Respiration-related control of abdominal motoneurons.

The abdominal muscles form part of the expiratory pump in cooperation with the other expiratory muscles, primarily the internal intercostal and triangularis sterni muscles. The discharge of abdominal muscles is divided into four main patterns: augmenting, plateau, spindle and decrementing. The patterns tend to be species-specific and dependent on the state of the central nervous system. Recent studies suggest that the abdominal muscles are more active than classically thought, even under resting conditions. Expiratory bulbospinal neurons (EBSN) in the caudal ventral respiratory group are the final output pathway to abdominal motoneurons in the spinal cord. Electrophysiological and anatomical studies indicated the excitatory monosynaptic inputs from EBSN to the abdominal motoneurons, although inputs from the propriospinal neurons seemed to be necessary to produce useful motor outputs. Respiration-related sensory modulation of expiratory neurons by vagal afferents that monitor the rate of change of lung volume and the end-expiratory lung volume (EELV) play a crucial role in modulating the drive to the abdominal musculature. Studies using in vitro and in situ preparations of neonatal and juvenile rats show bi-phasic abdominal activity, characterized by bursting at the end of expiration, a silent period during the inspiratory period, and another burst that occurs abruptly after inspiratory termination. Since the abdominal muscles rarely show these post-inspiratory bursts in the adult rat, the organization of the expiratory output pathway must undergo significant development alterations.