Vertebrate brain theory

4.8  The change in neural body models after the development of signal divergence

With the development of signal divergence in the nucleus olivaris, there was a strong change in the neuronal body models in the vertebrate brain. We recall that important neuronal nuclei and structures can be interpreted as body models, such as the nucleus olivaris, the cerebellum cortex, the cerebellar nuclei, the nucleus ruber, the thalamus nuclei or the cortex cortex.

The signal divergence had effects on the spatial arrangement and topology in the nucleus olivaris as well as in the spinocerebellum, in the cerebellum nuclei, the thalamus and the frontal cortex. The nucleus olivarisinitially had a kind of bilaterally symmetrical tubular shape, similar to the spinal cord, but also to the animal body.

Initially, a simple flexor joint had two muscles. If each of them initially had only one muscle spindle, two muscle spindles were assigned to this joint. Each of the two muscle spindles contacted a projection neuron in the nucleus olivaris.

If, in the course of evolution, the muscles involved produced several muscle spindles, this joint was represented in the nucleus olivaris by a double row of projection neurons arranged parallel to each other in a longitudinal direction. At the end of the double row, another double row of projection neurons began, which represented the next joint in the same way. This was followed by the next joint and so on. And since trunk muscles in particular were arranged around the entire trunk, there were further double rows for the remaining trunk muscles next to each double row of projection neurons.

For example, the surface of the nucleus olivaris consisted of double rows of projection neuronsrepresenting the muscle pairs of the joints in a motor body model. Because of its bilateral nature, the nucleus olivaris disintegrated into two symmetrical halves. This arrangement only changed when extremities were formed. Then the tubular structure got side tubeswhich represented the extremities.

The projection neurons of the nucleus olivaris contacted the projection neurons of the cerebellar nucleus, which thus also became a motor body model. And its projection neurons transferred this body model to the Purkinje cells of the cerebellum cortex. This bark now also consisted of double rows of Purkinje cells aligned parallel to each other, each pair of Purkinje cells representing two muscle spindles of a joint. Muscle spindles of the same joint followed each other directly. The direct projection of the spinocerebellum via the thalamus into the frontal cortex also transferred this body model to the cortex, where the double rows ran from bottom to top.

With the development of signal divergence, these double series were replaced by multiple series. The signals of the pair of neurons, each representing a pair of muscle spindles, were distributed by signal divergence in the nucleus olivaris to, for example, 3 projection neurons, later perhaps to 4, 5, 6, and many millions of years later perhaps to 100. Thus, the double series of projection neurons became triple series, quadruple series, quintuple series, sixfold series, and in the end one hundred parallel series of neurons. They were arranged lengthwise in the nucleus olivaris as before. Here, the nucleus olivaris had already divided into a left and a right half tube because of the bilaterality, so that this nucleus existed twice. The strong increase in the number of neurons led to a sack-like inflation of these structures.

Analogously, the double rows in the cerebellum were replaced by the multiple rows. The space requirement caused the cerebellum bark to expand and form folds which became visible as cerebellum wrappings. Each winding served a joint. The neurons resulting from the signal divergence were arranged in a transverse direction. In the longitudinal direction, on the other hand, the neurons that are assigned to the same muscle group are located first, followed by those of the following muscles and joints.

The frontal cortex was an image of this spinocerebellum, which also disintegrated into two bilateral parts. The multiple rows of Purkinje cells projected to multiple rows of cortex neurons, which here ran vertically. The same is true for the signals of the other receptors, which mostly reach the nucleus olivaris and the subsequent structures in an on and an off variant. Therefore, the different modalities in the frontal cortex are arranged in vertical modality strips and represent body images of the receptors of these modalities. The signal divergence has an effect here in the horizontal plane. This is the reason for the enormous spatial extension and the transition from the worm-like elongated to the hemispherical shape in the neuron nuclei as well as in the neuron layers of the cerebellum and the cortex.