Human brain theory

14.2 Body stability and the cerebellum

The spinocerebellum processes mainly motor signals that it receives from the contralateral nucleus ruber via the nucleus olivaris. It inverts these signals and sends the inverted signals to the motor counterparts. But what benefit does the living being derive from this algorithm?

The main result is to ensure body stability. The body retains its current shape even under external influences, such as gravity. Now it only moves when external signals act on it, the receptors register this and in turn send action potentials to the motor components. For example, the body will not be deformed by a water current in the same way as, say, seaweed, which constantly moves back and forth in the water.

What is the reason for this?

The muscle tension of a muscle is measured by the tendon organs and translated into a sequence of action potentials, which is inverted in the spinocerebellum. For this purpose, this signal inhibits a relatively frequency-constant mean signal originating from the motor mean nucleus of this floor, the formatio reticularis. The signal inversion inverts the firing rate. A strong signal is turned into a weak one by inversion. If the input signal is weak, the output signal from the spinocerebellum is strong. If the signal strength is equal to that of the mean signal, the input and output have the same firing rate. The output reaches the motor opponent, i.e. the muscle that opposes the original one.

This sets up a constant joint angle. Both muscles are tense and to a certain extent resist a change in their muscle tension. Theoretically, the affected joint would remain in this position for all eternity without changing the joint angle. Practically, this is not the case because the most diverse receptors of the body generate signals, very many of which have a motor effect.

However, if the signal generation by these receptors comes to a standstill, for example because the circadian signal from the nucleus suprachiasmaticus generates an inhibition of body movement, which we also call sleep, the body remains motionless in principle.

Here, the spinocerebellum often acts in two ways. This is because not only does a particular muscle send its signal to the spinocerebellum to invert, but also its motor counterpart. The body's stability is therefore doubly secured.

And since each muscle sends the signal of its tendon organs to both the cortex and the cerebellum, both signals are present simultaneously in the cortex. The direct signal rises from the tendon organ headwards and changes to the opposite side in the crossing floor. It reaches the sensory side of the frontal cortex and changes to the motor side on class 3 neurons, where it descends again to the spinal cord. Here it passes through the crossing floor again and changes to the original side of the body.

Thus, the contralateral hemisphere of the frontal cortex receives this muscle signal from the visual organ, whose muscle is located on the ipsilateral side of the body.

As this signal descends from the cortex and arrives back at the side of the body where the generating muscle is located, it passes the nucleus ruber. However, the nucleus not only sends the signal to the original muscle so that the muscle tension there is maintained, but also passes it on via the olive to the contralateral spinocerebellum. There, the signal inversion takes place. The inverted signal is destined for the motor counterpart and, if the counterpart is on the contralateral side of the body, simply descends down the spinal cord to it. This is almost always the case for the trunk muscles.

However, if the counterpart is on the same side of the body - because it is a finger muscle, for example - the signal from the spinocerebellum rises headwards. But the ascending cerebellum signals also switch body sides and thus reach the frontal cortex of the contralateral half of the body. We want to call this signal an indirect signal.

Thus, two types of signals arrive in the frontal cortex, the direct and the indirect. The direct excites the muscle of origin, the indirect the motor counterpart. The frontal cortex thus has two input layers of neuron class four on the sensory side. In one - the older and lower one - the direct signals from the tendon organs of the muscles arrive. In the second - the younger and upper one - the indirect signals from the spinicerebellum arrive. We assume that these cortical input neurons are topologically well ordered in such a way that a motor body image of the living being is created. But we go even further: we assume that the direct and indirect input neurons of class 4 of a muscle pair are spatially arranged together in such a way that the direct input neuron is located at the bottom and the indirect one just above it in layer four, if we think of the neuron layer as being oriented horizontally. One could also speak of both input neurons in the layer forming a cell column. This initially consists of only two vertically arranged input neurons.

And just as the frontal cortex input formed a bilayer of direct and indirect neurons, the nucleus olivaris also formed an arrangement of inut neurons in such bilayers.

Because if both a muscle and its counterpart were assigned to the same side of the body - and this applied to all limb muscles - the nucleus olivaris received both the direct and the indirect signal of the two involved tendon organs of the muscle and its counterpart simultaneously. Thus, as in the frontal cortex, the double layer of input neurons was formed. Since the associated input neurons received signals that were inverse to each other, we refer to the period of formation of these inverse double layers in the course of evolution as the phase of the formation of the mutually inverse double layers. These emerged not only in the olivary nucleus and the frontal cortex, but initially in all sensory and motor nuclei of the early cord ladder system or its successors. Since in the course of evolution such mutually inverse signal pairs also formed in non-motor receptors - think of the On and Off signals of the retina - the further development will generally have led to the formation of mutually inverse double layers of input neurons. Without the phase of the formation of the mutually inverse bilayers, the later signal divergence in the vertical and even later in the horizontal direction could not have taken place.

What ordering principle was responsible for arranging the two related input neurons vertically to each other in the neuronal layer of input neurons? In the motor system, it was the property that the input came from a common joint whose two muscles working against each other provided the two inverse signals. The attachment points of the two tendons, which contained the tendon organs for tension measurement, were located on the bone in close spatial proximity. If one imagines a body marker that is distributed in the body with a certain gradient gradient (concentration gradient), the two tendon organs involved of the flexor and the extensor were spatially quite close together. If the current concentration of the body marker was transported by the involved nerve cell from the point of origin of the signal recording to the axonal end point of the signal transfer, a gradient gradient of the marker analogous to the point of origin also arose at the destination, which enabled the involved axons to control the direction. Thus, the origin topology was transferred to the target structure. Therefore, the axons of two mutually inverse signals - regardless of whether they originated from the muscle spindles of a joint or from the retina - always ended up very close to each other. Since the cerebellar axons were only formed later, because the cerebellum formed quasi late during induvidual development, the inverse, cerebellar signals landed in their own neuronal layer, which was arranged above the previous, primary cortex layer.

If we consider the signal of the tendon organ of a muscle and the signal of the tendon organ of its motor counterpart as signals of two different modalities, their separation in two double layers of neurons can also be called separation of modalities. We want to call this basic neuronal principle of central nervous systems of vertebrates - certainly also found in earlier animal classes - the principle of separation of modalities. This means that two basic principles are directly coupled with each other. The phase of the formation of the mutually inverse bilayers is the realisation of the principle of the separation of modalities.

And both principles are not limited to the nucleus olivaris or the cortex, but in the course of evolution cover almost all neuronal structures. We find them in all head segments of the former rope ladder nervous system and there in all neuronal nuclei, both in sensory input nuclei, motor output nuclei, sensory side switch nuclei and motor side switch nuclei. The separation of modalities and the formation of mutually inverse double layers, once started, extended through all substructures of the central nervous system. In humans, traces of this development can even be found in the spinal cord, where the neurons that receive the signals from the flexor muscles merge spatially and separate clearly from those neurons that receive the signals from the extensor muscles. According to the author of this monograph, the separation of modalities first began in the motor lateral alternating nucleus of the entrance floor of the segmented cord ladder system, i.e. in the early nucleus olivaris, and led to the formation of the frontal cortex.

 

But evolution did not stand still.

With the development of signal divergence in the olivary nucleus and the cortex, a new stage of motor control began. Initially - when the double layer of input neurons still existed - there were also only two output neurons per simple muscle joint. These received the two input signals, which were inverses of each other, and passed the signals on to the motor side, where they descended to the motor targets.

The signal divergence led to the development of additional output neurons to increase transmission reliability. Now individual neurons could fail without endangering the overall system.

Let us imagine that each of the two input layers had its own output layer. In this output layer, there was exactly one output neuron for each input neuron. Thus, there were also two output layers.

As described in the chapter "Divergence modules with vertical signal mixing", the phase of vertical signal divergence began with the gradual emergence of additional output neurons in the two output layers. One of the two output layers was located exactly between the two input layers, the other was located above the second input layer.

However, the two inverse signals were signal-related to each other. The indirect signal arose in the spinocerebellum from the direct signal through signal inversion. The neurons that formed and transported the indirect signal were connected at the beginning of the neuron chain to the same input neuron that received the direct signal from the tendon organ of the muscle. In neuron chains, the signal relationship is passed on. Therefore, the direct input neurons of the bilayer were signal related to the indirect input neurons if they originated from the same joint.

The interneurons, which passed on the signals from the input neurons to the output neurons, only contacted signal-related input neurons. Therefore, on the one hand, the signal had to originate from the same joint. However, it did not matter whether it was the direct or the indirect signal. The output neurons of the lower output layer were arranged vertically among each other and thus formed a motor neuron column. Each output neuron there received both the direct input signal of the lower input layer and the signal-related, indirect, cerebellar signal of the upper input layer via interneurons.

It may have been the same in the upper outpouring layer. However, our preliminary view is that the upper outpouring layer was reduced over time. This was due to the competition between the signals. For every output signal from the lower outpouch layer, there was an identical signal from the upper outpouch layer, both competing as output signals via inhibitory interneurons. Since the lower output layer was formed with a time lead, it had an advantage, so that the upper output layer disappeared in the course of evolution.

This left only one output layer between the double layer of input neurons, in which, however, the number of output neurons increased in the course of evolution. This output layer was organised vertically in such a way that vertically arranged output neurons received the input of exactly one simple joint. The upper input neuron received the indirect, cerebellar signals of the joint, the lower one the direct signal of this joint.

Each neuron column formed a divergence module with vertical signal mixing. Due to the exponential attenuation in the signal propagation, which increased quadratically with the distance, a maximum coding of the signal strength ratio of direct and indirect signal was created in the output neuron column. This made it possible to display the joint angle in maximum coding. Exactly one output neuron in the vertical neuron column reached a signal strength maximum. If the direct signal was stronger, the active output neuron was located further down the column. If the signal strength of the direct signal decreased and that of the inverse, indirect signal increased, the excitation maximum in the column moved upwards. Each output neuron, if maximally excited, represented a different joint angle.

The path of the output signals remained the same during the transition from the double layer to the divergent module. All output neurons projected headwards into the frontal cortex. There too, instead of the double layer, there was now a module of one cell column per single joint. The number of neurons in the cortical column corresponded to the number of neurons in the origin structure. This could be either a sensory, a motor or a lateral switch nucleus. For motor signals, the olivary nucleus established itself as the side-switch nucleus for almost all motor signals.

However, the cortical cell column reversed the signal divergence. The number of neurons in the transmission path of the two signals had changed, but not the number of muscles at the responsible joint. Therefore, the frontal cortex again formed the two mutually inverse output signals from the maximum-encoded input signal of the cell column in order to deliver them to the two muscles of the joint. The output neurons again formed vertically organised cell columns.

Thus, in the early days of evolution, the frontal cortex was a double layer, but later the thickness of the outpouch layer increased and worked as a vertical convergence circuit.

We certainly find this type of circuit in reptiles as well as in birds. In the mammal, however, a further, additional signal divergence took place in the area, as described in the chapter "Modules with spatial signal propagation". Here, new output neurons were created not only exactly between the two input neurons of the original double layer, but also laterally in the area.

This now made it possible to analyse the position of joints with two degrees of freedom via maximum coding.

However, it is possible that the bird brain has managed to realise such an analytical capacity as well. It is possible that there are areas in the bird brain that also work as spatial divergence modules and spatial convergence modules. It would also be possible that the angular analysis of rotary joints is carried out via another, as yet unknown mathematical algorithm. Mathematicians may look for further possible solutions here.

However, we must now return to the original topic of body stability. It is ensured by transferring the direct signal of one muscle into the indirect signal of the counterpart. Both signals, descending via the cortex, reach the two muscles involved again and adjust the muscle tension exactly to the value that corresponds to the current joint angle. It is like power steering. The sensors are the tendon organs, their signal reaches the motor neurons. And each of the two tendon organs, with the help of the cerebellum, supplies two signals, one direct and one indirect, the latter supplying the motor counterpart.

In the course of evolution, special muscles even developed. They may even have been there first. They are muscles in miniature, extremely small, even tiny. They are called muscle spindles. They lie in line with the rest of the musculature.

They also have tendon organs that measure muscle tension. Their task is to stabilise the set joint angle.

If an external force acts on the joint and changes the joint angle, the tendon organ of the muscle spindle generates a correction signal which is used to additionally control the joint muscles. This additionally changes the tension of the joint muscles so that the original joint angle is resumed. The muscle spindles compensate for external forces acting on the body. They are the cause of several reflexes, such as the hamstring reflex, which neurologists use to analyse the state of motor control.

If the neuronal system is geared towards keeping the set joint angles stable, how does the movement of the living being come about?

Movements can only occur when external influences excite the receptors of the body and their excitation ultimately additionally excites the motoneurons of the system. Then the firing rate of the tendon organs changes, setting a new joint angle.

But how do movements like swimming, running or flying come about? Here, no external influences from the environment can be held responsible. The emergence of such periodic movements must come from the system itself, i.e. be actively caused by the animal. One might think that the animal must generate an inner drive, a kind of effort of will, possibly even trigger a conscious movement. This may be all, but at the beginning of all such movements were clock generators that made periodic movement such as swimming, running or flying possible. This is explained in more detail in the following chapter.