Location of neurones with cardiovascular and respiratory function, at the ventral surface of the cat's medulla.

A study has been made of the ventral surface of the medulla, to identify neurones with cardiovascular and respiratory functions. Experiments were performed on chloralose-anaesthetized, artificially ventilated cats. Ventral medullary neurones were stimulated by microinjections of excitant amino acid (which selectively activates cell bodies), and responses measured in blood pressure, heart rate, renal sympathetic and phrenic nerve activity. A small region of ventral medulla was found, corresponding to the "glycine-sensitive area", from which large increases in blood pressure and renal nerve activity were evoked by amino acid injections. More caudally, another cell group was localized lateral to the hypoglossal nerve roots, and these neurones depressed blood pressure and renal nerve activity. Two distinct regions were found to increase phrenic nerve activity: rostral to the pressor neurones, encroaching on the trapezoid body (roughly corresponding to area "M"), and a caudal group, close to the depressor neurones (i.e. lateral to the hypoglossal roots). No respiratory response could be evoked from medial to the hypoglossal roots (area "L") and stimulation of neurones in area "S" generally depressed phrenic activity. Neurones with cardiovascular and respiratory actions could be distinguished anatomically. Their locations have been mapped and compared with previous studies.