Neurophysiological studies on superficial medullary chemosensitive area for respiration

In cats anesthetized with chloralose-urethane (40 mg/kg chloralose; 200 mg/kg urethane) pH sensitivity of neurons in the caudal chemosensitive area on the ventrolateral surface of the medulla oblongata was examined while monitoring phrenic nerve activity simultaneously. pH was varied by superfusion of the ventral medullary surface with mock cerebrospinal fluid (CSF) of different pH (pH 7.4 control, 7.0 and 7.8). A total of 130 units from 21 cats changed their firing rate in response to CSF-pH changes. These were subdivided into 3 groups. In Group I, 31 respiratory pH sensitivie units increased their firing rate in response to decreased mock CSF-pH, noxious pinch, joint movement in contralateral forelimb, and increased inspired CO 2. These responses may have originated from respiratory center neurons. Group II consisted of 59 non-respiratory pH sensitive units whose firing rate changed in an inverse manner with CSF-pH changes. Of these, 30 responded to contralateral distal forelimb movement, 15 to hair manipulation, 9 to heavy pressure and 5 to noxious pinch. Increased inspired CO 2 (rebreathing) did not modify activity. The response to pH is believed to be from non-specific neurons. Group III consisted of 40 non-respiratory pH sensitive units responding to CSF-pH changes and to increased inspired CO 2. The firing rate was irregular, the interval distribution approaching an exponential function. It may be postulated that the impulse frequency of chemosensitive impulses may be irregular at the site of impulse generation, the irregularity decreasing by convergence during transmission to the respiratory centers. The time course of Group III chemosensitive units was similar to phrenic nerve responses.