Hyperpolarization of trigeminal primary afferents evoked by stimulation of lumbar sympathetic trunk and peripheral nerves

The gate control theory has emphasized primary afferent hyperpolarization (PAH) as a specific determinant in the perception of a stimulus as painful. Previous studies which indicated that primary afferent depolarization (PAD) could be elicited in the trigeminal system by nonsegmental inputs suggested that this might also be the case for primary afferent hyperpolarization. Decreased excitability of trigeminal primary afferent fibers, an indirect reflection of PAH, was elicited by stimulation of the lumbar sympathetic trunk, sural, and sciatic nerves in cats. This PAH has a magnitude and time course similar to that previously demonstrated in the trigeminal system by segmental noxious inputs. We postulate that PAH is a general phenomenon related to the control of information transmission across afferent synapses rather than a phenomenon specifically related to the perception of various stimuli as painful.