Formatio reticularis

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The reticular formation or Retikulärformation (from Latin formatio "design", "education" and reticular "net-like") refers to a broad, diffuse neural network in the brain stem , which from the medulla (medulla oblongata) to the midbrain (diencephalon) ranges. The reticular formation continues into the medial forebrain bundle.

The neurons are diffusely scattered like a mesh or condensed into nuclei . Well-defined nuclei within the reticular format are the raphe nuclei (median part) and the locus caeruleus . In addition, a large-cell nucleus group in the medial and a small-cell nucleus group in the lateral part must be distinguished. The neurons ascending to the higher brain centers have sensory functions, while those descending to the spinal cord ( efferents ) have motor functions.


In addition to interconnecting the cranial nerve nuclei, the reticular formation has the following tasks and coordination centers:

The brain pacemaker

Between the clearly delimited pathways and nuclei of the nervous system, groups of nerve cells and the associated fibers lie in a diffuse, network-like arrangement, the network substance that runs through the central nervous system from the spinal cord to the thalamus.

The task of these diffuse nerve networks consists in the temporal coordination, e.g. B. when using muscles in locomotion. Such networks for temporal coordination are detectable in all vertebrate brains, in insects and lower animals.

The ascending reticular activation system

The reticular formation plays a special role in mammals in connection with the development of the cerebral cortex. The cortical activities must also be coordinated with the whole system in terms of time. For this purpose go from the network substance of the thalamus connections to all parts of the cortex, the " A ufsteigendes R eticuläres A ktivierendes S ystem" (abbreviated ARAS) are called. Because the thalamus also transmits specific information from the sensory organs to the cortex, the ARAS connections are referred to as the “unspecific pathways”.

circuit diagram

Before these "unspecific pathways" from the thalamus reach the cortex, they make a loop to the so-called basal ganglia (nucleus caudatus, pallidum, putamen), which are an elongated core area between the thalamus and cortex. In this excitation loop, the duration of which can be finely regulated via many intermediate neurons, a variable rhythm is created in the thalamus, which is then directed from there to all areas of the cortex.

With a rhythmic excitation of the cortical pyramidal cells by the ARAS, what we call consciousness arises : Above a frequency of 6 Hz we become more and more awake, up to about 40 Hz. If the rhythm is slower than 6 Hz, the person sleeps, at 3 Hz he is in deep sleep or anesthesia , and the zero line in the EEG is regarded as a sure sign of death.

Another function of the ARAS is the modulation of a wake-up stimulus. We know the filtering effect of the thalamus, which only lets strong or “important” information into the consciousness. The thalamus has long been called the “gateway to consciousness” in anatomy, and the frequency of the ARAS determines how wide this gateway is open. Strong stimuli instantly accelerate the pacemaker rate to make you wide awake immediately.

Another function is the control of attention , which can be understood in this way: Because the information flowing in from the sensory organs and the reticular structures of the ARAS are in the immediate vicinity and connection in the thalamus, the sensory excitations there can influence the activity of the ARAS in this way that precisely those projection fields of the cortex are activated into which the strongest sensory impressions are projected.

Since efferent fibers also lead from the cortex to the thalamus, the cortex can influence the activity of the ARAS and direct attention to each cortical area independently of external stimuli.

Circulatory and respiratory center

The afferents reach the nucleus tractus solitarii (NTS) from peripheral baroreceptors . From there there is an efferent limb via the nucleus dorsalis nervi vagi and the nervus vagus (10th cranial nerve ) to the heart and leads to a lowering of blood pressure (negative inotropic ). A second, antagonistic leg runs via the reticular formation from the NTS to the caudal ventrolateral medulla oblongata , from there to the rostral ventrolateral medulla oblongata and finally into the spinal cord to the sympathetic centers. The sympathetic system then leads to the heart and arterial blood vessels to raise blood pressure. It is noteworthy that the central antihypertensive agent clonidine acts on α2-adrenoceptors of the rostral venterolateral medulla oblongata by inhibiting the sympathetic system.

To control breathing, the reticular formation contains inspiratory active and expiratory active neurons. If the vagus nerve conducts signals via the expansion of the lungs to the pre-Bötzinger complex of the reticular formation, the inspiratory neurons are more and more inhibited, the expiratory neurons uninhibited, until at some point the inspiration comes to a standstill and turns into expiration. This reflex is conditionally involuntary and can be modulated in certain areas through voluntary influence. It is called the Hering-Breuer reflex after its first description .

Extrapyramidal control of motor skills

The motor reticular formation is a section of the extrapyramidal motor system . It is divided into a facilitating and inhibiting area. The basin area covers a large part of the entire reticular formation from the medulla oblongata to the midbrain . It receives impulses from the cerebral cortex , the cerebellum , the vegetative nervous system and from sensory pathways. As a result, motor, sensory and vegetative excitation processes are integrated in it. The reticular formation takes up a small part of the ventral medulla oblongata.

Vomiting center

For vomiting center in addition to parts of the formation is one of the reticular postrema area and the nucleus of the solitary tract .

Pontine bladder emptying control

The function of the pontine micturition center, with descending pathways to the micturition center in the spinal cord (segments S1-4), is the coordination of somatic, sympathetic and parasympathetic influences on urination with the aim of enabling complete emptying of the bladder. This becomes clear in paraplegia , in which these descending tracts are severed. During the spinal shock (weeks to months after the injury), areflexia , i.e. the failure of the spinal reflexes, combined with incontinence . Since not only the closing mechanism of the bladder is disturbed, but also the bladder muscle, there is not only incontinence but also insufficient emptying of the bladder and residual urine . This stage turns into hyperreflexia : an involuntary contraction of the bladder muscle, which starts as a reflex when the bladder is filled, reduces the amount of residual urine. Some patients are able to trigger the reflex by tapping the bladder and thus regain a certain voluntary influence on the bladder function.

Modulation of the sensation of pain

A spinoreticulo-thalamic system ascends in the white matter of the spinal cord next to the anterior cord. These fibers end in the reticular formation and are further interconnected from there:


Due to the connection of hypothalamic nuclei and the limbic system , the reticular formation is also important for the affective coloring of sensory impressions (mesolimbic path from the ventral tegmental area to the accumbens nucleus ).


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