Epithalamus
Forming the roof of the diencephalon is a series of structures called the epithalamus. These structures include the habenular complex, the stria medullaris, and the pineal gland.
Habenular Complex and Stria Medullaris
The habenular complex consists of two sets of nuclei, a lateral and medial nucleus Fig. 13-8), which are connected anatomically on each side by a commissure called the habenular commissure (not shown on Fig. 13-8). Two pathways are associated with these nuclei. One arises from the medial nucleus and projects to the ventral aspect of the midline of the tegmentum (i.e., principally the interpe-duncular nucleus) and is called the habenulopeduncular tract (sometimes called the fasciculus retroflexus; Fig. 13-8). Although the habenulopeduncular tract can be easily visualized in sections taken through the midbrain-diencephalic juncture, its functions remain obscure. The second pathway, called the stria medullaris (Fig. 13-5B), contains principally habenular afferent fibers. Sources of afferent and efferent fibers linking the habenular nuclei with the lateral hypothalamus, preoptic region, substantia innominata (i.e., the region that lies immediate lateral to the preoptic region), septal area, and the anterior thalamic nucleus pass through the stria medullaris. When viewed collectively, the habenular complex appears to serve as a possible relay by which the limbic system and hypothala-mus can influence the activity of important midbrain structures, such as the reticular formation.
FIGURE 13-8 Photograph of a section taken through the caudal third of the thalamus through the level of the habenular and centromedian nucleus. Note the position of the fasciculus retroflexus, which passes around the parafascicular nucleus. In this section, one can also identify several other posterior thalamic nuclei, namely the lateral geniculate nucleus and pulvinar nucleus. At this borderline region of the midbrain-diencephalic juncture, the red nucleus, H field of Forel, substantia nigra, and subthalamic nucleus can also be seen. VPL = ventral posterolateral nucleus; VPM = ventral posterolateral nucleus; PF = parafascicular nucleus.
Pineal Gland
The pineal gland is conical in shape and is attached to the roof of the posterior aspect of the third ventricle (Fig. 13-1). It should be noted that the pineal gland has no direct connections with the CNS. However, this structure receives neural inputs from the sympathetic nervous system via the superior cervical ganglia. One of a number of unique features about the pineal gland is that, in animals, it displays a circadian rhythm to light with respect to its release of several hormones. This process is most likely due to the presence of an indirect pathway beginning with light-cycle information that is relayed to the suprachiasmatic nucleus directly from the retina. This information is then transmitted indirectly from the supra-chiasmatic nucleus downstream to the intermediolateral cell column of the thoracic cord. Light-cycle information is then relayed to the superior cervical ganglia, which maintains direct connections with the pineal gland.
The pineal gland also contains highly vascular connective tissue. Within the connective tissue meshwork are found specialized secretory cells called pinealocytes. A number of different pineal gland secretions have been identified, including biogenic amines, such as melatonin (synthesized within the gland from serotonin), serotonin, and norepinephrine. High concentrations of hypothalamic-releasing hormones, such as thyrotropin-releasing hormone, somatostatin, and luteinizing hormone-releasing hormone, have also been identified. The pineal gland likely affects CNS functions as a result of release of these hormones into the general circulation and into the brain through the blood-brain barrier.
Subthalamus
The subthalamus consists of several cell groups and fiber pathways, most of which are situated at the level of the posterior third of the diencephalon and which relate to motor functions associated with the basal ganglia. The nuclear groups include the subthalamic nucleus and the zona incerta (Fig. 13-8).
The subthalamic nucleus is oblong in shape and lies just off the ventromedial edge of the internal capsule. It maintains reciprocal connections with the globus pallidus and, therefore, plays an important role in the regulation of motor functions by the basal ganglia.
On the dorsomedial aspect of the subthalamic nucleus lies the zona incerta, whose functions are poorly understood. The zona incerta is formed by a thin band of cells separating the lenticular fasciculus from the thalamic fasciculus (see the following sections for a description of these pathways).
Hypothalamus
The hypothalamus extends anteriorly from the level of the optic chiasm posteriorly to the posterior commissure (Fig. 13-9). The preoptic area, which constitutes an anterior extension of the hypothalamus, extends rostrally to the level of the lamina terminalis (anterior end of third ventricle).
FIGURE 13-9 Sagittal view of the brain depicting the relative positions of the major nuclei of the hypothalamus. A1, A2, and A3 indicate the approximate levels at which the corresponding cross sections through the hypothalamus were taken in Figure 13-10.
The dorsal limit of the preoptic zone is the anterior commissure, whereas the dorsal limit of the rest of the hypothalamus is the hypothalamic sulcus and ventral thalamus. On the ventral surface of the brain just caudal to the optic chiasm lies the infundibulum to which the pituitary gland is attached. The hypothalamus includes a number of distinct groups of nuclei, regions, and fiber pathways. Superficially, the descending column of the for-nix divides the hypothalamus into two basic regions, a lateral hypothalamic area and a medial region. The hypothalamus is primarily concerned with CNS regulation of visceral, endocrine, autonomic, and emotional processes. Accordingly, the hypothalamus provides central control of temperature regulation; sympathetic and para-sympathetic events; endocrine functions of the pituitary; sexual behavior; feeding and drinking behavior; and affective processes, such as aggression and flight.
Lateral Hypothalamus
The lateral hypothalamic area, which is present throughout the rostro-caudal extent of the hypothalamus, consists of different groups of cells and diffusely arranged cells as well as a major fiber bundle called the medial forebrain bundle that contains fibers that pass in both rostral and caudal directions (Fig. 13-10). The lateral hypothalamus is associated with a number of behavioral processes, such as drinking and predation.
Medial Hypothalamus
The medial hypothalamus includes a number of significant nuclear groups as well as several fiber bundles. The medial hypothalamus contains most of the releasing hormones that control pituitary function. It also provides a mechanism that modulates feeding; generates affective processes, such as rage behavior, in animals and people; and its descending axons provide control over autonomic functions.
Anterior-Posterior Levels of Hypothalamus
Anterior Hypothalamus (and Preoptic Area). Important nuclei of the anterior hypothalamus include preoptic nuclei and suprachiasmatic, supraoptic, and paraventricular nuclei (Figs. 13-9 and 13-10). The preoptic region is basically divided into two regions: a lateral and medial preoptic nucleus. The medial preoptic region is principally associated with the regulation of endocrine function and temperature regulation. Functions of the lateral preoptic region are less well known. The suprachiasmatic nucleus is found in the ventromedial aspect of the anterior hypoth-alamus close to and at the preoptic area. It receives retinal inputs and is associated with diurnal rhythms for hormone release. The supraoptic nucleus contains large cells located near the lateral edge of the optic chiasm. The par-aventricular nucleus lies in the dorsomedial aspect of the anterior hypothalamus adjacent to the third ventricle. Both the supraoptic and paraventricular nuclei synthesize vasopressin and oxytocin, which are then transported along their axons to the posterior pituitary. The remainder of the anterior hypothalamus consists of medial and lateral regions that are poorly differentiated anatomically. Mid-Level of the Hypothalamus. At this level of hypothalamus, the optic chiasm has been replaced by the optic tract, which lies along the ventral surface of the diencephalon (Figs. 13-9 and 13-10). In addition, the suprachiasmatic nucleus has also been replaced by the arcuate nucleus, which lies in the most ventromedial aspect of the hypoth-alamus immediately above the infundibular stalk. The arcuate nucleus also lies close to the median eminence, and many of these neurons are dopaminergic. It has been suggested that these neurons may be the source of prolac-tin release-inhibiting hormone.
The ventromedial nucleus is a prominent structure that lies in the ventral aspect of the medial hypothalamus.
FIGURE 13-10 (A-C) Cross-sectional diagrams, the levels of which are depicted in Figure 13-9, illustrate the principal nuclei present at the anterior (A1), middle third (A2), and posterior (A3) aspects of the hypothalamus.
The expression of rage behavior, inhibition of feeding, and endocrine control are among the functions associated with the ventromedial nucleus. Immediately dorsal to the ven-tromedial nucleus lies the dorsomedial nucleus, which is a region composed of relatively smaller-sized cells that are situated close to the third ventricle and that share similar functions to the cells of the ventromedial nucleus. Neurons located near the third ventricle, such as the tuberal nuclei, release regulatory hormones (i.e., releasing or release-inhibiting hormones) that are transmitted through the portal system to the anterior pituitary gland and, thus, control the release of hormones from this region. Posterior Hypothalamus. At the level of the posterior hypoth-alamus, the lateral hypothalamus is still present but reduced in size. The descending fibers of the fornix have terminated within the mammillary bodies, which have now replaced the ventromedial nucleus in the ventrome-dial region of the hypothalamus at this level. Although the mammillary bodies receive many fibers from the for-nix, it is also the source of large numbers of axons that project to the anterior thalamic nucleus. This pathway can be seen at different levels of the hypothalamus and is referred to as the mammillothalamic tract (Fig. 13-10C). Functions associated with the mammillary bodies are not well understood but may be related to the functions of the Papez circuit.
Basal Ganglia
The basal ganglia play a key role in the regulation of motor functions. The primary components of the basal ganglia include the caudate nucleus, putamen, and globus pallidus. In addition, the substantia nigra and subthalamic nucleus are included as part of the basal ganglia because of the anatomical and functional relationships they share with the neostriatum (i.e., caudate nucleus and putamen) and glo-bus pallidus.
Principal Component Structures
Caudate Nucleus
The largest part of the caudate nucleus is the "head" and lies rostral to the thala-mus, forming the lateral border of the anterior horn of the lateral ventricle (Fig. 13-11). The internal capsule forms the lateral and ventral borders of the caudate nucleus, and its dorsal border is the corpus callosum. As the caudate nucleus extends caudally, it becomes progressively smaller in size. The region just caudal to the head of the caudate is the "body," and the narrowest part is the "tail." The tail, which is situated on the dorsal aspect of the inferior horn of the lateral ventricle, follows the ventricle in its initial ventral trajectory and then rostrally toward the amygdala.
Putamen
The putamen is a large structure that lies between the globus pallidus and the external capsule (Figs. 13-2, 13-4, 13-5, and 13-12). The putamen and globus pallidus lie lateral to the internal capsule; in contrast, the caudate nucleus lies medial to it. The putamen extends from the level of the head of the caudate nucleus, anteriorly, to the level of the posterior third of the thalamus, posteriorly (Figs. 13-2 and 13-11). Embryologically, the anterior limb of the internal capsule grows through the s triatal mass, partially separating the caudate medially and the putamen laterally.
FIGURE 13-11 The components of the caudate nucleus and their relationship to the thalamus, internal capsule, globus pallidus, putamen, and brainstem.
FIGURE 13-12 The major output pathways of the basal ganglia. The medial segment of the globus pallidus gives rise to two major efferent pathways of the basal ganglia: the ansa lenticularis and the lenticular fasciculus (H2 field of Forel). The fibers of the ansa lenticula-ris and lenticular fasciculus, together with axons from the cerebellum (den-tatothalamic fibers), merge en route to the thalamus as the H1 field of Forel.
At its most rostral position, the putamen is fused with the caudate because the internal capsule is no longer present at that level. The caudate nucleus and putamen are referred to as the neostriatum and, collectively, constitute the major sites of inputs into the basal ganglia from regions such as the cerebral cortex, thalamus, and substan-tia nigra. The claustrum, which is a narrow group of cells separated by the external and extreme capsules, is situated immediately lateral to the putamen (Fig. 13-4). However, the functions of this region are not clearly understood, and this region is generally not included as part of the basal ganglia.
