Fig. 7.

Lumbar motor-sensory loop as a neural substrate for mammalian locomotor–respiratory coupling. A, Spontaneous respiratory activity of phrenic nerve (Phr) and pharmacologically induced locomotor rhythmicity (L2, lumbar ventral root) in the absence of low-threshold lumbar afferent stimulation. The underlying central networks are schematized on theright. B, bottom left, Repetitive activation of low-threshold lumbar afferents (vertical bars: St.) in time with locomotor bursts immediately drives locomotor–respiratory coupling. The two sets oftraces in A and B are from the same experiment. Related synaptic events recorded from a single phrenic motoneuron are shown in B (upper left). B, right, Schematic representation of proposed circuitry involved in the locomotor–respiratory coupling (hindlimb extensor and flexor muscles, which were removed in our preparations, are included to complete thein vivo motor-sensory loop). Connections responsible for different synaptic influences (arrowheads) on phrenic motoneurons are also numbered. Phr Mn, Phrenic motoneuron; Lumbar Mn, lumbar motoneuron;Resp., respiratory; Loco., locomotor.