The Forebrain

Objectives

When you have completed this section, you should be able to

• name the three major components of the diencephalon and describe their locations and functions;

• identify the five lobes of the cerebrum;

• describe the three types of tracts in the cerebral white matter;

• describe the distinctive cell types and histological arrangement of the cerebral cortex; and

• describe the location and functions of the basal nuclei and limbic system.

The forebrain consists of the diencephalon and telencephalon. The diencephalon encloses the third ventricle and is the most rostral part of the brainstem. The telen-cephalon develops chiefly into the cerebrum.

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The Diencephalon

The embryonic diencephalon has three major derivatives: the thalamus, hypothalamus, and epithalamus.

The Thalamus

The thalamus28 (fig. 14.12) is about four-fifths of the diencephalon. It consists of an oval mass of gray matter that underlies each cerebral hemisphere, protrudes into the lateral ventricle, and medially protrudes into the third ventricle. In about 70% of people, a narrow intermediate mass connects the right and left halves to each other.

The thalamus is the "gateway to the cerebral cortex." Nearly all sensory input and other information going to

2sthalamus = chamber, inner room the cerebrum passes by way of synapses in the thalamus, and the thalamus receives input from several areas of the cerebrum involved in motor control. The thalamus has several nuclei that integrate sensory information originating throughout the body and direct it to the appropriate processing centers in the cerebrum. It is heavily interconnected with the limbic system (discussed later) and thus involved in its emotional and memory functions. It is also involved in arousal, eye movements, taste, smell, hearing, equilibrium, and the somesthetic senses. Its involvement in motor and sensory circuits is further discussed later in this chapter and in chapter 16.

The Hypothalamus

The hypothalamus (fig. 14.12) forms part of the walls and floor of the third ventricle. It extends anteriorly to the optic chiasm (ky-AZ-um), where the optic nerves meet, and posteriorly to a pair of humps called the mammil-

Thalamus

Intermediate mass

Optic Arousal

Optic chiasm Optic nerve

Thalamus

Intermediate mass

Hypothalamus

Optic chiasm Optic nerve

Pituitary gland

Lateral group

Medial group Ventral group

Anterior group

Somesthetic input to association areas; contributes to limbic system

Emotional input to prefrontal cortex; awareness of emotions

Somesthetic input to postcentral gyrus; signals from cerebellum and basal nuclei to primary motor and motor association areas

Part of limbic system

Lateral geniculate nucleus Visual signals to occipital lobe

Hypothalamus

Pituitary gland

Thalamus Gland

Mammillary body

Figure 14.12 The Diencephalon. Only some of the nuclei of the thalamus and hypothalamus are shown, and only some of their functions listed. These lists are by no means complete.

Mammillary body

Paraventricular nucleus

Anterior nucleus

Ventromedial nucleus

Preoptic nucleus

Supraoptic nucleus

Suprachiasmatic nucleus

Relays signals from limbic system to thalamus

Produces oxytocin (involved in childbirth, lactation, orgasm); autonomic motor effects; control of posterior pituitary

Thirst center

Satiety center (supresses hunger); emotion

Thermoregulation; control of female reproductive cycle; sexual behavior

Produces antidiuretic hormone (involved in water conservation); control of posterior pituitary

Biological clock; regulates circardian rhythms and female reproductive cycle

Figure 14.12 The Diencephalon. Only some of the nuclei of the thalamus and hypothalamus are shown, and only some of their functions listed. These lists are by no means complete.

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Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition lary29 bodies. The mammillary bodies relay signals from the limbic system to the thalamus. The pituitary gland is attached to the hypothalamus by a stalk (infundibulum) between the optic chiasm and mammillary bodies.

The hypothalamus is the major control center of the autonomic nervous system and endocrine system and plays an essential role in the homeostatic regulation of nearly all organs of the body. Its nuclei include centers concerned with a wide variety of visceral functions.

• Hormone secretion. The hypothalamus secretes hormones that control the anterior pituitary gland. Acting through the pituitary, the hypothalamus regulates growth, metabolism, reproduction, and stress responses. The hypothalamus also produces two hormones that are stored in the posterior pituitary gland, concerned with labor contractions, lactation, and water conservation. These relationships are explored especially in chapter 17.

• Autonomic effects. The hypothalamus is a major integrating center for the autonomic nervous system. It sends descending fibers to nuclei lower in the brainstem that influence heart rate, blood pressure, gastrointestinal secretion and motility, and pupillary diameter, among other functions.

• Thermoregulation. The hypothalamic thermostat is a nucleus that monitors blood temperature. When the temperature becomes too high or too low, the thermostat signals other hypothalamic nuclei—the heat-losing center or heat-producing center, respectively, which control cutaneous vasodilation and vasoconstriction, sweating, shivering, and piloerection.

• Food and water intake. Neurons of the hunger and satiety centers monitor blood glucose and amino acid levels and produce sensations of hunger and satiety. Hypothalamic neurons called osmoreceptors monitor the osmolarity of the blood and stimulate the hypothalamic thirst center when we are dehydrated. Dehydration also stimulates the hypothalamus to produce antidiuretic hormone, which conserves water by reducing urine output.

• Sleep and circadian rhythms. The caudal part of the hypothalamus is part of the reticular formation. It contains nuclei that regulate falling asleep and waking. Superior to the optic chiasm, the hypothalamus contains a suprachiasmatic nucleus that controls our 24-hour (circadian) rhythm of activity.

• Memory. The mammillary bodies of the hypothalamus lie in the pathway of signals traveling from the hippocampus, an important memory center of the

2amammill = nipple

Chapter 14 The Brain and Cranial Nerves 531

brain, to the thalamus. Thus they are important in memory, and lesions to the mammillary bodies result in memory deficits. (Memory is discussed more fully later in this chapter.)

• Emotional behavior. Hypothalamic centers are involved in a variety of emotional responses including anger, aggression, fear, pleasure, and contentment; and in sexual drive, copulation, and orgasm.

The Epithalamus

The epithalamus consists mainly of the pineal gland (an endocrine gland discussed in chapter 17), the habenula (a relay from the limbic system to the midbrain), and a thin roof over the third ventricle.

The Cerebrum

The embryonic telencephalon becomes the cerebrum, the largest and most conspicuous part of the human brain. Your cerebrum enables you to turn these pages, read and comprehend the words, remember ideas, talk about them with your peers, and take an examination. It is the seat of your sensory perception, memory, thought, judgment, and voluntary motor actions. It is the most challenging frontier of neurobiology.

Gross Anatomy

The surface of the cerebrum, including its gray matter (cerebral cortex) and part of the white matter, is folded into gyri that allow a greater amount of cortex to fit in the cranial cavity. These folds give the cerebrum a surface area of about 2,500 cm2, comparable to 4/2 pages of this book. If the cerebrum were smooth-surfaced, it would have only one-third as much area and proportionately less information-processing capability. This extensive folding is one of the greatest differences between the human brain and the relatively smooth-surfaced brains of most other mammals.

Some gyri have consistent and predictable anatomy, while others vary from brain to brain and from the right hemisphere to the left. Certain unusually prominent sulci divide each hemisphere into five anatomically and functionally distinct lobes, listed next. The first four lobes are visible superficially and are named for the cranial bones overlying them (fig. 14.13); the fifth lobe is not visible from the surface.

1. The frontal lobe lies immediately behind the frontal bone, superior to the orbits. Its posterior boundary is the central sulcus. The frontal lobe is chiefly concerned with voluntary motor functions, motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression.

2. The parietal lobe forms the uppermost part of the brain and underlies the parietal bone. Its rostral

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Labeled Forebrain Diencephalon
Figure 14.13 Lobes of the Cerebrum. The insula is not visible from the surface (see fig. 14.2a).

boundary is the central sulcus and its caudal boundary is the parieto-occipital sulcus, visible on the medial surface of each hemisphere (see fig. 14.1d). This lobe is concerned with the sensory reception and integration of somesthetic, taste, and some visual information.

3. The occipital lobe is at the rear of the head underlying the occipital bone. It is the principal visual center of the brain.

4. The temporal lobe is a lateral, horizontal lobe deep to the temporal bone, separated from the parietal lobe above it by a deep lateral sulcus. It is concerned with hearing, smell, learning, memory, visual recognition, and emotional behavior.

5. The insula30 is a small mass of cortex deep to the lateral sulcus, made visible only by retracting or cutting away some of the overlying cerebrum (see figs. 14.2a, 14.6c, and 14.16). It is not as accessible to study in living people as other parts of the cortex and is still little-known territory. It apparently plays roles in understanding spoken language, in the sense of taste, and in integrating sensory information from visceral receptors.

The Cerebral White Matter

Most of the volume of the cerebrum is white matter. This is composed of glia and myelinated nerve fibers that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers. These fibers are arranged in three types of tracts (fig. 14.14):

1. Projection tracts extend vertically from higher to lower brain or spinal cord centers and carry information between the cerebrum and the rest of the body. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a dense band called the internal capsule between the thalamus and basal nuclei (described shortly), then radiate in a diverging, fanlike array (the corona radiata31) to specific areas of the cortex.

2. Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures (COM-ih-shurs). The great majority of commissural fibers pass through the large C-shaped corpus callosum (see fig. 14.1d), which forms the floor of the longitudinal fissure. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the two sides of the cerebrum to communicate with each other.

3. Association tracts connect different regions of the same hemisphere. Long association fibers connect different lobes of a cerebral hemisphere to each other, whereas short association fibers connect

31 corona = crown + radiata = radiating

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Forebrain Archicortex

Figure 14.14 Tracts of Cerebral White Matter. (a) Left lateral aspect, showing association tracts; (b) frontal section, showing commissural and projection tracts.

What route can commissural tracts take other than the one shown here?

Figure 14.14 Tracts of Cerebral White Matter. (a) Left lateral aspect, showing association tracts; (b) frontal section, showing commissural and projection tracts.

What route can commissural tracts take other than the one shown here?

different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain; for example, they enable you to smell a rose, name it, and picture what it looks like.

The Cerebral Cortex

Neural integration is carried out in the gray matter of the cerebrum, which is found in three places—the cerebral cortex, basal nuclei, and limbic system. We begin with the cerebral cortex,32 a layer covering the surface of the hemispheres. Even though it is only 2 to 3 mm thick, the cortex constitutes about 40% of the mass of the brain and contains 14 to 16 billion neurons. It is composed of two principal types of neurons (fig. 14.15): (1) Stellate cells have spheroidal somas with dendrites projecting for short distances in all directions. They are concerned largely with receiving sensory input and processing information on a local level. (2) Pyramidal cells are tall and conical (triangular in tissue sections). Their apex points toward the brain surface and has a thick dendrite with many branches and small, knobby dendritic spines. The base gives rise to horizontally oriented dendrites and an axon that passes into the white matter. Pyramidal cells are the output neurons of the cerebrum— they transmit signals to other parts of the CNS. Their axons have collaterals that synapse with other neurons in the cortex or in deeper regions of the brain.

About 90% of the human cerebral cortex is a six-layered tissue called neocortex33 because of its relatively recent evolutionary origin. Although vertebrates have existed for about 600 million years, the neocortex did not develop significantly until about 60 million years ago, when there was a sharp increase in the diversity of mammals. It attained its highest development by

32cortex = bark, rind

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Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition

534 Part Three Integration and Control

Figure 14.15 Histology of the Neocortex. Neurons are arranged in six layers.

Are the long processes leading upward from each pyramidal cell body dendrites or axons?

far in the primates. The six layers of neocortex, numbered in figure 14.15, vary from one part of the cerebrum to another in relative thickness, cellular composition, synaptic connections, size of the neurons, and destination of their axons. Layer IV is thickest in sensory regions and layer V in motor regions, for example. All axons that leave the cortex and enter the white matter arise from layers III, V, and VI.

Some regions of cerebral cortex have fewer than six layers. The earliest type of cerebral cortex to appear in vertebrate evolution was a one- to five-layered tissue called paleocortex (PALE-ee-oh-cor-tex), limited in humans to part of the insula and certain areas of the temporal lobe concerned with smell. The next to evolve was a three-layered archicortex (AR-kee-cor-tex), found in the human hippocampus. The neocortex was the last to evolve.

The Basal Nuclei

The basal nuclei are masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus (fig. 14.16). They are often called basal ganglia, but the word ganglion is best restricted to clusters of neurons outside the CNS. Neuroanatomists disagree on how many brain centers to classify as basal nuclei, but agree on at least three: the caudate34 nucleus, putamen,35 and globus pallidus.36 The putamen and globus pallidus are also collectively called the lentiform37 nucleus, while the putamen and caudate nucleus are collectively called the corpus striatum after their striped appearance. The basal nuclei receive input from the substantia nigra of the mid-brain and motor areas of the cerebral cortex and send signals back to both of these locations. They are involved in motor control and are further discussed in a later section on that topic.

The Limbic System

The limbic38 system, named for the medial border of the temporal lobe, is a loop of cortical structures surrounding the corpus callosum and thalamus (fig. 14.17). It includes nuclei called the amygdala and hippocampus on the medial side of the temporal lobe, a tract called the fornix leading to the mammillary bodies of the hypothalamus, and a fold of cortex called the cingulate gyrus that arches over the corpus callosum. From the earliest investigations of this system in the 1930s, it was thought to be involved in emotion and smell; later, memory was added to the list of functions. Recently, some neuroanatomists have argued that the components of this system have so little in common that there is no point to calling it the limbic system. But whether or not this term is abandoned, it is still agreed that the amygdala is important in emotion and the hippocampus in memory. These functions are explored in later sections of this chapter.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

12. What are the three major components of the diencephalon? Which ventricle does it enclose?

13. What is the role of the thalamus in sensory function?

14. List at least six functions of the hypothalamus.

15. Name the five lobes of the cerebrum and describe their locations and boundaries.

34caudate = tailed, tail-like

35putam = pod, husk

3slimbus = border

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Six Layers Neocortex

Figure 14.15 Histology of the Neocortex. Neurons are arranged in six layers.

Are the long processes leading upward from each pyramidal cell body dendrites or axons?

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Superior

Cerebrum-

Corpus callosum-Lateral ventricle-

Horizontal Section Basal Nuclei

Figure 14.16 The Basal Nuclei (frontal section of the brain).

Superior

Hippocampus Commissural Association

Figure 14.16 The Basal Nuclei (frontal section of the brain).

Corpus callosum Cingulate gyrus-

Olfactory bulb-Olfactory tract -Hypothalamus -

Amygdala-

Temporal lobe -

Corpus callosum Cingulate gyrus-

Olfactory bulb-Olfactory tract -Hypothalamus -

Amygdala-

Temporal lobe -

Substantia Nigra Limbic System

Fornix

Third ventricle

Mammillary body

Hippocampus

Figure 14.17 The Limbic System.

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

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536 Part Three Integration and Control

16. Distinguish between commissural, association, and projection tracts of the cerebrum.

17. Where are the basal nuclei located? What is their general function?

18. Where is the limbic system located? What component of it is involved in emotion? What component is involved in memory?

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Essentials of Human Physiology

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