Functional role of ventral respiratory group expiratory neurons during vocalization

The activities of expiratory neurons of the caudal ventral respiratory group (cVRG) (n = 31) were recorded extracellularly during vocalization in unanesthetized, decerebrate cats. Vocalization was induced by electrical stimulation (0.2 ms, 20-80 microA, 100 Hz, lasting for 2-5 s) of the periaqueductal gray (PAG). The firing rates of more than two-thirds of the neurons (24/31) increased during periods of vocalization induced by stimulation of the PAG, with an increase in abdominal muscle activities. The axons of most of these neurons (19/24) projected to the contralateral lumbar spinal cord, and one-third of the bulbospinal neurons (6/19) were orthodromically activated by stimulation of the PAG. Even after sectioning of the axons of bulbospinal cVRG neurons by midsagittal lesion between the obex and the C1 spinal cord, stimulation of the PAG still induced adduction of the vocal cords and vocalization. However, activities of abdominal muscles during vocalization were abolished. These results indicate that the efferent signals for vocalization from the PAG may be relayed at the cVRG neurons and converted into adequate activation of these neurons, contributing to the activation of the abdominal muscles for vocalization.