Vertebrate brain theory

3.7  The signal interactions in the torus semicircularis

The described brain system of the segmented Bilateria, whose line led to the vertebrates, was successful because it allowed a signal interaction. We discuss here the signal interaction in the torus semicircularis. We already assigned this structure to the fourth segment of the segmented Bilateria, whose line led to the vertebrates. In this segment the signals of the hair cell systems arrived, which included the vestibular system, the lateral line system and later also the sense of hearing. Other senses that established themselves after the development of corresponding receptors were the sense of touch and the sense of pain. The structure of the torus semicircularis represents an important integration centre in vertebrates. It causes motor responses to the signals delivered by the various trunk receptors in response to external stimuli.

We consider the functioning of this structure at a very early evolutionary stage in the segmented Bilateria, whose lineage led to the vertebrates. Later, at the transition to vertebrates, this structure extends the algorithms of signal interaction. Even later, in the higher mammals, the cortex took over many of them, so that a strong reduction of the task areas occurred.

We recall that below the thalamus - i.e. below the second segment - there was a topological order of wellbeing of the class 4 and 5 connective axons of the neural tube, which we call modality rings. These, in turn, consisted of nested segmental rings each with the same modality, whose input came from the tail segment on the inside, but from the head segment on the outside, and was arranged between them according to segment numbers. This applied to both the sensory, upwardly projecting axons and the motor, downwardly projecting axons, with the head in this case being located at the top. The sensory axons were located in a modality half ring, the motor axons in the complementary half ring.

In each of the different sensory modality halfrings consisting of axons that were made up of associated segmental halfrings, each axon conducted its sensory excitation head-on to a class 4 connective neuron of the fourth segment - i.e. the torus semicircularis. Here we assume a topological order of these input neurons. The sensory connective neurons formed one layer per modality, which represented the curved surface of half a hollow cylinder. This cellular layer was a single layer. Each modality thus formed a single-layer cell layer of input neurons assigned to it. The order of the modalities of the half hollow cylinders corresponded to the order of the modal rings of the corresponding axons.

The input neurons in each half hollow cylinder were ordered by segments. So one can imagine as many half subcylinders stacked on top of each other as there were fuselage segments. The half subcylinder, which was assigned to the tail segment, was on top. Adjacent segments projected into adjacent half subcylinders.

Since all neurons are located inside the neural tube, the axons initially moved upwards on the outside until they were at the level of the responsible half subcylinder. Here, they bent approximately semicircularly towards the inside of the neural tube and moved towards the connective neuron of the respective half subcylinder of the modality assigned to them.

If there were several receptors of this modality in the respective body segment, their neighborhood relationships were transferred to the connective neurons in the model of the corresponding half subcylinder.

In the end, each half hollow cylinder represented a body model of the receptors of a modality assigned to it, in which the spatial position of the receptors on or in the body and their existing neighborhood relationships and segment assignments were represented by the associated class 4 connective neurons. And the different half hollow cylinders nested within each other were spatially synchronized with each other.

This meant that the signals of two receptors, which were assigned to different modalities, such as the sense of touch and the sense of pain, and which were spatially adjacent in the body, reached different neurons - in different layers - in the half hollow cylinder model, but these two neurons were also adjacent. They were therefore at about the same height and at about the same angular position.

Up to now, the topology of class 4 input neurons has been described in the torus semicircularis. They received the receptor signals of the trunk. But there were also class 5 downward projecting neurons in the torus, which were located in the complementary half of hollow cylinders. Here, however, there was only one modality, all projections were made into the motor neurons of the trunk. Therefore, there was only a single layer of projection neurons, but these were also sorted by segments and formed their own half subcylinders, which together formed the motor half cylinder. Therefore the motor half of the torus semicircularis is relatively inconspicuous.

Theorem of the topology of connective neurons in the torus semicircularis

The connective neurons of the fourth segment form the associative structure of the torus semicircularis. They form the interior of this structure. The sensory connective neurons form half hollow cylinders arranged according to modalities, which represent a single-layer layer of the connective neurons of the respective modality. Within the half hollow cylinders a sorting according to body segments takes place. Neighbourhood relationships between receptors of the same or different modalities in the real organism are preserved in the model of half hollow cylinders at the corresponding connective neurons.

The motor projection neurons of the fourth segment form - since they represent only one modality, the motor neurons - a half hollow cylinder with the same topology. Neighbouring output neurons project to muscle spindles of neighbouring muscles. In the torus semicircularis, the side assignment of the receptors and their projection neurons is also retained. Receptors of the left half of the body arrive in the left torus and those of the right half in the right torus. The same applies to the motor output.

Left and right torus as a whole form a kind of hollow cylinder - this shape is the eponym.

In mathematics, a body in the form of a swimming ring is called a torus. The Latin word semicircular means semicircular. The term torus semicircularis refers to the form of the sensory part of this striking structure in the brain stem of vertebrates, which consists of half hollow cylinders.

This special topology allowed well-ordered interactions between the different modalities on both the sensory and motor side.

The cause was the well-being of the neurons involved in the fourth segment.

We remember the rope ladder nervous system belonging to one half of the body of the segmented Bilateria. From the sensory center of the fourth segment the sensory part of the torus semicircularis developed. In the creatures whose line led to the vertebrates, a new type of mediating interneurons developed in the fourth segment. We assume here that these neurons were excitatory. If a receptor was stimulated at a certain body position, a corresponding connective neuron in the sensory part of the torus received this stimulation. Via excitatory interneurons, the projection neurons could now be excited, whose output controlled the motor neurons.

The result was quite simple from a motor point of view. If a stimulus was applied to the body at any point, the associated neuronal excitation reached the torus. This excitation was transmitted via interneurons to, among other things, the projection neurons of the muscle spindles, which were located in a neuron layer as a body model, well ordered next to the layer that received the original stimulus. Due to the well-orderedness in the torus system, the projection neurons were now excited to the muscle spindles, which were located in the immediate vicinity of the stimulus effect on the body. As a result, the muscles in the vicinity of the point of stimulus application received a neuronal excitation that led to the contraction of the trunk at this point. The body bent - roughly like a curved cucumber. This caused the body to move away from the point where the stimulus was applied to the body, such as a predator, obstacle or the like. The torus caused the body to avoid the stimulus and thus the self-protection of the living being. Much later, when the brain system had produced the Cerebellum, the living being attained a learning ability that should lead to the reduction of the torus.

In addition, the sensory and motor lateral change cores for contralateral inhibition are also located on this level. Since a separation of the modalities occurred in the course of evolution and the sense of hearing developed in addition to the vestibular sense, the side-line sense, and the electrosensory sense, there is an abundance of nuclei in higher vertebrates on this level that serve for signal evaluation. We will not discuss them further here, since it is precisely here that extensive remodelling took place during the course of evolution. The basic principle in the torus semicircularis consisted in the signal interaction (interference) of the different modalities via well-ordered body images, which were arranged in adjacent neuron layers and could influence the motor body image.

The signal interactions in the tectum opticum were described in the previous chapter and are based on the same principles, but there the visual body image is used for superimposition.