Chapter 15
The Diencephalon
Although it is considered by some investigators to be part of the brainstem, the diencephalon is treated here as a portion of the forebrain. The diencephalon includes the dorsal thalamus, hypothalamus, ventral thalamus, and epithalamus, and it is situated between the telencephalon and the brainstem. In general, the diencephalon is the main processing center for information destined to reach the cerebral cortex from all ascending sensory pathways (except those related to olfaction). The right and left halves of the diencephalon, for the most part, contain symmetrically distributed cell groups separated by the space of the third ventricle.
OVERVIEW
The dorsal thalamus, or thalamus as it is commonly called, is the largest of the four principal subdivisions of the diencephalon and consists of pools of neurons that collectively project to nearly all areas of the cerebral cortex. Some of the thalamic nuclei receive somatosensory, visual, or auditory input and transmit this information to the appropriate area of the cerebral cortex. Other thalamic nuclei receive input from subcortical motor areas and project to those parts of the overlying cortex that influence the successful execution of a motor act. A few thalamic nuclei receive a more diffuse input and accordingly relate in a more diffuse way to widespread areas of the cortex.
The hypothalamus is also composed of multiple nuclear subdivisions and is connected primarily to portions of the forebrain, brainstem, and spinal cord. This part of the diencephalon is involved in the control of visceromotor (autonomic) functions. In this respect, the hypothalamus regulates functions that are “automatically” adjusted (such as blood pressure and body temperature) without our being aware of the change. In contrast, conscious sensation and some aspects of motor control are mediated by the dorsal thalamus.
The ventral thalamus and epithalamus are the smallest subdivisions of the diencephalon. The ventral thalamus includes the subthalamic nucleus, which is linked to the basal nuclei of the forebrain and functions in the motor sphere; lesions in the subthalamus give rise to characteristic involuntary movement disorders. The epithalamus is functionally related to the limbic system.
DEVELOPMENT OF THE DIENCEPHALON
The cell groups that give rise to the diencephalon form in the caudomedial portion of the prosencephalon, bordering on the space that will become the third ventricle. The developing brain at this level consists initially of a roof plate and the two alar plates; it lacks a well-defined floor plate and basal plates.
A shallow groove appears in the wall of the third ventricle and extends rostrally from the developing cerebral aqueduct to the ventral edge of the interventricular foramen (Fig. 15-1A, B). This groove, the hypothalamic sulcus, divides the alar plate into a superior (dorsal) area, the future dorsal thalamus, and an inferior (ventral) portion, the future hypothalamus. The third ventricle, which is the vertically oriented, midline space of the diencephalon, is continuous with the paired lateral ventricles via the interventricular foramina (foramina of Monro) (Figs. 15-1B and 15-2). The dorsal thalamus on each side of the third ventricle increases rapidly in size and, in about 80% of patients, will partially fuse across the space of the third ventricle to form the massa intermedia, or interthalamic adhesion (Figs. 15-2 to 15-4). The epithalamus develops from the caudal portion of the roof plate (Fig. 15-1B). By the seventh week, a small thickening of the roof plate forms. It gradually increases in size and evaginates to form the epiphysis, which develops into the pineal gland of the adult (Figs. 15-2 to 15-4). The portion of the roof plate immediately rostral to the epiphysis gives rise to the habenula, a small thickening in which the habenular nuclei will develop (Figs. 15-2 and 15-4).
Figure 15-2. Axial T2-weighted magnetic resonance image showing the interventricular foramen, the massa intermedia, the area of the habenula, and the pineal in the superior cistern. The interventricular foramen is the space (containing a small portion of choroid plexus) located between the column of the fornix and the anterior tubercle of the thalamus. The major nuclei of the dorsal thalamus in an axial plane are shown in Figure 15-10.
Figure 15-3. Midsagittal view showing the locations of circumventricular organs.
Figure 15-4. Midsagittal view of the diencephalon and closely related structures. This is a drawing of the specimen shown in Figure 15-6.
Just anterior to the habenular region, the roof plate epithelium and adjacent pia mater give rise to the choroid plexus of the third ventricle, which, in the adult, remains suspended from the roof of this space (Fig. 15-1B-D). This choroid plexus is continuous through the interventricular foramina with that of the lateral ventricles. Elsewhere, in locations around the perimeter of the third ventricle, specialized patches of ependyma lie on the midline and form unpaired structures called the circumventricular organs. These structures include the subfornical organ, the organum vasculosum of the lamina terminalis, the subcommissural organ, and the pineal gland (Fig. 15-3). These cellular regions are characterized by the presence of fenestrated capillaries, which implies an absence of the blood-brain barrier. These structures are thought to release metabolites and neuropeptides into the cerebrospinal fluid or into the cerebrovascular system.
The development of the pituitary gland during the third week is linked to that of the diencephalon (Fig. 15-1B, C). A downward extension of the floor of the third ventricle, the infundibulum, meets the Rathke pouch, an upward outpocketing of the stomodeum, the primitive oral cavity. By the end of the second month, the Rathke pouch loses its connection with the developing oral cavity but maintains its attachment to the infundibulum. As development continues, the Rathke pouch gives rise to the anterior lobe (adenohypophysis) and pars intermedia of the pituitary gland, whereas the infundibulum differentiates into the posterior lobe of the pituitary gland, or neurohypophysis (Fig. 15-1B; see Chapter 30). A craniopharyngioma (Rathke pouch tumor) can arise from a portion of the Rathke pouch that fails to undergo proper migration and apposition to the infundibulum. These tumors mimic lesions of the pituitary gland and may cause visual problems, diabetes insipidus, and increased intracranial pressure.
BASIC ORGANIZATION
The junction between the diencephalon and midbrain lies along a line extending from the posterior commissure to the caudal edge of the mammillary body on the medial aspect of the hemisphere (Fig. 15-4). On the surface of the hemisphere, this interface is represented by a line starting at the caudal aspect of the mammillary body, extending anterolaterally over the edge of the crus cerebri, and following the caudal edge of the optic tract (Fig. 15-5). The boundary between the diencephalon and surrounding telencephalon is less distinct and is represented laterally by the internal capsule and rostrally by the interventricular foramen, lamina terminalis, and optic chiasm (Fig. 15-4).
The cavity of the diencephalon, the third ventricle, is a narrow, vertically oriented midline space located between the paired dorsal thalami and hypothalami of the two sides (Figs. 15-6 and 15-7). In addition to its connections with the lateral ventricles and the cerebral aqueduct, the third ventricle has small evaginations or recesses associated with the optic chiasm (supraoptic recess), the infundibulum (infundibular recess), and the pineal gland (pineal and suprapineal recesses) (Figs. 15-1 and 15-4).
Figure 15-6. Midsagittal view of the diencephalon. This view correlates with the drawing in Figure 15-3. Lines A through D indicate the planes of the stained sections in Figure 15-7.
Figure 15-7. Four levels of the forebrain from rostral (A) to caudal (D) showing the internal structure of the hemisphere with emphasis on the diencephalon. These levels correlate with those shown in Figure 15-6 and with the planes represented in the exploded view in Figure 15-10. Weil stain.
All four diencephalic subdivisions can be approximated in a midsagittal section of the forebrain (Figs. 15-4 and 15-6). The dorsal thalamus is located superior to the hypothalamic sulcus and extends from the interventricular foramen caudally to the level of the splenium of the corpus callosum. The hypothalamus lies inferior to the hypothalamic sulcus and is bordered rostrally by the lamina terminalis and caudally by a line that extends from the posterior aspect of the mammillary body superiorly to intersect with the hypothalamic sulcus. The only diencephalic structures visible on the inferior surface of the hemisphere are those related to the hypothalamus, including the optic chiasm, infundibulum, medial and lateral eminences, and mammillary bodies (Fig. 15-5). The ventral thalamus (subthalamus) does not border on the ventricle; rather, it occupies a position caudal to the hypothalamus, rostral to the diencephalon-midbrain junction, and lateral to the midline (Figs. 15-4 and 15-7B). Epithalamic structures are located posteriorly and caudally, in close apposition to the posterior commissure, and include the pineal gland, the habenular nuclei, and the main afferent bundle of these nuclei, the stria medullaris thalami.
DORSAL THALAMUS (THALAMUS)
The dorsal thalamus (or thalamus) (Figs. 15-4 and 15-7 to 15-9) is a massive collection of neuronal cell groups that participate in a widely diverse array of functions involving motor, sensory, and limbic systems. It receives a variety of ascending inputs and projects, via thalamocortical fibers to various cortical areas or gyri, and receives reciprocal connections, via corticothalamic fibers, from those cortical targets to which it sends projections. As a result, the thalamus is often regarded as the functional “gateway” to the cerebral cortex.
Figure 15-9. T2-weighted magnetic resonance image of the cerebral hemisphere in the axial plane. Emphasis in labeling is on diencephalic structures. Compare with Figure 15-12.
The thalamus is covered on its lateral aspect by a layer of myelinated axons, the external medullary lamina, which includes fibers that enter or leave the subcortical white matter (Fig. 15-7B, C). Within the external medullary lamina are clusters of neurons that form the thalamic reticular nucleus. The medial surface of the thalamus borders the third ventricle, and the external medullary lamina and thalamic reticular nucleus blend with the thalamic fasciculus and zona incerta, respectively, to form an interface between dorsal and ventral thalami (Fig. 15-7B).
An internal medullary lamina, also consisting of myelinated fibers, extends into the substance of the thalamus, where it forms partitions or boundaries that divide the thalamus into its principal cell groups (Figs. 15-9 and 15-10): the anterior, medial, lateral, and intralaminar nuclear groups. The last cell group is located in the portion of the internal medullary lamina that separates the lateral and medial nuclear groups. In addition, there are midline thalamic nuclei located just superior to the hypothalamic sulcus.
Figure 15-10. Exploded view of the dorsal thalamus illustrating the organization of thalamic nuclei. Compare with Figures 15-6 and 15-8.
Finally, attached to the caudolateral portion of the thalamus are the medial and lateral geniculate bodies (and their correspondingly named subjacent nuclei) (Figs. 15-7D, 15-8D, and 15-10). Although considered here as components of the lateral nuclear group, the geniculate nuclei are sometimes considered as a separate part of the thalamus, the metathalamus.
