ISBN
978-3-00-064888-5
Monograph of Dr. rer. nat. Andreas Heinrich Malczan
In the early primordial brain, striosomes enabled the recognition of movement. This was described in chapter 3.8.
Motion detection was achieved by forming a differential image in which the (time-delayed) past signals inhibited the exciting present signals in the thalamus.
One type of signal, however, could not be considered directly in this system. Olfactory signals did not reach the thalamus at all, but entered the uppermost cortical level (the temporal loop) in the early primordial brain. Therefore, motion detection for this type of signal had to be performed in a separate subsystem. This began at a time when the thalamic system (most likely) did not even exist.
The advantages were obvious: Movements of prey or predators could be detected via olfactory signals, and a detection of changes in signal strength made it possible to identify the movements of olfactory objects. This was a significant evolutionary advantage!
The basis for olfactory movement detection was the dopaminergic mean value centre, which was located on the seventh floor of the original rope ladder system, which was present bilaterally, i.e. on each half of the body. The location there was favourable in that a sufficient time delay could occur for action potentials when passing through this section due to the relatively large distance to the first floor.
In the course of evolution, this dopaminergic mean value centre split into two substructures, which are located directly next to each other. The substantia nigra pars compacta received the cortical signals, while the area tegmentalis ventralis received the olfactory (and later limbic) signals.
The area tegmentalis ventralis is the dopaminergic mean core of the olfactory (and later limbic) system. Originally, this core could be located directly in the olfactory floor. In the course of the competition between the mean value systems of the same transmitter affiliation, only the dopaminergic mean value nucleus remained on the initial level of the early primordial brain, the seventh level. The part that switched the olfactory signals of the first level of the early stranded conductor system to dopamine was therefore located directly next to the substantia nigra pars compacta near the nucleus ruber and is called the area tegmentalis ventralis.
The area tegmentalis ventralis receives its output from the output core of the olfactory system, the basal (parvocellular) core. And like any mean system, it projects activatingly back, but also into other subsystems.
The dopaminergic rear projection from the mean value centre (which in the course of time differentiated into two parts) already had inhibitory GABAergic target neurons in early primeval times, which it targeted. In the nonlimbic (cortical) system, the dopaminergic system projected into the striosome neurons. They emerged from the inhibitory interneurons of the cortical floor.
In the olfactory system an analogous development took place, only much earlier. In the amygdala there were also inhibiting interneurons which served to inhibit the lateral neighbours and thus to enhance contrast. These interneurons became the target of dopaminergic rear projection. In the course of evolution they became independent and formed their own subcore of the amygdala, which we call the central amygdala. It consists of GABAergic neurons, which as switch neurons receive the dopaminergic input from the VTA. They possess the dopamine receptors D2 and are therefore excited by the VTA.
Thus the entire output of the parvocellelar basal nucleus of the amygdala - the output nucleus - is switched to the inhibitory transmitter GABA via the detour of the VTA in the central amygdala. Because of the longer detour, this output has a time delay. Thus it forms the inhibitory part of a time-sensitive differential mapping in the amygdala.
The excitatory output for differential imaging comes directly from the amygdala. Via the input nucleus, the lateral amygdala, it excitably reaches the neurons in the basal amygdala. This is also where the inhibitory output of the central amygdala arrives. Point by point, the inhibitory past signal is superimposed on the excitatory present signal, forming the differential signal. Thus, the neurons in the accessory basal nucleus are the differential neurons of the motion detection system.
And, as in the early basal ganglia system, motion detection is based not only on time delay but also on the D2 receptors of the inhibitory switch neurons - here in the central nucleus of the amygdala and there in the striosome neurons. This great analogy shows that a proven solution is applied several times in the nervous system.
It is even more astonishing that, as briefly described above, the ascending axons from the VTA also contacted the neurons of the striatum in addition to the contacts in the central amygdala once these structures were able to form. They moved directly through this area and contacted their own GABAergic neurons with the D2 receptors. However, these no longer projected to the amygdala but, as before, directly into the thalamus, where further differential imaging was also possible because this ventral thalamus also received the descending olfactory signals. Thus there were two systems of motion detection for olfactory (and later limbic) signals. However, it should be noted that the thalamic system was established much later.
The part of the striatum whose striosomal neurons exclusively switched the olfactory (and later the limbic) signals to GABA later separated and formed an independent area called nucleus accumbens. The differentiation of the striatum from the nucleus accumbens is mainly due to the different signal origins, because the neuronal structure is almost identical.
In later evolutionary times, when D1 receptors were also formed in the nucleus accumbens and in the striatum, the matrix was formed in the striatum next to the striosomes. In the nucleus accumbens, however, neurons with D2 receptors formed the nucleus (core), while those with D1 receptors formed the shell. Thus, the striatum and the nucleus accumbens still differed in their cytoarchitectonic structure.
The nucleus (core) of the nucleus accumben receives the output of the basal (parvocellular) amygdala, which was switched to dopamine in the VTA. Its GABAergic neurons possess the dopamine receptor D2 and are excited by these signals. Therefore this nucleus is a switch nucleus from dopamine to GABA. On the dopaminergic detour, the signals suffer a time delay. In the thalamus, its output inhibits precisely those neurons that receive the same signals directly from the basal (parvocellular) amygdala without transmitter switching. This results in a time-sensitive differential mapping in the thalamus, which realizes the changes in olfactory signals and thus the movement of olfactorically perceived objects in the early stages.
Thus the brain has two systems for motion detection of olfactory perceivable objects.
Motion detection for olfactory perceptible objects is achieved by differential mapping, in which inhibitory past signals and excitatory present signals are superimposed in differential neurons.
The output signals of the lateral amygdala reached the dopaminergic VTA via the basal amygdala, where they were switched to dopamine and returned to the central amygdala with a time delay to excite GABAergic switch neurons. These provided the time-delayed and inhibitory component to the differential neurons in the basal amygdala subcore. The excitatory component reaches these differential neurons from the lateral amygdala via the basal amygdala.
This resulted in a time-sensitive differential mapping in the amygdala for motion detection of olfactorically perceivable objects.
After formation of the thalamic system and the striatum in the course of evolution, the dopaminergic signals of the VTA also reached the striatum, led to the formation of nucleus accumulation there and could contribute to the formation of a further differential mapping of olfactory (and later limbic) signals in the thalamus, because these signals also reached the thalamus areas contacted by the striatum in descending order.
Thus the living being had two systems for motion detection in olfactory and limbic signals respectively.
In the course of evolution, the olfactory system should evolve and produce new structures, which are dealt with in the relevant chapters of this monograph.
Monograph of Dr. rer. nat. Andreas Heinrich Malczan