Demonstration of axon terminals of projection fibers from the periaqueductal gray onto neurons in the nucleus raphe magnus which send their axons to the trigeminal sensory nuclei.

Axon terminals of projection fibers from the periaqueductal gray (PAG) onto neurons in the nucleus raphe magnus (NRM) sending their axons to the principal sensory trigeminal nucleus (Vp) or caudal spinal trigeminal nucleus (Vc) were demonstrated light microscopically: In the rats which were injected with Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) respectively into the PAG and trigeminal sensory complex (Vp or Vc), presumed axon terminals labeled anterogradely with PHA-L appeared to make synaptic contacts with NRM neurons labeled retrogradely with WGA-HRP.