Sensory modalities conveyed in the hindlimb somatic afferent input to nucleus tractus solitarius

To determine the somatic sensory modalities conveyed by hindlimb somatic af ferent inputs, the discharge of neurons in the nucleus tractus solitarius w as recorded in anesthetized rats after electrical stimulation of either the contralateral sciatic nerve or L-6 spinal nerve, which innervates the hind limb. The discharge of seven of eight cells was increased (P < 0.05) by cap saicin injected into the arterial supply of the hindlimb. Discharge was una ltered in 19 neurons tested for sensitivity to nonnoxious (40 degrees C) an d noxious (47 degrees C) heating of the hindlimb skin. In contrast, lightly stroking the skin elicited discharge in 2 of 14 cells, whereas noxious pin ching increased activity in 4 other cells. Rhythmic (1- to 3-s) muscle cont raction (MC) increased (P < 0.05) discharge in >60% of neurons tested (11 o f 18). Static (10- to 30-s) MC significantly (P < 0.05) increased discharge in four cells, two of which were also responsive to rhythmic MC. Rhythmic and sustained muscle stretch increased discharge (P < 0.05) in three of eig ht neurons tested. These data indicate that nucleus tractus solitarius neur ons receive input from low- and high-threshold cutaneous mechanoreceptors, respond to capsaicin delivered into the hindlimb arterial supply, lack ther mal sensitivity, and respond to activation of mechanosensitive as well as m etabosensitive endings in skeletal muscle.