Monosynaptic excitation of medullary inspiratory neurons by bulbospinal inspiratory neurons of the ventral respiratory group in the cat.

In Nembutal-anesthetized, immobilized, and artificially ventilated cats, we studied the connectivity of medullary collaterals of bulbospinal inspiratory (BS-I) neurons in the ventral respiratory group (VRG). BS-I neurons which projected to the contralateral spinal cord were isolated extracellularly and intracellular recordings were made from the respiratory neurons in the contralateral VRG. The intracellular membrane potentials were averaged using extracellular spikes of the BS-I neurons as triggers (spike-triggered averaging method). Fast-rising and short-lasting depolarizing potentials locked to the triggering spikes were obtained and shown to be unitary EPSPs induced monosynaptically by the medullary collaterals of BS-I neurons. A total of 137 pairs were analyzed and unitary EPSPs were found in 11 pairs. Four BS-I neurons and 7 inspiratory vagal motoneurons received EPSPs from the medullary collaterals of BS-I neurons. These findings suggest that 1) BS-I neurons in the VRG drive medullary motoneurons of accessory respiratory muscles and phrenic or intercostal motoneurons simultaneously, 2) BS-I neurons on both sides synchronize via the excitatory connections, and 3) the augmenting firing pattern of BS-I neurons might partly be produced by this reexcitatory connection within the population of BS-I neurons.