ABSTRACT
This chapter outlines trigeminal brainstem mechanisms related to orofacial somatosensory transmission and its modulation by segmental influences and by descending influences from higher brainstem regions. The brainstem projections of single vibrissal afferents are organized into columns or cylinders that run rostrocaudally and are somatotopically parcellated in the transverse plane, in the main sensory nucleus, and in the subnuclei interpolaris and caudalis. The spatiotemporal properties of the nociceptive-specific and wide dynamic range neurons in subnucleus caudalis have been extensively studied, and both types of neurons have been shown in anesthetized preparations to possess a graded response as the intensity of noxious stimulation is progressively increased or as more of the receptive field is stimulated. Neurons in the spinal somatosensory system are subject to powerful descending modulatory influence. Although these particular descending influences appear to be preferentially directed at nociceptive neurons, non-nociceptive transmission may be affected.
