Vertebrate brain theory

ISBN 978-3-00-064888-5

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

3.2. The nervous system of segmented Bilateria

From the simple unsegmented Bilateria of replication level 2 the segmented Bilateria of replication level 3 emerged. This was achieved by the intermediate step of colony formation by budding with no separation of the bud. Each bud formed a body segment, initially these segments were completely identical to each other. The resources were shared. In the end there was a jointly used digestive system, as well as a common circulation and later a certain division of labour.

The nervous system initially continued to exist in each segment as it did in the unsegmented Bilateria. There were bilateral sensory centers, motor centers and mean value centers. Class 3 neurons transmitted the receptor signals from the sensory to the ipsilateral motor centre of the same segment. There motor neurons controlled the target muscles in the same segment. There was lateral inhibition in each neuronal center to enhance contrast. There was also contralateral inhibition in each segment, which affected the sensory, motor and mean value centers. Both the sensory and motor centers projected ipsilaterally into the different mean nuclei, which were rear-projected for pre-excitation.
We observe an exchange of signals between the segments in real existing segmented bilateria. Just as the circulatory system or the digestive system functioned across segments, so did neuronal interaction across segments. Projection neurons developed, which linked similar neuronal centres with each other. There were two signal directions: An afferent projection was headward, an efferent projection was tailward. In this way, identical structures (centers) of two adjacent segments were linked. We assign two new neuron classes to the projection neurons crossing the segments. Projection neurons that project afferently across segments are assigned to neuron class 4. Projection neurons that project efferently across segments are assigned to neuron class 5. We postulate that the class 4 neurons transmit sensory signals, whereas the class 5 neurons transmit motor signals.

Thus, there were a total of 6 neuron classes in these animals that acted as projection neurons. These six neuron classes are still found in the human brain today, but also in all vertebrates.

With segmented bilateria, we distinguish the internal signals from the external signals in each segment. Own signals derive directly or indirectly (via neuron chains) from the receptors of this segment, no matter whether they represent sensory signals, motor signals or signals from the mean value centers of this segment.

External signals originate from the other segments.

The ascending (afferent) projection is performed in such a way that the input arriving at the sensory center - both that from the sensory receptors and that from the mean value centers of this segment - is added to the sensory center of the head-side neighboring segment of the same half of the body. Class 4 projection neurons are responsible for this. Thus, the amount of input in the head direction increases from segment to segment, especially since the cross-segment output is transferred to separate, independent neurons, so that the number of neurons in the head direction increases from segment to segment.

Here we assume a topological order with respect to the signal-receiving neurons. Each segment inserts its information in the superordinate segment in such a way that it is received by neurons that attach themselves laterally to the previous neurons.

Thus, each segment receives its own input and an external input. The own input comes from the own segment, i.e. its receptors and neuronal nuclei. The external input comes from the other segments and was passed on to this segment.

This has the advantage that the head segment contains the complete sensory information about the entire organism. Therefore the brain was formed at the head part of the animal.

What happened to the input that arrived in each segment and on each side of the body in each sensory center?

Firstly, it was completely passed on to the adjacent head-side segment via class 4 projection neurons, but this was no longer possible in the head segment and was not done because there was no other superior segment.

Secondly, the input was passed on completely to the motor centre on the same side of the body. Once there, the own input took a different path than the external input.

This is because the own input came from the same segment. It was still used to control the motor neurons of this segment and caused the motor response in this segment.

The external input came from the segments allocated lower in the tail direction. It had ascended from the original segment to class 4 neurons, switched to the motor side and now reached the original segment again via class 5 neurons. Here it docked to the assigned motor neurons and caused corresponding motor responses.

Thus, the sensory output of any segment reached the motor centre several times. Once directly via class 3 neurons, which moved horizontally to it. Secondly, it was sent headwards on the axons of neuron class 4, passed from segment to segment, sent in each of these segments via the horizontal projection of class 3 to the sensory center and from there again via neurons of class 5 downwards to the target segment, where it ended at the motor neurons. The more segments were located head first above a segment, the more signal paths there were for a sensory signal to reach the target neuron of the associated segment.

Thus, there was a nerve cord on each side of the body, consisting of class 4 neurons, which realized an afferent projection from segment to segment. On the other side there was a nerve cord consisting of class 5 neurons, which realized an afferent projection from segment to segment, the corresponding signals were of motor type.

If we imagine the animal with the head on top and the tail below, there was a horizontal projection from the sensory to the motor centre in each segment, which was caused by the class 3 neurons. Therefore this nervous system of one half of the body had the appearance of a rope ladder.

 

Simple rope ladder nervous system without mean centres

Figure 1 - Simple rope ladder nervous system without mean centres

Explanation of figure 1:

From the left the input of the receptors arrives in each segment and reaches the sensory center of each segment.

It is transmitted horizontally to the motor centre by the axons of the class 3 neurons.

Furthermore, axons of class 4 neurons also transmit this excitation to the higher-level segment.

The class 5 axons in the motor centre of each segment transmit the incoming excitation to the motor neurons of the segment on the one hand and project it to the motor centre of the subordinate segment on the other.

The mean value centers have been omitted in this presentation for the sake of clarity.

Each half of the body has its own rope ladder nervous system. Both are connected by commissures.

This rope ladder system was available twice, once on the left and once on the right side of the body, due to the bilateral nature of the system. However, these two rope ladder systems were not independent of each other. In each segment there was the projection of class 2 neurons, which on the one hand linked the motor centre of one side with the other side, ended there at inhibitory interneurons and realised a lateral inhibition for contrast enhancement. Thus, all motor centers of one side of the body were in competition with the other. But the sensory centers were also linked in this way. Thus every neuronal structure of one side of the body was in neuronal competition with the same neuronal structure of the opposite side (in the same segment). This also applies to the signals from the mean value centres.

Therefore, the entire organism possessed a nervous system consisting of four nerve cords, which were connected in each segment by transverse commissures of neuron class 3 and by transverse cross commissures of neuron class 2. This is called the tetraneural nervous system.

With regard to the mean value centers, the signal course can be described easily. On the one hand, each mean value centre of a segment projected excitatory signals into the class 1 neurons of the same segment, which were located in both the sensory and motor centre. In addition, it projected excitatory into the class 1 neurons in all segments located at the head, with the interposition of the afferently projecting class 4 neurons. These neurons were also located in the motor and sensory centers of each head-side segment.

Thus, the mean value excitation of each mean value core used for preactivation reached all segments that were at the same or greater height.

And due to the lateral inhibition caused by interneurons in the sensory and motor centers, all similar mean systems were in competition with each other, both with regard to the segments of one half of the body and with regard to both halves of the body.

This is because we assume that lateral inhibition was initially limited to signals that were signal-related, e.g. originated from the same type of receptor or from similar mean value centers.

Similarly, in each segment, the class 6 neurons projected not only into the mean centers of the same segment, but also into all tail-side segments.

The neuronal competition of signals from different segments of one half of the body should have serious consequences for the body structure of the segmented animals. These become visible through increasing specialisation, as a result of which a division of labour develops between the segments, so that each segment is no longer responsible for everything.

Theorem of neuronal organization of segmented bilateria

In segmented Bilateria, whose line led to the vertebrates, each segment initially had the same neural architecture. Each segment had a left and a right sensory center, a left and a right motor center and different mean centers, also arranged bilaterally left and right.

In every segment except the head segment, there was an afferent projection on each side of the body from the sensory center of the segment to the sensory center of the segment above. This projection was realized by class 4 neurons, whose axons formed the longitudinal connections called sensory connective tissue. The axons of these neurons have a constant length in each segment and reach exactly to the sensory center of the segment above.

In every segment except the tail segment there was an efferent projection from the motor centre of one segment to the motor centre of the segment below. This projection was produced by class 5 neurons whose axons also formed longitudinal connections called motor connective tissue. The axons of these neurons have a constant length in each segment and reach exactly to the motor center of the segment below.

Each sensory centre received the excitation of the ipsilateral receptors of the same segment (intrinsic signals) as well as, via class 4 neurons, the excitation of the ipsilateral receptors of all deeper, tail-side segments (external signals). In addition, the class 1 neurons received the pre-excitation from the ipsilateral mean value centers of the same segment and, via class 4 neurons, the ipsilateral mean value excitation of all deeper, tail-side segments. The class 1 neurons transferred this pre-excitation to the class 3 neurons, and also to the class 4 neurons.

The axons of the class 3 neurons formed the cross-connections called commissures, they terminated for the segment's own signals at the segment's ipsilateral motoneurons. Foreign signals reach the class 5 neurons in the ipsilateral motor centre via the axons of the class 3 neurons and are passed on in descending order to the target segments.

Each motor centre projected via class 2 neurons (cross-commissures) excitatory into inhibitory interneurons of the motor centre of the opposite side of the same segment, where it caused contralateral motor inhibition at segmental level.

In the same way, each sensory center projected excitatory via class 2 neurons (cross-commissures) into inhibitory interneurons of the sensory center of the opposite side of the same segment, causing contralateral sensory inhibition at segmental level.

Similarly, excitation of the motor centre by class 6 neurons reached the equilateral mean centres of the segment and, if present, the corresponding mean centres of the segments below.

The excitation of the sensory center also reached the equilateral mean value centers of the segment and, if present, the corresponding mean value centers of the segments below via class 6 neurons.

Each mean centre of one side of the body projected into the class 1 neurons of the equilateral sensory and motor centre of the same side of the body, giving them a mean-dependent pre-excitation, and via class 4 neurons into class 1 neurons of the ipsilateral sensory and motor centres of all segments located above them on the head side.

There were inhibitory interneurons in both the sensory and motor centers of each side of the body and each segment, which caused lateral inhibition and led to contrast enhancement of stronger signals and attenuation of weaker signals. The same was true for all mean nuclei.

The nervous system of the segmented Bilateria, whose line led to the vertebrates, consisted of two rope ladder systems, one for each half of the body. The sensory centers corresponded to the ganglia of the sensory ladder, the motor centers to the ganglia of the motor ladder. The sensory ganglia were connected to the motor ganglia of the same side of the body by commissures.

The sensory and motor ganglia were connected to those of the opposite side by cross-commissures, which served as contralateral inhibition.

The axons of the different neuron classes ran approximately at right angles to each other, this also applies to the cross commissure.

In later evolutionary times, the axons of class 5 and 6 neurons were able to multiply their length, which originally extended exactly to the next segment, so that their axon length could (later) extend over several segments.

To understand the topological organization, it is necessary to consider the growth directions of the axons of the different neuron classes, which are most likely controlled by the concentration gradients of neuronal marker substances.

directional theorem of the rope ladder nervous system

In an idealized rope ladder nervous system, the axons of the connective neurons form right angles with the axons of the commissure neurons and span a plane to which the axons of the mean neurons are also perpendicular. The dendrites orient themselves approximately opposite to the axons. Spatial deformations of the body can cause systematic topological deviations from the ideal directions.

The different projection neurons transmit signals on their axons.

Since we can assign neuron classes to the projection neurons in the nervous system, it is also possible to divide the signals into signal classes. The signal class can be derived from the neuron class of the neuron that transports this signal on its axons. Therefore, there are also six signal classes.

Then for example the following statements are equivalent:

-         The class 6 neurons project into the centre of the mean value.

-         Signal class 6 reaches the mean value centre.

We imagine the rope ladder nervous system in an arrangement in which the head is on top and the tail is below. It was advantageous to have the axons run evenly and without crossing each other, separated as far as possible according to the different classes of neurons.

As shown symbolically in the following figure, the connective neurons of classes 2 and 3 are each located inside next to the corresponding commissure neurons of classes 4 and 5, because they make the horizontal connection and are thus located at the position that ensures short conduction paths.

In contrast, the upward and downward projecting commissure neurons of class 4 and 5 are located on the outside, so that a crossing-free course of their axons is also possible.

The class 6 neurons that project to the mid-anterior centers are located on the motor side on the far side, because their targets are located outside the rope ladder nervous system. The location of the class 1 neurons inside the sensory side is hypothetical.

The long axons of the class 4 head-projecting neurons ran in the sensory part of the rope ladder system on its outer side. The same was true for the tail-projecting axons of the class 5 neurons. These two classes of axons were particularly long, each of which had to bridge a segment length of the body. Therefore, they were initially quite thick so that the signal attenuation did not bring the action potentials to a standstill. At the same time, these larger axon diameters served to achieve a higher propagation speed of the action potentials. In simple bilaterias these thick axon bundles, which run from the head to the tail and are partly called giant fibers, still exist today. Later, myelin formed as an electrical shield of the axons of the class 4 and 5 connective neurons, which are also located on the outside of the spinal cord of vertebrates, where they form the white matter.

In contrast, the axons of the commissular neurons of class 2 and 3 ran in the inner part of the rope ladder system, they moved horizontally to the target neurons of class 4 and 5. Because of their shortness, no myelin layer had to form here, these axons thus belonged to the grey matter.

 

Spatial arrangement of neuron classes

Figure 2 - Spatial arrangement of neuron classes

 

Figure 2 shows only one rope ladder of the tetraneural nervous system. The second rope ladder system of the other half of the body is connected to it via the class 2 neurons, with the cross-commissures of the class 2 neurons running in both directions. The mean systems are not shown.

Theorem of contralateral inhibition of neuronal centers at segmental level

Each neuronal centre (sensory as well as motor centre, mean value centre) in each segment and on each side of the body projects excitatory into inhibitory interneurons of the same neuronal centre of the opposite side of the body. Both are in competition with each other.

The projection to the contralateral side of the same segment was made via switched-on intermediate neurons, which were located approximately at the (imaginary) separation plane of the two halves of the body. One reason was the change of the marker which marked the side of the body. Each half of the body probably had its own body side marker. During the transition from one half of the body to the other, the body side marker changed, so that an intermediate neuron was needed at the border point, which now responded to the contralateral side marker and whose axon led to the corresponding neuronal center.

In the course of evolution, the interneurons, which were also projection neurons, formed their own nuclei, which we call side-change nuclei. There was one sensory and one motor lateral change nucleus per floor on each side of the body.

paging kernel theorem

In each segment, one sensory and one motoric side-change core was formed on each side of the body. The sensory core received the output of the sensory core of this side of the body and projected excitatory into the sensory core of the opposite side, where it ended at inhibitory interneurons and caused the contralateral inhibition. The motor received the output of the motor core of this side of the body and excitably projected into the motor core of the opposite side, where it ended at inhibitory interneurons and also caused contralateral inhibition.

 

The side change cores were located near the parting plane between the two halves of the body. The input reached the side-change nuclei via class 2 neurons. We assume that the neurons in the side-change nucleus also belong to class 2 neurons, but responded to the contralateral side marker.

Whether the contralateral inhibition of the mean nuclei was also performed by such side-changing nuclei will not be further discussed here in view of the small number of neurons in these nuclei.


Monograph of Dr. rer. nat. Andreas Heinrich Malczan