Anatomy of the Nervous System PDF

Chapter 3 Anatomy of the Nervous System Systems, Structures, and Cells That Make Up Your Nervous System Mike Kemp/Tetra Images/Alamy Stock Photo Chapter Overview and Learning Objectives General Layout of the LO 3.1...

Chapter 3 Anatomy of the Nervous System Systems, Structures, and Cells That Make Up Your Nervous System Mike Kemp/Tetra Images/Alamy Stock Photo Chapter Overview and Learning Objectives General Layout of the LO 3.1 List and describe the major divisions of the nervous system. Nervous System LO 3.2 Describe the three meninges and explain their functional role. LO 3.3 Explain where cerebrospinal fluid is produced and where it flows. LO 3.4 Explain what the blood–brain barrier is and what functional role it serves. Cells of the Nervous LO 3.5 Draw, label, and define the major features of a multipolar System neuron. 72 M03_PINE1933_11_GE_C03.indd 72 22/01/2021 10:42  Anatomy of the Nervous System 73 LO 3.6 Briefly describe four kinds of glial cells. Neuroanatomical LO 3.7 Compare several neuroanatomical research techniques. Techniques and Directions LO 3.8 Illustrate the neuroanatomical directions. Anatomy of the Central LO 3.9 Draw and label a cross section of the spinal cord. Nervous System LO 3.10 List and discuss the five major divisions of the human brain. LO 3.11 List and describe the components of the myelencephalon. LO 3.12 List and describe the components of the metencephalon. LO 3.13 List and describe the components of the mesencephalon. LO 3.14 List and describe the components of the diencephalon. LO 3.15 List and describe the components of the telencephalon. LO 3.16 List and describe the components of the limbic system and of the basal ganglia. In order to understand what the brain does, it is first neces- Divisions of the Nervous System sary to understand what it is—to know the names and loca- tions of its major parts and how they are connected to one LO 3.1 List and describe the major divisions of the another. This chapter introduces you to these fundamentals nervous system. of brain anatomy. The vertebrate nervous system is composed of two Before you begin this chapter, we want to apologize for ­divisions: the central nervous system and the peripheral the lack of foresight displayed by early neuroanatomists in nervous system (see Figure 3.1). Roughly speaking, the their choice of names for neuroanatomical s­ tructures—but ­central nervous system (CNS) is the division of the ner- how could they have anticipated that Latin and Greek, vous system located within the skull and spine, and the universal languages of the educated in their day, would ­peripheral nervous system (PNS) is the division located not be compulsory university fare in our time? To help outside the skull and spine. you, we have provided the literal English meanings of The central nervous system is composed of two divi- many of the neuroanatomical terms, and we have kept sions: the brain and the spinal cord. The brain is the part this chapter as brief, clear, and to the point as possible, of the CNS located in the skull; the spinal cord is the part covering only the most important structures. The payoff located in the spine. for your effort will be a fundamental understanding of The peripheral nervous system is also composed of two the structure of the human brain and a new vocabulary divisions: the somatic nervous system and the autonomic to discuss it. 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 General Layout of the so on, to the central nervous system and efferent nerves Nervous System that carry motor signals from the central nervous system to the skeletal muscles. The autonomic nervous system (ANS) In this module, we’ll cover the general layout of the ner- is the part of the peripheral nervous system that regulates vous system. We’ll begin by discussing its two main divi- the body’s internal environment. It is composed of affer- sions. Then, we’ll look at the roles of meninges, ventricles, ent nerves that carry sensory signals from internal organs and cerebrospinal fluid. We’ll conclude with a look at the to the CNS and efferent nerves that carry motor signals blood–brain barrier. from the CNS to internal organs. You will not confuse the M03_PINE1933_11_GE_C03.indd 73 22/01/2021 10:42  74 Chapter 3 other neurons (second-stage neurons) that carry the signals Figure 3.1 The human central nervous system (CNS) and peripheral nervous system (PNS). The CNS is represented in the rest of the way. However, the sympathetic and para- red; the PNS in orange. Notice that even those portions of sympathetic systems differ in that the sympathetic neurons nerves that are within the spinal cord are considered to be project from the CNS synapse on second-stage neurons at part of the PNS. a substantial distance from their target organs, whereas the parasympathetic neurons project from the CNS synapse Central nervous near their target organs on very short second-stage neurons system (see Appendix I). The conventional view of the respective functions of Peripheral nervous system the sympathetic and parasympathetic systems stresses three important principles: (1) sympathetic nerves stimu- late, organize, and mobilize energy resources in threat- ening situations, whereas parasympathetic nerves act to conserve energy; (2) each autonomic target organ receives opposing sympathetic and parasympathetic input, and its activity is thus controlled by relative levels of sympathetic and parasympathetic activity; and (3) sympathetic changes are indicative of psychological arousal, whereas parasym- pathetic changes are indicative 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 excep- tions: 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 traveling to and from the gut. The 12 pairs of cra- nial 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. terms afferent and efferent if you remember that many words The functions of the various cranial nerves are com- that involve the idea of going toward something—in this monly assessed by neurologists as a basis for diagnosis. case, going toward the CNS—begin with an a (e.g., advance, Because the functions and locations of the cranial nerves approach, arrive) and that many words that involve the idea are specific, disruptions of particular cranial nerve func- of going away from something begin with an e (e.g., ­exit, tions provide excellent clues about the location and extent embark, escape). of tumors and other kinds of brain pathology. The autonomic nervous system has two kinds of Figure 3.2 summarizes the major divisions of the ner- efferent nerves: sympathetic nerves and parasympathetic vous system. Notice that the nervous system is a “system nerves. The sympathetic nerves are autonomic motor of twos.” nerves that project from the CNS in the lumbar (small of the back) and thoracic (chest area) regions of the spinal cord. The Meninges ­parasympathetic nerves are those autonomic motor nerves LO 3.2 Describe the three meninges and explain their that project from the brain and sacral (lower back) region functional role. of the spinal cord. See Appendix I. (Ask your instructor to specify the degree to which you are responsible for material The brain and spinal cord (the CNS) are the most protected in the appendices.) All sympathetic and parasympathetic organs in the body. They are encased in bone and covered nerves are two-stage neural paths: The sympathetic and by three protective membranes, the three meninges (pro- parasympathetic neurons project from the CNS and go only nounced “men-IN-gees”; see Coles et al., 2017). The outer part of the way to the target organs before they synapse on meninx (which, believe it or not, is the singular of meninges) M03_PINE1933_11_GE_C03.indd 74 22/01/2021 10:42  Anatomy of the Nervous System 75 Figure 3.2 The major divisions of the nervous system. Nervous system Central Peripheral nervous nervous system (CNS) system (PNS) Spinal Somatic Autonomic Brain cord nervous nervous system (SNS) system (ANS) Afferent Efferent Afferent Efferent nerves nerves nerves nerves Sympathetic Parasympathetic nervous system nervous system is a tough membrane called the dura mater (tough mother). or small blood vessels that protrude into the ventricles from Immediately inside the dura mater is the fine arachnoid the pia mater), and the excess cerebrospinal fluid is con- membrane (spider-web-like membrane). Beneath the tinuously absorbed from the subarachnoid space into large arachnoid membrane is a space called the subarachnoid blood-filled spaces, or dural sinuses, which run through the space, which contains many large blood vessels and cere- dura mater and drain into the large jugular veins of the brospinal fluid; then comes the innermost meninx, the deli- neck. However, there is growing appreciation that cere- cate pia mater (pious mother), which adheres to the surface brospinal fluid production and absorption are more com- of the CNS. plex than was originally thought (see Brinker et al., 2014). Figure 3.4 illustrates the absorption of cerebrospinal fluid Ventricles and Cerebrospinal Fluid from the subarachnoid space into the large sinus that runs along the top of the brain between the two cerebral LO 3.3 Explain where cerebrospinal fluid is produced hemispheres. and where it flows. Occasionally, the flow of cerebrospinal fluid is Also protecting the CNS is the cerebrospinal fluid (CSF), blocked by a tumor near one of the narrow channels which fills the subarachnoid space, the central canal of the that link the ventricles—for example, near the cerebral spinal cord, and the cerebral ventricles of the brain. The aqueduct, which connects the third and fourth ventricles. central canal is a small central channel that runs the length The resulting buildup of fluid in the ventricles causes of the spinal cord; the cerebral ventricles are the four large the walls of the ventricles, and thus the entire brain, internal chambers of the brain: the two lateral ventricles, the to expand, producing a condition called hydrocephalus third ventricle, and the fourth ventricle (see Figure 3.3). The (water head). Hydrocephalus is treated by draining the subarachnoid space, central canal, and cerebral ventricles excess fluid from the ventricles and trying to remove the are interconnected by a series of openings and thus form a obstruction. single reservoir. The cerebrospinal fluid supports and cushions the brain. Patients who have had some of their cerebrospinal Journal Prompt 3.1 fluid drained away often suffer raging headaches and expe- Hydrocephalus is often congenital (present from birth). rience stabbing pain each time they jerk their heads. What do you think might be some of the long-term According to the traditional view, cerebrospinal fluid is effects of being born with hydrocephalus? produced by the choroid plexuses (networks of capillaries, M03_PINE1933_11_GE_C03.indd 75 22/01/2021 10:42  76 Chapter 3 Figure 3.3 The cerebral ventricles and central canal. Lateral ventricles Third ventricle Third ventricle Cerebral aqueduct Cerebral aqueduct Fourth ventricle Fourth ventricle Lateral ventricles Central canal Figure 3.4 The absorption of cerebrospinal fluid (CSF) from the subarachnoid space (blue) into a major sinus. Note the three meninges. Scalp Skull Dura mater meninx Arachnoid meninx Subarachnoid space Pia mater meninx Cortex Artery Sinus Blood–Brain Barrier of certain kinds of chemicals. Fortunately, a mechanism impedes the passage of many toxic substances from the LO 3.4 Explain what the blood–brain barrier is and blood into the brain: the blood–brain barrier. This barrier what functional role it serves. is a consequence of the special structure of cerebral blood The brain is a finely tuned electrochemical organ whose vessels. In the rest of the body, the cells that compose the function can be severely disturbed by the introduction walls of blood vessels are loosely packed; as a result, most M03_PINE1933_11_GE_C03.indd 76 22/01/2021 10:42  Anatomy of the Nervous System 77 molecules pass readily through them into surrounding tis- The blood–brain barrier does not impede the passage sue. In the brain, however, the cells of the blood vessel walls of all large molecules. Some large molecules that are criti- are tightly packed, thus forming a barrier to the passage cal for normal brain function (e.g., glucose) are actively of many molecules—particularly proteins and other large transported through cerebral blood vessel walls. Also, the molecules (see Chow & Gu, 2015). The degree to which ther- blood vessel walls in some areas of the brain allow certain apeutic or recreational drugs can influence brain activity large molecules to pass through them unimpeded. Many depends on the ease with which they penetrate the blood– CNS disorders are associated with impairment of the brain barrier (see Interlandi, 2013; Siegenthaler, Sohet, & blood–brain barrier (see Bentivoglio & Kristensson, 2014; Daneman, 2013). Jiang et al., 2018). Scan Your Brain This is a good place to pause and scan your brain to check 7. The _______ nerve is a purely sensory nerve that t­ransfers your knowledge of the CNS. Fill in the following blanks with visual information from the retina of the eye to the brain. the most appropriate terms. The correct answers are provided 8. The _______ nerve is the nerve cell that extends directly at the end of the exercise. Before proceeding, review material from the brain to the gut. related to your errors and omissions. 9. The _______ is a channel that connects the third and fourth ventricles in the brain. 1. The somatic nervous system includes _______ nerves that carry motor signals from the central nervous system 10. The ventricles of patients with a congenital condition to the muscles. called _______ build up fluid as a result of blocked channels in the brain. 2. The _______ is the part of the peripheral nervous system that regulates the body’s internal environment. 11. Many toxic substances that are present in the ­ loodstream are prohibited from entering the brain by b 3. The brain and the spinal cord are the only organs that a mechanism called the _______ where cells of blood are protected with three layers of protective membranes vessel walls are tightly packed, forming a barrier to the called _______. passage of large proteins. 4. _______ or “tough mother” is the outer meninx. 12. Unlike large toxic molecules, _______, which is ­critical 5. The _______ nervous system is activated when you for the function of the brain, is actively transported encounter a threatening information such as a bear through the vessel walls. attacking you. This system is essential for the initiation of fight-or-flight responses. (11) blood–brain barrier, (12) glucose. 6. Motor nerves that project from the brain and (8) vagus, (9) cerebral aqueduct, (10) hydrocephalus, the lower region of the spine are called _______ (4) Dura mater, (5) sympathetic, (6) parasympathetic, (7) optic, nerves. Scan Your Brain answers: (1) efferent, (2) ANS, (3) meninges, NEURON CELL MEMBRANE. The neuron cell membrane Cells of the Nervous is composed of a lipid bilayer, or two layers of fat molecules (see Figure 3.7). Embedded in the lipid bilayer are numer- System ous protein molecules that are the basis of many of the cell membrane’s functional properties. Some membrane pro- Most of the cells of the nervous system are of two fun- teins are channel proteins, through which certain molecules damentally different types: neurons and glial cells. Their can pass; others are signal proteins, which transfer a signal ­anatomy is discussed in the following two sections. to the inside of the neuron when particular molecules bind to them on the outside of the membrane. Anatomy of Neurons CLASSES OF NEURONS. Figure 3.8 illustrates a way of LO 3.5 Draw, label, and define the major features of a classifying neurons based on the number of processes (pro- multipolar neuron. jections) emanating from their cell bodies. A neuron with Recall that neurons are cells that are specialized for the more than two processes extending from its cell body is reception, conduction, and transmission of electrochemi- classified as a multipolar neuron; most neurons are mul- cal signals. They come in an incredible variety of shapes tipolar. A neuron with one process extending from its cell and sizes (see Sharpee, 2014; Shen, 2015; Underwood, body is classified as a unipolar neuron, and a neuron with 2015); however, many are similar to the one illustrated in two processes extending from its cell body is classified as a Figures 3.5 and 3.6, which detail the major external and bipolar neuron. Neurons with a short axon or no axon at all internal features of a neuron, respectively. are called interneurons; their function is to integrate neural M03_PINE1933_11_GE_C03.indd 77 22/01/2021 10:42  78 Chapter 3 Figure 3.5 The major external features of a neuron. 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. 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. Axon. Initial segment. Cell body. The metabolic center of the neuron; also called the soma. 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. NEURONS AND NEUROANATOMICAL STRUCTURE. In general, there are two kinds of gross neural structures in the nervous system: those composed primarily of cell bodies activity within a single brain structure, not to conduct sig- and those composed primarily of axons. In the central nals from one structure to another. Classifying neurons is a nervous system, clusters of cell bodies are called nuclei complex task, and neuroscientists still don’t agree on the best (singular nucleus); in the peripheral nervous system, they method of classification (see Cembrowski & Menon, 2018; are called ganglia (singular ganglion). (Note that the word Wichterle, Gifford, & Mazzoni, 2013; Zeng & Sanes, 2017). nucleus has two different neuroanatomical meanings; it M03_PINE1933_11_GE_C03.indd 78 22/01/2021 10:42  Anatomy of the Nervous System 79 Figure 3.6 The major internal features of a neuron. Endoplasmic reticulum. A Nucleus. The spherical Mitochondria. Sites of aerobic system of folded membranes in DNA-containing structure of the (oxygen-consuming) energy the cell body; rough portions cell body. release. (those with ribosomes) play a role in the synthesis of proteins; smooth portions (those without ribosomes) play a role in the synthesis of fats. 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 molecules throughout the neuron. Synaptic vesicles. Membrane packages that store neurotransmitter molecules ready to release near synapses. Neurotransmitters. Molecules that are released from active neurons and influence the activity of other cells. M03_PINE1933_11_GE_C03.indd 79 22/01/2021 10:42  80 Chapter 3 is a structure in the neuron cell Figure 3.7 The cell membrane is a lipid bilayer with signal proteins and channel proteins embedded in it. body and a cluster of cell bodies in the CNS.) In the central ner- vous system, bundles of axons Channel Signal are called tracts; in the peripheral protein protein nervous system, they are called nerves. Glia: The Forgotten Cells LO 3.6 Briefly describe four kinds of glial cells. Neurons are not the only cells in the nervous system; there are about as many glial cells, or glia (pronounced “GLEE-a”). It is commonly said that there are 10 times as many glia as neurons in the human brain, but this is incor- Lipid bilayer rect: There are roughly two glia for every three neurons in your brain (see Nimmerjahn & Bergles, 2015; von Bartheld, 2017). There are several kinds of glia. Oligodendrocytes, for example, are glial cells with extensions that wrap around the axons of some neurons Figure 3.8 A unipolar neuron, a bipolar neuron, a multipolar neuron, and an of the central nervous system. interneuron. These extensions are rich in myelin, a fatty insulating substance, and the myelin sheaths they form increase the speed of axonal conduction. A similar function is performed in the peripheral nervous system by Schwann cells, a second class of glia. Oligodendrocytes and Dendrites Schwann cells are illustrated in Figure 3.9. Notice that each Dendrites Schwann cell constitutes one myelin segment, whereas each oligoden- Cell body drocyte provides several myelin segments, often on more than one axon. Another important difference Cell body between Schwann cells and oligo- dendrocytes is that only Schwann Axon Axon cells can guide axonal regeneration (regrowth) after damage. That is why effective axonal regeneration in the mammalian nervous system Unipolar Bipolar Multipolar Multipolar is restricted to the PNS. Neuron Neuron Neuron Interneuron Microglia make up a third class of glia. Microglia are smaller than M03_PINE1933_11_GE_C03.indd 80 22/01/2021 10:42  Anatomy of the Nervous System 81 Figure 3.9 The myelination of CNS axons by an oligodendrocyte and the myelination of PNS axons by Schwann cells. Myelination in the Central Myelination in the Peripheral Nervous System Nervous System Nucleus Axon Axon Nucleus Oligodendrocyte Schwann cell other glial cells—thus their name. They respond to injury or with nutrition, clear waste, and form a physical matrix disease by multiplying, engulfing cellular debris or even to hold neural circuits together (glia means “glue”). But entire cells (see Brown & Neher, 2014), and triggering this limited view of the role of glial cells has changed, inflammatory responses (see Smith & Dragunow, 2014). thanks to a series of remarkable findings. For example, Astrocytes constitute a fourth class of glia. They are astrocytes, the most studied of the glial cells, have been the largest glial cells, and they are so named because they shown to exchange chemical signals with neurons and are star-shaped (astro means “star”). The extensions of other astrocytes (Araque et al., 2014; Montero & Orel- some astrocytes cover the outer surfaces of blood ves- lana, 2015; Yoon & Lee, 2014), to control the establishment sels that course through the brain; they also make contact and maintenance of synapses between neurons (Baldwin with neurons (see Figure 3.10). These particular astrocytes & Eroglu, 2017), to modulate neural activity (Bouzier- appear to play a role in allowing the passage of some Sore & Pellerin, 2013), to form functional networks with chemicals from the blood into CNS neurons and in block- neurons and other astrocytes (Gittis & Brasier, 2015; Haim & ing other chemicals (see Paixão & Klein, 2010), and they ­Rowitch, 2017; Lee et al., 2014; Perea, Sur, & Araque, 2014), have the ability to contract or relax blood vessels based to control the blood–brain barrier (Alvarez, Katayama, & on the blood flow demands of particular brain regions Prat, 2013; Cabezas et al., 2014), to respond to brain injury (see Howarth, 2014; Mishra et al., 2016; Muoio, Persson, (Khakh & Sofroniew, 2015), and to play a role in certain & Sendeski, 2014). forms of cognition (e.g., Dallérac & Rouach, 2016; Martin- For decades, it was assumed that the function of glia Fernandez et al., 2017). Microglia have also been shown to was mainly to provide support for neurons—provide them play many more roles in brain function than had previously M03_PINE1933_11_GE_C03.indd 81 22/01/2021 10:42  82 Chapter 3 Figure 3.10 Astrocytes (shown in pink) have an affinity Neuroanatomical Techniques for blood vessels (in red) and they also make contact with neurons (in blue). LO 3.7 Compare several neuroanatomical research techniques. The major problem in visualizing neurons is not that they are minute. The major problem is that neurons are so tightly packed and their axons and dendrites so intricately inter- twined that looking through a microscope at unprepared neural tissue reveals almost nothing about them. The key to the study of neuroanatomy lies in preparing neural tissue in a variety of ways, each of which permits a clear view of a different aspect of neuronal structure, and then combining the knowledge obtained from each of the preparations. This point is illustrated by the following widely used neuroana- tomical techniques. GOLGI STAIN. The greatest blessing to befall neurosci- ence in its early years was the accidental discovery of the GUNILLA ELAM/Science Source Golgi stain by Camillo Golgi (pronounced “GOLE-jee”), an Italian physician, in the early 1870s. Golgi was trying been thought (see Pósfai et al., 2018); for example, they to stain the meninges, by exposing a block of neural tis- have been shown to play a role in the regulation of cell sue to potassium dichromate and silver nitrate, when he death (Wake et al., 2013), synapse formation (Parkhurst noticed an amazing thing. For some unknown reason, the et al., 2013; Welberg, 2014), and synapse elimination (Wake silver chromate created by the chemical reaction of the two et al., 2012). substances Golgi was using invaded a few neurons in each Research on the function of glia, although still in its slice of tissue and stained each invaded neuron entirely early stages, is creating considerable excitement. There black. This discovery made it possible to see individual is now substantial evidence that the physiological effects neurons for the first time, although only in silhouette (see of glia are both numerous and much more important Figure 3.11). Golgi stains are commonly used to discover the than anyone might have imagined two decades ago. For overall shape of neurons. example, some researchers have suggested that glial net- works may be the dwelling places of thoughts (see Verkh- NISSL STAIN. Although the Golgi stain permits an ratsky, Parpura, & Rodríguez, 2010). One final important excellent view of the silhouettes of the few neurons that discovery about glial cells is that they are much more take up the stain, it provides no indication of the number varied than implied by the four types that we have just of neurons in an area. The first neural staining procedure described: oligodendrocytes, Schwann cells, microglia, to overcome this shortcoming was the Nissl stain, which and astrocytes. For example, a new type of glial cell was was developed by Franz Nissl, a German psychiatrist, recently discovered (see Fan & Agid, 2018); and at least in the 1880s. The most common dye used in the Nissl fifteen different kinds of astrocytes have been identified, method is cresyl violet. Cresyl violet and other Nissl dyes each with its own structure, physiology, and specific loca- penetrate all cells on a slide, but they bind to molecules tions in the brain (Chai et al., 2017; Clarke & Liddelow, (i.e., DNA and RNA) that are most prevalent in neuron 2017; Lin et al., 2017). Sorting out the functions of each cell bodies. Thus, they often are used to estimate the type is not going to be easy. number of cell bodies in an area, by counting the num- ber of Nissl-stained dots. Figure 3.12 is a photograph of a slice of brain tissue stained with cresyl violet. Notice that only the layers composed mainly of neuron cell bodies are densely stained. Neuroanatomical Techniques and Directions ELECTRON MICROSCOPY. A neuroanatomical technique that provides information about the details of This module first describes a few of the most common neu- neuronal structure is electron microscopy (pronounced roanatomical techniques. Then, it explains the system of “my-CROSS-cuh-pee”). Because of the nature of light, directions that neuroanatomists use to describe the location the limit of magnification in light microscopy is about of structures in vertebrate nervous systems. 1,500 times, a level of magnification insufficient to reveal M03_PINE1933_11_GE_C03.indd 82 22/01/2021 10:42  Anatomy of the Nervous System 83 Figure 3.11 Neural tissue that has been stained by the Figure 3.12 The Nissl stain. Presented here is a Golgi method. Because only a few neurons take up the stain, ­ issl-stained section through the rat hippocampus, at two N their silhouettes are revealed in great detail, but their internal ­levels of magnification to illustrate two uses of Nissl stains. details are invisible. Under low magnification (top panel), Nissl stains provide a gross indication of brain structure by selectively staining groups of neural cell bodies. Under higher magnification (­bottom panel), one can distinguish individual neural cell ­bodies, and thus, count the number of neurons in various areas. Martin M. Rotker/Science Source the fine anatomical details of neurons. Greater detail can be obtained by first coating thin slices of neural tissue with an electron-­absorbing substance that is taken up by different parts of neurons to different degrees, then passing a beam of electrons through the tissue onto a photographic film. The result is an electron micrograph, which captures neuronal structure in exquisite detail. A scanning electron microscope provides spectacular electron micrographs in three dimensions (see Figure 3.13), but it is not capable of as much magnification as conventional electron microscopy. The strength of electron microscopy is also a weakness: Because the images are so detailed, they can make it difficult to visualize general aspects of neuroanatomical structure. Carl Ernst/Brian Christie/University of British Columbia Department of Psychology NEUROANATOMICAL TRACING TECHNIQUES. Neuro­ anatomical tracing techniques are of two types: anterograde (forward) tracing methods and retrograde (backward) trac- Figure 3.13 A color-enhanced scanning electron m ­ icrograph ing methods (see Figure 3.14). Anterograde tracing methods of a neuron cell body (green) studded with terminal buttons are used when an investigator wants to trace the paths of (orange). Each neuron receives numerous synaptic contacts. axons projecting away from cell bodies located in a particu- lar area. The investigator begins by injecting one of several chemicals commonly used for anterograde tracing into the cell body. It is then taken up by cell bodies and transported forward along their axons to their terminal buttons. Then, after a few days, the investigator removes the brain and slices it. Those slices are then treated to reveal the locations of the injected chemical. Retrograde tracing methods work in the reverse manner; they are used when an investigator wants to trace the paths of axons projecting into a particular area. The investigator begins by injecting one of several chemicals commonly used for retrograde-­tracing into an area of the brain. These chem- icals are taken up by terminal buttons and then transported Photo Researchers/Science History Images/Alamy Stock Photo M03_PINE1933_11_GE_C03.indd 83 22/01/2021 10:42  84 Chapter 3 Figure 3.14 One example of anterograde tracing (A) and one example of retrograde tracing (B). A. Anterograde Tracing B. Retrograde Tracing backward along their axons to their Figure 3.15 (a) Anatomical directions in representative vertebrates, my (JP) cell bodies. After a few days, the cats Sambala and Rastaman. (b) Anatomical directions in a human. Notice that the investigator removes the brain ­directions in the cerebral hemispheres are rotated by 90° in comparison to those in and slices it. Those slices are then the spinal cord and brain stem because of the unusual upright posture of humans. treated to reveal the locations of the injected chemical. Directions in the DORSAL Vertebrate Nervous MEDIAL System LATERAL POSTERIOR ANTERIOR LO 3.8 Illustrate the neuroanatomical directions. It would be difficult for you to VENTRAL develop an understanding of the layout of an unfamiliar city with- (a) out a system of directional coordi- nates: north–south, east–west. The DORSAL same goes for the nervous system. Thus, before introducing you to the locations of major nervous sys- ANTERIOR POSTERIOR tem structures, we will describe the three-dimensional system of VENTRAL directional coordinates used by MEDIAL neuroanatomists. LATERAL Directions in the vertebrate nervous system are described in ANTERIOR relation to the orientation of the spinal cord. This system is straight- forward for most vertebrates, as VENTRAL DORSAL Figure 3.15a indicates. The ver- tebrate nervous system has three POSTERIOR axes: anterior–posterior, dorsal– ventral, and medial–lateral. First, (b) M03_PINE1933_11_GE_C03.indd 84 22/01/2021 10:42  Anatomy of the Nervous System 85 anterior means toward the nose end (the anterior end), Figure 3.16 Horizontal, frontal (coronal), and sagittal and posterior means toward the tail end (the posterior planes in the human brain and a cross section of the human end); these same directions are sometimes referred to spinal cord. as rostral and caudal, respectively. Second, dorsal means toward the surface of the back or the top of the head (the dorsal surface), and ventral means toward the surface of Sagittal plane the chest or the bottom of the head (the ventral surface). Third, medial means toward the midline of the body, and lateral means away from the midline toward the body’s lateral surfaces. Horizontal plane Humans complicate this simple three-axis (anterior– posterior, ventral–dorsal, medial–lateral) system of neu- roanatomical directions by insisting on walking around on our hind legs. This changes the orientation of our cerebral hemispheres in relation to our spines and brain stems. Frontal You can save yourself a lot of confusion if you remem- plane Cross ber that the system of vertebrate neuroanatomical direc- section tions was adapted for use in humans in such a way that the terms used to describe the positions of various body surfaces are the same in humans as they are in more typical, nervous system, proximal means closer to the CNS, and non-upright vertebrates. Specifically, notice that the top of distal means farther from the CNS. Your shoulders are the human head and the back of the human body are both proximal to your elbows, and your elbows are proximal referred to as dorsal even though they are in different direc- to your fingers. tions, and the bottom of the human head and the front of In the next module, you will see drawings of sections the human body are both referred to as ventral even though (slices) of the brain cut in one of three different planes: they are in different directions (see Figure 3.15b). To circum- horizontal sections, frontal sections (also termed coronal vent this complication, the terms superior and inferior are sections), and sagittal sections. These three planes are illus- often used to refer to the top and bottom of the primate trated in Figure 3.16. A section cut down the center of the head, respectively. brain, between the two hemispheres, is called a midsagittal Proximal and distal are two other common directional section. A section cut at a right angle to any long, narrow terms. In general, proximal means “close,” and distal structure, such as the spinal cord or a nerve, is called a means “far.” Specifically, with regard to the peripheral cross section. Scan Your Brain This is a good place for you to pause to scan your brain. Are you ready to proceed to the structures of the brain and spinal cord? Test your grasp of the preceding modules of this chapter by drawing a line between each term in the left column and the appropri- ate word or phrase in the right column. The correct answers are provided at the end of the exercise. Before proceeding, review material related to your errors and omissions. 1. myelin 8. synaptic vesicles a. gaps h. protein synthesis 2. soma 9. astrocytes b. cone-shaped region i. the forgotten cells 3. axon hillock 10. ganglia c. packaging membranes j. CNS myelinators 4. Golgi complex 11. oligodendrocytes d. fatty substance k. black 5. ribosomes 12. Golgi stain e. neurotransmitter storage l. largest glial cells 6. synapses 13. dorsal f. cell body m. caudal 7. glial cells 14. posterior g. PNS clusters of cell bodies n. top of head Scan Your Brain answers: (1) d, (2) f, (3) b, (4) c, (5) h, (6) a, (7) i, (8) e, (9) l, (10) g, (11) j, (12) k, (13) n, (14) m. M03_PINE1933_11_GE_C03.indd 85 22/01/2021 10:42  86 Chapter 3 many of their synaptic terminals are in the dorsal horns Anatomy of the Central of the spinal gray matter. In contrast, the neurons of the ventral root are motor (efferent) multipolar neurons with Nervous System their cell bodies in the ventral horns. Those that are part of the somatic nervous system project to skeletal muscles; In the first three modules of this chapter, you learned about those that are part of the autonomic nervous system proj- the divisions of the nervous system, the cells that compose ect to ganglia, where they synapse on neurons that in it, and some of the neuroanatomical techniques used to turn project to internal organs (heart, stomach, liver, etc.). study it. This final module focuses exclusively on the anat- See Appendix I. omy of the CNS. Your ascent through the CNS will begin with a focus on the spinal cord, and then you will move up to the brain. Five Major Divisions of the Brain LO 3.10 List and discuss the five major divisions of the human brain. Spinal Cord A necessary step in learning to live in an unfamiliar city LO 3.9 Draw and label a cross section of the spinal cord. is learning the names and locations of its major neighbor- In cross section, it is apparent that the spinal cord com- hoods or districts. Those who possess this information can prises two different areas (see Figure 3.17): an inner easily communicate the general location of any destination H-shaped core of gray matter and a surrounding area in the city. This section of the chapter introduces you to the of white matter. Gray matter is composed largely of cell five “neighborhoods,” or divisions, of the brain—for much bodies and unmyelinated interneurons, whereas white the same reason. matter is composed largely of myelinated axons. (It is the To understand why the brain is considered to be myelin that gives the white matter its glossy white sheen.) composed of five divisions, it is necessary to understand The two dorsal arms of the spinal gray matter are called its early development. In the vertebrate embryo, the tis- the dorsal horns, and the two ventral arms are called the sue that eventually develops into the CNS is recognizable ­ventral horns. as a fluid-filled tube (see Figure 3.18). The first indica- Pairs of spinal nerves are attached to the spinal cord— tions of the developing brain are three swellings that one on the left and one on the right—at 31 different levels occur at the anterior end of this tube. These three swell- of the spine. Each of these 62 spinal nerves divides as it ings eventually develop into the adult forebrain, midbrain, nears the cord (see Figure 3.17), and its axons are joined and hindbrain. to the cord via one of two roots: the dorsal root or the Before birth, the initial three swellings in the neural ­ventral root. tube become five (see Figure 3.18). This occurs because All dorsal root axons, whether somatic or autonomic, the forebrain swelling grows into two different swell- are sensory (afferent) unipolar neurons with their cell ings, and so does the hindbrain swelling. From anterior bodies grouped together just outside the cord to form to posterior, the five swellings that compose the develop- the dorsal root ganglia (see Figure 3.17). As you can see, ing brain at birth are the telencephalon, the diencephalon, the mesencephalon (or midbrain), the metencephalon, and the myelencephalon (encephalon means “within the head”). These swellings ultimately develop into the five divisions Figure 3.17 A schematic cross section of the spinal cord, of the adult brain. As students, we memorized their order and the dorsal and ventral roots. by remembering that the telencephalon is on the top and Gray matter the other four divisions are arrayed below it in alphabeti- Dorsal horn White matter cal order. Dorsal Figure 3.19 illustrates the locations of the telencepha- Dorsal root Unipolar sensory lon, diencephalon, mesencephalon, metencephalon, and Dorsal root neuron myelencephalon in the adult human brain. Notice that in ganglion humans, as in other higher vertebrates, the telencepha- lon (the left and right cerebral hemispheres) undergoes the greatest growth during development. The other four divisions of the brain are often referred to collectively Multipolar as the brain stem—the stem on which the cerebral hemi- Ventral root motor Ventral spheres sit. The myelencephalon is often referred to as Spinal Ventral neuron horn nerve the medulla. M03_PINE1933_11_GE_C03.indd 86 22/01/2021 10:42  Anatomy of the Nervous System 87 Figure 3.18 The early development of the mammalian brain illustrated in schematic horizontal sections. Compare with the adult human brain in Figure 3.19. Telencephalon (cerebral hemispheres) Forebrain Diencephalon Midbrain Mesencephalon (midbrain) Metencephalon Hindbrain Myelencephalon (medulla) Spinal cord Spinal cord 100 tiny nuclei that occupies the central core of the brain Figure 3.19 The five divisions of the adult human brain. stem from the posterior boundary of the myelencephalon Forebrain to the anterior boundary of the midbrain. It is so named Telencephalon because of its netlike appearance (reticulum means “little Diencephalon net”). Sometimes, the reticular formation is referred to as the reticular activating system because parts of it seem to play a role in arousal. However, the various nuclei Midbrain Mesencephalon of the reticular formation are involved in a variety of functions—including sleep, attention, movement, the maintenance of muscle tone, and various cardiac, circulatory, and respiratory reflexes. Accordingly, Hindbrain referring to this collection of nuclei as a s­ ystem can be Metencephalon misleading. Myelencephalon Metencephalon LO 3.12 List and describe the components of the Now that you have learned the five major divisions of metencephalon. the brain, it is time to introduce you to their major struc- The metencephalon, like the myelencephalon, houses tures. We begin our survey of brain structures in the myel- many ascending and descending tracts and part of the encephalon, then ascend through the other divisions to the reticular formation. These structures create a bulge, called telencephalon. the pons, on the brain stem’s ventral surface. The pons is one major division of the metencephalon; the other is the Myelencephalon cerebellum (little brain)—see Figure 3.21. The c­ erebellum is the large, convoluted structure on the brain stem’s LO 3.11 List and describe the components of the dorsal surface. It is an important sensorimotor structure; myelencephalon. cerebellar damage eliminates the ability to precisely Not surprisingly, the myelencephalon (or medulla), the control one’s movements and to adapt them to changing most posterior division of the brain, is composed largely conditions. However, the fact that cerebellar damage also of tracts carrying signals between the rest of the brain and produces a variety of cognitive deficits (e.g., deficits in the body. An interesting part of the myelencephalon from decision making and in the use of language) suggests a psychological perspective is the reticular formation that the functions of the cerebellum are not restricted to (see Figure 3.20). It is a complex network of about sensorimotor control. M03_PINE1933_11_GE_C03.indd 87 22/01/2021 10:42  88 Chapter 3 Figure 3.20 Structures of the human myelencephalon Figure 3.21 The human mesencephalon (midbrain). (medulla) and metencephalon. Superior colliculus Superior colliculus Periaqueductal Inferior gray colliculus Dorsal Mesencephalic Tectum reticular formation Cerebral aqueduct Tegmentum Red nucleus Pons Substantia Ventral nigra Cerebellum Reticular Medulla formation colorful structures of particular interest to biopsycholo- gists: the periaqueductal gray, the substantia nigra, and the red nucleus (see Figure 3.21). The periaqueductal gray is the gray matter situated around the cerebral Mesencephalon aqueduct, the duct connecting the third and fourth LO 3.13 List and describe the components of the ventricles; it is of special interest because of its role in mesencephalon. mediating the analgesic (pain-reducing) effects of opioid drugs. The substantia nigra (black substance) and the The mesencephalon, like the metencephalon, has two red nucleus are both important components of the sen- divisions. The two divisions of the mesencephalon are the sorimotor system. tectum and the tegmentum (see Figure 3.21). The tectum (roof) is the dorsal surface of the midbrain. In mammals, the tectum is composed of two pairs of bumps, the colliculi Diencephalon (little hills). The posterior pair, called the inferior colliculi, have an auditory function. The anterior pair, called the LO 3.14 List and describe the components of the superior colliculi, have a visual-motor function; more spe- diencephalon. cifically, to direct the body’s orientation toward or away The diencephalon is composed of two structures: the thala- from particular visual stimuli (see Gandhi & Katnani, mus and the hypothalamus (see Figure 3.22). The thalamus 2011). In lower vertebrates, the function of the tectum is is the large, two-lobed structure that constitutes the top of entirely visual-motor, and it is sometimes referred to as the brain stem. One lobe sits on each side of the third ven- the optic tectum. tricle, and the two lobes are joined by the massa intermedia, The tegmentum is the division of the mesencephalon which runs through the ventricle. Visible on the surface of ventral to the tectum. In addition to the reticular forma- the thalamus are white lamina (layers) that are composed of tion and tracts of passage, the tegmentum contains three myelinated axons. M03_PINE1933_11_GE_C03.indd 88 22/01/2021 10:42  Anatomy of the Nervous System 89 motivated behaviors (e.g., eating, sleep, and sexual Figure 3.22 The human diencephalon. behavior). It exerts its effects in part by regulating the Right release of hormones from the pituitary gland, which Left thalamus dangles from it on the ventral surface of the brain. The Bands of thalamus myelinated literal meaning of pituitary gland is “snot gland”; it was axons first discovered in a gelatinous state behind the nose of a cadaver and was incorrectly assumed to be the main source of nasal mucus. In addition to the pituitary gland, two other struc- tures appear on the inferior surface of the hypothalamus: the optic chiasm and the mammillary bodies (see Figure 3.23). The optic chiasm is the point at which the optic nerves from each eye come together and then decussate (cross over to the other side of the brain) (see Chapter 6). The decussating fibers are said to be contralateral (pro- jecting from one side of the body to the other), and the nondecussating fibers are said to be ipsilateral (staying on the same side of the body). The mammillary bodies, Figure 3.23 The human hypothalamus (in color) in ­relation to the optic chiasm and the pituitary gland. Hypothalamus The thalamus comprises many different pairs of nuclei, most of which project to the cortex. The general organiza- tion of the thalamus is illustrated in Appendix V. The most well-understood thalamic nuclei are the ­sensory relay nuclei—nuclei that receive signals from sensory receptors, process them, and then transmit them to the appropriate areas of sensory cortex. For example, the lateral geniculate nuclei, the medial geniculate nuclei, and the ventral posterior nuclei are important relay stations in the visual, auditory, and somatosensory systems, respectively. Sensory relay nuclei are not one- way streets; they all receive feedback signals from the very areas of cortex to which they project (Zembrzycki Mammillary Optic body et al., 2013). Although less is known about the other tha- chiasm lamic nuclei, the majority of them receive input from areas of the cortex and project to other areas of the cortex (see Sherman, 2007). The hypothalamus is located just below the ante- Pituitary rior thalamus (hypo means “below”)—see Figure 3.23. gland It plays an important role in the regulation of several M03_PINE1933_11_GE_C03.indd 89 22/01/2021 10:42  90 Chapter 3 which are often considered to be part of the hypothala- Figure 3.24 The major fissures of the human cerebral mus, are a pair of spherical nuclei located on the inferior cortex. surface of the hypothalamus, just behind the pituitary. Lateral Longitudinal Corpus The mammillary bodies and the other nuclei of the hypo- ventricle fissure callosum thalamus are illustrated in Appendix VI. Central fissure Third Telencephalon ventricle LO 3.15 List and describe the components of the telencephalon. The telencephalon, the largest division of the human brain, Lateral mediates the brain’s most complex functions. It initiates vol- fissure untary movement, interprets sensory input, and mediates complex cognitive processes such as learning, speaking, and problem solving. CEREBRAL CORTEX. The cerebral hemispheres are covered by a layer of tissue called the cerebral cortex Hippocampus (cerebral bark). Because the cerebral cortex is mainly com- posed of small, unmyelinated neurons, it is gray and is often referred to as the gray matter. In contrast, the layer Central beneath the cortex is mainly composed of large myelin- fissure ated axons, which are white and often referred to as the Lateral white matter. fissure In humans, the cerebral cortex is deeply convoluted (furrowed)—see Figure 3.24. The convolutions have the effect of increasing the amount of cerebral cortex without increasing the overall volume of the brain. Not all mammals have convoluted cortexes; most mammals are lissencephalic (smooth-brained). It was once believed that the number and size of corti- cal convolutions determined a species’ intellectual capaci- ties; however, the number and size of cortical convolutions appear to be related more to body size. Every large mammal has an extremely convoluted cortex. hemisphere-­connecting tracts are called cerebral commis- sures. The largest cerebral commissure, the corpus callo- sum, is clearly visible in Figure 3.24. As Figures 3.24 and 3.25 indicate, the two major land- Journal Prompt 3.2 marks on the lateral surface of each hemisphere are the Why do you think only large mammals have extremely ­central fissure and the lateral fissure. These fissures par- convoluted cortices? tially divide each hemisphere into four lobes: the frontal lobe, the parietal lobe (pronounced “pa-RYE-e-tal”), the temporal lobe, and the occipital lobe (pronounced “ok- SIP-i-tal”). Among the largest gyri are the precentral gyri, The large furrows in a convoluted cortex are called the postcentral gyri, and the superior temporal gyri in the f­ issures, and the small ones are called sulci (singular frontal, parietal, and temporal lobes, respectively. sulcus). The ridges between fissures and sulci are called It is important to understand that the cerebral lobes gyri (singular gyrus). It is apparent in Figure 3.24 that are not functional units. It is best to think of the cere- the cerebral hemispheres are almost completely sepa- bral cortex as a flat sheet of cells that just happens to be rated by the largest of the fissures: the longitudinal divided into lobes because it folds in on itself at certain ­fissure. The cerebral hemispheres are directly connected places during development. Thus, it is incorrect to think by a few tracts spanning the longitudinal fissure; these that a lobe is a functional unit, having one set of functions. M03_PINE1933_11_GE_C03.indd 90 22/01/2021 10:42  Anatomy of the Nervous System 91 lobe: The postcentral gyrus analyzes sensations from the Figure 3.25 The lobes of the cerebral hemisphere. body (e.g., touch), whereas the remaining areas of cortex Longitudinal in the posterior parts of the parietal lobes play roles in per- fissure ceiving the location of both objects and our own bodies and in directing our attention. The cortex of each tempo- ral lobe has three general functional areas: The superior temporal gyrus is involved in hearing and language, the inferior temporal cortex identifies complex visual patterns, and the medial portion of temporal cortex (which is not visible from the usual side view) is important for certain kinds of memory. Lastly, each frontal lobe has two distinct functional areas: The precentral gyrus and adjacent frontal cortex have a motor function, whereas the frontal cortex