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Lab 11 - The Limbic System

Hypothalamic Nuclei (continued)

Each of the hypothalamic regions is subdivided into medial and lateral zones. The medial zone mostly contains nuclear structures, while the lateral zone predominantly contains fiber bundles passing through to other brain regions. In photomicrographic slides, it is virtually impossible to identify individual hypothalamic nuclei with the exception of the medial mammillary nuclei in the mammillary bodies. Learn to identify the approximate locations of these nuclei by learning their positions relative to other well-defined landmarks. Be certain that you can identify the region containing the nuclei. Look for the structures in bold type.

Figure 1, Layer A: We are in the anterior region of the hypothalamus. The anterior region itself is divided into two parts. The more rostral area is defined by the presence of the anterior commissure. Locate the preoptic nuclei. These nuclei are bounded inferiorly by the optic chiasm and superiorly by the anterior commissure (an important landmark for identifying the location of these nuclei). The preoptic nuclei are poorly differentiated, and form the gray area surrounding the most rostral part of the third ventricle (known as the supraoptic recess). The recess is the space directly above the optic chiasm. The suprachiasmatic nucleus, a small group of cells, lies immediately superior to the optic chiasm (frankly, not easy to see; identify only the general location).

Figure 1, Layer B: Identify the approximate locations of the nuclei in more caudal areas of the anterior region of the hypothalamus (note the absence of the anterior commissure). The supraoptic nucleus is located along the superior surface of the optic tract. Caudal to the optic chiasm (i.e., at the level of the optic tract), the anterior hypothalamic nucleus replaces the preoptic nucleus. The anterior nucleus consists of clusters of cells that are scattered in the medial zone above the supraoptic and below the paraventricular nuclei. The paraventricular nucleus consists of a fairly broad plate of cells near the wall of the third ventricle, superior to the anterior nucleus. It extends rostrally almost to the anterior commissure and superiorly approaches the hypothalamic sulcus. A thin sheet of small cells located just underneath the ependymal lining of the third ventricle is collectively known as the periventricular nucleus. The periventricular nuclei are connected to the midbrain periaqueductal gray through the dorsal longitudinal fasciculus. This pathway provides an indirect link between the hypothalamus and the autonomic nervous system (important!)

In the lateral zone of the anterior region, fibers of the medial forebrain bundle (MFB) appear slightly darker, because they are thinly myelinated. The MFB passes through the lateral area and interconnects the hypothalamus with rostral and caudal brain regions. The lateral nuclei are clusters of cells scattered throughout the lateral area. The MFB also provides a link between the hypothalamus and the autonomic nervous system (important!)

Figure 1, Layer C: We are now in the tuberal region of the hypothalamus. In the tuberal region, the hypothalamus reaches its greatest width. You should be able to identify this region by the appearance of the median eminence and infundibular stem. The medial zone of the tuberal region contains the dorsomedial, ventromedial and arcuate ("Infundibular") nuclei, while the lateral zone contains the lateral and tuberal nuclei. At this level, the postcommissural fornix passes caudally on its way to terminate in the mammillary bodies. This prominent fiber bundle divides all regions of the hypothalamus into medial and lateral zones (use the fornix as a landmark to distinguish between the two zones).

The paraventricular nuclei are absent or reduced in size at this level of the hypothalamus. The dorsomedial and ventromedial hypothalamic nuclei, which are merged in humans, replace the anterior and paraventricular nuclei. In most mammals, the arcuate nucleus is situated ventrolaterally, one on each side of the floor of the third ventricle. In humans, however, these nuclei merge to form a single unpaired structure, situated medially at the base of the third ventricle.

Figure 1, Layer D: We are now in the posterior (or mammillary) region. Appropriately enough, the Posterior Hypothalamic Nucleus can be seen here. This is the most caudal region of the hypothalamus. The lateral zone in this region is reduced in size. The medial zone contains the medial mammillary nucleus. The neurons of the posterior hypothalamic region give rise to descending fibers, which project via the dorsal longitudinal fasciculus to the central gray and the tectum in the midbrain, a link to the autonomic nervous system.

The postcommissural fornix, a large fiber bundle originating in the subiculum of the hippocampal formation, terminates in the lateral part of each medial mammillary nucleus. The mammillothalamic tract is the dark fiber bundle located on the superior surface of the mammillary nucleus. It contains mammillary nucleus axons that are traveling to the anterior thalamic nucleus.

NOTE: The term mammillary bodies refers to the bulging structures seen outside, on the ventral aspect of the brain. The mammillary nucleus, especially the medial mammillary nucleus, the larger of the two mammillary nuclei, can be seen only in sections cut through the mammillary body, as shown in the photomicrographic slides.