). The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).
1. Multipolar neurons usually have several dendrites and one axon ( Figure 12.3a ). Most neurons in the brain and spinal cord are of this type, as well as all motor neurons (described shortly). 2. Bipolar neurons have one main dendrite and one axon ( Fig- ure 12.3b ). They are found in the retina of the eye, the inner ear, and the olfactory area (olfact = to smell) of the brain. 3. Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body ( Figure 12.3c ). These neurons are more appropriately called pseudounipolar neurons (soo′-dō-ū′-ni-PŌ-lar) because they begin in the embryo as bipolar neurons. During development, the dendrites and axon fuse together and become a single process. The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone for nerve impulses in a unipolar neuron is at the junction of the dendrites and axon ( Figure 12.3c ). The impulses then propagate toward the synaptic end bulbs. The cell bodies of most unipolar neurons are located in the ganglia of spinal and cranial nerves.
In addition to the structural classification scheme just described, some neurons are named for the histologist who first described them 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 pyramid-shaped cell bodies ( Figure 12.4 ).
Axon terminal
Cell body
Dendrites
Axon
(a) Purkinje cell (b) Pyramidal cell
FIGURE 12.4 Two examples of CNS neurons. Arrows indicate the direction of information flow.
The dendritic branching pattern often is distinctive for a particular type of neuron.
Q How did the pyramidal cells get their name?
