Vertebrate brain theory

3.12  The origin of the origonal Spinocerebellum

From a certain stage of development, the nucleus olivaris also received the descending signals of all available trunk and head senses, but also the mean signals of these modalities. Initially, these signals were all integrated into the contralateral inhibition. At the latest with the change to the signal inversion of the vestibular signals, this signal transformation was also applied to the trunk and head senses. Vestibular, spinal and cranial signals were treated equally and were transmitted from the nucleus olivaris to the purkinjekern of the opposite side. Its output was inverted at the excitatory mean neurons of the developing cerebellar nucleus. Since these new signals were quite numerous, the neurons involved formed independent structures. Thus, a new structure was created next to the vestibulocerebellum, which we call spinocerebellum. The associated cerebellar nucleus for signal inversion is the nucleus interpositus.

On closer examination, it consists of two nuclei, the nucleus emboliformis and the nucleus globosus. Both serve the spinocerebellum as output nuclei. This dichotomy could be a consequence of the division of the input nucleus of the primordial brain, which also broke down into the subnuclei, the nucleus cuneatus and the nucleus gracilis, one of which was responsible for the upper and the other for the lower half of the fuselage. The latter had developed only later, before that there was the state of the cephalopods (Cephalopoda), which only had to develop an initial nucleus.

Also the original Spinocerebellum had only Purkinje cells. Only in the course of further evolution cerebellar interneurons such as basket cells, star cells, golgi cells and granule cells as well as their parallel fibres developed.

Since the output of the cerebellar nucleus was excitatory and projected into the contralateral nucleus ruber, the original contralateral inhibition was (gradually) replaced by the inverse excitation of the motor opponents. The original nucleus ruber was the motor nucleus of the primordial brain. Thus, the original contralateral inhibition at brainstem level was replaced by the inverse excitation of the motor opponents.

Theorem of the transition from contralateral inhibition to inverse excitation of the motor antagonist muscles

The descending motor signals to the corresponding motor neurons on one side of the body were additionally transmitted via the nucleus ruber to the nucleus olivaris, from where they reached the Purkinje cells of the developing, primordial spinocerebellum, which switched them to the inhibitory transmitter GABA. The GABAergic signals reached the neurons of the nucleus fastegii, which were permanently excited by the mean value signals of the Formatio reticularis, where they caused the signal inversion. Within the cerebellar nucleus, inhibitory interneurons caused lateral inhibition to enhance contrast between the output signals. The output signals of the cerebellar nucleus reached the cortex via thalamic nuclei and descended to the motor neurons on the opposite side to excite them inversely.

As a consequence, each excitation of a motor neuron (via whatever receptors) simultaneously led to an inverse contralateral excitation of the antagonist muscles.

The original principle of contralateral inhibition of the antagonist muscles of the trunk or inhibition of the ipsilateral antagonist muscles of the extremities (if present) at the spinal cord level was replaced by the principle of inverse excitation of these muscles at the brainstem level. This brought a considerable advantage. Because under the influence of gravity it was no longer sufficient to simply inhibit the antagonist muscles. A basic stability of the whole body against external influences could only be achieved if both muscles of a joint were tense, one more, the other less. The signal strength ratio of the motoneurons of the muscles involved ultimately resulted in the angle of flexion or the angle of rotation of the joint. The basic tension compensated for the gravitational load and other forces acting on the joint, such as those that occurred during locomotion in water. The excitation of a motor neuron and an inverse excitation of the antagonist neuron (co-activation) enabled the stable adjustment of any joint angle even under external load. This was an essential prerequisite for the original water-dwelling organisms to be able to leave the water and for the group of tetrapods, i.e. agricultural vertebrates, to develop. This form of inverse excitation of the antagonist muscles in a joint is also referred to as co-contraction in the literature [1] (Kandel, Schwarz, Jessell, Neurosciences, Spektrum Akademischer Verlag, page 532).

The development of inverse excitation also led to the first motor pacemakers. One cause was the small time delay with which the contralateral inverse excitation occurred. The second cause was that the spinocerebellum for both muscles formed an inverse excitation signal for the opponent, which led to feedback. Time-delayed feedback is the basis for motor pacemakers. As a simplified model, two pendulums connected by a longer cord, in the middle of which a small weight causes the coupling of both pendulums. When one pendulum is struck, the oscillation is gradually transferred to the second pendulum, while the first pendulum comes to rest. The second pendulum then transfers its oscillation energy to the first pendulum, whereby its oscillation intensity decreases. In this way, both pendulums oscillate with a time delay and alternately. Similarly, the meandering motion occurs, with the spinocerebellum realizing the coupling.

We consider that the nucleus ruber, which itself projected into the spinocerebellum via the nucleus olivaris, received its input from the (motor) cortex. This input was on the contralateral side due to the passage of the crossing floor.

Theorem of the function of early spinocerebellum

The early spinocerebellum realized the signal inversion of the output of the contralateral primal brain and used this output for the inverse excitation of the motor opponents, so that the contralateral inhibition was replaced by an inverse excitation.

It should be remembered that the Formatio reticularis also projected as a mean value nucleus excitatory (activating) into the motoneurons of the spinal cord and was involved in maintaining the basic tension of all muscles. Thus, there were two excitation paths: the basic tension excitation of the reticular format, which affected all muscles, and the inverse excitation, which only affected the antagonist muscles. It should be noted that almost every muscle of the trunk and later extremities had an antagonist muscle.

The nucleus fastegii received the necessary mean value signals from the reticular format. From the Purkinje nucleus it received the signals to be inverted, which originated from the olivaris nucleus. These were signal-related to the mean value signals, since the latter were also obtained from them. Therefore, contact was established between them. Thus, the climbing fibers also contacted the cerebellar nucleus and thus stabilized its permanent excitation. This principle - it had probably already been developed in the vestibulocerebellum - manifested itself and was adopted in all substructures of the cerebellum, including the later pontocerebellum.

Theorem of excitation of cerebellar nuclei by climbing fiber signals

In all cerebellum structures (vestibulo-, spino- and pontocerebellum) the climbing fiber signals contribute to the mean excitation of the cerebellar nuclei and stabilize their function as inversion nuclei.

In the course of evolution, the Formatio reticularis was no longer able to provide the mean value signals required for signal inversion for the spinocerebellum that was forming. This was no longer a tragedy, because the signals descending from the first segment, i.e. cortical signals, included not only the signals of class 5 but also those of class 6. Those of class 5 still originated from the neural tube or spinal cord, where they ascended on the sensory side, reached the top level and there, via class 3 neurons, made the lateral change to the motor side and from there descended to the nucleus ruber. The mean class 6 neurons were also located in the uppermost cortical level on the motor side and formed signal mean values by tapping the nearby class 5 signal neurons.

These mean value neurons naturally also projected in a descending order to the output nucleus of the primordial brain, i.e. to the nucleus ruber. From there they reached the mean value systems of the different levels to contribute to mean excitation. In this way, new body mean values emerged from the segment mean values, which were much more suitable for controlling life processes.

When descending from the cortical level, these mean signals also passed through the crossing level and thus reached the spinocerebellum, where they docked at exactly those output neurons of the nucleus interpositus that served to invert the descending signal class 5. Thus, the spinocerebellum received in its cerebellar nuclei - over a longer period of time - instead of the average signals from the reticular format, those from the cortical areas.

Theorem of the conversion of the mean value supplier for the spinocerebellum

In the course of a longer conversion process, the output neurons of the cerebellar nuclei of the spinocerebellum were no longer supplied primarily by the reticular format with the mean signals required for signal inversion, but by the cortical mean neurons of class 6, which reached the spinocerebellum via the crossing level.

The output of the spinocerebellum could not descend to the spinal cord with respect to the non-motor head senses, because there were no signal-related targets there. The only signal-related targets were located headward, in the original levels of the cephalic and trunk senses. Thus, the path of these signals inverted by the spinocerebellum was predetermined. They moved headwards. The motor signals also followed them. As they passed through the thalamic level, the axons contacted their own projection neurons and formed new, modality-ordered independent thalamic nuclei, the nuclei intralaminaris. They form a truncothalamic core group, the largest of which is called the nucleus centromedianus (Centre médianus Luys). We assume here that the nucleus centromedianus (predominantly) receives the motor output of the spinocerebellum, i.e. the inverted motor signals of the opposite side.

However, it must be said that the number of muscle spindles of the body and thus the number of these primary thalamic projection of the nucleus fastegii remains relatively small compared to the axon number of the third expansion stage of the cerebellum described in later chapters. Therefore, this projection of the nucleus fastegii is usually not explicitly mentioned in the literature. Werner Kahle and Michael Frotscher name in [9] on page 183 this projection of the nucleus emboliformis cerebelli to the nucleus centromedianus of the thalamus and state that the nucleus centromedianus also receives excitations from the homolateral reticular formatio reticularis and from the inner pallidum as well as fibers from the precentral cortex (Area 4). They also state that the nucleus centromedianus projects into the striatum.

Theorem of the extension of the functional range of the mean value systems

After the development of signal inversion in the vestibulo- and spinocerebellum, the range of functions of the neuronal mean value systems was extended, which now also served to provide mean values for signal inversion.

Theorem of cortical projection of the early spinocerebellum

The nucleus interpositus of the spinocerebellum projects into the intralaminar nuclei of the thalamus, mainly into the nucleus centromedianus. Via class 4 neurons, these signals (in the early rope ladder system) reach the uppermost, cortical turning level and are switched there to class 3 neurons, which project to the motor side into class 5 neurons. The latter project downward to the motor neurons. Thus, a new cortical structure is formed, which receives the output of the early spinocerebellum as input. This input is the inverted output of the receptors of the trunk senses of the contralateral half of the body. This new structure is the frontal turning loop from which the frontal cortex of vertebrates originated. The output of this structure reaches the contralateral half of the body again via the turning floor.

The spinocerebellum as a new structure produced a new, previously non-existent type of output: the inverted output of the nucleus ruber.

This new type of output ended in new thalamic nuclei, which in turn projected into a new turning loop of the rope ladder system, which we call the frontal turning loop. It is a new formation and is linked to the formation of the spinocerebellum. From this turning loop, the frontal cortex is formed in vertebrates.

Theorem of the input of the frontalcortex

The frontal cortex formed when the spinocerebellum was formed. It received its output and was used for inverse excitation of the antagonist muscles. It was present bilaterally.

After the formation of the spinocerebellum, there were two motor signals for each muscle tension receptor and for each motor neuron: The signal originally produced by the muscle tension receptor, whose signal moved headwards, was possibly amplified in the association areas of the torus semicircularis and the optical tectum by receptors of the head senses and moved downwards again in the cortical turning area to reach the motor neuron of the muscle from the nucleus ruber. This signal is called the primary motor signal.

At the same time, the signal of the contralateral muscle tension receptor of the motor antagonist reached the contralateral nucleus ruber in the same way and moved to the motor neuron of the antagonist muscle. However, each motor output neuron of the ruber nucleus also projected via the olivar nucleus to the opposite side, where the spinocerebellum inverted this signal to excite the antagonist muscles with the inverted signal. This signal is called the secondary motor signal. The signal-carrying axon of the cerebellar output nucleus reached the corresponding half of the neural tube on the opposite side and was able to generate and contact three types of neurons there.

First, they contacted the corresponding output neurons of the nucleus ruber, which projected to the motor neurons via commissure neurons of class 5. In this way, the motoneuron of each muscle received, on the one hand, the sensorily modified output signal of its own muscle tension receptor in an exciting way, but secondly, the inverted and also exciting signal of the muscle tension receptor of its antagonist muscle. The latter could of course also be sensorily modified if it was previously subjected to additional excitation in the torus or tectum.

Secondly, the output axons of the cerebellum nucleus in the neural tube could contact class 4 connective neurons that projected headwards. Here a new input nucleus, the nucleus centromedianus, was formed in the thalamic plane. The primary motor signals originating from the trunk were represented in the thalamic nucleus ventralis lateralis, the secondary signals inverted by the cerebellum in the nucleus centromedianus of the thalamus. Both projected to the motor cortex, which is called frontal cortex in vertebrates.

Thirdly, the neurons of the cerebellum nucleus in the neural tube could also transfer their excitation to the neurons of class 6, which functioned as mean value neurons and supplied the Formatio reticularis present at this segment height with mean value signals. Thus the cerebellum output could be included in the mean value excitation.

And because mean nuclei generally develop a rear projection, the cerebellar nucleus and, through it, the spinocerebellum also received the output of the Formatio reticularis for the purpose of pre-excitation.

The nucleus centromedianus should play an important role in the development of the basal ganglia. This is derived in the following chapters. But first we will deal with the development of the interneurons in the primordial cerebellum in the following chapter.

Neuron nuclei that receive permanent excitation from average nuclei, such as the nucleus interpositus, run the risk of being overloaded by the permanent excitation. Therefore, inhibitory feedback is appropriate. The nucleus interpositus as cerebellar nucleus in its function as an inversion nucleus received the input to be inverted via the nucleus Purkinje from the nucleus olivaris. Therefore, it proved to be useful to establish inhibitory connections to this nucleus in order to avoid a too strong permanent excitation. With increasing brain size, the distance these inhibitory signals had to travel from the cerebellar nucleus to the olive also increased. This resulted in a time-delayed inhibition, which caused short pauses in the tonic continuous signal. This inhibitory projection of all cerebellar nuclei to the olive became standard in the vertebrate brain.

Theorem of the inhibitory projection of the cerebellar nuclei into the nucleus olivaris

The nucleus interpositus projected inhibitingly into the nucleus olivaris so that its tonic excitation could be limited and, if the brain size was sufficient, interrupted by short pauses. This principle was retained in all extensions of the cerebellum and was adopted in all cerebellar nuclei that were later formed.

This obstructive rear projection was given further specifications in the course of evolution.

If we summarize the vestibulocerebellum and the spinocerebellum at this stage of development to the early primordial cerebellum, we find that there were no granule cells, star cells or basket cells in the early primordial cerebellum. The only input came from the nucleus olivaris. This input is called climbing fibre input by neurologists.

Theorem of the early original rebellion

The early Urcerebellum was used for signal inversion of the vestibular and spinal input. The only input came from the nucleus olivaris via axons, which were later called climbing fibres. In the beginning there were no cerebellar interneurons at all. Signal inversion in the cerebellar nuclei was achieved by relative inhibition of the mean excitation of nuclear neurons by the inhibitory output of Purkinje cells. The mean excitation of the neurons of the cerebellar nuclei originated from the Formatio reticularis, which functioned as the mean nucleus of this level.