Concurrent inhibition and excitation of phrenic motoneurons during inspiration: Phase-specific control of excitability

The movements that define behavior are controlled by motoneuron output, whi ch depends on the excitability of motoneurons and the synaptic inputs they receive. Modulation of motoneuron excitability takes place over many time s cales. To determine whether motoneuron excitability is specifically modulat ed during the active versus the quiescent phase of rhythmic behavior, we co mpared the input-output properties of phrenic motoneurons (PMNs) during ins piratory and expiratory phases of respiration. In neonatal rat brainstem-spinal cord preparations that generate rhythmic r espiratory motor outflow, we blocked excitatory inspiratory synaptic drive to PMNs and then examined their phase-dependent responses to superthreshold current pulses. Pulses during inspiration elicited fewer action potentials compared with identical pulses during expiration. This reduced excitabilit y arose from an inspiratory-phase inhibitory input that hyperpolarized PMNs in the absence of excitatory inspiratory inputs. Local application of bicu culline blocked this inhibition as well as the difference between inspirato ry and expiratory firing. Correspondingly, bicuculline locally applied to t he midcervical spinal cord enhanced fourth cervical nerve (C4) inspiratory burst amplitude. Strychnine had no effect on C4 output. Nicotinic receptor antagonists neither potentiated C4 output nor blocked its potentiation by b icuculline, further indicating that the inhibition is not from recurrent in hibitory pathways. We conclude that it is bulbospinal in origin. These data demonstrate that rapid changes in motoneuron excitability occur during behavior and suggest that integration of overlapping, opposing synap tic inputs to motoneurons is important in controlling motor outflow. Modula tion of phasic inhibition may represent a means for regulating the transfer function of PMNs to suit behavioral demands.