Ascending Activator Trellis System: Features, Parts and Functions

Reticular formation is a brain system involved in many functions, all of which are fundamental to our survival and proper functioning.

It consists of two subsystems, one of them the ascending activator trellis system, a multicore circuit which is involved in changes in wakefulness, attention and sleep-wakefulness.

This fascinating set of hearts is responsible for our attention and, in fact, is one of the areas that are currently activated as we read this article. Learn more about such an interesting system.

    What is the ascending activator trellis system?

    The Ascending Reticular Activation System or ARAS, known simply as the Ascending Reticular Activation System (ARAS) is one of the two main subunits of the reticular formation.

    The reticular formation consists of a series of interconnected nuclei that lie along the brainstem. The RAAS is with the other subsystem, which are the descending pathways leading to the spinal cord, via the reticulospinal pathways.

    The ascending activating reticular system is a fundamental part of the vertebrate nervous system because is responsible for regulating wakefulness and sleep-wake transitions. This structure is mainly composed of nuclei in the thalamus and a large number of dopaminergic, noradrenergic, serotoninergic, histaminergic, cholinergic and glutamatergic brain nuclei.

    Structure and parts

    The ascending reticular activating system consists of several circuits that connect the dorsal part of the posterior midbrain (midbrain) and the anterior part of the trunk-brain bridge or Varolio. These circuits are directed to the cerebral cortex by means of different pathways protruding through the thalamus and hypothalamus.

    As a whole, the RAAS is made up of more than 20 nuclei on each side of the upper brainstem, Varolio bridge, spinal cord, and posterior hypothalamus. These neurons release different transmitters, among which we can find dopamine, norepinephrine, serotonin, histamine, acetylcholine and glutamate. These neurons exert influence at the cortical level via direct axonal projections and indirectly via links at the thalamic level.

    The thalamic pathway consists primarily of cholinergic neurons in the pontine roof, while the hypothalamic pathway is primarily composed of monoamine-releasing neurons. They are: dopamine, norepinephrine, serotonin and histamine.

    As mentioned, there are also neurons in the ascending activator reticular system that release glutamate. These neurons have been identified relatively recently and have been linked to monoaminergic and cholinergic nuclei. The glutamatergic component of SARA consists of one nucleus in the hypothalamus and several in the brainstem.

    Next, we will go into more detail about the parts that make up each of the nuclei specializing in a different type of neurotransmitter.

    1. Dopaminergic nuclei

    We have the dopaminergic nuclei in the ventral tegmental zone and in the compact part substantia nigra.

    2. Noradrenergic nuclei

    Among the noradrenergic nuclei we have the Locus coeruleus and the noradrenergic nuclei of the brainstem.

    3. Serotonergic nuclei

    The serotonergic nuclei are the dorsal and medial nuclei of the raft.

    4. Histaminergic nucleus

    The histaminergic nucleus is the tuberomamilary nucleus.

    5. Cholinergic nuclei

    Among the cholinergic nuclei we have the cholinergic nuclei of the forebrain and the nuclei of the pontine roof, in particular the laterodorsal and pedunculopontine nuclei.

    6. Glutamatergic nuclei

    At the level of the brainstem, we have the parabrachial nucleus, the precoerule and the pedunculopontin tegmental nucleus. As for the thalamic level, we have the supramillary nucleus.

    7. Thalamic nuclei

    In the thalamus we have the thalamic reticular nucleus and the intralaminar nucleus, in which the centromedial is included.

    the functions

    The reticular system of ascending activation is a very important network of nuclei, because its functions are of vital importance for the body to remain active and be aware of how it works.

    1. Consciousness

    Consciousness, understood in the sense of be aware of his condition, of his existenceThis is a human and animal capacity due to the operation of SARA.

    2. Regulation of sleep-wake transitions

    The ascending activating reticular system is responsible for physiological changes from a deep sleep state to a waking state, Reversible and necessary transitions for the body.

    The ventrolateral preoptic nucleus of the hypothalamus inhibits the neural circuits responsible for wakefulness. When this same nucleus is activated, it causes a state of sleep.

    During sleep state, SARA neurons have a lower rate of fire, While it is higher when you are awake. In order to be able to enter deep sleep, it is necessary to give a reduction in the ascending afferent activity which arrives at the crust, which occurs by suppression of the ascending reticular system.

    3. Attention

    The ascending reticular activating system is also involved in transitions a relaxed and distracted state of consciousness at times of high attention.

    This is due to an increase in blood flow to the regions where the nuclei of this subsystem are located, which blood flow indicates that there is an increase in neuronal activity in the reticular formation of the midbrain and intralaminar nuclei of the thalamus.

    clinical significance

    Knowing the functions just seen, it is possible to understand the great clinical importance of the ascending activating reticular system, especially when massive damage occurs in the system. Damage to this set of nuclei can lead to reduced states of consciousness, Such as coma or brain death, in addition to absolute death.

    With regard to pathologies, we can emphasize that ARDS is primarily affected by age. As we age, this set of nuclei deteriorates and its neural activity becomes more dysfunctional. Among the diseases that affect this system, we can highlight the following:


    narcolepsy it can occur due to lesions in the pedonculopontino and laterodorsal tegmental nuclei. In these nuclei, there is a downregulation of the activation of their neurons, as well as a loss of orexin peptides, causing the daytime sleepiness so characteristic of the disease.

      Parkinson’s disease

      Parkinson’s disease is a medical condition that affects the brain, particularly with regard to dopaminergic production. However, cholinergic nuclei are also affected. It should be noted that the ARDS is one of the systems previously damaged by the onset of the disease.

      Bibliographical references:

      • Young, GB. (2014). Ascending Reticular Activation System (ARAS). 10.1016 / B978-0-12-385157-4.00311-0.
      • Purves, Dale (2011). Neurosciences (5th ed.). Sunderland, Massachusetts: Sinauer. pages 390-395. ISBN 978-0-87893-695-3.
      • Augustine JR (2016). “Chapter 9: Reticular formation”. Human Neuroanatomy (2nd ed.). John Wiley and sons. pages 141-153. ISBN 9781119073994. Archived
      • Mohan Kumar V, Mallick BN, Chhina GS, Singh B (October 1984). “Influence of the ascending reticular activation system on preoptic neuronal activity.” Exp. Neurol. 86 (1): 40-52. doi: 10.1016 / 0014-4886 (84) 90065-7

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