Vertebrate brain theory

1.3 The substructures of the mesencephalon of vertebrates

A large part of the facts presented here are non-wordly quotations from the work "Wie einzigartig ist der Mensch" by Gerhard Roth, [48], pages 180 to 185. They would not gain substance by a reformulation.

The mesencephalon (midbrain) of vertebrates consists of

• the isthmic region
• the tegmentum
• the dorsally preceding torus and tectum region .

Isthmic region:

• contains the nucleus isthmi (called nucleus parabigeminalis in mammals), which is reciprocally and topographically (with a point-to-point connection) connected to the tectum (Collicului superiores of mammals) in all vertebrates and coordinates the activity of the two tectum hemispheres, for example in connection with the orientation of eye, head and hand movements and with depth perception.

• The ventral part contains the motor nuclei of the oculomotor nerve (III cranial nerve) and the trochlear nerve (IV cranial nerve).
• In the dorsal part are the nucleus of the Fasciculus longitudinalis mediales and the dorsal tegmental nucleus, which have close connections with the tectum and the colliculus superior respectively and have vestibular functions, especially in the coordination of head movements.
• The tegmentum also contains a number of other nuclei such as the dorsal and ventral tegmental nuclei and the pedunculopontinous nucleus. They form an important link between the limbic centres of the diencephalon and the endbrain, the periaqueductal grey, the reticular formation and the visceral areas in the medulla oblongata in connection with the emotional and vegetative control of the body and behaviour.
• The tegmentum contains the substantia nigra and the area tegmentalis ventralis, i.e. the ventral tegmental area. Both contain dopamine-producing projection neurons. The VTA projects predominantly to the ventral striatum, more precisely to the nucleus accumbens and is closely integrated into the limbic system. The substantia nigra, however, projects to the dorsal corpus striatum. The dopaminergic system is related to the reward and motivation system.
• The tegmentum contains the nucleus ruber, which is already detectable in the earliest vertebrates. In humans it consists of a parvocellular and a magnocellular part, receives input from the motor cortex via the tractus corticorubralis and via the tractus cerebellorubralis from the cerebellum. The output reaches the spinal cord via the tractus rubrospinalis and via the tractus rubroolivaris the inferior olive.
• In the dorsal torus and tectum region lies the torus semicircularis of vertebrates, which corresponds to the colliculus inferior of mammals. It lies below the midbrain roof (tectum mesencephali) and is an important switch point of the auditory, mechano- and electroreceptive system between the medulla oblongata, cerebellum and diencephalon. It consistently shows lamination or nucleation, with different layers or nuclei receiving or processing different sensory modalities. The lamination shown by Roth in the aforementioned work "How unique is man" on page 182 in Figure 27 and its exact description was the key to the author's understanding of the interaction of signals of different modalities in the so-called "interference layers", which could ultimately explain motor or intellectual activities of vertebrates at all. However, the theoretical element of "interference layers" or "interference nuclei" can only be explained and understood if the cytoarchitectonic structure of such layered or nuclear systems is explained and understood.
• According to Roth, the tectum mesencephali is the most important somatosensory, visual and auditory integration centre of the brain in all anamneses, all sauropsides and many mammals. In most vertebrates, it exhibits a laminar organisation in which cell and fibre layers alternate, with different inputs and outputs. The number of layers varies depending on the importance of the different sensory functions for the specific animal species in the context of its particular way of life.
• In mammals, the tectum mesencephali is formed by the four-mound plate (Corpora quadrigemina). It consists of the anterior and posterior mounds (Colliculi superiores and Colliculi inferiores). The superior colliculi correspond to the tectum of the other vertebrates and play an essential role in visually and auditory triggered eye and head movements as well as in directed hand and arm movements. The inferior colliculi are important centers of the hearing system. The tetum mesencephali of mammals is relatively small compared to that of other vertebrates and probably reduced because the isocortex has taken over corresponding tasks.

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

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