Excitation of upper cervical inspiratory neurons by inspiratory neurons of the nucleus tractus solitarius in the cat.

Experiments were conducted to investigate the connection from inspiratory neurons in the ventrolateral region of the nucleus tractus solitarius to the inspiratory neurons in the contralateral upper cervical (C1-C2) spinal cord. Microstimulation within the region usually produced long-latency (10 to 30 ms) synaptic activation of the upper cervical inspiratory neurons. Antidromic mapping of axons of the ventrolateral nucleus tractus solitarius in C1 demonstrated the existence of collateral arbors in the region of the upper cervical inspiratory neurons (7 of 15). Cross-correlation histograms showed the existence of short-term synchronization of firing for 12 of 74 pairs of ventrolateral nucleus tractus solitarius and upper cervical inspiratory neurons. Cross-correlation combined with antidromic activation showed that the short-term synchronization was due to a direct connection in two cases and a common input in one case. It was concluded that at least some ventrolateral nucleus tractus solitarius inspiratory neurons may directly excite upper cervical inspiratory neurons, probably via monosynaptic connections. Both groups of neurons may also share common inputs.