CHANGES IN OUTFLOW TO RESPIRATORY PUMP MUSCLES PRODUCED BY NATURAL VESTIBULAR STIMULATION

1. Activity was recorded from abdominal (expiratory) and phrenic (insp iratory) nerves during natural vestibular stimulation in multiple vert ical planes and the horizontal plane in decerebrate cats. Vestibular s timulation was produced by rotating the head in animals whose upper ce rvical dorsal roots were transected to remove inputs from neck recepto rs; the upper airway and carotid sinus were denervated, and the vagus nerves were transected to assure that the head rotations did not elici t visceral or pulmonary inputs. 2. The plane of head rotation that pro duced maximal modulation of respiratory nerve activity (response vecto r orientation) was measured at one or more frequencies between 0.05 an d 0.5 Kz. The dynamics of the response were then studied with sinusoid al (0.05-2 Hz) stimuli aligned with this orientation. In some animals, sinusoidal horizontal rotations of the head at 0.5 and 1 Hz or static head tilts in the pitch and roll planes were also delivered. 3. Typic ally, maximal modulation of abdominal nerve outflow was elicited by he ad relations In a plane near pitch; nose-up rotations produced increas ed outflow, and nose-down relations reduced nerve discharges. The gain s of the responses (relative to stimulus position) remained relatively constant across stimulus frequencies, and the phases were consistentl y near stimulus position, like regularly firing otolith afferents. Sta tic nose-up tilt produced elevated abdominal nerve activity throughout the stimulus period, providing further evidence that pitch-sensitive otolith receptors contribute to the response. Horizontal head rotation s had little influence on abdominal nerve discharges. 4. The abdominal nerve responses to head rotation were abolished by chemical or aspira tion lesions or the medial and inferior vestibular nuclei, which is co ncordant with the responses resulting from activation of vestibular re ceptors. Transections of axons arising from bulbospinal neurons in the ventral respiratory group, which are known to be the predominant sour ce of expiratory signals to the spinal cord, reduced but did not aboli sh the vestibulo abdominal reflex. Thus it is likely that nonrespirato ry neurons also participate in generating this response. 5. Nose-up pi tch of the head, and in particular large (50 degrees) static tilts, pr oduced small increases in phrenic nerve activity. Ear-down tilt and ho rizontal rotation of the head produced no responses in the phrenic ner ve. 6. The existence of vestibular inputs to some respiratory motoneur ons suggests that the vestibular system has influences on muscles in a ddition to those typically considered to have antigravity roles, and p articipates globally in adjusting muscle activity during movement and changes in posture.