Anatomy of the Nervous System Systems, Structures, and Cells That Make Up Your Nervous System

Anatomy of the Nervous System Systems, Structures, and Cells That Make Up Your Nervous System

3.1 General Layout of the Nervous System

3.2 Cells of the Nervous System

3.3 Neuroanatomical Techniques and Directions

3.4 Spinal Cord

3.5 Five Major Divisions of the Brain

3.6 Major Structures of the Brain

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Biopsychology, Eighth Edition, by John P.J. Pinel. Published by Allyn & Bacon. Copyright © 2011 by Pearson Education, Inc.

In order to understand what the brain does, it is firstnecessary to understand what it is—to know the namesand locations of its major parts and how they are con- nected to one another. This chapter introduces you to these fundamentals of brain anatomy.

Before you begin this chapter, I want to apologize for the lack of foresight displayed by early neuroanatomists in their choice of names for neuroanatomical structures— but, then, how could they have anticipated that Latin and Greek, universal languages of the educated in their day, would not be compulsory university fare in our time? To help you, I have provided the literal English meanings of many of the neuroanatomical terms, and I have kept this chapter as brief, clear, and to the point as possible, covering only the most important structures. The payoff for your effort will be a fundamental understanding of the structure of the human brain and a new vocabulary to discuss it.

3.1 General Layout of the Nervous System

Divisions of the Nervous System The vertebrate nervous system is composed of two divi- sions: the central nervous system and the peripheral nerv- ous system (see Figure 3.1). Roughly speaking, the central nervous system (CNS) is the division of the nervous system that is located within the skull and spine; the peripheral nervous system (PNS) is the division that is located out- side the skull and spine.

The central nervous system is composed of two divi- sions: the brain and the spinal cord. The brain is the part of the CNS that is located in the skull; the spinal cord is the part that is located in the spine.

The peripheral nervous system is also composed of two divisions: the somatic nervous system and the autonomic nervous system. The somatic nervous system (SNS) is the part of the PNS that interacts with the external environ- ment. It is composed of afferent nerves that carry sensory signals from the skin, skeletal muscles, joints, eyes, ears, and so on, to the central nervous system, and efferent nerves that carry motor signals from the central nervous system to the skeletal muscles. The autonomic nervous system (ANS) is the part of the peripheral nervous system that regulates the body’s internal environment. It is com- posed of afferent nerves that carry sensory signals from in- ternal organs to the CNS and efferent nerves that carry motor signals from the CNS to internal organs. You will not confuse the terms afferent and efferent if you remem- ber that many words that involve the idea of going toward

something—in this case, going toward the CNS—begin with an a (e.g., advance, approach, arrive) and that many words that involve the idea of going away from something begin with an e (e.g., exit, embark, escape).

The autonomic nervous system has two kinds of effer- ent nerves: sympathetic nerves and parasympathetic nerves. The sympathetic nerves are those autonomic motor nerves that project from the CNS in the lumbar (small of the back) and thoracic (chest area) regions of the spinal cord. The parasympathetic nerves are those autonomic motor nerves that project from the brain and sacral (lower back) region of the spinal cord. See Appendix I. (Ask your instructor to specify the degree to which you are responsible for material in the appendices.) All sym- pathetic and parasympathetic nerves are two-stage neural paths: The sympathetic and parasympathetic neurons project from the CNS and go only part of the way to the

513.1 ■ General Layout of the Nervous System

Central nervous system

Peripheral nervous system

FIGURE 3.1 The human central nervous system (CNS) and peripheral nervous system (PNS). The CNS is represented in red; the PNS in yellow. Notice that even those portions of nerves that are within the spinal cord are considered to be part of the PNS.

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target organs before they synapse on other neurons (sec- ond-stage neurons) that carry the signals the rest of the way. However, the sympathetic and parasympathetic sys- tems differ in that the sympathetic neurons that project from the CNS synapse on second-stage neurons at a sub- stantial distance from their target organs, whereas the parasympathetic neurons that project from the CNS synapse near their target organs on very short second- stage neurons (see Appendix I).

The conventional view of the respective functions of the sympathetic and parasympathetic systems stresses three important principles: (1) that sympathetic nerves stimulate, organize, and mobilize energy resources in threatening situations, whereas parasympathetic nerves act to conserve energy; (2) that each autonomic target organ receives opposing sympathetic and parasympa- thetic input, and its activity is thus controlled by relative levels of sympathetic and parasympathetic activity; and (3) that sympathetic changes are indicative of psycholog- ical arousal, whereas parasympathetic changes are indica- tive of psychological relaxation. Although these principles are generally correct, there are significant qualifications and exceptions to each of them (see Guyenet, 2006)—see Appendix II.

Most of the nerves of the peripheral nervous system project from the spinal cord, but there are 12 pairs of exceptions: the 12 pairs of cranial nerves, which project from the brain. They are numbered in sequence from front to back. The cranial nerves include purely sensory nerves such as the olfactory nerves (I) and the optic nerves (II), but most contain both sensory and motor fibers. The longest cranial nerves are the vagus nerves (X), which contain motor and sensory fibers travel- ing to and from the gut. The 12 pairs of cranial nerves and their targets are illustrated in Appendix III; the functions of these nerves are listed in Appendix IV. The autonomic motor fibers of the cranial nerves are parasympathetic.

The functions of the various cranial nerves are commonly assessed by neu- rologists as a basis for diagnosis. Because the functions and locations of the cranial nerves are spe- cific, disruptions of particular cranial nerve functions provide excellent clues about the location and extent of tumors and other kinds of brain pathology.

Figure 3.2 summarizes the major divisions of the nerv- ous system. Notice that the nervous system is a “system of twos.”

52 Chapter 3 ■ Anatomy of the Nervous System

Clinical Clinical Implications Implications

Brain Spinalcord Somatic nervous system

Autonomic nervous system

Afferent nerves

Efferent nerves

Afferent nerves

Efferent nerves

Sympathetic nervous system

Parasympathetic nervous system

Nervous system

Central nervous system

Peripheral nervous system

FIGURE 3.2 The major divisions of the nervous system.

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Biopsychology, Eighth Edition, by John P.J. Pinel. Published by Allyn & Bacon. Copyright © 2011 by Pearson Education, Inc.

Meninges, Ventricles, and Cerebrospinal Fluid The brain and spinal cord (the CNS) are the most protected organs in the body. They are encased in bone and covered by three protective membranes, the three meninges (pro- nounced “men-IN-gees”). The outer meninx (which, believe it or not, is the singular of meninges) is a tough membrane called the dura mater (tough mother). Immediately inside the dura mater is the fine arachnoid membrane (spider- weblike membrane). Beneath the arachnoid membrane is a space called the subarachnoid space, which contains many large blood vessels and cerebrospinal fluid; then comes the innermost meninx, the delicate pia mater (pious mother), which adheres to the surface of the CNS.

Also protecting the CNS is the cerebrospinal fluid (CSF), which fills the subarachnoid space, the central canal of the spinal cord, and the cerebral ventricles of the brain. The central canal is a small central channel that runs the length of the spinal cord; the cerebral ventricles are the four large internal chambers of the brain: the two lateral ventricles, the third ventricle, and the fourth ven- tricle (see Figure 3.3). The subarachnoid space, central canal, and cerebral ventricles are interconnected by a series of openings and thus form a single reservoir.

The cerebrospinal fluid supports and cushions the brain. Patients who have had some of their cerebrospinal fluid drained away often suffer raging headaches and ex- perience stabbing pain each time they jerk their heads.

Cerebrospinal fluid is continuously produced by the choroid plexuses—networks of capillaries (small blood vessels) that protrude into the ventricles from the pia mater. The excess cerebrospinal fluid is continuously ab- sorbed from the subarachnoid space into large blood- filled spaces, or dural sinuses, which run through the dura mater and drain into the large jugular veins of the neck. Figure 3.4 on page 54 illustrates the absorption of cere- brospinal fluid from the subarachnoid space into the large sinus that runs along the top of the brain between the two cerebral hemispheres.

Occasionally, the flow of cerebrospinal fluid is blocked by a tumor near one of the narrow channels that link the ventricles—for example, near the cerebral aqueduct, which connects the third and fourth ventricles. The re- sulting buildup of fluid in the ventricles causes the walls of the ventricles, and thus the entire brain, to expand, producing a condition called hydrocephalus (water head). Hydrocephalus is treated by draining the excess fluid from the ventricles and trying to remove the obstruction.

Blood–Brain Barrier The brain is a finely tuned electrochemical organ whose function can be severely disturbed by the introduction of certain kinds of chemicals. Fortunately, there is a mecha- nism that impedes the passage of many toxic substances from the blood into the brain: the blood–brain barrier

533.1 ■ General Layout of the Nervous System

Clinical Clinical Implications Implications

Lateral ventricles

Third ventricle

Fourth ventricle

Central canal

Cerebral aqueduct

Lateral ventricles

Third ventricle

Fourth ventricle

Cerebral aqueduct

FIGURE 3.3 The cerebral ventricles.

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(see Banerjee & Bhat, 2007). This barrier is a consequence of the special structure of cerebral blood vessels. In the rest of the body, the cells that compose the walls of blood ves- sels are loosely packed; as a result, most molecules pass readily through them into surrounding tissue. In the brain, however, the cells of the blood vessel walls are tightly packed, thus forming a barrier to the passage of many mol- ecules—particularly proteins and other large molecules (Abbott, Rönnbäck, & Hannson, 2005). The degree to which therapeutic or recreational drugs can influence brain activity depends on the ease with which they penetrate the blood–brain barrier (Löscher & Potschka, 2005).

The blood–brain barrier does not impede the passage of all large molecules. Some large molecules that are crit- ical for normal brain function (e.g., glucose) are actively transported through cerebral blood vessel walls. Also, the blood vessel walls in some areas of the brain allow certain large molecules to pass through them unimpeded.

54 Chapter 3 ■ Anatomy of the Nervous System

FIGURE 3.4 The absorption of cerebrospinal fluid from the subarachnoid space (blue) into a major sinus. Note the three meninges.

Scalp

Skull

Arachnoid meninx

Artery Sinus

Cortex

Pia mater meninx

Dura mater meninx

Subarachnoid space

This is good place for you to scan your brain: Are you ready to learn about the cells of the nervous system? Test your grasp of the first section of this chapter by filling in the following blanks with the most appropriate terms. The correct answers are provided at the end of the exercise.

Before proceeding, review material related to your errors and omissions.

1. The ______ system is composed of the brain and the spinal cord.

2. The part of the peripheral nervous system that regu- lates the body’s internal environment is the ______ system.

3. Nerves that carry signals away from a structure, such as the CNS, are ______ nerves.

4. The ANS nerves that project from the thoracic and lum- bar regions of the spinal cord are part of the ______ system.

5. ______ nerves stimulate, organize, and mobilize energy resources in threatening situations.

6. The vagus nerves are the longest ______. 7. The olfactory nerves and optic nerves are the only two

purely sensory ______. 8. The innermost meninx is the ______. 9. The cerebral ventricles, central canal, and subarachnoid

space are filled with ______. 10. ______ is continuously produced by the choroid

plexuses. 11. A tumor near the ______ can produce hydrocephalus. 12. The ______ blocks the entry of many large molecules

into brain tissue from the circulatory system.

Scan Your Brainanswers: (1) central nervous, (2) autonomic nervous, (3) efferent, (4) sympathetic nervous, (5) Sympathetic, (6) cranial nerves, (7) cranial nerves, (8) pia mater, (9) cerebrospinal fluid, (10) Cerebrospinal fluid, (11) cerebral aqueduct, (12) blood–brain barrier.

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Biopsychology, Eighth Edition, by John P.J. Pinel. Published by Allyn & Bacon. Copyright © 2011 by Pearson Education, Inc.

Anatomy of Neurons As you learned in Chapter 1, neurons are cells that are specialized for the reception, conduction, and transmis- sion of electrochemical signals. They come in an incredi- ble variety of shapes and sizes (see Nelson, Sugino, & Hempel, 2006); however, many are similar to the one illustrated in Figures 3.5 and 3.6 (on page 56).

553.2 ■ Cells of the Nervous System

Cell membrane. The semipermeable membrane that encloses the neuron.

Dendrites. The short processes emanating from the cell body, which receive most of the synaptic contacts from other neurons.

Cell body. The metabolic center of the neuron; also called the soma.

Axon hillock. The cone-shaped region at the junction between the axon and the cell body.

Axon. The long, narrow process that projects from the cell body.

Myelin. The fatty insulation around many axons.

Nodes of Ranvier (pronounced “RAHN-vee-yay”). The gaps between sections of myelin.

Buttons. The buttonlike endings of the axon branches, which release chemicals into synapses.

Synapses. The gaps between adjacent neurons across which chemical signals are transmitted.

3.2 Cells of the Nervous System

Most of the cells of the nervous system are of two funda- mentally different types: neurons and glial cells. Their anatomy is discussed in the following two subsections.

FIGURE 3.5 The major external features of a typical neuron.

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Endoplasmic reticulum. A system of folded membranes in the cell body; rough portions (those with ribosomes) play a role in the synthesis of proteins; smooth portions (those without ribosomes) play a role in the synthesis of fats.

Mitochondria. Sites of aerobic (oxygen-consuming) energy release.

Nucleus. The spherical DNA-containing structure of the cell body.

Cytoplasm. The clear internal fluid of the cell.

Ribosomes. Internal cellular structures on which proteins are synthesized; they are located on the endoplasmic reticulum.

Golgi complex. A connected system of membranes that packages molecules in vesicles.

Microtubules. Tubules responsible for the rapid transport of material throughout neurons.

Synaptic vesicles. Spherical membrane packages that store neurotransmitter molecules ready for release near synapses.

Neurotransmitters. Molecules that are released from active neurons and influence the activity of other cells.

FIGURE 3.6 The major internal features of a typical neuron.

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External Anatomy of Neurons Figure 3.5 is an illus- tration of the major external features of one type of neu- ron. For your convenience, the definition of each feature is included in the illustration.

Internal Anatomy of Neurons Figure 3.6 is an illus- tration of the major internal features of one type of neu- ron. Again, the definition of each feature is included in the illustration.

Neuron Cell Membrane The neuron cell membrane is composed of a lipid bilayer (Piomelli, Astarita, & Rapaka, 2007), or two layers of fat molecules (see Figure 3.7). Em- bedded in the lipid bilayer are numerous protein mole- cules that are the basis of many of the cell membrane’s functional properties. Some membrane proteins are channel proteins, through which certain molecules can pass; oth- ers are signal proteins, which transfer a signal to the inside of the neuron when particular molecules bind to them on the outside of the membrane.

Classes of Neurons Figure 3.8 on page 58 illustrates a way of classifying neurons that is based on the number of processes (projections) emanating from their cell bodies. A neuron with more than two processes extending from its cell body is classified as a multipolar neuron; most neu- rons are multipolar. A neuron with one process extending from its cell body is classified as a unipolar neuron, and a neuron with two processes extending from its cell body is classified as a bipolar neuron. Neurons with a short axon or no axon at all are called interneurons; their function is to integrate the neural activity within a single brain structure, not to conduct signals from one structure to another.

Neurons and Neuroanatomi- cal Structure In general, there are two kinds of gross neural structures in the nervous sys- tem: those composed primarily of cell bodies and those com- posed primarily of axons. In the central nervous system, clusters of cell bodies are called nuclei (singular nucleus); in the peripheral nervous system, they are called ganglia (singular ganglion). (Note that the word nucleus has two different neu- roanatomical meanings; it is a

structure in the neuron cell body and a cluster of cell bod- ies in the CNS.) In the central nervous system, bundles of axons are called tracts; in the peripheral nervous system, they are called nerves.

Glial Cells: The Forgotten Cells Neurons are not the only cells in the nervous system; glial cells are found throughout the system. Although they have been widely reported to outnumber neurons 10 to 1, this view has been challenged by recent research. Glial cells do predominate in some brain structures, but over- all the numbers of glial cells and neural cells are approxi- mately equal (Azevedo et al., 2009).

There are several kinds of glial cells (Fields & Stevens- Graham, 2002). Oligodendrocytes, for example, are glial cells with extensions that wrap around the axons of some neurons of the central nervous system. These extensions are rich in myelin, a fatty insulating substance, and the myelin sheaths that they form increase the speed and effi- ciency of axonal conduction. A similar function is per- formed in the peripheral nervous system by Schwann cells, a second class of glial cells. Oligodendrocytes and Schwann cells are illustrated in Figure 3.9 on page 58. Notice that each Schwann cell constitutes one myelin segment, whereas each oligodendrocyte provides several myelin segments, often on more than one axon. Another important difference between Schwann cells and oligodendrocytes is that only Schwann cells can guide axonal regeneration (regrowth) after damage. That is why effective axonal regeneration in the mammalian nervous system is restricted to the PNS.