Periaqueductal Gray : A gray matter region located around the cerebral aqueduct in the midbrain. It is a critical center for the modulation of pain; stimulation of the PAG can cause powerful analgesia (pain relief). It is also involved in the defensive behaviors (e.g., freezing, fight-or-flight) and autonomic regulation.
Substantia Nigra pars compacta : Contains dopamine-producing neurons that project to the striatum. These projections are vital for the initiation and control of voluntary movement. The degeneration of these neurons is the primary pathology in Parkinson's disease.
Substantia Nigra pars reticulata : Functions as an output nucleus of the basal ganglia. It sends inhibitory projections to the thalamus, superior colliculus, and other brainstem regions, playing a key role in motor output and the control of head and eye movements.
Raphe Nuclei : A collection of nuclei located along the midline (raphe) of the brainstem. These nuclei are the primary source of serotonin (5-HT) for the entire central nervous system. Serotonin is involved in regulating mood, sleep, appetite, and pain processing.
Reticular Formation : A diffuse, intricate network of nuclei and nerve fibers running through the core of the brainstem (midbrain, pons, and medulla).
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