Functional associations among simultaneously monitored lateral medullary respiratory neurons in the cat. II. Evidence for inhibitory actions of expiratory neurons.

Arrays of extracellular electrodes were used to monitor simultaneously several (2-8) respiratory neurons in the lateral medulla of anesthetized, paralyzed, bilaterally vagotomized, artificially ventilated cats. Efferent phrenic nerve activity was also recorded. The average discharge rate as a function of time in the respiratory cycle was determined for each neuron. Most cells were tested for spinal or vagal axonal projections using antidromic stimulation methods. Cross-correlational methods were used to analyze spike trains of 480 cell pairs. Each pair included at least one neuron most active during the expiratory phase. All simultaneously recorded neurons were located in the same side of the brain stem. Twenty-six percent (33/129) of the expiratory (E) neuron pairs exhibited short time scale correlations indicative of paucisynaptic interactions or shared inputs, whereas 8% (27/351) of the pairs consisting of an E neuron and an inspiratory (I) cell were similarly correlated. Evidence for several inhibitory actions of E neurons was found: 1) inhibition of I neurons by E neurons with both decrementing (DEC) and augmenting (AUG) firing patterns; 2) inhibition of E-DEC and E-AUG neurons by E-DEC cells; 3) inhibition of E-DEC and E-AUG neurons by E-AUG neurons; and 4) inhibition of E-DEC neurons by tonic I-E phase-spanning cells. Because several cells were recorded simultaneously, direct evidence for concurrent parallel and serial inhibitory processes was also obtained. The results suggest and support several hypotheses for mechanisms that may help to generate and control the pattern and coordination of respiratory motoneuron activities.