Activation of spinobulbar lamina I neurons by static muscle contraction.

Spinal lamina I neurons are selectively activated by small-diameter somatic afferents, and they project to brain stem sites that are critical for homeostatic control. Because small-diameter afferent activity evoked by contraction of skeletal muscle reflexly elicits exercise-related cardiorespiratory activation, we tested whether spinobulbar lamina I cells respond to muscle contraction. Spinobulbar lamina I neurons were identified in chloralose-anesthetized cats by antidromic activation from the ipsilateral caudal ventrolateral medulla. Static contractions of the ipsilateral triceps surae muscle were evoked by tibial nerve stimulation using parameters that avoid afferent activation, and arterial blood pressure responses were recorded. Recordings were maintained from 13 of 17 L(7) lamina I spinobulbar neurons during static muscle contraction, and 5 of these neurons were excited. Three were selectively activated only by muscle afferents and did not have a cutaneous receptive field. Spinobulbar lamina I neurons activated by muscle contraction provide an ascending link for the reflex cardiorespiratory adjustments that accompany muscular work. This study provides an important first step in elucidating an ascending afferent pathway for somato-autonomic reflexes.