Decrementing expiratory neurons of the Bötzinger complex. II. Direct inhibitory synaptic linkage with ventral respiratory group neurons.

In Nembutal-anesthetized, immobilized and artificially ventilated cats, decrementing expiratory (E-DEC) neurons which were excited by lung inflation were isolated in the vicinity of the Bötzinger complex. Then intracellular recordings were made from the respiratory neurons in the contralateral ventral respiratory group (VRG). The intracellular membrane potentials were averaged using extracellular spikes of the E-DEC neurons as triggers (spike-triggered averaging method). Hyperpolarizing potentials locked to the triggering spikes were obtained and they were shown to be unitary IPSPs since their polarity was reversed when averaged during passage of hyperpolarizing current. The latencies of antidromic activation of the E-DEC neurons from the area of intracellular recordings were shorter by about 0.2 ms than those of unitary IPSPs. This showed that the connections were monosynaptic. A total of 47 pairs were analyzed and unitary IPSPs were found in 12 pairs. The E-DEC neurons inhibited both inspiratory and expiratory neurons, including bulbospinal inspiratory neurons, propriobulbar inspiratory neurons, and vagal motoneurons with expiratory activity. These inhibitory E-DEC neurons, receiving excitatory inputs from the stretch receptors of the lungs, presumably intervene in reflex loops such as the Hering-Breuer reflex and may make some contribution to normal breathing.