The Diencephalon 1 The Thalamus

Analysis of detail and synthesis of effector patterns are carried out in the cerebral cortex to a degree unequalled by other parts of the CNS. A close competitor is the thalamus, a diencephalic structure between the

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Figure 9 Basal ganglia: the massive putamen is continuous medially in many places with the fishlike caudate nucleus and inferiorly with the amygdala. Medial to the putamen and visible only in view c is the cone-shaped globus pallidus with its outer and inner divisions. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press Inc. Used by permission of Oxford University Press, Inc. (illustration by Emeline M. Angevine).

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Figure 9 Basal ganglia: the massive putamen is continuous medially in many places with the fishlike caudate nucleus and inferiorly with the amygdala. Medial to the putamen and visible only in view c is the cone-shaped globus pallidus with its outer and inner divisions. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press Inc. Used by permission of Oxford University Press, Inc. (illustration by Emeline M. Angevine).

cerebral hemispheres, surrounded by them, and during development bound to them by thalamocortical fibers (Fig. 12). The thalamus thus is physically coherent with the cerebrum. It is a bilateral ovoid gray mass with a prominent pole (the pulvinar) facing posteriorly on each side. During development, its halves bulge to form a small interthalamic adhesion. In aging, this adhesion may atrophy and disappear. (Descriptively, "thalamus" often refers to one half-of the thalamus, ''thalamic function'' to both halves collectively. Such dual usage also applies to the hypothalamus.)

The intimate association with the optic tract is a metaphor of thalamic function. It is the portal of the cerebral cortex. All of the sensory tracts, and many nonsensory tracts, converge on the thalamus. It integrates this information, then passes it to the cortex for additional integration, correlation, and compar ison. Olfaction is an exception. It is first processed in the olfactory bulb, a region of old cortex. This sensory information eventually reaches the thalamus after passing through other way stations.

Messages to the thalamus concern discriminative aspects of sensation (location, quality, intensity) and affective aspects (pleasant, unpleasant). Such inputs dominate the caudal part of the thalamus. Other messages, however, have but an indirect relation to sensation; they come from the basal ganglia, cerebellum, and limbic system. As noted, the basal ganglia and cerebellum act as consultants to the cortex in synthesizing movement patterns, the former offering complex programming of movements and the latter regulating their direction and force. Results of these consultations go to the rostral part of the thalamus (Fig. 10, VA, VL) and are projected to motor cortex,

Circuitry Motor Cortex

Figure 10 Motor system: an important feature of this complex circuitry is that the basal ganglia lie between the entire cerebral cortex and the motor cortex, by way of pallidothalamic feedback. The same is true for the cerebellum. Although the motor system includes many structures and connections, what emerges is refined motor control. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Jay B. Angevine, Jr.).

Figure 10 Motor system: an important feature of this complex circuitry is that the basal ganglia lie between the entire cerebral cortex and the motor cortex, by way of pallidothalamic feedback. The same is true for the cerebellum. Although the motor system includes many structures and connections, what emerges is refined motor control. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Jay B. Angevine, Jr.).

where movement patterns are synthesized. Limbic input comes from the hypothalamus and orbitofrontal cortex: visceral afferent data from the former, highly refined integration of the widest range of function imaginable from the latter. As before, thalamic projections to specific cortical areas permit the necessary cortical interplay.

The human thalamus comprises two egg-shaped halves containing some 15 major nuclei and many small cell clusters. Space constraints preclude their coverage, but the structural and functional unity of the thalamus is more important. Over the last 50 years, neurophysiology has shown five functional attributes of thalamocortical connections that are especially illuminating.

Thalamic overlap and fusion blend modalities and submodalities of sensation, e.g., all sensations of the index finger are processed by neurons in a particular region of the thalamic somesthetic nucleus. Orderly representations of sensory or other functional domains form topographic maps of the retina, cochlear duct, and cutaneous surface, in tracts leading to the thalamus, and in certain thalamic nuclei. Multiple representations of venues are exemplified by the lateral geniculate nucleus (Fig. 12). It receives the fibers of the optic tract. Its six cell layers provide six tightly registered retinal maps of the contralateral half of the binocular visual field, the map in each layer representing the ipsilateral or contralateral hemiretina serving the half visual field in alternation. Reciprocity of function is evident in the equally numerous and precise thalamocortical and corticothalamic projections. These inhibit or facilitate thalamic neurons, perhaps for selective attention or further integration. Finally, specificity of thalamic neurons: David Hubel and Torsten Wiesel, Nobel laureates of 1981, showed that

Figure 11 The limbic system: major limbic centers and connections, based on a computer reconstruction of limbic structures traced and digitized from whole brain serial sections. From Cheryl A. Cotman, Jay B. Angevine, Jr., and Kevin Head (illustration by Cheryl A. Cotman; commissioned for this article by the author). (See color insert in Volume 1).

Figure 11 The limbic system: major limbic centers and connections, based on a computer reconstruction of limbic structures traced and digitized from whole brain serial sections. From Cheryl A. Cotman, Jay B. Angevine, Jr., and Kevin Head (illustration by Cheryl A. Cotman; commissioned for this article by the author). (See color insert in Volume 1).

nerve cells in the visual cortex respond to particular features of sensory information and only that feature.

2. The Hypothalamus

The hypothalamus (Fig. 13) lies at the heart of the limbic system. With dual outputs, it plays key roles in short-term and long-range homeostasis. Its neural output is effected by descending tracts in the core of the neuraxis and its endocrine output by pituitary hormones. It regulates vegetative functions: control of body temperature, caloric input, water-osmolar balance, and sleep-wakefulness. It selects and integrates autonomic responses. It controls the release of adeno-hypophyseal and neurohypophyseal hormones that exert far-ranging and, in some cases (growth hormone), enduring effects on the body.

The hypothalamus lies just below the thalamus. It is minute: about one-three hundredth to one four-hundredth of total brain weight (4 out of 1400 g). Yet it has over a dozen nuclei and receives, emits, or gives passage to as many tracts and to and from as many brain regions, both upstream and downstream. Neu-roanatomist Walle Nauta saw it as the instrument panel or the "dashboard" of the brain. It monitors and regulates (not on dials or by switches but from receptors and via diverse connections) visceral and endocrine activities for homeostasis, offensive-defensive or trophic responses, and conative pursuits of long-range goals.

C. The Midbrain

The cylindrical midbrain (Fig. 7) separates the dience-phalon and pons. Hardly more than 20 mm in diameter, its layout of structures reflects its embryonic tubular plan. Massive tracts run through it, some ascending from the spinal cord and cerebellum and others descending from the cerebral cortex. The cerebral aqueduct in its upper part is a vital, yet narrow, vulnerable conduit of cerebrospinal fluid (CSF) and a useful landmark. Above it, in the tectum (Latin: "roof"), lie two small hills of gray matter: the superior and inferior colliculi, which are visual and auditory way stations. They receive 10% of the fibers of the optic nerve and 100% of the auditory projection, respectively. They show cortical features of organization, perform multimodal integration, and mediate complex responses to sights and sounds.

Beneath the aqueduct is the tegmentum (Latin: "covering"). It covers basal parts of the midbrain, the cerebral peduncles, and substantia nigra. It houses

Thalamus

Thalamus

Path Visual Fibers Into The Thalamus

Figure 12 The thalamus, gateway to the cerebral cortex: a large mass of gray matter, comprising over a dozen recognizable nuclei, in the upper diencephalon, separated by the third ventricle into right and left halves. Its strategic central position permits a broad range of upstream input and cortical interconnections. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Jay B. Angevine, Jr.).

Figure 12 The thalamus, gateway to the cerebral cortex: a large mass of gray matter, comprising over a dozen recognizable nuclei, in the upper diencephalon, separated by the third ventricle into right and left halves. Its strategic central position permits a broad range of upstream input and cortical interconnections. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Jay B. Angevine, Jr.).

cranial nerve nuclei mediating eye movements and pupillary constriction and provides the rostral part of the brain stem reticular formation, a fabric of gray and white matter forming a continuous core of the midbrain, pons, and medulla oblongata. It exerts strong facilitatory and inhibitory influences on virtually all activities up and down the neuraxis. In the reticular formation on each side is a large round mass of neurons: the highly vascularized red nucleus, a motor center distributing cerebellar output to nuclei in the brain stem and thalamus.

Basal components are corticospinal and cortico-pontine fiber bundles surmounted by a gray cushion, the substantia nigra. Its compact upper part, where some neurons contain melanin pigment, makes dopa-mine and delivers it by axonal transport to the caudate nucleus and putamen for use in their complex neuronal interactions related to motor control. The lower layer of the nigra, as stated, serves as an output center of the basal ganglia, along with the inner segment of the globus pallidus.

Injuries to the midbrain have devastating results: almost total loss of sensation, paralysis, severe extensor spasms from the interruption of key connections of the motor system, and, from damage to the reticular formation, lasting coma if not death.

D. The Cerebellum

In humans, the cerebellum is half-hidden by the overlying occipitotemporal regions of the cerebral hemispheres (Fig. 7). It is bilobed, with a narrower median part, the vermis, connecting two large, ovoid hemispheres. These are gracefully fissurated in a curving way, a feature resulting in closely apposed, almost sinusoidal folia coated by cortical gray matter. The geometrical organization of this cortex, the self-evident design of its elegant neurons, and the swift, reliable contributions to motor control make it a natural wonder. The cerebellum is found in all vertebrate brains, with tremendous variations in size, shape, numbers of neurons, and neuronal idiosyncrasies. But the usual neuronal types are always there and the basic plan of cortical connectivity is instantly recognizable in all.

The cerebellum, like a computer, regulates the rate, range, and force of movements and contributes to muscle tone and posture. Like the basal ganglia, it works in concert with the cerebral cortex. It has ties to pontomedullary vestibular centers and also input from the spinal cord. Unlike the cerebrum, it does not play a major role in perception of sensation or initiation of volitional movement. With damage to the cerebellum,

Basal Systems Tract

Figure 13 Hypothalamus: afferents (upper view) are corticohypothalamic fibers (1), medial forebrain bundle (2), fornix (3), stria terminalis (4) periventricular fiber system (5), mammillary peduncle (6), ventral amygdalofugal pathway (7), and retinohypothalamic fibers (8). Efferents (lower view) are the hypothalamohypophysial tract (1), tuberoinfundibular tract (2), principal mammillary fasciculus (including the mammillothalamic tract) (3), and dorsolateral fasciculus (4). These connections are largely reciprocal and form integral parts of the limbic and neuroendocrine systems of the brain. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by W. J. H. Nauta and V. B. Domesick).

Figure 13 Hypothalamus: afferents (upper view) are corticohypothalamic fibers (1), medial forebrain bundle (2), fornix (3), stria terminalis (4) periventricular fiber system (5), mammillary peduncle (6), ventral amygdalofugal pathway (7), and retinohypothalamic fibers (8). Efferents (lower view) are the hypothalamohypophysial tract (1), tuberoinfundibular tract (2), principal mammillary fasciculus (including the mammillothalamic tract) (3), and dorsolateral fasciculus (4). These connections are largely reciprocal and form integral parts of the limbic and neuroendocrine systems of the brain. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by W. J. H. Nauta and V. B. Domesick).

Medulla Bulb

Figure 14 Base of the brain: low power view of same brain shown in Fig. 5 to show the inferior surface of the cerebellum, pons, and medulla oblongata, together with other structures. From J. Nolte and J. B. Angevine, Jr., The Human Brain. In Photographs and Diagrams, 2nd ed., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

Figure 14 Base of the brain: low power view of same brain shown in Fig. 5 to show the inferior surface of the cerebellum, pons, and medulla oblongata, together with other structures. From J. Nolte and J. B. Angevine, Jr., The Human Brain. In Photographs and Diagrams, 2nd ed., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

conscious sensation is largely spared, but dexterity and smooth execution of movement (especially skilled movements of the upper extremities) are impaired, muscle tone and strength are diminished, and posture and equilibrium are deficient. The degree of deficits depends on the location, severity, and duration of the insult. In time, cerebellar circuitry often brings improvement, but damage to its outflow path, the superior cerebellar peduncle, usually effects lasting dysfunction.

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