Stimulation of the trunk area of the motor cortex elicits straining in dogs

To clarify the cortical portion of dogs which corresponds to the portion to perform voluntary straining in human, the posterior sigmoid gyrus of anesthetized dogs was systematically stimulated. When reflex-straining was elicited at constant intervals (10-20 s) by stimulation of pelvic afferents, extra-straining (cortical straining) intervening between reflex-straining was elicited after a latent period (0.2-1.0 s) by pulse train stimulation of a focal portion in the trunk area of the motor cortex (M1). Cortical straining reset cycles of reflex-straining. The minimum threshold intensity was observed in the fifth or sixth layer of the focal portion. No difference was recognized between the firing patterns exhibited during cortical and reflex-straining by the phrenic and rectus abdominis muscle nerves, and by the nerves innervating the glottis adductors and external urethral sphincter. Pulse train stimulation of the focal portion, however, interrupted reflex-straining when it was applied during reflex-straining. Pulse train stimulation of the focal portion also elicited burst firings in the phrenic and abdominal muscle nerves after short latencies (20 and 12 ms). The short-latency bursts disappeared after severance of the medulla pyramis ipsilateral to the stimulated focal portion, but cortical straining induced by stimulation of the same cortical site persisted. Stimulation of the focal portion still elicited straining after severance of the ipsilateral hypothalamus, but not after severance of the cerebral peduncle at the midbrain level. However, the cortical stimulation intensified to about three times occasionally induced straining after severance of the peduncle. From these results, it may be concluded that stimulation of the focal portion in the trunk motor area induces straining through the cerebral peduncle, midbrain tegmentum, and pontine straining reflex center.