Vertebrate brain theory

1.2  The substructures of the vertebrate metencephalon

The main component of the metencephalon is the cerebellum (the cerebellum). Some authors also count the pons (the bridge) to the metencephalon. 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 176 to 180. They would not gain substance by a reformulation.

The Cerebellum is a paired organ, only the middle part itself is only present once, is called Corpus cerebelli (in mammals as vermis "worm") and according to Roth only occurs in the jaw-bearing vertebrates.

Originally, the cerebellum processed only the information from the vestibular system and the mechanosensory system and consisted of the vestibulocerebellum and the spinocerebellum. To this day, the author has not been able to find a reasonable explanation in the literature for the origin and, above all, the functioning of the vestibulo- and spinocerebellum. Here he is willing to present his own theory on this open problem. But before that, the basic facts have to be compiled.

Much of the information processed in the vestibulo- and spinocerebellum originates from the medulla oblongata and is supplied to the cerebellum via both the moss fibres and the climbing fibres. This information comes from

• the trigeminal and trochlear nerves
• the vestibular-nuclear complex (responsible for balance)
• the glossopharyneus (taste)
• the sensorimotor lower olive
• the primary and secondary afferences from the somatosensory system

The input reaches the cerebellum via two separate signal paths:

• Moss fibres provide input to the cerebellar nuclei and, via the granule cells, to the parallel fibres of the cerebellar cortex, the parallel fibres represent the axons of the granule cells.
• Climbing fibers provide input to the cerebellar nuclei and cerebellar lum neurons, with all types of neurons and interneurons of the cerebellum being reached by the climbing fibers. The climbing fibers are the axons of the projection neurons of the inferior olive nucleus, which is also called the olive nucleus.

Descending pathways of the vestibulo- and spinocerebellum usually lead from the nuclei of the cerebellum, but sometimes also directly into the lateral medulla oblongata and reach

• primarily the vestibular-core complex
• but also descend to the spinal cord
• a small but important projection moves to the area of Oculomotorius in the midbrain fragment
• Reciprocal connections exist with the nucleus ruber and are important for sensorimotor integration in posture, body motion control, prey capture control and the involuntary (optokinetic) eye movements that occur when turning the head.

A cerebellar new formation is the Cerebrocerebellum, also called Neocerebellum or Pontocerebellum. According to Roth "it occupies the largest, lateral part of the cerebellar hemispheres and was formed at the same time as the cerebral cortex of mammals. This part receives information from the motor and premotor cortex via the bridge nuclei and is responsible for fine motor skills and their 'smooth' operation". According to Roth, the simultaneous development of cerebrocerebellum and cerebral cortex in mammals requires the consideration of these facts in a theory of the brain and the cerebellum.

There are differences in size between the cerebellums of different vertebrate species, depending on the stage of development of the different senses and the use of the limbs. However, the basic structure of the cerebellum is very conservative and very uniform. Even the mammalian cerebrocerebellum has the same basic structure as its temporal predecessors.