Respiration-related Neurons in the Ventral Medulla of Newborn Rats in Vitro

In brainstem-spinal cord preparations isolated from newborn rats, we examined functions of the ventral medulla in respiratory rhythm generation, and located respiratory neurons in that region. Removal of the dorsal half of the medulla caused only modest reduction of the rate of inspiratory bursts from the cervical (C4 or C5) ventral root and moderate changes in the burst pattern. We describe here two types of respiratory neurons; Pre-I neurons that are presumably crucial in primary rhythm generation, and inspiratory (I) neurons that we presume to be important in inspiratory pattern generation. Pre-I neurons were located close to phenylethanolamine N-methyltransferase (PNMT)-immunoreactive (IR) neurons that are common in the reticular formation of the rostral ventrolateral medulla (RVL). Distributions of I neurons and Pre-I neurons overlapped in the RVL, and I neurons were also near the nucleus ambiguus in the more caudal part of the ventrolateral medulla. The results indicate that the ventral medulla is essential to inspiratory pattern generation as well as rhythm generation. It is suggested that the RVL is an important site in rhythm generation. The region of inspiratory pattern generation may extend more caudally in the ventral medulla.