Neuroanatomy PDF - Chapter 2

Neuroanatomy O U T L I N E Module 1: Cells of the Nervous System Neurons and Glia: Structure...

Neuroanatomy O U T L I N E Module 1: Cells of the Nervous System Neurons and Glia: Structure and Function Communication within the Neuron: The Action Potential Communication between Neurons: The Synapse Neurotransmitters Module 2: The Nervous System Positional Terms Divisions of the Nervous System The Spinal Cord Divisions of the Brain Connections between the Two Halves of the Brain Cranial Nerves Blood Supply Protection If the human brain were so simple that we could understand it, we would be so simple that we couldn’t. —EMERSON M. PUGH From Chapter 2 of Neuropsychology: Clinical and Experimental Foundations, First Edition. Lorin J. Elias, Deborah M. Saucier. Copyright © 2006 by Pearson Education, Inc. All rights reserved. 27  Neuroanatomy MODULE 1 Cells of the Nervous System We must start somewhere, so let’s start small. Every living thing is made up of cells. What makes the human a higher-functioning organism is the fact that humans have aggregates of specialized cells that perform specialized functions. Although the brain is composed of many parts, all of which have multiple functions, the larger compo- nents of the brain are made up of individual cells, which are the focus of this unit. Neurons and glia are the specialized cells of the nervous system, and they are special- ized in both structure and function. As you will see in this unit, glia provide support functions, and neurons are the communicators. Neurons react and respond to stim- uli, and they are the basis of behavior. Neurons also learn and store information about their external environment. Before we can investigate higher functions, it is impor- tant to consider what a neuron is and what a neuron does to achieve all of this. We are going to begin this module with a discussion of the types of WHERE WE cells (neurons and glia) that make up the nervous system. We will ARE GOING discuss the means by which information is transmitted within the neuron and how neurons communicate with each other. The second module will focus on the major divisions of the nervous system and the structures and systems within these di- visions. Finally, we will discuss how the brain is protected from damage. Neurons and Glia: Structure and Function GROSS ANATOMY OF THE NEURON. Although there are many types of neurons, most are similar to the one depicted in Figure 1. Perhaps the most distinctive structural fea- ture of a neuron is its shape. As you will see, the neuron’s shape is closely related to its function: to receive, conduct, and transmit signals—to collect information and send it on (or not). The neuron consists of three main components: (1) the dendrites, which receive incoming information from other neurons; (2) the soma, or cell body, which contains the genetic machinery and most of the metabolic machinery needed for com- mon cellular functions; and (3) the axon, which sends neural information to other neurons. Information is passed from the axon to the dendrite across a gap (about 20–50 nanometers wide), which is called a synapse. On the basis of their positions relative to the synapse, you will often see events that occur in the axon referred to as presynaptic and events that happen in the dendrite referred to as postsynaptic. Dendrites essentially increase the surface area available for the reception of signals from the axons of other neurons. The extent of branching of the dendrites gives an in- dication of the number of connections or synapses it makes with incoming axons. In some cases, dendrites from one neuron can receive as many as 100,000 inputs. All of this information is sent to the rest of the neuron in the form of an electrical charge, or action potential. Dendrites are often covered with tiny spines, which grow and retract in response to experience. The spines themselves can form synapses with other neurons. The axon is commonly thought of as an information sender. The neuron has only one axon, although an axon can divide at its far end into many branches (thereby 28  Major Features of the Neuron Figure 1 Cell body. The metabolic center of the neuron; also called the soma. 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. Cell membrane. The semipermeable membrane that encloses the neuron. Myelin. The fatty insulation around many axons. Nodes of Ranvier (pronounced “RAHN-vee-yay”). The gaps between sections of myelin. Buttons. The button-like endings of the axon branches, which release chemicals into synapses. Synapses. The gaps between adjacent neurons across which chemical signals are transmitted. Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. 29  Neuroanatomy increasing the number of synapses it can form). The axon is essentially a long thin fiber or wire that can pass its message along to many different cells simultaneously. Consistent with the wire analogy, many axons in the mammalian nervous system are covered with insulation, called myelin. Myelin helps to speed the rate of information transfer and to ensure that the message gets to the end of the axon. The end of an axon is called the terminal button. Information is sent from the terminal button across the synapse to the dendrite. Information that passes from the axon across the synapse is in the form of a neurochemical message (by substances referred to as neurotrans- mitters), which may be transformed into an electrical message within the dendrite. INTERNAL ANATOMY OF THE NEURON. Like all animal cells, the neuron is covered with a membrane. There is nothing obvious that sets neural cell membranes apart from other animal cell membranes. The plasma membrane consists of a bilayer of contin- uous sheets of phospholipids that separate two fluid (H2O) environments—one in- side the cell (cytoplasm) and the other outside the cell. Within this membrane are proteins and channels that allow the passage of materials into and out of the neuron. Inside the main cell body, small components of the cell, called organelles, form a com- plex environment in which organelles perform the various genetic (nucleus), synthetic (ribosomes, endoplasmic reticulum), and metabolic (mitochondria) processes that keep the neuron functioning. The cell nucleus is probably the most recognizable organelle under the microscope. Functionally, the nucleus packages and controls the genetic information contained in DNA (deoxyribonucleic acid). In two very crucial steps, the nucleus processes the ge- netic information needed to complete a series of events that form a path from the recipe that the genetic information provides to form proteins that the neuron needs. The nu- cleus also contains all of the genetic information needed to code proteins such as those for eye or hair color, as well as those that are thought to underlie complex processes such as linguistic ability. STRUCTURE AND FUNCTION OF NEURONS. Neurons can be classified according to struc- ture and function. The variety of patterns of branching in both axons and dendrites aids in our classification of neurons into different functional and structural classes. In the nervous system, structure and function are often related. Structurally, some com- mon neurons are labelled as unipolar, bipolar, and multipolar (the most common). Unipolar neurons have only one process emanating from the cell body; bipolar neurons have two processes; and multipolar have numerous processes extending from the cell body. Neurons with no axons or only very short axons are called inter- neurons, and they tend to integrate information within a structure rather than send- ing information between structures. Functionally, neurons can be classified by the type of signals that they process. For instance, the signals that motor neurons convey may represent muscle contrac- tion. Sensory neurons process information elicited from sensory-type stimuli, whereas interneurons make connections between cells, enabling a sort of convergence and com- bination of behavioral responses. Thus, the type of information that is represented by neural activity relates to the function of the neuron. Neurons can also be classed 30  Neuroanatomy as being afferent (bringing information to the central nervous system or structure) or efferent (sending information from the brain or away from a structure). Nonetheless, it is important that you appreciate that neurons do vary in size, shape, and function and that a neuron can change shape as a result of experience. GLIA. As was mentioned in the introduction to this module, neurons are not the only type of cell in the nervous system; there are also glia, which perform an essential role in the functioning of the central nervous system. Generally, glia perform support func- tions, different types of glia providing different types of support. (Support cells out- side of the brain and spinal cord are called satellite cells.) There are at least three different types of glia: astrocytes, oligodendrocytes, and microglia. Astrocytes are the largest glia and are named astrocytes because they tend to be star-shaped. Astrocytes fill the space between neurons, resulting in close contact be- tween neurons and astrocytes. (There is often as little as 20 nanometers between neu- rons and astrocytes.) It is thought that this close contact between astrocytes and neurons can affect the growth of neurons (Sheppard, 1994). As you will see in the next module, astrocytes are involved in the blood–brain barrier, a protective system that keeps the brain separate from the rest of the body. Astrocytes also perform nu- tritive and metabolic functions for neurons. Astrocytes are also essential for the reg- ulation of the chemical content of the extracellular space; that is, because they envelop the synapse, they can regulate how far neurotransmitters and other substances released by the terminal button can spread. Similarly, astrocytes are important in the storage of neurotransmitters. It is clear that we do not know all of the functions of the astro- cyte, as recent evidence suggests that astrocytes may even play a role in the transmis- sion of information in the nervous system. Oligodendrocytes, however, have one very clear function: to make myelin (Peters, Palay, & Webster, 1991). Oligodendrocytes wrap their processes around most axons in the brain and spinal cord (Figure 2). These processes are made of myelin, which is a fatty substance that acts to insulate the axon. Axons outside of the brain and spinal cord are also frequently myelinated, with the myelin provided by Schwann cells. Beyond their location in the nervous system, one major difference between Schwann cells and oligodendrocytes is that Schwann cells provide only one segment of myelin to an axon, whereas oligodendrocytes can contribute many segments to many axons. Microglia are named with reference to their size—they are the smallest of the glia. Microglia are phagocytes that remove debris from the nervous system. Debris can ac- cumulate in the brain as a result of injury, disease, infection, or aging. Microglia are very different from the other cells of the nervous system: They are made outside of the brain and spinal cord by macrophages. Excessive activation of microglia has been implicated in neurodegenerative diseases such as multiple sclerosis and Alzheimer’s disease. We have discussed the structure and functions of neurons and glia. WHERE WE Neurons are the communicators of the nervous system, whereas glia HAVE BEEN tend to perform support functions. Neurons and glia can be catego- rized by structure or by function. 31  Neuroanatomy Myelination of Peripheral and Central Nervous System Axons Figure 2 Myelination in the central Myelination in the peripheral nervous system nervous system Nucleus Axon Axon Nucleus Oligodendrocyte Schwann cell Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. We will now discuss the means by which information is transmitted WHERE WE within the neuron and how neurons communicate with each other. ARE GOING The second module will focus on the major divisions of the nervous system and the structures and systems within these divisions. Finally, we will discuss how the brain is protected from damage. Communication within the Neuron: The Action Potential When a neurotransmitter diffuses across the synapse to interact with the postsynap- tic site, a series of electrical events can occur, some of which act to send information to other neurons and some of which inhibit sending information to other neurons. 32  Neuroanatomy The electrical events that underlie the transmission (or inhibition) of information rely on the balance of ions between the inside of the neuron (intracellular) and the out- side of the neuron (extracellular). When the neuron is at rest, it maintains an electri- cal charge of about –70 millivolts (mV), which means that the electrical charge on the inside of the neuron is 70 mV less than the charge on the outside. This initial state of the neuron is called the resting potential. The resting potential of the neuron depends on the difference between the con- centrations of ions across the neuron membrane. Neurons contain a variety of ions, although the ones that are important for understanding the electrical properties of the neuron are sodium ions (Na+) and potassium ions (K+). At rest, the extracellular fluid contains high concentrations of Na+, and the intracellular fluid contains high concentrations of K+. In simple solutions, ions are distributed homogeneously, that is, they are found in equal amounts throughout the solution. However, in the brain, ions are concentrated in either the extracellular or intracellular fluid. The neuron has two properties that promote the uneven distribution of ions. The first property relates to the permeability of the cell membrane that covers the neuron. The membrane is not permeable to all types of ions. Ions cross the membrane through proteins embedded in the membrane, which are known as ion channels. At rest, K+ read- ily crosses the membrane, whereas Na+ cannot easily enter the neuron. However, given enough time, enough Na+ would sneak into the cell and enough K+ would leak out of the neuron that there would be homogeneous distribution of the ions. Thus, the sec- ond property of the neuron that promotes uneven distribution of ions is the neuron active transport of ions by the neuron. Neurons actively import K+ and actively ex- port Na+ through a transport mechanism known as a sodium–potassium pump. The sodium–potassium pump requires the neuron to use energy, thereby ensuring that the uneven distribution of ions is maintained. The sodium–potassium pump exchanges three Na+ ions inside the cell for two K+ ions that are outside the cell. When a neurotransmitter diffuses across the synapse, it can open ion channels that allow the rapid influx (inflow) of Na+ into the neuron and the rapid efflux (out- flow) of K+ from the neuron. The opening of the sodium channels allows Na+ to rap- idly enter the neuron, which makes the intracellular space more positive. When the change in the membrane potential moves from its resting state of about –70 mV to about +50 mV (this change in the membrane potential is called depolarization), an action potential occurs. When an action potential occurs, neurotransmitters are re- leased from the terminal buttons. Thus, although action potentials occur entirely within one neuron, they result in neurotransmitter release that results in communi- cation between neurons. However, Na+ entering the neuron is not the entire story. As the neuron becomes depolarized, K+ channels open, and K+ ions rapidly leave the neuron. The efflux of K+ triggers the closing of the sodium channels, and eventually, the neuron returns to its resting state of –70 mV, also called repolarization (Figure 3). Because the K+ chan- nels take longer than necessary to close, some additional K+ leaks out, which results in a temporary change in the membrane beyond –70 mV (called hyperpolarization). There are several features of an action potential that must be considered. The first is that there are times when an action potential cannot be triggered. For instance, when 33  Neuroanatomy Phases of the Action Potential Figure 3 +60 Sodium +50 channels Rising phase close +30 Membrane potential Repolarization Potassium (millivolts) +10 channels Hyperpolarization –10 open –30 Sodium Potassium channels channels –50 open start to close –70 1 2 3 4 5 Time (milliseconds) Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. the neuron is strongly depolarized, sodium channels close and cannot be reopened. Because action potentials require the movement of Na+, the inability to open sodium channels results in a period of time during which an action potential cannot be triggered (known as the absolute refractory period). The second feature of action po- tentials is that they are “all or none.” That is, once the neuron becomes sufficiently depolarized, sodium channels open and an action potential occurs. The features of the sodium channels also result in similar levels of depolarization, which ensures that all action potentials are the same size. Another feature of action potentials relates to the myelination of axons. Myelin is not uniformly located on the axon; there are a number of small gaps in the myelin, known as nodes of Ranvier. In myelinated neurons, ion channels and sodium– potassium pumps occur only at the nodes of Ranvier. Thus, in myelinated axons, ions can cross the membrane only at the nodes of Ranvier. When an action potential first reaches the axon, it is passively propagated to the first node of Ranvier. This initial depolarization results in the production of a new action potential at the node of Ranvier. This depolarization jumps to the next node of Ranvier, and the sequence of events occurs again. The jumping of the action po- tential from one node of Ranvier to another is called saltatory conduction, and this series of events occurs down the entire length of the axon. Each node of Ranvier ac- tively generates a new action potential, resulting in an action potential that is of uni- form size. Furthermore, because the action potential is actively propagated, neural transmission in myelinated neurons is faster than transmission in neurons without myelination. 34  Neuroanatomy We have discussed the structure and functions of neurons and glia. WHERE WE Neurons use electrical signals to send information internally. These HAVE BEEN electrical signals are called action potentials, and action potentials tend to be all or none, tend to be equivalent in size, and are actively propagated by neu- rons. Myelin is the insulation on the neuron that speeds up neurotransmission. The next section will focus on communication between neurons and WHERE WE the chemicals (or neurotransmitters) that are used to send messages ARE GOING across the synapse. The second module will focus on the major di- visions of the nervous system and the structures and systems within these divisions. Finally, we will discuss how the brain is protected from damage. Communication between Neurons: The Synapse Within the neuron, communication is largely electrical, relying on the action poten- tial to transmit information. However, between neurons, communication is largely chemical. This section will focus on the details of how neurons communicate with each other across the synapse. Although most synapses are axodendritic (see Figure 4), that is, they consist of axons that form synapses with dendritic spines, there are other types of synapses. Axosomatic synapses are made up of axons forming synapses with the soma of the neurons, and they are also very common. There are also den- drodendritic synapses (dendrites forming synapses with other dendrites) and axo- axonic synapses (axons forming synapses with other axons). For the purposes of this chapter, we will consider axodendritic synapses, beginning with the presynaptic events and ending with the postsynaptic events that result in an action potential. The terminal button of an axon contains dozens of small packages (vesicles) that contain neurotransmitters (Figure 4). Often the neurotransmitters are located next to active zones, which are areas of protein accumulation on the membrane that allow the vesicle to deposit its contents into the synapse. Neurotransmitter release is trig- gered by the arrival of an action potential at the terminal button of the axon. The ac- tion potential causes calcium (Ca2+) channels to open, and Ca2+ rushes into the neuron. The increase in concentration of Ca2+ causes the neurotransmitter to be released into the synapse by a process known as exocytosis. During exocytosis, the membrane of the vesicle fuses with the axonal membrane (at the active zone), which results in an opening in the vesicle (or pore), allowing the neurotransmitter to flow into the synapse. Once the neurotransmitter has been released, it diffuses across the synapse to pro- duce postsynaptic effects. Postsynaptic effects occur when the neurotransmitter binds to a protein embedded in the postsynaptic membrane known as a receptor. For the most part, receptors are specific; that is, only one type of neurotransmitter can bind to a given receptor (although there are many subtypes of receptors for the same neuro- transmitter). Commonly, this specificity is described by using the analogy of a lock and key. That is, you may have many keys to many locks, but only one key opens a given lock (hopefully!). Two types of receptors are located on the postsynaptic membrane: transmitter-gated ion channels and G-protein-coupled receptors. Often 35  Neuroanatomy The Synapse Figure 4 Microtubules Synaptic vesicles Button Synaptic cleft Golgi complex Mitochondrion Dendritic spine Presynaptic Postsynaptic membrane membrane Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. dendrites will have a mixture of the two types of receptors located in their membranes. Additionally, these different receptors (located on the same cell membrane) frequently bind different neurotransmitters. Transmitter-gated ion channels (Figure 5) or ionotropic receptors are proteins that control an ion channel. When a neurotransmitter binds to a transmitter-gated ion chan- nel, the channel changes conformation (either opening or closing). Ionotropic recep- tors result in quick changes in ionic concentrations and often appear in situations in which a fast response is required. The functional consequence of receptor binding often depends on the ion that is controlled by the receptor. For instance, if the receptor controls a channel that is permeable to Na+, the net effect will be to depolarize the dendrite (often resulting in 36  Neuroanatomy Neurotransmitter Receptors an action potential). If the receptor controls Figure 5 a channel that is permeable to chloride An ionotropic receptor (Cl–), which is highly concentrated in the extracellular fluid, the net effect will be to Ion hyperpolarize the dendrite. When a dendrite Neurotransmitter is depolarized (moved toward producing an Ionotropic receptor action potential) by the release of a neuro- Closed transmitter from the presynaptic site, we ion call the electrical event an excitatory post- channel synaptic potential (EPSP). Conversely, when a dendrite is hyperpolarized (moved away from producing an action potential) by the release of a neurotransmitter from the presynaptic site, we call the electrical event an inhibitory postsynaptic potential (IPSP). Unlike action potentials, postsynaptic po- Some neurotransmitter molecules bind to receptors on ion channels. tentials are not actively propagated; they get When a neurotransmitter molecule binds to an ionotropic receptor, smaller the farther they travel, and they can the channel opens (as in this case) or closes, thereby altering the flow of ions into or out of the neuron. differ in the degree to which they depolar- ize or hyperpolarize the neuron. G-protein-coupled receptors or me- A metabotropic receptor tabotropic receptors (Figure 5) produce Neurotransmitter slower, more diverse, and more sustained re- Metabotropic receptor sponses than transmitter-gated ion channel Signal receptors do. Metabotropic receptors also protein occur more frequently in the nervous system than do transmitter-gated ion channel recep- tors. Metabotropic receptors use a multistep process to produce their responses, which begins with the neurotransmitter binding to the receptor. Once the neurotransmitter is G-protein bound, a subunit of the G-protein breaks away and can either move along the inside of the membrane and bind to an ion chan- Some neurotransmitter molecules bind to receptors on membrane signal proteins, which are linked to G-proteins. When a neurotransmitter nel or trigger the synthesis of other chemi- molecule binds to a metabotropic receptor, a subunit of the cals. Thus, binding of G-protein receptors G-protein breaks off into the neuron and either binds to an ion channel or stimulates the synthesis of a second messenger. can result in IPSPs or EPSPs, or they can Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, result in changes in gene expression. Thus, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. G-protein receptors can have more diverse effects than ionotropic receptors. As a final note, there are also neurotransmitter receptors on the presynaptic mem- brane (autoreceptors). Autoreceptors are metabotropic receptors that are located on the presynaptic cell membrane and bind the neurotransmitter released by the pre- synaptic axon. It is thought that their primary function is to regulate and monitor the amount of neurotransmitter in the synapse. 37  Neuroanatomy There must be some mechanism to terminate the action of a neurotransmitter bind- ing to a receptor; otherwise, once neurons were activated, they would remain active. Once the neurotransmitter is bound to the receptor, it will break away from the re- ceptor and diffuse back into the synapse. However, the neurotransmitter must be re- moved from the synaptic cleft, or it will rebind with the receptor. Two mechanisms are responsible for terminating the activity of neurotransmitters: reuptake and enzy- matic degradation. Reuptake is more common and involves the presynaptic neuron reabsorbing the neurotransmitter from the synapse and repackaging it in vesicles to be used again. Enzymatic degradation is when a neurotransmitter is broken down into an inactive form by an enzyme present in the synapse. Often the inactive forms are absorbed into the presynaptic neuron to be resynthesized into the neurotransmitter. We have discussed how neurons communicate between themselves. WHERE WE When neurotransmitters bind to ionotropic receptors, postsynaptic HAVE BEEN potentials (either EPSPs or IPSPs) are generated. Unlike action po- tentials, postsynaptic potentials are not propagated and can vary in size. Metabotropic receptors can also produce EPSPs or IPSPs, although they also produce a variety of more general responses in the neuron. The next section will examine a number of common neurotrans- WHERE WE mitters in the nervous system. The second module will focus on the ARE GOING major divisions of the nervous system and the structures and systems within these divisions. Finally, we will discuss how the brain is protected from damage. Neurotransmitters There are a variety of neurotransmitters, which often are observed in specific types of neurons and are associated with specific behaviors. Commonly, neurotransmitters are divided into small and large molecule neurotransmitters. Within the small- molecule neurotransmitter group are four classes of neurotransmitters: acetylcholine, monoamines, soluble gases, and amino acids. Within the large-molecule group, there is only one class of neurotransmitter: neuropeptides. Most neurotransmitters are ei- ther excitatory or inhibitory (although there are some exceptions that depend on re- ceptors). Small-molecule neurotransmitters tend to be released in a directed fashion, activating either ionotropic or metabotropic receptors that act directly on ion chan- nels. Large-molecule neurotransmitters tend to be released diffusely, activating metabotropic receptors, and produce either metabolic or genetic alterations within the neuron. Thus, small-molecule neurotransmitters are associated with fast responses (either excitatory or inhibitory), whereas large-molecule neurotransmitters are asso- ciated with slower, longer-lasting responses. ACETYLCHOLINE. Acetylcholine (ACh) was the first neurotransmitter to be identified, and neurons that release this neurotransmitter are called cholinergic. ACh is the neu- rotransmitter that is used by all motor neurons in the brain and spinal cord (although other neurons in the nervous system also use ACh). ACh is synthesized by enzymatic 38  Neuroanatomy conversion from choline, which is commonly found in vegetables and egg yolks. ACh is deactivated into choline and acetic acid by acetylcholinesterase (AChE). The rate of degradation is very fast (among the fastest in the nervous system), and choline is reabsorbed presynaptically. Drugs that inhibit AChE prevent the breakdown of ACh and are often used as insecticides and as nerve gases in chemical warfare. Effects of these drugs include decreases in heart rate, blood pressure, and respiration and death. There are two types of receptors for ACh: muscarinic and nicotinic receptors (named for the exogenous ligands, muscarine and nicotine, that bind to them). The most common receptor subtype is muscarinic, which is a metabotropic receptor. Muscarinic receptors are commonly found throughout the brain and in cardiac and smooth muscle (e.g., stomach). Conversely, nicotinic receptors are ionotropic and ex- citatory, and their activity can be blocked by the poison curare. Although nicotinic receptors are found in all striated muscles, they occur in only a few locations within the brain (Feldman, Meyer, & Quenzer, 1997). MONOAMINES. The monoamine class of neurotransmitters is derived from a single amino acid, of which there are two groups: those derived from tryptophan (indole- amines) and those derived from tyrosine (catecholamines). Both tryptophan and tyrosine are readily available in the diet. (Common sources include meat and dairy products.) Serotonin (abbreviated as 5-HT) is the only indoleamine neurotransmitter, and it is relatively rare in the nervous system. 5-HT-containing neurons tend to be involved in brain systems that regulate eating, sleep, and emotional behavior. 5-HT is removed from the synapse by reuptake into the presynaptic neuron. Drugs that affect the rate at which 5-HT is reabsorbed are potent antidepressants (e.g., Prozac). Almost all 5-HT receptors are metabotropic, and many different subtypes of receptors are cur- rently known (labeled as 5-HT1A, 5-HT21B, 5-HT2, etc.). There are three catecholaminergic neurotransmitters: dopamine (DA), norepineph- rine (NE), and epinephrine (E). Catecholamine-containing neurons are numerous in the nervous system and tend to be involved in brain systems that regulate movement, mood, motivation, and attention. Catecholamines are converted by using different enzymes from the original amino acid tyrosine to a compound called dopa, which is then converted into DA. DA can be converted into NE (also known as noradrena- line, or NA), and NE can be converted into E (also known as adrenaline, or A). Dopaminergic neurons are located in the areas of the brain involved with move- ment and reward, and all known DA receptor subtypes are metabotropic. Depletion of DA in the brain occurs in individuals with the movement disorder Parkinson’s dis- ease. Replacement of DA (by using the drug L-dopa) often results in an improvement of the symptoms of Parkinson’s disease. Drugs that stimulate the release of DA (e.g., amphetamines) are very addictive, suggesting that DA also plays a role in the neural systems underlying addiction. Adrenergic neurons (neurons that use either NE or E) are located throughout the brain, although many originate in the locus coeruleus. In addition to acting as a neuro- transmitter in the brain, E also acts as a hormone that is released by the adrenal glands. 39  Neuroanatomy All known adrenergic receptors are metabotropic and appear to play a broad role in mediating the hormonal effects of catecholamines (mainly E). Drugs that are used to treat the acute symptoms of asthma affect adrenergic neurons and act by causing the stimulation of adrenergic receptors, which results in the relaxation of the bronchial muscles (widening airways) and the contraction of the smooth muscles in the bronchi (reducing inflammation). SOLUBLE GASES. The soluble gases are the most recently discovered group of neuro- transmitters. These neurotransmitters are actually the gaseous molecules nitric oxide (NO) and carbon monoxide (CO). Both NO and CO are rapidly synthesized within the nervous system and undergo very rapid degeneration. Because NO is small, it can easily cross the neural membrane and does not need a receptor to produce effects. The functions of NO are better known than those of CO. (In fact, some researchers are not sure whether or not CO is a neurotransmitter.) NO is thought to play a role as a retrograde messenger, as NO is released from the postsynaptic site and acts on the presynaptic site (Feldman et al., 1997). The most famous drug that affects NO is Viagra™, although in the brain, NO plays an important role in the brain’s ability to learn. AMINO ACIDS. There are at least four amino acids that act as neurotransmitters: as- partate, glutamate, glycine, and gamma-aminobutyric acid (GABA). For the most part, the receptors for the amino acids are all ionotropic, which suggests that amino acid neurotransmitters are involved in all fast responses within the nervous system. Indeed, receptors for the amino acid neurotransmitters are located throughout the brain and are thought to be involved in a variety of neural activity, including learning and mem- ory. The most prevalent excitatory amino acid neurotransmitter is glutamate, and the most prevalent inhibitory amino acid neurotransmitter is GABA. NEUROPEPTIDES. Over fifty peptides qualify as neurotransmitters, including endor- phins, substance P, cholycystokinin, and insulin (Feldman et al., 1997). Endorphins are known to play a role in pain mediation. Drugs such as codeine and heroin act on endorphin receptors, providing potent relief from pain (among other things). Substance P is a neurotransmitter that plays a significant role in sensory transmis- sion, especially related to the transmission of touch, temperature, and pain. Capsaicin (a compound that is found in chili peppers) produces a strong “hot” feeling in the mouth when it is ingested, which is due to the stimulation of cells in the mouth that release substance P. Cholycystokinin and insulin are examples of peptide neurotrans- mitters that are involved in the regulation of hunger and ingestion. Neuropeptides are different from standard neurotransmitters not only because they are large molecules. For the most part, peptides are made, stored, and transported differently from the small-molecule neurotransmitters. However, more important, pep- tide neurotransmitters tend to modulate the responses of neurons. That is, they tend not to induce action potentials in neurons by themselves; rather, they adjust the sen- sitivity of neurons. Thus, the primary function of a neuropeptide appears to be re- lated to its ability to modulate the effects of other neurotransmitters. 40  Neuroanatomy Self-Test 1. Name three differences between EPSPs and action potentials. 2. Complete this chart with the following words: fewer, greater, less, longer, more, shorter (some words may be used twice). Ionotropic Receptors Metabotropic Receptors __________ response latency __________ response latency __________ common __________ common __________ response duration __________ response duration __________ range of actions __________ range of actions 3. What three actions can a metabotropic receptor produce when it is bound to a neurotransmitter? 4. Compare the actions and functions of small-molecule and peptide neurotransmitters. The means by which the cells of the nervous system communicate WHERE WE within themselves and among themselves have been discussed. HAVE BEEN Within the neuron, the primary form of communication is electric (either action potentials or postsynaptic potentials). Between neurons, neurotransmitters are used to communicate (by diffusing across the synapse and binding to receptors). There are a variety of neurotransmitters, and the type of receptor that they bind to often affects their impact in the nervous system. The second module in this chapter will focus on the major divisions WHERE WE of the nervous system and the structures and systems within these ARE GOING divisions. Finally, we will discuss how the brain is protected from damage. MODULE2 The Nervous System The human brain has been called the most complicated object in the known universe. These are not exactly encouraging words for the student who is sitting down to ex- plore its anatomy for the first time! It can be similarly discouraging for the textbook writer who aspires to summarize the most critical elements of neuroanatomy in a few short pages. When lecturing to my introductory neuropsychology class, I find this topic particularly frustrating. Why? There are two main reasons, listed next. Providing them might appear as though I am complaining (probably because I am complaining), but my primary goal in listing them is to help you learn neuroanatomy. If you are aware of some of the pitfalls of learning this content, you might be able to avoid them. My top two frustrations with teaching neuroanatomy are as follows: 41  Neuroanatomy 1. Its Complexity. The brain is not a particularly heavy object (approximately 1300 grams, or 3 pounds), but it contains 50 to 100 billion neurons. These cells are interconnected, and some people estimate that there are over 1 quadrillion con- nections within an average brain. The brain’s complexity is not simply the prod- uct of the number of its parts. It is also a result of how the brain evolved. The brain’s structure and function were shaped by evolution, a slow and clumsy process based on random mutations in genes. Therefore, the brain is not organized as if it were engineered. If it were, your visual cortex would be behind your eyes, not at the back of the head. (A large amount of space and energy is wasted transmit- ting visual information to the very back of your head, only to have it start head- ing toward the eyes again as it is processed in more and more complex ways.) Similarly, the primary motor and somatosensory areas would not be at the very top of your head, as far away as possible from your spinal cord! The nervous sys- tem is extremely complex in its design, and its layout often does not appear to make much sense. 2. Its Inconsistencies. There is considerable variability between human brains (pre- sumably reflecting differences in genetics and environment between people); but in addition to the brain’s own variability, there are inconsistencies in how it has been described. Some names for brain structures refer to what an object looks like. For example, the convoluted outer surface of the brain is called cortex, which means “bark.” A small, almond-shaped object in the temporal lobe is called the amygdala, which, surprisingly enough, means “almond-shaped object.” A nearby structure is called the hippocampus, which means “sea horse,” but it does not look much like a sea horse to me! Other structures are named according to where they are. The superior temporal gyrus is not named superior because it is better than the other gyri of the temporal lobe; instead, the term superior refers to the gyrus closest to the top. Other structures are named after the investigators who described them. For example, the nucleus basalis of Meynert was named after Theodore Meynert, an Austrian neurologist in the 1800s. Structures are also named according to their function. The sensory cortices (cortices is the plural of cortex) are often referred to by function, such as the primary visual cortex. In addition to the many different types of names, some structures have more than one name. To make matters worse, it isn’t just the small, obscure nuclei buried deep in the recesses of your brain that have multiple names; even the major land- marks have multiple aliases. For example, the large fissure that runs between the temporal lobe and frontal/parietal lobes can be called the Sylvian fissure, Sylvian sulcus, lateral fissure, lateral sulcus, fissure of Sylvius—you get the idea. In ad- dition to the anatomical terms used to describe brain areas, they can also be re- ferred to by their location relative to other structures, corresponding Brodmann area, or even function. The terms primary auditory cortex, the posterior portion of the superior temporal gyrus, Heschl’s gyrus, and Brodmann’s area 41 all refer to the same structure! There are also different systems of grouping neuroanatom- ical structures together. For example, in this chapter, we discuss the hypothala- mus as part of the diencephalon, whereas other sources often discuss it as part 42  Neuroanatomy of the telencephalon (the limbic system, to be more exact—more on that later). Some say that the brain has four lobes, whereas others claim that there are five! When I first encountered these inconsistencies as a student, I wondered which name was correct or which system of classification was correct. I quickly discov- ered that although there were no “correct” answers, there certainly are incorrect answers (to my knowledge, there is no system that classifies the amygdala as a hindbrain structure). Instead, there are a variety of systems of nomenclature, each of which has its own merits. After all, the brain is an extremely interconnected structure, and defining borders between regions is often an arbitrary exercise rather than an exact science. Where does the parietal lobe end and the occipital lobe begin? It depends on whom you ask! These inconsistencies plague both the student and instructor of neuroanatomy. To simplify matters somewhat, we have chosen to provide what we consider the most common name for each structure (not listing all possible names but occa- sionally listing popular synonyms), and we have provided an account of the most commonly taught system of subdivisions for these structures. However, depend- ing on when and where they were taught, your instructor(s) might prefer differ- ent terms (such as fissure of Rolando instead of central fissure) or a different system for dividing the structures into a hierarchy. The best advice we can offer is to ask your instructor when you encounter inconsistent information. We are going to begin this module with a description about how lo- WHERE WE cations in the nervous system are described. Then we will examine ARE GOING the major divisions of the nervous system and the structures and sys- tems within these divisions. Finally, we will discuss how the brain is protected from damage. Positional Terms Learning the terms to describe the relative position of parts of the nervous system serves two functions. First, it allows you to describe a location quite precisely. For example, if someone suffers a lesion to part of his or her temporal lobe, the part run- ning along the bottom surface that curls up underneath toward the midline of the brain, these terms allow you to describe the location precisely and economically (us- ing few words). The location of the temporal lesion described above can also be re- ferred to as medial inferior temporal (using three words instead of sixteen). There is a second advantage to learning these terms: As was previously mentioned, many parts of the nervous system are named after where they are. Therefore, the names some- times are directions to the location of these objects. Neuroanatomical directions are always given in relation to the spinal cord. This system is particularly convenient when describing structures in four-legged animals (quadrupeds), such as the crocodile in Figure 6. In such animals, the spinal cord is connected to the most rearward portion of the brain. If you could draw an imaginary line that extended through the spinal cord and up through the front of brain, this line would be relatively straight. This imaginary line is called the neuraxis. Straight neuraxes are the rule for quadrupeds, but for us bipeds, our neuraxis has a large 43  Neuroanatomy Terms Used to Describe Anatomical Directions Figure 6 Neuraxis Dorsal Dorsal Rostral or Caudal or anterior posterior Lateral Lateral Medial Medial Ventral Ventral Dorsal Dorsal Rostral or anterior Lateral Lateral Neuraxis Medial Medial Ventral Dorsal Ventral Caudal or Caudal or posterior posterior Source: Neil R. Carlson, Physiology of Behavior, 8e. Published by Allyn and Bacon, Boston, MA. Copyright © 2004 by Pearson Education. Reprinted by permission of the publisher. (almost 90 degree) bend in it. This has major implications for some (but not all) of the directional terms used to describe the human brain versus the human spinal cord. The part of your spinal cord closest to your back is referred to as dorsal (if you speak some French, the easy way to remember this is to remind yourself that dos means “back”). Conversely, the part of the spinal cord closest to your front is referred to as ventral. However, because of the bend in the human neuraxis, the part of your head closest to your back is not the dorsal portion. Instead, the dorsal portion of the brain runs along the top of your head. (See Figure 6 to clarify this, and remember that all positional terms are given relative to the spinal cord, not the brain.) Similarly, the part of your brain closest to your front is not the ventral portion. Instead, ventral refers 44  Neuroanatomy to the part of the brain running along the bottom surface and the part of the spinal cord closest to your belly. The term anterior refers to objects located toward the head. Therefore, in the human nervous system, objects in the brain that are closest to one’s nose are relatively anterior, and parts of the spinal cord that are closest to the brain are also anterior. Conversely, the term posterior refers to objects toward one’s behind. Therefore, in the human brain, the back of the head is the most posterior, as is the lower portion of the spinal cord. Other sources might refer to anterior objects as ros- tral and posterior objects as caudal. Two other commonly used positional terms are superior (above or topmost) and inferior (below or bottommost). For all the terms described in the preceding paragraphs, the differences between the directions relevant to the spinal cord and those referring to the human brain are easier to remember if you imagine a human in a position in which the person does not have a 90 degree bend in the neuraxis. What is this position? When teaching my students, I illustrate this by getting up on all fours on a table at the front of the class- room and tilting my head back as if I were a quadruped (see Figure 6). In this posi- tion, the dorsal portion of my spinal cord and the dorsal portion of my brain are now parallel to one another. Other directional terms refer to the relative distance of objects from the neuraxis. The term medial refers to objects that are located close to the neuraxis (midline), whereas the term lateral refers to objects that are relatively farther from the midline. There are two other convenient words for describing location relative to the midline, but instead of describing whether objects are close to the midline, they refer to different sides of the midline. The term ipsilateral refers to two objects, lesions, or behaviors that are located on the same side (right or left) of the body. Conversely, the term contralateral refers to two objects, lesions, or behaviors that are localized to opposite sides of the body. Relative to the human brain, here is a summary of the positional terms in plain English: neuraxis: an imaginary line running along the length of the nervous system, extend- ing from the bottom of the spinal cord to the most frontward portion of the brain dorsal: toward the back (top of the brain, back of the spinal cord) ventral: toward the front (bottom of the brain, front of the spinal cord) anterior: toward the head (front of the brain, top of the spinal cord) posterior: toward the tail (back of the brain, bottom of the spinal cord) superior: above or topmost inferior: below or bottommost medial: toward the middle lateral: away from the middle (toward the outside) ipsilateral: on the same side contralateral: on opposite sides In addition to these terms, it is useful to describe three planes into which the cen- tral nervous system is usually “cut” (see Figure 7). Of course, there are almost an in- finite number of planes in which any object can be cut. (If you have every tried to cut up a mango efficiently, you know this firsthand.) However, the standard approach to slicing the brain is to use one of three planes. These terms are useful not only for de- scribing dissected nervous systems. Medical procedures such as magnetic resonance 45  Neuroanatomy Planes of the Nervous System Figure 7 Dorsal Transverse plane (frontal section) Horizontal Sagittal plane plane Ventral Dorsal Rostral Caudal Transverse plane Ventral (cross section) Dorsal Rostrall Caudal Ventral Source: Neil R. Carlson, Physiology of Behavior, 8e. Published by Allyn and Bacon, Boston, MA. Copyright © 2004 by Pearson Education. Reprinted by permission of the publisher. imaging allow the visualization of the brain and spinal cord in these three planes as well. Further, many of the drawings and scans that you see of brains and spinal cords will show the nervous system cut into one of these three planes. Horizontal sections are slices through the neuraxis taken in the plane parallel to the horizon. Sagittal sec- tions are those taken parallel to a line cut down the center of the brain, between the two hemispheres (the line right down the center is called midsagittal). Sagittal sec- tions that are not midsagittal do not cross the neuraxis. Sections taken across the neur- axis running parallel to one’s face (perpendicular to the ground, such as a section cut between both ears from above) are coronal (sometimes called frontal or transverse). 46  Neuroanatomy Relative to the human brain, here is a summary of the three planes: Horizontal section: a brain slice taken parallel to the ground Sagittal section: a brain slice taken parallel to the side of the brain Coronal section: a brain slice taken parallel to the face; also known as frontal or transverse We have described the manner in which locations in the nervous sys- WHERE WE tem are described and the planes in which slices of the nervous HAVE BEEN system are examined. We are going to examine the major divisions of the nervous system WHERE WE and the structures that are located within these divisions. We will ARE GOING mention the function of these structures only briefly. Divisions of the Nervous System The following sections detail the major divisions of the nervous system and the sys- tems and structures within the divisions. Within the nervous system, structures or sys- tems within the same division have similar locations and functions. For example, struc- Self-Test Match the following definitions with the tures within the basal ganglia are more simi- appropriate terms. lar in function and location than are, for neuraxis away from the middle (toward the example, the tectum (a midbrain structure) outside) and the cerebellum (a hindbrain structure). dorsal below or bottommost Thus, knowing the division(s) in which a ner- ventral on the same side vous system structure or system falls provides information about its location and function. anterior on opposite sides There are two major divisions in the posterior toward the back (top of the brain, nervous system of the human (and other ver- back of the spinal cord) tebrates). The central nervous system (CNS) superior toward the front (bottom of the is the part of your nervous system that is brain, front of the spinal cord) encased by bone. This includes your brain inferior toward the tail (back of the brain, (protected by your skull) and spinal cord (pro- bottom of the spinal cord) tected by the spinal column). Because humans medial toward the head (front of the brain, are endoskeletal (i.e., we wear our skeleton top of the spinal cord) on the inside), some of our nervous system lateral above or topmost exists outside of protection from bone. This ipsilateral toward the middle part is called the peripheral nervous system (PNS). contralateral an imaginary line running along The PNS itself has two major divisions. the length of the nervous system, extending from the bottom of the The autonomic nervous system (ANS) is pri- spinal cord to the most frontward marily responsible for regulating internal states, portion of the brain such as temperature. Therefore, it contains nerves that convey information to the CNS 47  Neuroanatomy from internal organs. Nerves that convey information to the CNS are called afferent nerves. The ANS also contain nerves from the CNS, projecting motor information. These projections are called efferent nerves. Therefore afferent nerves go toward the CNS, whereas efferent nerves carry information from the CNS (an easy way to remember the distinction is to think “e” is for “exit” and “a” is for “approach”). The efferent nerves of the ANS are of two types. Sympathetic nerves form a network that serves to prepare the body for vigorous activity (e.g., in response to a threatening or exciting situation) whereas parasympathetic nerves form a network that sustains nonemergency behaviors. More specifically, sympathetic and parasympathetic projections tend to op- pose one another, helping to maintain a balance. When environmental circumstances warrant a shift in this balance, one of the two systems becomes relatively more active. The second division of the PNS is primarily responsible for interacting with the external environment. This division is called the somatic nervous system (SNS). It per- haps comes as no surprise to you that this system is composed largely of afferent pro- jections. After all, to interact with the external environment, one must receive sensory signals from organs such as the eyes, ears, nose, skin, muscles, and joints. The ANS also projects efferents that convey motor signals from the CNS. Figure 8 summarizes the components of the CNS and PNS. Summary of the Components of the Nervous System Figure 8 Brain Central nervous system Spinal cord Nervous Afferent system nerves Somatic nervous system Efferent nerves Peripheral nervous system Afferent nerves Autonomic Parasympathetic nervous nervous system system Efferent nerves Sympathetic nervous system Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. 48  Neuroanatomy The Spinal Cord The CNS (all of which is encased by bone) has two major divisions: the brain and the spinal cord. Superficially, these two divisions look very different. One is very long and skinny, weighing about 35 grams; the other is a larger, roundish structure, weigh- ing 40 times as much. Furthermore, the brain is a grayish color on the outside with more white on the inside, whereas the spinal cord is gray on the inside and white on the outside. The gray matter is mostly composed of cell bodies (including somas) and some blood vessels; white matter is mostly composed of myelinated axons. As you learned in the previous module, myelin is a fatty substance, which is also why white matter has a white and glossy appearance. In cross section, the spinal cord appears to have a gray H-shape in the middle. The spinal cord has thirty-one segments, and each of these segments has a pair of spinal nerves (one on the left and one on the right) attached to it. According to the Bell-Magendie law, the dorsal projections entering the spinal cord (efferents) carry sensory information, whereas the ventral projections from the spinal cord (afferents) carry motor information to muscles and glands. The thirty-one segments are divided into five groups. The eight most anterior segments are cervical segments, followed by twelve thoracic segments, five lumbar segments, five sacral segments, and one coccy- geal segment. If the spinal cord is damaged at a given segment, the brain loses both sensation and control from that segment downward. Divisions of the Brain The other division of the CNS, the brain, has three major divisions within it. These three divisions are the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). THE HINDBRAIN. The hindbrain is the interface between the brain and the spinal cord and is divided into the metencephalon and myelencephalon. The myelencephalon is a heavily myelinated region that houses tracts conveying signals between the brain and the rest of the body. The lowest portion of the hindbrain is the medulla oblon- gata, which helps to regulate basic or “vegetative” functions such as one’s breathing and heartbeat. Therefore, damage to the medulla oblongata is often fatal. Just superior to the myelencephalon is the metecephalon. This division contains two major structures: the pons and the cerebellum. The pons (which means “bridge”) looks like a bulge above the medulla, and its function is relaying sensory information from the spinal cord to the cerebellum and other brain structures, mostly via the thal- amus. Posterior to the pons is the cerebellum (which means “little brain”). It looks like a miniature version of the rest of the brain, with very small ridges called gyri and grooves called sulci. Traditionally, the cerebellum has been regarded as a structure that is responsible for coordinating and initiating movement. However, recent evidence has implicated it in a wide variety of other behaviors, such as language processing. THE MIDBRAIN. The midbrain (or mesencephalon) also has two subdivisions: the tectum and tegmentum (Figure 9). The tectum (which means “roof”) is primarily in- 49  Neuroanatomy The Cerebellum and Brain Stem Figure 9 Superior colliculus Inferior colliculus Superior colliculus Periaqueductal Dorsal gray Tectum Mesencephalic reticular formation Cerebral aqueduct Tegmentum Red nucleus Substantia nigra Ve n tral Source: Neil R. Carlson, Physiology of Behavior, 8e. Published by Allyn and Bacon, Boston, MA. Copyright © 2004 by Pearson Education. Reprinted by permission of the publisher. volved with relaying visual and auditory sensory information. It looks like four small bumps on the dorsal surface of the midbrain. The bottom two bumps are the infe- rior colliculi, and they relay auditory information and help to control auditory re- flexes (such as orienting oneself to a loud sound). The top two bumps are the superior colliculi, and they relay visual information and control simple visual reflexes (such as blinking). The tegmentum (meaning “covering”) is located ventral to the tectum. The tegmentum is a very vigorously studied portion of the brain because of the nuclei (groups of cells of similar shape and function) located within it. The substantia nigra 50  Neuroanatomy (meaning “black substance”) and red nucleus are motor nuclei, and Parkinson’s dis- ease is associated with the degeneration of the substantia nigra. The other nucleus of interest in the tegmentum is the periaqueductal gray, which is the gray matter sur- rounding the central canal. The periaqueductal gray appears to play a role in pain perception. Electrical stimulation of this area can relieve even severe pain in some cases. The periaqueductal gray contains large numbers of opioid and cannabinoid recep- tors, which might help to explain the analgesic (pain-relieving) characteristics of drugs that affect these neurotransmitter systems. THE FOREBRAIN. The forebrain (prosencephalon) makes up the bulk of the human brain. It has two major divisions: the diencephalon and the telencephalon. The diencephalon is a relatively small portion of the forebrain containing the thalamus and hypothala- mus. The thalamus is an egg-shaped structure that is located right over the top of the midbrain (thalamus means “inner chamber”). It serves as the main sensory and mo- tor relay station in the brain. More specifically, most afferents to or efferents from the rest of the forebrain pass through the thalamus. It contains quite a number of nuclei, which are specialized for relaying specific types of sensory or motor information. For example, the lateral geniculate nucleus, located on the lateral posterior surface of the thalamus, is the primary relay for visual information. Just posterior and toward the midline, the medial geniculate nucleus relays auditory sensory information. Somatosensory information is relayed via the ventral posterior nucleus, pulvinar nu- cleus, and lateral posterior nucleus. Olfactory information is relayed via the dorsal me- dial nucleus. Motor information is relayed via the ventrolateral nucleus. The two lobes of the thalamus are connected through a structure called the massa intermedia. The other, smaller component of the diencephalon within the forebrain is the hy- pothalamus. Although the hypothalamus is often referred to as a unitary structure, it is, like the thalamus, composed of a number (about twenty-two, in this case) of smaller nuclei (see Figure 10). The hypothalamus is more of a cluster of these small nuclei rather than the more homogeneous-looking egg-shaped thalamus, with its clearly defined borders. As is implied by its name, the hypothalamus is located un- derneath (hence “hypo”) the thalamus. The nuclei of the hypothalamus are relatively small in comparison to the rest of the brain (they make up 0.3% of the brain, accord- ing to Hoffman and Swaab, 1994), but their influence on a wide variety of behaviors is considerable. The hypothalamus is a very famous structure, and you probably recall learning about it in your introductory psychology class. One of the functions of the hypothal- amus that makes it so famous is its role in satiety—the feeling of being full after eat- ing. If the lateral nuclei of the hypothalamus are damaged, an animal will eat much more than it normally would. (Most introductory psychology students are presented with a picture of a very fat rat with a lateral hypothalamus lesion sitting beside a normal-looking rat of the same age and strain to emphasize this point.) In addition to its role in satiety, the hypothalamus has a much broader role: It controls both the autonomic and endocrine systems. Its control of the endocrine system is accomplished through hormones and its control of the pituitary gland, the so-called master gland of the body, which hangs below the hypothalamus. The hypothalamus also regulates 51  Neuroanatomy The Hypothalamus behaviors that appear to be critical Figure 10 to all mammals, the so-called four Fs: fighting, fleeing, feeding, and, um, well, fornicating! (Every in- structor of neuropsychology makes this same joke.) The other main division of the forebrain is the telencephalon. It contains the cerebral cortex, the limbic system, and the basal gan- glia. The cerebral cortex is the largest part of the brain (filling most of the skull), and it covers most of the other parts of the brain, including the other parts of the tel- encephalon. It is usually simply called the cortex (meaning “bark”). Its crumpled appearance allows it to have a relatively large surface area (2500 square centimeters, 2.5 square feet) in a small space, much as crumpling a piece of paper al- lows one to fit it into a space that is smaller than its original dimen- sions. The folds in the cortical sur- face produce ridges called gyri (the singular of which is gyrus) and Mammillary sulci (the singular of which is sul- body cus). Depending on the depth of the Optic chiasm sulcus, it may also be referred to as a fissure. Fissures are sulci that reach so far into the cortex that they touch one of the ventricles (more about ventricles later in this Pituitary module). Not all sulci are this deep; gland therefore, although all fissures are sulci, not all sulci are fissures. There are three fissures that you Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. should be able to locate and name: Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. the lateral fissure, central fissure, and longitudinal fissure (see Figure 11). Two of these fissures serve as borders between different lobes of the brain, whereas the other divides the brain into right and left halves called hemispheres. There are four (reasonably) anatomically and functionally distinct regions of the brain called lobes. Some of the borders between these regions can be easily identified; 52  Neuroanatomy Lobes and Fissures of the Brain others are somewhat arbitrary. Figure 11 The frontal lobe is bordered by the Longitudinal central fissure and lateral fissure. fissure The major gyri and sulci of the Parietal lobe frontal lobe can be seen in Figure 12. The major gyri are the superior Frontal frontal gyrus, middle frontal gyrus, lobe Occipital inferior frontal gyrus, orbital gyrus, lobe and precentral gyrus. The major sulci are the superior frontal sulcus, middle frontal sulcus, inferior frontal sulcus, and precentral sul- cus. Functionally, the frontal lobe performs a wide variety of func- tions, including the panning and control of movement, memory, in- hibition, regulating complex social behavior, and influencing (if not Precentral Central Postcentral subserving) personality. gyrus fissure gyrus The lateral fissure and an imaginary line called the parieto- occipital sulcus border the tem- poral lobe. In most people, this is Parietal not really a sulcus at all, but rather lobe an imaginary line that extends be- Frontal lobe tween the preoccipital notch and the transverse occipital sulcus (see Figure 13). Within the temporal Occipital lobe, the major gyri are the superior Temporal lobe temporal gyrus, middle temporal lobe gyrus, and inferior temporal gyrus Lateral (see Figure 13). The most ante-rior fissure section of the temporal lobe is called the temporal pole. The ma- Superior jor sulci of the temporal lobe are temporal the superior temporal sulcus, mid- gyrus Cerebellum dle temporal sulcus, and inferior Source: John P. J. Pinel, Biopsychology, 5e. Published by Allyn and Bacon, Boston, MA. temporal sulcus. The temporal lobe Copyright © 2003 by Pearson Education. Reprinted by permission of the publisher. is mostly involved in language pro- cessing and memory, but it also plays a role in complex object recognition and emotion. 53  Major Gyri of the Brain Figure 12 Postcentral sulcus Lateral sulcus Postcentral gyrus (Sylvian fissure) Central sulcus Interparietal sulcus (Rolandic fissure) Precentral gyrus Supramarginal gyrus Precentral sulcus Angular gyrus Superior frontal gyrus Inferior parietal lobule Superior frontal sulcus Transverse occipital sulcus Middle frontal gyrus Inferior frontal sulcus Lateral occipital gyri Inferior frontal gyrus: Pars opercularis Pars triangularis