Vertebrate brain theory

3.16. The first expansion phase of the original Pontocerebellum

In the first expansion phase of the spinocerebellum, a moss fiber projection and the formation of interneurons in the spinocerebellum occurred. A similar developmental phase was observed in the forming pontocerebellum. We therefore call it the first extension phase of the primitive pontocerebellum.

Each class 6 cortical mean neuron received its input from the neighbouring class 5 neurons in its environment. We call this environment a cortex cluster. Thus, each class 6 mean neuron received the output of the class 5 neurons from its cortex cluster.

Cluster theorem of the cortical floor

Each class 6 mean neuron of the cortical floor receives its input from the class 5 neurons of its catchment area, the associated cortex cluster. The mean neurons project into the mean centre of the thalamic floor and via the nucleus ruber and the nucleus olivaris into the climbing fibres of the pontocerebellum.

Hint:

The columnar structure of the cortex cortex is based (according to the author) on clustering.

The cortical mean neurons projected into the subthalamic nucleus, which was the mean center of the thalamic floor. The axons of the output neurons of this nucleus reached the nucleus ruber in descending order and also had no motor target neurons below this level, but reached the mean centers of the lower segments. The nucleus ruber forwarded them - as all its input - to the nucleus olivaris so that they could generate new Purkinje cells in the Pontocerebellum that was forming.

These new Purkinje cells received via the moss fibre system exactly the cortical input that was originally included in the mean value excitation. This signal facilitated target finding and resulted in the Pontocerebellum being a topologically organized cortex model.

Theorem of the Pontocerebellum as cortex model

The cerebellum bark of the Pontocerebellum was originally a model of the cortex bark. The cerebellar cells corresponded to class 5 cortical signal neurons, their topological arrangement corresponded to that of the cortical floor.

Initially, the Purkinje cells corresponded to the cortical mean neurons of class 6, which arrived in the nucleus olivaris after passing through the nucleus subthalamicus and from there reached the Purkinje cells as climbing fibres.

Initially, the spatial arrangement of the neurons of the pontocerebellum corresponded to the arrangement in the cortex, neighbourhood relationships were transferred from the cortex model to the cerebellum model. Class 5 cortical signal neurons of a cortex cluster projected into the granule cells of an associated cerebellum cluster. The mean class 6 neuron of a cortex cluster projected via the nucleus subthalamicus, the nucleus ruber and the nucleus olivaris into the single purkinje cell of the associated cerebellum cluster.

Each mean value centre causes an activation of its signal suppliers. This property can be passed on in signal chains because the following neurons ultimately also represent mean value neurons. The mean value property of the class 6 neurons is quasi "inherited" by the Purkinje cells of the Pontocerebellum.

Now, mean-value neurons are generally capable of tapping passing axons and including their excitation in the generated mean value. This is especially true when there is a signal relationship. In Pontocerebellum, the mean neuron was represented by the Purkinje cell, whereas the cortex neurons were represented by the granule cells that received their signals.

Therefore, synaptic contacts between these neurons occurred. Thus, each Purkinje cell in the Pontocerebellum was excited by the granule cells that received their signals from the assigned cortex cluster. All signals from a cortex cluster converged via the moss fibres and the granule cells to exactly the Purkinje cell that received the mean signal from this cluster.

The Purkinje cells of the Pontocerebellum represented (initially) the cortical mean neurons of class 6. in each cerebellum cluster there was exactly one Purkinje cell, which with its large dendrite tree absorbed the excitation from the cortex cluster, which it reached via the granule cells.

Thus the mean value system was implemented twice: The Purkinje cells worked as mean-value neurons. This had advantages: If the cortical mean value neuron failed, its signal mean value was still present in the corresponding Purkinje cell.

Theorem of mean value generation by Purkinje cells in early Pontocerebellum

In the early Pontocerebellum, each Purkinje cell received the mean signal from the corresponding cortex cluster via the climbing fibre. At the same time, it formed the same mean value signal from the input of the granule cells, which received the class 5 averaging signals from the cortex via the bridge nuclei.

The Purkinje cells in turn transferred their mean value property to the output neurons of the nucleus dentatus, because all these neurons are coupled together in signalling chains. Therefore, the output neurons in the nucleus dentatus of the original Pontocerebellum also represent mean value neurons. For this reason, they were also excited by all moss fibres that carried the signals from the associated cortex cluster.

Just as the pontocerebellum spatially separated from the spinocerebellum during evolution and became independent, so the cerebellar nucleus of the pontocerebellum separated from the cerebellar nucleus of the spinocerebellum and formed an independent nuclear structure called nucleus dentatus. Initially, this nucleus had only a few neurons whose number corresponded to the number of mean neurons of the first, cortical level of the early primordial brain. Only much later did the number of neurons increase, since the number of receptors and receptor types also increased, and with it the number of mean value neurons.

Theorem of mean excitation of the neurons of the nucleus dentatus in the early Pontocerebellum

In the early Pontocerebellum, each excitatory output neuron of the nucleus dentatus received the mean signal from the associated cortex cluster via the climbing fiber. At the same time, it formed the same average signal from the input of the granule cells, which received the class 5 averaging signals from the cortex via the bridge nuclei.

Since each Purkinje cell inhibited its assigned excitatory output neuron in the nucleus dentatus, and both excitations were equally strong, both would theoretically cancel each other out. Then the output of the Pontocerebellum would be the zero signal everywhere. This would not be an obstacle, only an unnecessary consumption of resources. But as in the spinocerebellum, there were interactions between the Purkinje cells and the existing - inhibiting - interneurons, the basket and star cells, in the Pontocerebellum that was formed. These had an inhibitory effect on the Purkinje cells with their transmitter GABA, so that their excitation was smaller than that of the corresponding output neuron in the nucleus dentatus. Therefore, there was a residual output whose strength depended on the inhibitory effect of the cerebellar interneurons. In addition, neuronal phenomena such as long-term depression (LTD) and long-term potentiation (LTP) were to be established, which modified this output of the pontocerebellum in such a way that new advantages for the organisms concerned resulted. This will be described in later chapters.

The mean value centres were in signal exchange with each other. The first mean value centre reached by the cortical mean value neurons is the nucleus subthalamicus. Further mean value nuclei are also supplied by this nucleus so that their signal mean values do not form segment mean values but body mean values.

Theorem of the origin of the Pontocerebellum

The original Pontocerebellum was created as a descendant of the Spinocerebellum when it was reached by the cortical mean signals of class 6. These were also transferred to the nucleus olivaris when passing through the nucleus ruber, whose axons as climbing fibres in the former spinocerebellum created a separate area from which the Pontocerebellum emerged.

The cortical projection neurons of class 5, which had originally contributed to the mean value of a specific class 6 neuron, reached in the Pontocerebellum via the moss fibres and granule cells exactly that Purkinje cell where the axon of the mean neuron ended as a climbing fibre axon. They also activated the output neurons of the developing nucleus dentatus.

With the development of extremities, the worm-like structure of the cerebellum got additional lateral bulges, which also had a mean value stripe that can be attributed to the pontocerebellum.

The neurons of the Pontocerebellum were descendants of the corresponding neurons of the Spinocerebellum, the only difference was in the class of neurons.

Theorem of the difference between Spinocerebellum and Pontocerebellum

The climbing fibre input of the primordial spinocerebellum is assigned to neuron class 5. The input comes from the motor side of the cortical floor and reaches the contralateral spinocerebellum exclusively via the nucleus ruber, which projects into the Purkinje cells via the nucleus olivaris. This input is inverted by the spinocerebellum in the associated cerebellar nuclei and serves to inversely excite the motor opponents. The moss fiber input originates from the neuron class 5 of the cortical level and, after passing through the crossing level, reaches the cerebellar nuclei of the spinocerebellum, where it also contributes to the necessary mean excitation. Furthermore, it models the output of the Purkinje cells as parallel fiber input, the associated algorithm ensures the body's own protection and the transfer of the reflexes of the autologous apparatus of the neural tube or spinal cord into the spinocerebellum. The main part of the mean excitation of the cerebellar nuclei originates in the spinocerebellum of the Formatio reticularis.

The climbing fibre input of the primordial Pontocerebellum is assigned to neuron class 6. Via the nucleus ruber it reaches the nucleus olivaris, where it changes sides to the nucleus olivaris and ends at the Purkinje cells and the cerebellar nucleus. The Purkinje cells project into an independent cerebellar nucleus, the nucleus dentatus. Its output neurons additionally receive the same cortical output of the neuron class 5 via the moss fibre system. The input is cluster-faithful with respect to both the climbing fibers and the granule cells, each cortex cluster projecting into an associated cerebellum cluster of the pontocerebellum. The main contribution to the mean excitation of the nucleus dentatus no longer comes from the Formatio reticularis, but is provided by the climbing fibers, whose signals themselves represent mean values of cortex clusters.

In terms of circuitry, there was not much difference between the Spinocerebellum and the Pontocerebellum. The climbing fiber input ended at the neurons of the respective cerebellar nucleus as well as at the Purkinje cells. The moss fibre input also reached both the neurons of the respective cerebellar nucleus and the Purkinje cells. In all cases, the granule cells were the mediators between moss fibers and the Purkinje cells. There were also interneurons as in the spinocerebellum, their importance will be discussed in later chapters.

Only the climbing fibre input was of different origin. In the spinocerebellum, the climbing fibres were supplied by neuron class 5 of the cortex, whereas in the pontocerebellum they were supplied by neuron class 6.

Initially, the cortical mean values that the Purkinje cells of the early Pontocerebellum reached via the climbing fibers and also via the moss fibers were inverted in the cerebellar nucleus and served to inversely excite the contralateral mean centers. The necessary continuous signal for inversion originated from the reticular format. At that time, the cortex clusters were relatively small and the fire rate of the mean signals was still relatively low. As they became larger, the rate of fire of the mean value signals increased significantly.

However, the corresponding output nucleus of the forming pontocerebellum split off early and formed the nucleus dentatus. The connection to the Formatio reticularis was reduced, the contribution of the moss fiber signals to the mean excitation of the inversion neurons in the nucleus dentatus increased during evolution.

Since each mean value neuron projects excitatory into the regions of origin of its excitations, and the neurons of the nucleus dentatus took over this mean value property, they also projected into the cortical regions of origin in the sense of an activation. Each output neuron of the dentate nucleus contacted an associated class 4 projection neuron, which switched the signal to a class 3 neuron. Its projection axon in turn established synaptic connections with all class 5 neurons that were originally included in the averaging process for the responsible class 6 neuron.

In this way, each excitatory dentate neuron excited the cortex neurons of its cluster.

Theorem of derived mean projections of the Pontocerebellum

Each Purkinje cell and each output neuron of the nucleus dentatus received exactly the class 5 cortex signals assigned to this cortex cluster via the moss fibre projection or the granule cells. Each excitatory output neuron in the nucleus dentatus of the early pontocerebellum projected back exactly into the cortex cluster that provided the mean signal for the associated Purkinje cell and excited all class 5 signal neurons of this cluster. The projection was made from the nucleus dentatus ascending through class 4 neurons to the sensory part of the cortical floor. There the signals were transferred to class 3 neurons, which projected to the motor part of the cortex. On the motor side, these incoming mean signals were again distributed to those class 5 neurons that belonged to the corresponding cortex cluster. Thus, the class 5 cluster neurons were activated by the associated dentate neuron.

The spinocerebellum already showed a cortical projection. Thus, there were closed signal circuits in which the signals could rotate, and a rotational memory manifested itself in the cerebellum. The supplied signals rotated until new input entered these loops via receptors and inhibited old signals via lateral inhibition. This is why lateral inhibition was so important, it allowed the switching off of signals that were no longer current.

Rotational memory was important for motor function: a muscle whose motoneurons do not receive any signals immediately goes limp. If a certain joint angle is to be maintained under stress, constant contraction commands in the form of action potentials are required. The signal rotation between cortex and cerebellum was able to secure these. However, the signal rotation in the pontocerebellum was only activating and not yet specific. All cortex neurons of the corresponding cluster were activated in the same way. This was to change later when long-term depression and long-term potentiation began to develop. The prerequisites were in place: The Purkinje cells of the Pontocerebellum were tonically excited via the climbing fibres by global mean signals, while at the same time certain cortex neurons in the cluster were active and excited the parallel fibres via moss fibre projection. If synaptic alterations by LTP or LTD occurred, the activating signal rotation could be transformed into a specific signal rotation. This in turn required a lateral inhibition to be interrupted.

Among the neurons of the nucleus dentatus there was, as between the excitatory output neurons of the remaining cerebellar nuclei, a lateral inhibition by interneurons. In the nucleus dentatus this lateral inhibition is realized to enhance the contrast of the output signals of glycinergic interneurons.