Assignment: Nervous System Systems

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

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.