BOTZINGER-COMPLEX EXPIRATORY NEURONS MONOSYNAPTICALLY INHIBIT PHRENICMOTONEURONS IN THE DECEREBRATE RAT

We examined respiratory neurons in the Botzinger complex of the medull a oblongata in 18 vagotomized, paralyzed, ventilated, and decerebrated rats and tested the hypothesis that bulbospinal expiratory neurons in this region monosynaptically inhibit phrenic motoneurons. First, we s urveyed the types of respiratory neurons found in the Botzinger comple x; only 11 of the 98 (similar to 11%) examined were bulbospinal, and a ll discharged only during late expiration (E2), usually with an augmen ting discharge frequency (AUG). Then, we examined the spinal projectio ns of 34 E2-AUG neurons using antidromic activation and found that all projected as far as the C4 or C5 segments of the spinal cord but no f urther caudally. Most (30, similar to 88%) had only unilateral project ions, the majority (25, similar to 83%) ipsilateral, but 4 neurons (si milar to 12%) had bilateral projections. Their axons could be antidrom ically activated at low currents (less than 10 mu A) in the dorsal-lat eral part of the spinal cord at the C2-3 border; 0.5-1.2 mm (mean +/- SD 0.84 +/- 0.23 mm) below the dorsal surface and 0.7-1.5 mm (1.19 +/- 0.25 mm) lateral from the midline. We sought evidence for connections from bulbospinal E2-AUG neurons to 118 phrenic motoneurons by computi ng spike-triggered averages (STAs) of their intracellular potentials t riggered by the action potentials of 38 unilaterally-projecting E2-AUG neurons. Resting phrenic motoneuron membrane potentials ranged from - 40 to -75 mV (-56 +/- 8 mV) and fluctuations with the respiratory cycl e from 7 to 20 mV (14 +/- 4 mV). Of the 118 STAs computed, hyperpolari zations were evident in 18 (similar to 15%) STAs, evoked by 11 of 38 ( similar to 29%) E2-AUG neurons. Their amplitudes varied from 35 to 550 mu V (105 +/- 113 mu V), 10-90% fall times from 0.4 to 0.9 ms (0.63 /- 0.17 ms), and half-amplitude widths from 1.3 to 3.2 ms (2.0 +/- 0.5 2 ms). Most (16/95, similar to 17%) of the STAs that displayed hyperpo larizations were associated with ipsilateral trigger neurons but some (2/23, similar to 9%) resulted from contralateral trigger neurons. We conclude that Botzinger-complex, expiratory neurons project to the C4 and/or C5 segments of the cervical spinal cord but no further caudal. Their axons are located dorsolaterally in the upper cervical segments of the spinal cord, and they monosynaptically inhibit phrenic motoneur ons during the late part of expiration.