Reticular activating system

The ascending reticular activating system ( ARAS ) is in neurobiology, a part of the reticular formation.

The brain pacemaker

The brain researcher Rolf Hassler (1914-1984) wrote in 1971 a contribution to the anthology brain research and psychiatry in the Colloquium Verlag Berlin with the headline: regulation of mental activity. In it he described reticular the sleep-wake system or " non-specific activation system " of the formation.

Thus, there are two activation effects of the reticular formation, a " tonic " and " phasic ". The tonic system in the reticular formation of the midbrain regulated by the hypothalamus, the excretion of neurotransmitters ( nor- ) adrenaline and serotonin, leading to long -lasting, tonic activation and attenuation of cortical activity and influence, for example the day -night rhythm.

The phasic system for short-term activations of individual cow shares has its center in the medial thalamus. As a shell are the reticular structures to the specific nuclei of the sensory organs that result from the thalamus to the sensory projection centers of the cerebral cortex. The unspecific reticular structures are here convergent informed by secondary lines of all sensory systems and set in motion, especially of pulses from the skin and from the sense organ of equilibrium.

These non-specific switching units of the thalamus, a lower general effect on the corresponding cerebral hemisphere than the reticular formation of the midbrain. They act much more selectively to individual cortical fields and can even simultaneously shield other cortical fields.

Of the reticular thalamic nuclei is known that they do not lead directly to the cerebral cortex, but come on intermediate stations only after 20-40 ms in the cortical fields for action. One explanation for this delay is Hassler in studies on fiber directions, which demonstrate a detour of the excitations to specific nuclei of the basal ganglia, from which it is delayed by a number of intermediate neurons return to specific thalamic nuclei to influence now on faster cable routes the cerebrum fields.

The non-specific central nuclei of the thalamus lead to the caudate nucleus and putamen for which both have a regulatory effect on motor function.

From the shell layer of the thalamus, connections to the globus pallidus. Of these pale basal ganglia nuclei a line going back to the specific thalamic nuclei. Only after this loop through the basal ganglia, the non-specific stimuli reach the cerebral cortex and act here on stellate cells on the ladder- shaped dendrites of pyramidal cells.

As a result of nonspecific arousal loops through the basal ganglia Hassler sees the basic electrical activity in the cerebral cortex, which is registerable in the EEG and affects the mental activity from sleep to tense excitement.

The thalamo - cortical circuit diagram

This sketch is drawn according to the description Hassler and to make the relationships between the thalamus and the cortex clear. It is limited for reasons of clarity on one of the billions of connections that run to the cerebrum and from this back to the thalamus from the thalamus.

The entitlement to such simplification results from the similarity, guided by the various nuclei of the thalamus their fibers in all parts of the cortex. In the cortex, the type of interconnection in all regions is similar if one considers etc. the ratio of the pyramidal cells, stellate cells in detail.

The functional unit of the cerebral cortex of a million times woven into the basic pattern of pyramidal cells to be seen. These cells are found only in the cerebral cortex and form the stellate cells whose specific architecture.

This functional unit consists not only of the pyramidal cell and its cortical connections, but it also encompasses compounds from the thalamus to the cortex. But in between are still the basal ganglia, which impose the excitations of the reticular formation that strange detour, who returns to the thalamus and then only with 20-40 msec delay in the specific projection areas of the cortex "fires".

The function of the neural oscillator

If a communications engineer wants to understand the circuit diagram of a device, it starts with its input in order to keep track of the signals. All sense organs send their signals via the thalamus to the cortex, shown here with the red arrow on the input dark blue Thalamuszelle. The dark blue cell sends the result of the previous analysis to one of the many pyramidal cells in the cortex.

At the same time this Thalamuszelle activated via a side branch of the left reticular, red cell formation, which first sends the impulses to the basal ganglia. This line is by Hassler very slow, because it goes through a number of intermediate neurons, and it makes a loop back to the thalamus (right red cell), before they can activate their special pyramidal cell in the cortex from there.

Here it must be asked what the point of this loop may be declining for the modifications of the reticular activating system. Why this detour, why the reticular signals are not passed on directly to the cortex? To find a plausible answer to this question, it is good that the result of reticular activity in the cerebral cortex already by the many studies of brain waves (EEG ) is known: So long as man is alive, his cerebral cortex is through the reticular activation system puts into electrical oscillations, which increase from sleep to tense attention in frequency, decrease in the amplitude. Since the investigations of William Grey Walter is also known that these waves are influenced by external stimuli can be ( triggered). Thus, the " wake-up function " of the reticular system is indeed explained. Strong stimuli wake us from sleep, can " scare " us by via the reticular formation of the brain cortex in a higher activation state on.

If such stimuli occur rhythmically in the frequency range of brain waves, there would quickly be a synchronization between stimulus rhythm and brain waves. So there must be present, which creates a variable, influenced over the sensory input vibration in this system between the thalamus and cortex, a type of oscillator.

If the circuit is viewed from this perspective, one can easily construct such a vibration generator so. It is sufficient to have an inhibitory cell turn into the reticular cycle, as it was shown here in the form of the dark red switch cell between the starting cell and the target cell of the thalamic reticular formation in the image.

If the cell is activated by the right red cell, it inhibits the left red cell and interrupting the signal power until the right red cell is no longer energized and the input release again. The result is, as a result of the right red cell from the continuous input, a rhythmic sequence passes from there into the cortex. Your period corresponds to the time required for the nerve impulses for the loop. About the variable conductivity of the intermediate neurons the duration of the loop can be very finely controlled and varied. Thus the emergence of the variable EEG waves with the simple circuit diagram can be roughly outlined.

The sense of rhythmic excitation opens up about the events of the pyramidal cells. They should be excited by the input of the sense organs and connect to the same excited cells to complexes. Each pyramidal cell is but "wired" countless connections with all parts of the nervous system, so that a single excitation would be like a bush fire quickly spread to the whole brain, when the cells were always in excitable state. Thus, each differentiated perception or action would be impossible.

When the membrane potential of all the pyramidal cells is raised and lowered jointly by means of a rhythmic pulse train as arise in excitable them moments and moments unerregbare in constant alternation. This can take no action potential or limitless spread, each excitation is immediately destroyed by a deletion to make a new exciting place.

The ability to raise and lower the membrane potential arises in the pyramidal cells of the rope-ladder- like contacts with the stellate cells, which are excited by the pulse sequence of the reticular formation rhythmic. This ladder- shaped ( John Carew Eccles calls them " cartridge belt- shaped" ) synaptic contacts are at birth does not exist, they form particularly strong in the first years of life, and thus appear to be necessary for maturation of the brain associated with learning processes. It is conceivable that the rhythmic excitations from the ARAS by the ladder- transfer to the pyramidal cells in the strength and in the temporal behavior via neurotransmitters can be adjusted very finely.

With these considerations, the purpose of the EEG waves can be recognized: the thalamic reticular formation rises and falls in rhythm variable, the membrane potential of cortical pyramidal cells. It will therefore stimulated by the strength of the input stimuli. Whenever the membrane potential of pyramidal cells reached the " firing threshold ", then the incoming of the sense organs specific sensory stimuli can be excited together and cause -specific neural networks that create a whole because of the self-similar penetration of all sense organs of many sub- complexes; a holistic connected structure that is newly formed approximately ten times per second.

In the diagram Hassler's description are drawn according to the new retrograde connections from the pyramidal cell in the specific nuclei of the thalamus. This can be understood that the brain is not dependent on input signals to be active. Even if a person lives isolated from all sensations, he has ideas and thoughts, sometimes even hallucinations because its Cortex wishes and generates activity. In sleep or in the anesthesia, the brain waves become slower, so the distance to the " firing threshold " for the pyramidal cells is so great that only very strong " Weckreize " lead to the activation of perception.

The pacemaker mechanism described is thus shown useful from many sides. But he also makes possible incidents that may occur in two pathological variants:

• The " brain pacemaker " can strongly excite the pyramidal cells through dysregulation, so that their " firing threshold " without specific sensory stimuli of many neurons is exceeded. An epileptic seizure would result.
• The opposite extreme is thus conceivable that the rhythmic excitation is too weak and the " firing threshold " is rarely achieved. Inhibition or slowing of movement, which can be determined, inter alia, Parkinson's disease, as in many disorders of the nervous system, reticular, then it must be the result.

Artificial brain pacemaker

Since a few years, therapeutic trials with electronic stimulation in the basal ganglia and in the nucleus can be carried out mainly in the subthalamic Parkinson's disease, bring good results in some cases.

2007 American neurosurgeons a 38 - year-old man who spent after an accident six years in a minimally conscious state (MCS ) - clearly distinguishable from coma - a pacemaker in thalamus implanted with the effect of the patient immediately for six years closed could open his eyes and speak a few words.