Vertebrate brain theory

ISBN 978-3-00-064888-5

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

3.15  The first expansion phase of the original spinocerebellum

On each side of the neural tube, from about the height of the entrance level with the nucleus ruber, the nucleus olivaris, the reticular format and the spinocerebellum, both the primary (spinal) and secondary (cerebellar) motor signals of each muscle moved headwards (muscle spindle signals, signals from the tendon organs, etc.). In the spinocerebellum a special situation was present. The only cells of the cerebellum bark at that time were the Purkinje cells. There were (initially) no cerebellar interneurons at all.

We know that new neuron layers in the neural tube always attach themselves to the outside of the existing ones. Therefore, all Purkinje cells were located as an outer layer above the already existing ones. Even further out, the axons of the upwardly projecting connective neurons ran even further out.

Thus, right next to the area of the Purkinje nucleus, all the axons of the signals of the muscle tension receptors, which we call primary signals, passed by the trunk. The Purkinje cells represented the secondary signals. Both types of signals were signal-related. The primary signals belonged to the ipsilateral side of the body, but the secondary signals belonged to the motor counterparts of the opposite side. The latter had been obtained from the primary signals by signal inversion. Hence the signal relationship.

The Purkinje cells were located inside and the sensory axons outside. They practically ran past the Purkinje cells.

It is therefore not surprising that the small dendrite trees of Purkinje cells began to make synaptic contact with these ascending axons. However, this contact was not made directly, but was mediated by excitatory interneurons. In this monograph we refer to this phase as the first expansion phase of the primordial spinocerebellum.

The neurons of the primary projection contacted - as in the original rope ladder system - class 3 commissure neurons, which in turn sent their axons towards the cerebellum structure. These axons are called moss fibers and transported the signals of the primary projection. On the one hand, they reached the nuclei of the cerebellum (at that time there was the vestibular nucleus fastegii and the spinal nucleus interpositus). There they contacted the nuclear neurons of the cerebellum and contributed to their necessary mean excitation. This increased by the amount that this primary projection represented. This increased the rate of fire of the output neurons of the cerebellum.

At the same time, the moss fibres excited a newly emerging class of excitatory interneurons, the granule cells. These made synaptic contact with the Purkinje cells and excited them. As a result, the Purkinje cells were more strongly excited, and they in turn inhibited the output neurons of the cerebellum by exactly the same amount as the primary projection.

The additional excitation of the cerebellar nuclei by the moss fibres was thus cancelled by the additional inhibition of the cerebellar nuclei by the Purkinje cells, everything remained in the previous state. That this resulted in an extremely strong benefit for the vertebrate, which should lead to the development of higher intelligence, was not yet foreseeable at this point and will be described in later chapters of this monograph.

In the course of neuronal evolution, it is more often the case that a new development is initially result-neutral, as in the case of moss fibre projection onto the cerebellar nuclei and the granule cell population. Non-harmful developments can, however, manifest themselves at the latest when an advantage arises from them.

Theorem of the origin of granule cell projection

In the Cerebellum a moss fibre projection was created. The signals of the head-projecting class 4 connective neurons of this floor were (among other things) switched to commissure class 3 neurons, whose axons formed the moss fibres. These contacted a new emerging population of excitatory interneurons, called granule cells, in the cerebellum. Their axons (later) formed the parallel fibres. They had an excitatory effect on the Purkinje cells, whose output became stronger as a result. This led to a higher inhibitory effect on the output neurons of the cerebellar nuclei. The resulting loss of excitation in the cerebellar nuclei was compensated by axon collaterals of the moss fibers, which now also additionally excited the neurons of the cerebellar nuclei, so that the moss fiber input initially had no further effects.

  It is a question of interpretation, from which point on the moss fibre projection into the cerebellum should be assigned to neuron class 2 or 3. Since class 3 neurons project from sensory to motor centers and the cerebellum (initially) processes mainly motor signals, we classify the moss fiber projection here as class 3 neurons.

Hint:

Although the axons of the granule cells initially formed small, roundish dendrite trees, we will already refer to them here as parallel fibres, as this term has become established in the scientific literature.

Theorem of the stabilization of the mean excitation of cerebellar nuclei by the moss fiber system

On their way to the granule cells, the moss fibres also contacted the neurones of the cerebellar nuclei, where they contributed with their signals to the stabilisation of mean excitation, which was essential for signal inversion.

The formation of inhibitory interneurons in the cerebellum bark

In the course of evolution, more and more new types of receptors were created, and this also applied to the trunk. Touch and pain receptors, in particular, proved to be extremely useful when the stable outer shell of early animal creatures regressed because it no longer offered sufficient protection to the increasing forces of the jaws of predators. Instead, the now exposed body surface was equipped with a myriad of touch and pain receptors. The inner strength of the body was now secured by a stable inner skeleton with spine - if these animals belonged to the vertebrates. And this was true at the latest when they had developed a cerebellum, because this is a special characteristic of vertebrates.

  The signals of the tactile and pain receptors already developed suitable interactivities with the motor signals at the level of the neural tube or spinal cord. They were delivered to those motor neurons whose contractions reduced the strength of the tactile and pain signals. This resulted in an expansion of the autonomic apparatus of the neural tube or the spinal cord. The necessary signal transmission took place via the commissure neurons of classes 3 and 2, which were already present in the rope ladder nervous system.

Since the signals of the new receptors also moved headward on the axons of class 4 connective neurons, they were treated like all other signals with this property. The axons passed at the level of the cerebellum and formed class 3 analog commissure connections, which, as moss fiber bundles, both contacted the cerebellar nuclei and generated an associated granule cell population. These new granule cells sent their axons into the outer cerebellar cortex, where star cells, basket cells and golgi cells were later formed.

With the influx of excitatory signals into the cerebellar cortex, the possibility arose to apply the principle of lateral inhibition for contrast enhancement also in the cerebellar cortex. Therefore, new interneurons were created there, which had an inhibitory effect. The neuronal competition between the cortex signals was passed on to the moss fibre projection and in the course of evolution also reached the Purkinje cells, which were excited by it.

Theorem of cerebellar interneurons

Three types of inhibitory interneurons were formed in the cerebellum:star cells, basket cells and golgi cells. Star cells and basket cells are excited by the cerebellum cells and in turn inhibit the Purkinje cells. Golgi cells are excited by granule cells and inhibit the signal flow between moss fibres and granule cells.

At this point, the directional theorem of the rope ladder nervous system and the neural tube nervous system as well as the classification theorem for neurons should be recalled. We want to apply these theorems to the neurons and interneurons of the cerebellum and its substructures.  

classification theorem for the neurons of the cerebellum system

The neurons, whose axons are called moss fibres, belong to neuron class 3. They end at the granule cells, which belong to neuron class 4. These in turn end at the Purkinje cells, which thus belong to neuron class 5.

The neurons of the cerebellar nuclei belong to neuron class 6 of the mean neurons. They derive the major part of their mean excitation (initially) from the Formatio reticularis.

The remaining neurons of the cerebellum system are local interneurons.

The following follows for the axon course in the cerebellum:

-         Parallel fibres and the dendrites of Purkinje cells form right angles to each other.

-        Moss fibres and the ascending parts of the granule cells (before division into two T-shaped parts) form right angles to each other.

-        The axons of Purkinje cells and the parallel fibres also form right angles to each other.

-         The axons of Purkinje cells and the moss fibres also form right angles to each other.

It seems that the Purkinje cells have taken over an important property of their signal receivers: the huge dendrite tree of mean value neurons. Thus, the Purkinje cells are possibly a neuronal mixed type: type 5 and type 6 seem to be united here. The orientation of the dendrite tree follows the orientation rules for type 5 neurons, but its size follows the blueprint rules for mean value neurons.

Theorem of the effect of touch and pain signals on cerebellum output

Touch and pain signals arriving in the cerebellum led to additional excitation of star cells, which were mainly located in the outer layer. These now had a stronger inhibitory effect on the Purkinje cells. Therefore, the corresponding output neurons of the cerebellum nucleus were less inhibited, i.e. more strongly excited. The output now reached the motor neurons of the motor opponents. These contracted more strongly and moved the body part in question away from the influence. As a result, the strength of the tactile and pain signals decreased.

Thus, the cerebellum took over exactly the same tasks that the so-called spinal apparatus had previously performed. Thus, the cerebellum served to automate movement sequences and reflexes and to protect the living being.

  The spinocerebellum, which initially served purely for signal inversion and provided the antagonist muscles with an inverted but excitatory input, could be regarded as a body model.

Theorem of the body model of the early spinocerebellum

Each half of the bilateral early spinocerebellum was a body image of one half of the body in which the motor neurons of the opposite side were represented by Purkinje cells, while the granule cells represented the tactile, pain and muscle tension receptors of the ipsilateral side.

Thus, the spinocerebellum formed an elongated body, which resembled the neural tube, but was considerably shortened in proportion. Activation of the pain and tactile stimuli of one side led via the cerebellar interneurons to activation of the motor neurons, which, however, controlled the opposite side and thus served to protect the living being from external influences.

It must be pointed out at this point that at this stage of evolutionary development the dendrite trees of Purkinje cells had a round shape, but were already more branched to reach the axons of the granule cells. Initially, the granule cell axons only formed elongated structures in order to be able to reach several Purkinje cells so that the signals from muscles of other segments could influence the Purkinje cells. They also did not run below the molecular layer but laterally from it and ran parallel to the surface without forming a T-shape. This is still observed today in the valvula cerebelli of teleosteers and electrosensory fish [48]. The shift of the granule cells below the molecular layer and the T-shaped splitting of their axons into the typical present-day parallel fiber structure is evolutionary later.

The reason for the T-shaped splitting of the axons of the granule cells had solid reasons. With the elimination of the outer skeleton and the formation of an inner skeleton, it became necessary to analyse the entire body surface for mechanical effects from outside. This is why touch and pain receptors were formed in such large numbers that the number of muscle spindles or muscle tension receptors and also of motor neurons was very small in comparison. If the tactile and pain stimuli were to reach the corresponding Purkinje cells - which were the representatives of the contralateral motor neurons - longer axons were required. These initially spread in all directions, but at the latest with the second expansion phase of the spinocerebellum - which will be described in detail later - the granule cell axons began to align themselves parallel to each other and at right angles to the plane of the Purkinje cells.

This monograph attempts to analyze the functioning of the vertebrate brain on the basis of signal images of the body and the internal and external environment. We focus primarily on the signals and their significance in the signal images. Like a Christmas parcel that was transported by the post office over a long distance, we ask about the contents of the parcel - which we interpret as a signal parcel - and about its sender, and not so much about the route it took or the (last) place from which it was delivered to us. We are mainly interested in the sender of the packet and its contents. The question of which route the parcel took is of secondary importance. The latter is easy to find out on the Internet today, as the transport companies make it possible to track shipments on the Internet. Many signalling pathways in the vertebrate brain have already been well researched.

So the primary question in brain research is not (for us) "What does the nucleus centromedianus do in the brain", but - according to our interpretation - "Who is the sender of the signals reaching the nucleus centromedianus?

The secondary question is then "What intermediate stations have the signals reaching the nucleus centromedianus passed through in the meantime and what changes have they undergone in the process?

We assume that the various subsystems of the vertebrate brain through which the signals of a sender pass have an effect on the signals passing through or arriving in them and question the associated algorithms.

Theorem of the elucidation of the functioning of the vertebrate brain

The functioning of the vertebrate brain can be elucidated by attributing to each substructure an influence on the signals arriving in or passing through it, always taking into account who the original sender of the signals is.

Most signals are part of an image of the body and the environment, in which the receptors transform the strength of the impact on them into a rate of fire. The neighborhood relationships of the receptors, even of different modalities, are retained in the signal images. The vertebrate brain transforms these signal images from lower to higher levels and forms interactions between them through associations.

Monograph of Dr. rer. nat. Andreas Heinrich Malczan