Frontal Section Through the Rostral Thalamus
In the myelin staining, the anterior and me-dial nuclear groups are clearly distinguishable from the lateral nuclear group by their poor and delicate myelination. The dorsally located anterior nuclear group (green) (A – C1) bulges against the interventricular foramen (foramen of Monro) (AB2) and forms the thalamic eminence. Themedial nuclear group (red) is enveloped by the internal medullarylamina (B3) and the intralaminar nuclei (C4)which separate it from the lateral portion. Within the medial nuclear group, a medial magnocellular portion (AC5) and a lateral parvocellular portion (AC6) surrounding the medial one can be distinguished.
The largest part of the thalamus is formed by the lateroventral nuclear group (blue), which surrounds the medial portion like a broad shell. It contains considerably more myelin, and a difference between its dorsal and ventral regions can be recognized in the myelin stained section (B). As compared to the dorsal region (lateral dorsal nucleus) (A – C7), the ventral nuclear region has more prominent and coarser myelin fibers. Its di-vision into a medial and a lateral segment can be easily recognized in the overview. The section shows the ventral lateral nu-cleus. In its medial segment (A–C8) termi-nate fibers from the midbrain tegmentum. Lateral to it is seen the rostral part of the nu-cleus (A – C9) where the fiber bundles of the superior cerebellar peduncle terminate; its projection to the precentral area (area 4) re-veals a somatotopic organization.
The lateral surface of the thalamus is formed by the reticular nucleus of the thalamus (A – C10). As a narrow layer of cells, this nucleus laterally surrounds the entire thalamus like a shell and extends from the rostral pole, where it is widest, to the pulvi-nar and to the lateral geniculate nucleus. It is separated from the lateral nuclear com-plex by a lamella of myelin fibers, the exter-nal medullary lamina (B11). The relation-ships between cerebral cortex and reticular nucleus vary for different nuclear segments: the frontal cortex is connected with the ros-tral portion of the nucleus, the temporal cortex with the middle portion, and the occipital cortex with the caudal portion. The functional significance of this nucleus is un-known. Its neurons send many collaterals to the other thalamic nuclei.
Fiber relationships between the thalamic nuclei and certain cortical areas have been established by experimentally destroying the cortical segments or by severing the fibers. The neurons of the respective nuclei undergo retrograde degeneration once their axons have been severed. Neurons of the re-ticular nucleus, however, are thought to un-dergo transneuronal degeneration rather than retrograde degeneration, that is, they not degenerate because their axons have been severed but because they have lost af-ferent fibers terminating on them. This would mean that the cortex projects to the reticular nucleus, while the latter does not project to the cortex.