The Caudal Diencephalon

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The pretectum (alar p1) is the caudalmost forebrain region. It is characterized by the posterior commissure, whose fibers cross the pretectal dorsal midline and then course transversally through the alar plate, just in front of the diencephalomesencephalic limit, before spreading longitudinally in the basal plate (pc in Fig. 3). The function of the posterior commissure is unclear. This region contains various pretectal nuclei involved in visual processing, including the centers for the pupillary and optokinetic eye reflexes. The subcommissural organ is a dorsal midline specialization that secretes glycoprotein (Reissner's fiber) into the ventricular fluid. The basal plate has dopamine-containing neurons that form part of the substantia nigra in addition to diverse reticular cell populations involved in motor circuits, like the parvocellular nucleus ruber (origin of the rubroolivary tract) and the interstitial nucleus of Cajal. The latter is a rostral source of descending preoculomotor axons in the medial longitudinal fasciculus, a tract that coordinates eye movements.

Anterior of the pretectum lies the dorsal thalamus and epithalamus complex (alar p2). Its dorsal midline includes the epiphysis (pineal gland), the habenular commissure, and choroid plexus rostrally. The epitha-lamus (ET or habenula) is the most dorsal nuclear complex; these neurons relate to the longitudinal stria medullaris tract (sm), which crosses the dorsal midline in the habenular commissure (hc in Fig. 3). The habenular nuclei produce the retroflex tract (habenu-lointerpeduncular tract). This compact transverse fascicle is a landmark that remains just anterior to the p1/p2 boundary throughout its dorsoventral extent; near the diencephalic floor it bends caudally (retroflects), continuing longitudinally across p1 and

Figure 3 Schema of subdivisions of the caudal diencephalon and relevant anatomical landmarks in lateral view (rostral to the left; alar/basal limit as in Fig. 2): The pretectum (PT) in alar pi contains the posterior commissure (pc); the dorsal thalamus (DT) and epithalamus (ET) in alar p2 contain to the retroflex tract (rt). The zona limitans (zl) separates dorsal and ventral thalami (VT) and seems to contain an expansion of the basal plate (according to gene expression data). The stria medullaris tract (sm) courses longitudinally near the roof choroidal plexus (cp). (Right) A schematic cross section through the dorsal thalamus, whose section level is indicated in the schema on the left, shows various nuclear groups (each of them is further subdivided into several nuclei), separated by the axon-rich internal medullary lamina (iml). See Table I for other abbreviations.

Figure 3 Schema of subdivisions of the caudal diencephalon and relevant anatomical landmarks in lateral view (rostral to the left; alar/basal limit as in Fig. 2): The pretectum (PT) in alar pi contains the posterior commissure (pc); the dorsal thalamus (DT) and epithalamus (ET) in alar p2 contain to the retroflex tract (rt). The zona limitans (zl) separates dorsal and ventral thalami (VT) and seems to contain an expansion of the basal plate (according to gene expression data). The stria medullaris tract (sm) courses longitudinally near the roof choroidal plexus (cp). (Right) A schematic cross section through the dorsal thalamus, whose section level is indicated in the schema on the left, shows various nuclear groups (each of them is further subdivided into several nuclei), separated by the axon-rich internal medullary lamina (iml). See Table I for other abbreviations.

Ventrobasal Schema

Figure 4 This schema highlights subdivisions in the rostral diencephalon in the context of the overlying telencephalon and caudal diencephalon. We call ''hypothalamus'' the basal part of the rostral diencephalon and ''prethalamus'' the alar part. These are both divided into three prosomeres (p4-p6) and various smaller areas. See Table I for abbreviations.

Figure 4 This schema highlights subdivisions in the rostral diencephalon in the context of the overlying telencephalon and caudal diencephalon. We call ''hypothalamus'' the basal part of the rostral diencephalon and ''prethalamus'' the alar part. These are both divided into three prosomeres (p4-p6) and various smaller areas. See Table I for abbreviations.

the midbrain into the interpeduncular nuclear complex in the isthmic floor plate (rt in Fig. 3).

The dorsal thalamus (DT) lies ventral to the epithalamus and consists of an elaborate complex of nuclei that generally project to the telencephalon (targeting both subcortical and cortical structures) (Fig. 3 and arrow 11 in Fig. 5A). These nuclei serve as relay centers for numerous telencephalopetal pathways with diverse functional implications. The tele-ncephalic cortex sends topographically ordered projections to the respective thalamic relay centers. The main subdivisions of the DT contain groups of related nuclei. The intralaminar nuclei lie within the fiber-rich internal medullary lamina, which separates the other nuclear groups (iml in Fig. 3). These intralaminar nuclei form a separate projecting system with a modulatory role over the cortex and basal ganglia (embodying the final stage of the ascending activating system for mental arousal). The sensory nuclei process somatosensory and viscerosensory information (ventrobasal complex), visual input (dorsal lateral geniculate nucleus), and auditory input (medial geniculate nucleus), which are then relayed by these nuclei to the primary sensory areas of the isocortex via axons in the internal capsule (telence-phalic peduncle; arrow 11 in Fig. 5A). Other thalamic nuclei are part of the motor control system (ventral anterior and ventrolateral nuclei); these process inputs from the globus pallidus and the cerebellar dentate nucleus and send efferents to the motor and premotor cerebral cortex. The anterior and periventricular thalamic nuclei are the limbic part of the thalamus, receiving hypothalamic (mammillary and other) projections and projecting to the limbic cingulate cortex, hippocampus, septum, and amygdala. Another important group of dorsal thalamic nuclei is conceived as the associative group (lateral dorsal, lateral posterior, pulvinar, and medial nuclei). These nuclei receive their major inputs from parts of the secondary sensorimo-tor, limbic, and associative cortex and project again into higher order cortical areas of the frontal, parietal, and temporal lobes. The associative nuclei are particularly developed and become secondarily subdivided and specialized in man.

The basal plate of p2 is poorly understood; it resembles that of p1 in that it contains a part of the dopaminergic neurons of the substantia nigra and of the median ventral tegmental area, in addition to some reticular populations participating in preoculomotor functions (i.e., the rostral interstitial nucleus).

The transition from p2 to p3 is characterized by several major changes in molecular and developmental properties. The p2/p3 limit appears as a fiber-rich gap between dorsal and ventral thalami; it is known as the zona limitans intrathalamica, or external medullary lamina (zl in Figs. 3 and 4).

Alar p3 is also known as the ventral thalamus for historical reasons, even though it is anterior to the dorsal thalamus (VT in Figs. 2 and 3; compare resulting position in bent forebrain, as in Fig. 1D). The choroid plexus of p2 extends through p3 and p4 into the telencephalon (cp; dotted roofdomain in Figs. 1F, and 3 and 4; striped roof in Fig. 2). A choroid plexus is a neuroepithelial specialization where blood plasma is filtered into the brain ventricles as cerebral spinal fluid, which circulates and then flows out from holes in the hindbrain roof. Alar p3 consists of the ventral lateral geniculate nucleus (a superficial reflex visual center), the reticular thalamic nucleus (traversed by the thalamocortical fiber bundles), and a periven-tricular zona incerta. The latter two nuclei contain many inhibitory neurons, whose projections spread widely ipsi- and contralaterally in the alar and basal plates of the caudal and rostral diencephalon, perhaps reaching the midbrain. Inhibition exerted by the reticular nucleus on the dorsal thalamus organizes rhythmic electrical activity characteristic of sleep.

The basal plate of p3 consists of the so-called posterior hypothalamic area and the retromammillary area, which is frequently named the "supramammil-lary area.'' The latter is characterized by some dopaminergic neurons and other reticular cell populations.

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