They project widely to the SCN and to the limbic system. This area of the basal forebrain acts as an ascending pathway of the reticular activating system and provides a ventral extra-thalamic relay to the cerebral cortex. It is active in wakefulness and in REM sleep, but inactive in NREM sleep. Adenosine accumulates in the extracellular spaces, attaches to A1 receptors and hyperpolarizes (inhibits) the cholinergic basal forebrain neurones, promoting NREM sleep.

Limbic system
The limbic system regulates both the autonomic nervous system and emotional reactions to external stimuli and memories, thereby making these systems more flexible and adaptive. It regulates mental state, motor activity – for instance related to fight or flight – and sexual activity, and responds to other stimuli such as those associated with thirst and hunger.

The most important areas within the cerebral cortex are the anterior cingulate gyrus, the para-hippocampal gyrus and the hippocampal formation in the temporal lobe, as well as the orbitofrontal region of the prefrontal cortex. The limbic cortex gives representation for autonomic function such as cardiovascular and gastro-intestinal control and is involved in behaviour related to emotion. It is inactive in NREM sleep, but active in REM sleep.

The non-cortical components of the limbic system include the amygdala, nucleus accumbens, part of the basal forebrain (basal nucleus of Meynert), and parts of the hypothalamus and some thalamic nuclei. In the midbrain the limbic system comprises the periaqueductal grey matter whose descending impulses influence motor nuclei, respiratory activity and cardiovascular aspects of autonomic function, particularly sympathetic responses.