Evidence for direct reciprocal interactions between the central rhythm generators for spinal "respiratory" and locomotor activities in the rabbit.

Rhythm generators for locomotion and respiration have been previously identified in the high spinal rabbit treated with nialamide and DOPA. In curarized preparations, with no sensory feedback, simultaneous recordings of motor commands from the nerves to the diaphragm and to several hindlimb nerves have demonstrated that central (intraspinal) interactions exist between these respiratory and locomotor activities. The purpose of the present study was to investigate the nature of these interactions. Two main possibilities existed: "direct" interactions taking place between the rhythm generators; the activity of one of the rhythm generators modifying the other generator's activity at its "output" (at the interneuronal or motoneuronal level). The present analysis of the timing (and resetting) of activities in the phrenic, hindlimb extensor (gastrocnemius medialis) and flexor (tibialis anterior) nerves suggests a strong direct interaction between the two sets of rhythm generators. Each new locomotor cycle thus only begins at the termination of a "long-lasting phrenic burst" and a respiratory burst can only occur at certain parts of a locomotor cycle.