Neurones in the midbrain periaqueductal grey send collateral projections to nucleus raphe magnus and the rostral ventrolateral medulla in the rat.

Projections to the rostral ventrolateral medulla (RVLM) and nucleus raphe magnus (NRM) appear to originate from neurones with overlapping distributions in the periaqueductal grey (PAG) as demonstrated by the retrograde transport of red and green fluorescent latex microspheres. Furthermore, double-labelling studies demonstrated collateral projections from individual neurones in the PAG to the RVLM and NRM. This anatomical arrangement may allow interactions between descending control systems during specific behaviours.