Respiration in vitro: II. Electrical stimulation.

The present report describes electrical-stimulation-induced activity recorded from intercostal muscles in the neonatal rat rib-attached, in vitro brain stem-spinal cord preparation. The muscle bursts induced by electrical stimulation included a short-latency twitch contraction and a long-latency modulated contraction similar to that observed during spontaneous respiratory-like activity. Multilevel recordings showed a cephalocaudal order of recruitment of intercostal muscles similar to that observed during spontaneous activity. The optimal parameters of stimulation were 2-msec pulses delivered at 0.1-0.2 Hz. Trains of pulses also were effective. These movements could be induced following stimulation of various sites within each segment of the spinal cord, with the lowest threshold sites located in the ventrolateral funiculus and intermediate gray. Stimulation of every cervical segment was effective in inducing respiratory-like activity, with the lowest-threshold segments being C1, C2, and C5. These results suggest that low-frequency, long-duration pulses applied directly to the spinal cord can induce respiratory-like activity similar to that observed during spontaneous activity in the neonatal rat, rib-attached in vitro brain stem-spinal cord preparation. The ability to elicit a coordinated respiratory pattern even after a high spinal transection suggests that such stimulation may be effective in inducing respiratory-like activity in the absence of descending brain stem connections.