Vertebrate brain theory

ISBN 978-3-00-064888-5

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

3.11  The formation of the vestibulocerebellum for the inversion of minimum coded vestibular signals of the neovestibular sense

The output of the vestibular system was supposed to trigger motor corrective movements, so it reached the motor nucleus of the primordial brain, the nucleus ruber. The signals were divided into left and right signals that reached the nucleus ruber of the respective side of the body.

In paleovestibular sense - a grain of sand rolled to the lowest point in the statocysteine - the vestibular sense was maximum coded and served to actively control the motor neurons to trigger suitable corrective movements. For this purpose, a signal crossing in the crossing level of the primordial brain was necessary so that the vestibular signals reached the muscles of the opposite side. Only in this way was a position correction possible.

After the conversion the signal was minimum coded. The muscle that was supposed to contract received the zero signal, while the others were more or less strongly excited. This led to false reactions.

One way to restore the original reactions was the signal inversion of the vestibular signals. Then the signal minimum, which could not generate a correction movement, would be transformed into a usable signal maximum. A signal inversion inverted the monotony, minima became maxima and vice versa. We also call this extreme value inversion.

The signal inversion takes place in three steps.

1. switching to inhibiting transmitters (GABA).

2. providing an exciting average signal.

3. relative inhibition of the average signal by the inhibiting signal from step 1.

The signal inversion of the vestibular signals occurred twice, in two independent subsystems. This allowed one subsystem to fail without losing vestibular control.  

The first signal inversion of the vestibular signals occurred in the vestibular system itself. The continuous signal necessary for inversion was provided by the Nucleus Deiters, which functioned as the mean value nucleus of this floor. It supplied the necessary mean value signal.

The second signal inversion took place at the level of the seventh segment.

The first step for both variants was taken by the Purkinjekern. It received all excitatory vestibular signals that travelled via the nucleus ruber to the nucleus olivaris and from there to the nucleus Purkinje of the opposite side and switched them to the inhibitory transmitter GABA (the same signals naturally travelled on the class 5 connective neurons to the equilateral motor neurons).

Some of the output signals of the Purkinje nucleus were signal related to the output of the vestibular system, they were the inverted vestibular signals. They moved on the axons of the Purkinje cells again in the direction of the vestibular system. There they contacted (in the course of a longer lasting evolutionary process) the mean value neurons of the nucleus deiter, which at that time was a pure mean value nucleus, and inhibited their excitation relatively. This caused a signal inversion

, which resulted in excitatory signals. The signal inversion converted the minimum coded vestibular signals into maximum coded and excitation signals. These were thus very useful for motor control of the motoneurons and replaced the original vestibular signals until only they were used. It should be pointed out that the ascending and descending vestibular signals must pass through the crossing floor

 and thus change sides.

The continuous signal necessary for inversion is generated within the nucleus deiter by very conspicuous giant neurons (diameter 40-70 μm). These neurons absorb the input of the nucleus and form a permanent signal. They are still silent witnesses of the original mean activity of this nucleus.

Theorem of the functional change of the nucleus deiter

The Nucleus Deiters is used for the signal inversion of the vestibular signals of the neovestibular sense, so that they are maximum coded. The necessary mean signal is formed by the nucleus itself from its input, it was originally the vestibular mean nucleus.

We call the nucleus deiter an inversion nucleus because it causes a signal inversion of the input.

In the course of evolution, a second inversion circuit formed at the level of the seventh segment. The corresponding mean nucleus at the level of the nucleus ruber was already present and is called Formatio reticularis.

Theorem of the Formatio reticularis as mean centre of the brainstem

The Formatio reticularis is a mean value centre of the input segment of the primordial brain and generates a stable mean value signal.

The inhibitory axons of the Purkinje nucleus ran at the level of the nucleus ruber to the mutual nucleus ruber, where they had an inhibitory effect. In the course of evolution, they were able to occupy new mid-level neurons of the reticular format, which they relatively inhibited. The output signals of these neurons were now the inverted vestibular signals, they were now maximum coded. Therefore they were (also) suitable for vestibular position correction. These signals only had to find the way to the motoneurons. Since these signals were derived from sensory, vestibular signals, they were also treated as sensory signals in the neural tube. They contacted (newly formed) class 4 connective neurons, which projected in the head. In the uppermost, cortical segment, these signals changed to the motor side via the commissure class 3 neurons. There they contacted motor connective neurons of class 5, which projected downwards to the motor neurons. Thus the vestibular position correction was complete and functional as a circuit.

Thus, the cortical level also had the control signals of the neovestibular sense and could feed them to further evaluation.

Remarkable is the inhibiting projection of the Purkinje nucleus into the nucleus deiter. When the Cerebellum developed from the Purkinje nucleus, this projection remained one of the few where the Purkinje cells as projection neurons projected directly inhibiting into another, relatively distant structure.

In the course of evolution, the Formatio reticularis disintegrated into two independently acting subcores. The first nucleus remained to provide the mean signals and continued to form the Formatio reticularis. The second nucleus consisted of those mean-value neurons that were relatively inhibited by the neurons of the Purkinje nucleus. This nucleus became the nucleus fastegii. On the one hand it received its mean value signals from the reticular format and on the other hand the inhibitory input from the Purkinje nucleus. Its output were the inverted vestibular signals. This nucleus fastegii was the second step on the way to a cerebellum. The first step was the formation of the Purkinje nucleus, which would develop into the later cerebellum. The nucleus fastegii became the first and earliest cerebellum nucleus.

However, these inverted vestibular signals were on the wrong side of the body due to the olive side change. There, the output axons of the nucleus fastegii reached the inner side of the neural tube and were treated in the same way as all sensory input axons, contacting new class 4 connective neurons and joining the ascending axons of the remaining connective neurons. As is well known, these moved upwards towards the cortical level, having previously passed through the crossing level where they changed sides and crossed to the contralateral side. In this way the vestibular signal crossing was reversed at the level of the nucleus olivaris. The incoming, inverted and excited vestibular signals moved to the cortical turning level. On their way there, they passed the previous nucleus deiter and docked to exactly those connective neurons that already represented the inverted and excitatory vestibular signals. Thus, these signals were available there quasi twice.

The Nucleus Deiters (Nucleus vestibularis lateralis) became the initial nucleus of the vestibular system, its output signals are maximum coded. It projects (among others) via the tractus vestibulothalamicus over the thalamus to the cortical turning floor. Via connective neurons the signals reach the motor cortex side and from there descending via the nucleus ruber to the motoneurons. These are now controlled with the inverted (and excited) vestibular output and lead to meaningful corrective movements.

If a differentiation of neurons takes place in an existing nucleus and a group of differentiated neurons spatially splits off from this nucleus, we call the newly formed nucleus a descendant of the old nucleus. For a more precise specification, we can also indicate from which neurons the new nucleus is a descendant. This allows us to make the following statements.

Theorem of the derivatives of the nucleus ruber and the nucleus reticularis

The Purkinje nucleus is a descendant of the inhibitory interneurons of the nucleus ruber. It receives its input from the nucleus olivaris and projects inhibitingly to the opposite side into the nucleus fastegii.

The nucleus fastegii is a descendant of the mean neurons of the reticular nucleus and is an inversion nucleus. It receives the input to be inverted from the Purkinje nucleus and the required mean excitation from the nucleus reticularis and projects excitatory partly ascending to the thalamus and partly to the nucleus ruber.

Theorem of the formation of the vestibulocerebellum

The original vestibulocerebellum was formed from the Purkinje core and the nucleus fastegii. Its input came from the nucleus olivaris, which received its input from the vestibular system via the nucleus ruber of the opposite side. The nucleus fastegii received its mean signal from the Formatio reticularis and passed it on to each of its inversion neurons, which in turn were inhibited by the vestibular signals, since these had previously been switched to GABA in the olive. The output of the nucleus fastegii was the inverted output of the neovestibular sense. It was maximum coded by the signal inversion and was used for motor correction control of body position.

The original vestibulocerebellum consisted only of Purkinje cells. Nothing more was needed for signal inversion. The formation of basket cells, star cells, golgi cells and granule cells including parallel fibres occurred at a much later evolutionary time. However, the reason for this was already given: In all neuronal nuclei inhibitory interneurons were formed, which served to enhance the contrast between the signals.

Theorem of signal inversion in the nucleus fastegii

The nucleus fastegii serves the signal inversion of the signals from the Purkinje nucleus.

Therefore we call the nucleus fastegii also the inversion nucleus.

The inverted vestibular signals were now available twice: first as output of the Nucleus Deiter and second as output of the Nucleus fastegii. The latter was to become important for future vertebrates, since its signals reached the cortex. When new trunk and extremity muscles were formed much later, they were able to interact with the vestibular signals in the cortex, even though it was no longer possible to take these signals into account in the nucleus deiter, because this nucleus had become a very conservative structure in the meantime and could no longer change.


Monograph of Dr. rer. nat. Andreas Heinrich Malczan