INSPIRATORY DRIVE AND PHASE DURATION DURING CAROTID CHEMORECEPTOR STIMULATION IN THE CAT - MEDULLARY NEURON CORRELATIONS

1. This study addressed the hypothesis that there is a parallel proces sing of input from carotid chemoreceptors to brainstem neurones involv ed in inspiratory phase timing and control of inspiratory motor output amplitude. Data were from fifteen anaesthetized, bilaterally vagotomi zed, paralysed, artificially ventilated cats. Carotid chemoreceptors w ere stimulated by close arterial injection of 200 mu l of CO2-saturate d saline solution. 2. Planar arrays of tungsten microelectrodes were u sed to monitor simultaneously up to twenty-two neurones in the nucleus tractus solitarii (NTS) and ventral respiratory group (VRG). Spike tr ains were analysed with two statistical tests of respiratory modulatio n, cycle-triggered histograms, peristimulus-time histograms, cumulativ e sum histograms and cross-correlograms. 3. In NTS, 16 of 26 neurones with respiratory and 12 of 27 without respiratory modulation changed f iring rate during carotid chemoreceptor stimulation. In the VRG 72 of 112 respiratory and 14 of 48 non-respiratory neurones changed firing r ate during stimulation. 4. The spike trains of 85 of 1276 pairs (6.7%) of cells exhibited short time scale correlations indicative of paucis ynaptic interactions. Ten pairs of neurones were each composed of a ro stral VRG phasic inspiratory neurone that responded to carotid chemore ceptor stimulation with a decline in firing rate and a caudal VRG phas ic inspiratory neurone that increased its firing rate. Cross-correlogr ams from two of the pairs had features consistent with excitation of t he caudal neurones by the rostral cells. A decrease in the duration of activity of the rostral VRG neurones was paralleled by the decrease i n inspiratory time of phrenic nerve activity. Caudal VRG inspiratory n eurones increased their activity as phrenic amplitude increased. Spike -triggered averages of all four neurones indicated post-spike facilita tion of phrenic motoneurones. 5. The results support the hypothesis th at unilateral stimulation of carotid chemoreceptors results in paralle l actions. (a) Inhibition of rostral VRG I-Driver neurones decreases i nspiratory duration. (b) Concurrent excitation of premotor VRG and dor sal respiratory group inspiratory neurones increases inspiratory drive to phrenic motoneurones. Other data suggest that responsive ipsilater al neurones act to regulate contralateral neurones.