Summary ARAS – Activating Reticulate Ascending System
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Course
Physiology
Institution
Humanitas University
Explore the neuroscience of wakefulness, attention, and cognition with "ARAS – Activating Reticulate Ascending System." Discover:
The thalamus's role in sensory processing and wakefulness.
The ARAS's impact on cortical activity and alertness.
Neurotransmitter pathways (dopamine, adrenaline, ac...
ARAS – Activating Reticulate Ascending System
The cerebral cortex receives two types of input: information and regulatory inputs.
All information reaches the cortex from the thalamus, that collects, elaborates and relays information
from the ascending sensory paths, the cerebellum, the basal nuclei.
A number of other ascending paths reach the cortex bypassing the thalamus, in a diffuse manner; most
of them originate from the brainstem; they do not carry information; they rather have a regulatory
function and may change the intensity and mode of information processing by the cortex.
They are referred to as the activating reticular ascending system (ARAS).
The difference between what passes and what doesn’t (by the thalamus) is like what you can say about a
radio device: it has an antenna but also a number of knobs. The antenna receives information that
will be reproduced, while the knobs will change the volume, the way of hearing, the pitches... It will
modulate but it will not alter the information.
In the same way the thalamus conveys information, ARAS modifies the way the cortex elaborates the
information that it receives (but does not carry information).
A change in the function of the thalamus produces an interference in sensory data flow to the cortex
(and possibly cerebellar and basal nuclei feedback), so that the cortex may be driven to elaborate
information that it already contains; this tends to occur when the mind wanders in a fully relaxed person,
or during sleep.
A. The thalamus functioning mode defines the source of the data and the way the cortex
elaborates them and produces wake or sleep.
B. Vice versa, the ARAS determines the intensity of cortical activity, the alertness, arousal,
and attention level: if the brainstem is damaged and the ARAS silenced, coma ensues,
not sleep.
The main difference between the 2 kinds of information clinically consists in the fact that if you cut
information from thalamus, you cut information from the surrounding body, so the cortex will only
elaborate internal information; the activity will not be reduced, but the attention will be shifted to the
internal part (which is what happens during sleep, isolating the cortex from sensory information). If you
were to stop projections from the brainstem to cortex, the activity might even wane completely, resulting
in a coma (shown by a flat EEG in the worst case, depending on the severity of the coma, so on the
extent of the suppression of the brainstem). Coma is very frequent with increased intracranial pressure,
hemorrhage or shock/contusion because of the anatomy of the skull: the brain ends into the brainstem,
which is enclosed in the tentorium in a position in which, if the brain is shaken, the brainstem would be
subject to pressure and mechanical strain.
If intracranial pressure is increased, you will press down on the exit of the skull, impairing the function
of the brainstem, which will thus be one of the first structures to suffer, and produce coma.
The ARAS is also known as the extrathalamic control modulatory system or simply the reticular
activating system (RAS), “a set of connected nuclei in the brains of vertebrate that is responsible for the
regulation of wakefulness and sleep-wake transitions. It includes:
- Noradrenergic projections,
- Cholinergic projections,
- Serotonergic projections,
- Dopaminergic projections,
- (Possibly) histaminergic projections
All coming from various structures of the brainstem either to specific areas of the cortex or diffuse to
most of the cortex.
183 Body At Work II
, Enrico Tiepolo
Dopamine
Dopamine is the simplest catecholamine. There are four major dopaminergic paths in the brain; three
of them originate from the midbrain and are involved in hedonic (pleasure) evaluation of brain activities:
• Nigro-striatal path from the Substantia nigra pars compacta (SNpc) to the dorsal striatum:
o D1 facilitatory receptors on dynorphin-SP positive medium spiny neurons (direct path);
o D2 inhibitory receptors on neurotensin-endorphin positive MSN (indirect path)
• Meso-limbic path from the ventral tegmental area (VTA) of the midbrain to the ventral
striatum (n. accumbens), involved in producing the perception of pleasure - hedonic (“reward
path”), so it’s also called the reward pathway. When activated it induces a feeling of pleasure,
whose kind will depend on the other activities going on in the cortex but the kind of emotion is
the same. Whether you ate something good, had sex or passed an exam, pleasure is pleasure,
then you characterise it in various ways.
• Meso-cortical path – the one most properly considered as part of the ARAS, as it reaches
the cortex from the brainstem (VTA) – that brings the same kind of information (hedonic
information) to the prefrontal cortex. The prefrontal cortex is the most cognitive part of the
brain and it is the one that produces strategical behavior and cognitive strategies. It can
use information similar to that which reaches the nucleus accumbens except it will not use it to
produce pleasure but to generate motivation (evaluate if something must be pursued or not).
• The tubero-infundibular path carries dopamine from the hypothalamus to the portal
circulation of the anterior hypophysis to inhibit prolactin release (dopamine is a PIF,
prolactin inhibiting factor). Not a classical “neural function” as it has to do mainly with the
endocrine system.
184 Body At Work II
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