Forebrain

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The larger component of the forebrain (see Figure 8-38), the cerebrum, consists of the right and left cerebral hemispheres as well as certain other structures on the underside of the brain. The central core of the fore-brain is formed by the diencephalon.

The cerebral hemispheres (Figure 8-40) consist of the cerebral cortex, an outer shell of gray matter covering myelinated fiber tracts, which form the white matter. This in turn overlies cell clusters, which are also gray matter and are collectively termed the subcortical nuclei. The fiber tracts consist of the many nerve fibers that bring information into the cerebrum, carry information out, and connect different areas within a

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

TABLE 8-8 The Cranial Nerves

Name

Fibers

Comments

I. Olfactory

Afferent

Carries input from receptors in olfactory (smell) neuroepithelium. Not true nerve.

II. Optic

Afferent

Carries input from receptors in eye. Not true nerve.

III. Oculomotor

Efferent Afferent

Innervates skeletal muscles that move eyeball up, down, and medially and raise upper eyelid; innervates smooth muscles that constrict pupil and alter lens shape for near and far vision.

Transmits information from receptors in muscles.

IV. Trochlear

Efferent Afferent

Innervates skeletal muscles that move eyeball downward and laterally. Transmits information from receptors in muscle.

V. Trigeminal

Efferent Afferent

Innervates skeletal chewing muscles.

Transmits information from receptors in skin; skeletal muscles of face, nose, and mouth; and teeth sockets.

VI. Abducens

Efferent Afferent

Innervates skeletal muscles that move eyeball laterally. Transmits information from receptors in muscle.

VII. Facial

Efferent Afferent

Innervates skeletal muscles of facial expression and swallowing; innervates nose, palate, and lacrimal and salivary glands. Transmits information from taste buds in front of tongue and mouth.

VIII. Vestibulocochlear

Afferent

Transmits information from receptors in ear.

IX. Glossopharyngeal

Efferent Afferent

Innervates skeletal muscles involved in swallowing and parotid salivary gland. Transmits information from taste buds at back of tongue and receptors in auditory-tube skin.

X. Vagus

Efferent Afferent

Innervates skeletal muscles of pharynx and larynx and smooth muscle and glands of thorax and abdomen. Transmits information from receptors in thorax and abdomen.

XI. Accessory

Efferent

Innervates neck skeletal muscles.

XII. Hypoglossal

Efferent

Innervates skeletal muscles of tongue.

hemisphere. The cortex layers of the two cerebral hemispheres, although largely separated by a longitudinal division, are connected by a massive bundle of nerve fibers known as the corpus callosum (Figure 8-40).

The cortex of each cerebral hemisphere is divided into four lobes: the frontal, parietal, occipital, and temporal (Figure 8-41). Although it averages only 3 mm in thickness, the cortex is highly folded, which results in an area for cortical neurons that is four times larger than it would be if unfolded, yet does not appreciably increase the volume of the brain. The cells of the cerebral cortex are organized in six layers. The cortical neurons are of two basic types: pyramidal cells (named for the shape of their cell bodies) and nonpyramidal cells. The pyramidal cells form the major output cells of the cortex, sending their axons to other parts of the cortex and to other parts of the central nervous system.

The cerebral cortex is the most complex integrating area of the nervous system. It is where basic afferent information is collected and processed into meaningful perceptual images, and where the ultimate refinement of control over the systems that govern the movement of the skeletal muscles occurs. Nerve fibers enter the cortex predominantly from the diencephalon, specifically from a region known as the thalamus (see below), other regions of the cortex, and the reticular formation of the brainstem. Some of the input fibers convey information about specific events in the environment, whereas others have as their function controlling levels of cortical excitability, determining states of arousal, and directing attention to specific stimuli.

The subcortical nuclei are heterogeneous groups of gray matter that lie deep within the cerebral hemispheres. Predominant among them are the basal ganglia, which play an important role in the control of movement and posture and in more complex aspects

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Corpus callosum

Cerebral cortex -

Lateral ventricle

White -matter

Corpus callosum

Lateral ventricle

Cerebral cortex -

Cross Section The Cerebral Cortex

Caudate nucleus

Globus pallidus

Putamen

FIGURE 8-40

(a) Coronal (side-to-side) section of the brain. (b) The dashed line indicates the location of the cross section in a.

Caudate nucleus

Globus pallidus

Putamen

Basal ganglia

Occipital Lobe Function

- Hypothalamus_

FIGURE 8-40

(a) Coronal (side-to-side) section of the brain. (b) The dashed line indicates the location of the cross section in a.

Frontal lobe

Parietal lobe

Frontal lobe

Parietal lobe

Figura Coronal Side Side

Occipital lobe

Temporal lobe

FIGURE 8-41

A lateral view of the brain. The outer layer of the forebrain (the cortex) is divided into four lobes, as shown. %

Occipital lobe

Temporal lobe

FIGURE 8-41

A lateral view of the brain. The outer layer of the forebrain (the cortex) is divided into four lobes, as shown. %

of behavior. (Note that the name "basal ganglia" is an exception to the generalization that ganglia are neuronal cell clusters that lie outside the central nervous system.)

The diencephalon, which is divided in two by the slitlike third ventricle, is the second component of the forebrain. It contains two major parts: the thalamus and the hypothalamus (see Figure 8-40). The thalamus is a collection of several large nuclei that serve as synaptic relay stations and important integrating centers for most inputs to the cortex. It also plays a key role in nonspecific arousal and focused attention.

The hypothalamus (see Figure 8-40) lies below the thalamus and is on the undersurface of the brain. Although it is a tiny region that accounts for less than 1 percent of the brain's weight, it contains different cell groups and pathways that form the master command center for neural and endocrine coordination. Indeed, the hypothalamus is the single most important control area for homeostatic regulation of the internal environment and behaviors having to do with preservation of the individual—for example, eating and drinking—

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

Septal nuclei

Frontal lobe

Olfactory bulbs

Septal nuclei

Frontal lobe

Olfactory bulbs

Pituitary Gland And Olfactory Bulbs

Thalamus

Hippocampus Spinal cord

Thalamus

Hippocampus Spinal cord

FIGURE 8-42

Structures of the limbic system are shown shaded in violet in this partially transparent view of the brain.

Redrawn from BRAIN, MIND, AND BEHAVIOR by Floyd E. Bloom and Arlyne Lazerson. Copyright 1985, 1988 by Educational Broadcasting Corporation. Reprinted by permission of W. H. Freeman and Company.

and preservation of the species—reproduction. The hypothalamus lies directly above the pituitary gland, an important endocrine structure, to which it is attached by a stalk (Chapter 10).

Thus far we have described discrete anatomical areas of the forebrain. Some of these forebrain areas, consisting of both gray and white matter, are also classified together in a functional system, termed the limbic system. This interconnected group of brain structures includes portions of frontal-lobe cortex, temporal lobe, thalamus, and hypothalamus, as well as the circuitous fiber pathways that connect them (Figure 8-42). Besides being connected with each other, the parts of the limbic system are connected with many other parts of the central nervous system. Structures within the lim-bic system are associated with learning, emotional experience and behavior, and a wide variety of visceral and endocrine functions. In fact, much of the output of the limbic system is coordinated by the hypothalamus into behavioral and endocrine responses.

The functions of the major parts of the brain are listed in Table 8-9.

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