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 .
These
three structures can be further subdivided.
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
tegmentum contains mainly premotor functions and contains the following
substructures:
-
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.