The reticular formation is a diffuse network of nuclei threaded through the brainstem core, spanning midbrain to medulla. The ascending reticular activating system (ARAS) projects broadly to the Thalamus and cortex, maintaining wakefulness and modulating attention. Bilateral lesions cause coma.
It’s less a discrete region than a distributed system — the raphe nuclei (serotonin), locus coeruleus (norepinephrine), and cholinergic nuclei are all components of it.