The relation between end-tidal CO2 and discharge patterns of sympathetic preganglionic neurons

In 11 Nembutal-anesthetized, vagotomized, thoracotomized, paralyzed and artificially ventilated cats, the electrical activity of 32 single sympathetic preganglionic neurons (SPNs), dissected from the cervical nerve was recorded at various end-tidal CO2 levels, together with the activity of the phrenic nerve. Seven of these neurons were insensitive to CO2 changes, within a range of end-tidal CO2 values from 1;0 to 10;0%. All 7 had a background firing pattern without respiratory modulation, even at the highest CO2 levels tested, i.e., had the same firing frequency in both phases of the phrenic nerve activity cycle. Seventeen units were silent at low CO2 levels, began to discharge at particular CO2 levels (on the average, at 2.3% CO2) and increased their firing frequency (on the average, by 0.9 spikes/sec/% CO2) as end-tidal CO2 was raised above the threshold level. Their background discharge pattern was characterized by firing only in the inspiratory phase of the phrenic nerve activity cycle. Three units had firing which was CO2-independent within a range of low CO2 concentrations and which increased as CO2 concentration was increased above this range. These units fired throughout the phrenic nerve activity cycle but had their peak frequency in inspiration. Five units had a firing frequency which was highest at low CO2 and which decreased with increasing CO2 levels. These units had their peak frequency in expiration. These results show that the output of this SPN population is strongly influenced by CO2 within the range of concentrations tested. The finding that sensitivity to CO2 changes is a property only of SPNs with respiratory-modulated firing pattern suggests that the CO2-dependent input is relayed to these SPNs via the respiratory center; A comparison of data obtained under hypocapnic conditions with data obtained in previous studies in normocapnic cats with mid-cervical spinal cord transections suggests that brain stem inspiratory neurons represent a major excitatory input to this SPN pool.