Fiber Connections of the Poorly Myelinated Hypothalamus
Numerous connections transmit olfactory, gustatory, viscerosensory, and somatosensory impulses to the hypothalamus. These are usually loose and divergent systems, and almost all of them are bidirectional. The following are the most important ones.
The medial forebrain bundle (medial telen-cephalic fasciculus), containing the olfacto-hypothalamotegmental fibers (B3), con-nects almost all hypothalamic nuclei with the olfactory centers and with the reticular formation of the midbrain. The bundle con-tains a large number of peptidergic fibers (VIP [vasoactive intestinal peptide], enkephalin, somatostatin).
The terminal stria (B4) runs in an arch around the caudate nucleus and connects the amyg-daloid body (B5) (pp. 226, 228) with the pre-optic region (B6) and the ventromedial nu-cleus (B7). It is rich in peptidergic fibers.
The fibers of the fornix (BC8) originate from the pyramidal cells of the hippocampus (BC9) and the subiculum and terminate in the mamil-lary body (BC10). A large portion of the for-nix fibers are peptidergic. At the level of the anterior commissure, fibers branch off from the fornix to the preoptic region and to the tuberal nuclei (B7).
The dorsal longitudinal fasciculus, or Schütz’s bundle (B11), is the most impor-tant component of an extensive periventric-ular fiber system. At the transition to the midbrain, fibers collect to form a compact bundle that connects the hypothalamus with the nuclei of the brain stem.
Connections with thalamus and pallidum: Thehypothalamic nuclei are connected by peri-ventricular fibers with the medial nuclei ofthalamus (B12), the fibers of which projectto the frontal cortex. In this way, an indirect connection is established between hypo-thalamus and frontal cortex. The pallidohy-pothalamic fasciculus runs from the globuspallidus to the tuberal nuclei (ventromedial nucleus).
Commissures: The commissures in the regionof the hypothalamus contain almost no fibers from hypothalamic nuclei; fibers from the midbrain and pons cross in the dorsal supraoptic commissure (Ganser’s commissure) and in the ventral supraoptic commissure (Gudden’s commissure), while fibers from subthalamic nuclei cross in the supramamillary commissure.