We examined the synaptic connections from ventral-group bulbospinal inspiratory neurons to upper-cervical inspiratory neurons and phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Inspiratory neurons were recorded in the medulla (n=38) at the level of the obex and from the upper-cervical segments (C1 and C2) of the spinal cord (n=29) in 18 vagotomized, paralyzed. ventilated and decerebrated rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the spinal cord at C7, Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time scale synchronizations indicative of synaptic connections. Cross-correlation histograms (n=20), computed between the activities of ventral-group bulbospinal inspiratory neurons and the phrenic nerve. all showed peaks (mean half-amplitude width+/-SD, 1.1+/-0.3 ms) tit short latencies (mean latency+/-SD, 2.0+/-0.6 ms) suggestive of monosynaptic excitation. cross-correlation histograms (n=33), computed between the activities of ventral-group bulbospinal inspiratory neurons and upper-cervical inspiratory neurons, displayed four (12%) peaks (mean half-amplitude width+/-SD, 0.9+/-0.1 ms) at shea latencies (mean latency+/-SD, 1.8+/-0.6 ms) suggestive of monosynaptic excitation, and six (18%) peaks (mean half-amplitude width+/-SD, 1.4+/-0.4 ms) at latencies near zero suggestive of excitation from a common source, Cross-correlation histograms (n=34), computed between the activities of ventral-group bulbospinal inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2-8), showed six (18%) peaks (mean half-amplitude width+/-SD. 2.5+/-0.5 ms) at shore latency (mean latency+/-SD, 4.5+/-1.1 ms) suggestive of oligosynaptic connections. Cross-correlation histograms (n=42) computed between activities of intercostal nerves at various levels of the thoracic spinal cord showed central peaks suggestive of excitation from a common source, although the size of the peaks decreased with segmental separation, the displacement of the peaks from time zero did not increase with segmental separation (mean displacement+/-SD, 0.6+/-0.6 ms) as would be expected if the common excitation resulted from a descending monosynaptic excitation by a source such as the ventral-group bulbospinal inspiratory neurons. We conclude that all ventral-group bulbospinal inspiratory neurons make monosynaptic connections to phrenic motoneurons. a few make monosynaptic connections to upper-cervical inspiratory neurons, but connections to intercostal motoneurons are made via interneurons.
