Synaptic Inhibition of Cat Phrenic Motoneurons by Internal Intercostal Nerve Stimulation

Abstract

Intracellular recordings from 65 phrenic motoneurons (PMNs) in the C5 segment and recordings of C5 phrenic nerve activity were made in 27 pentobarbitone-anesthetized, paralyzed, and artificially ventilated adult cats. Inhibition of phrenic nerve activity and PMN membrane potential hyperpolarization (48/55 PMNs tested) was seen after stimulation of the internal intercostal nerve (IIN) at a mean latency to onset of 10.3 ± 2.7 ms. Reversal of IIN-evoked hyperpolarization ( n = 14) by injection of negative current or diffusion of chloride ions occurred in six cases, and the hyperpolarization was reduced in seven others. Stimulation of the IIN thus activates chloride-dependent inhibitory synaptic inputs to most PMNs. The inhibitory phrenic nerve response to IIN stimulation was reduced by ipsilateral transection of the lateral white matter at the C3 level and was converted to an excitatory response by complete ipsilateral cord hemisection at the same level. After complete ipsilateral hemisection of the spinal cord at C3 level, stimulation of the IIN evoked both excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in PMNs ( n = 10). It was concluded that IIN stimulation can evoke both excitatory and inhibitory responses in PMNs using purely spinal circuitry, but that excitatory responses are normally suppressed by a descending pathway in intact animals. Fifteen PMNs were tested for possible presynaptic convergence of inputs in these reflex pathways, using test and conditioning stimuli. Significant enhancement (>20%) of IPSPs were seen in seven of eight IIN-evoked responses using pericruciate sensorimotor cortex (SMC) conditioning stimuli, but only one of five IIN-evoked responses were enhanced by superior laryngeal nerve (SLN) conditioning stimuli. The IIN-evoked IPSP was enhanced in one of two motoneurons by stimulation of the contralateral phrenic nerve. It was concluded that presynaptic interneurons were shared by the IIN and SMC pathways, but uncommonly by other pathways. These results indicate that PMNs receive inhibitory synaptic inputs from ascending thoracocervical pathways and from spinal interneurons. These inhibitory reflex pathways activated by afferent inputs from the chest wall may play a significant role in the control of PMN discharge, in parallel with disfacilitation following reduced activity in bulbospinal neurons projecting to PMNs.