Lateral reticular formation

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Caption = Taken from "The Brainstem Reticular Formation and its Significance for Autonomic and Affective Behavior". The above diagram illustrates the reticular nuclei in the brainstem in a tiered fashion


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Moving caudally from the rostral midbrain, at the site of the rostral pons and the midbrain, the medial RF becomes less prominent, and the lateral RF becomes more prominent.

Existing on the sides of the medial reticular formation is its lateral cousin, which is particularly pronounced in the rostral medulla and caudal pons.

Out from this area spring the cranial nerves, including the very important vagus nerve.

The Lateral RF is known for its ganglions and areas of interneurons around the cranial nerves, which serve to mediate their characteristic reflexes and functions.

Ascending reticular activation system

The dorsolateral reticular formation shoots long ascending axons to the thalamus, which relays their signals to the cortex, forming the Ascending reticular activation system or ARAS.

This is part of the ascending reticular activation system pictured above.

These axonal projections are both cholinergic and noradrenergic, the former of which projects to the sensory nucleus of the thalamus and the reticular nucleus of the thalamus.

The reticular nucleus of the thalamus (not pictured) has nothing to do with the reticular formation; its naming was a coincidence.

This nucleus wraps around the thalamus, forming a thin net, for which it is named (reticular means netlike or an intricate network). The reticular nucleus of the thalamus, when active, inhibits the sensory nucleus with GABA.

The sensory nucleus is positively stimulated by acetylcholine, while the reticular nucleus is inhibited by acetylcholine.

This means that when the Ascending Reticular Activation System is active, as during waking hours, the inhibitory actions of the reticular nucleus are inhibited.

The thalamus is never fully stimulated to an action potential via these projections from the ARAS, but it is sensitized by them [Weedman Molavi, Diana PhD. SLEEP AND LANGUAGE, the Washington University School of Medicine. (http://thalamus.wustl.edu/course/sleep.html).] .

During sleep, when the ARAS shuts down, the reticular nucleus is free to inhibit the sensory nucleus of the thalamus. This is how the reticular formation mediates attention and wakefulness.

This makes sense because during wakefulness, it is easy to take in sensory stimuli.

Once the ARAS system begins to shut down, the world seems duller, and it is much harder to take in information from the outside world.

The chemical equivalent of this would be if the thalamus was being partially inhibited by GABA, making it more difficult to relay information to the cortex.

Nuclei

* nucleus reticularis lateralis
* nucleus reticularis paramedianus
* nucleus reticularis tegmenti pontis

References