Inspiratory on-switch evoked by stimulation of mesencephalic structures: a patterned response.

The effects of brief tetanic electrical stimulation (50 to 100 ms) of the mesencephalic central gray matter and reticular formation on the inspiratory "on-switch" mechanism were studied during expiration (E phase) in cats anaesthetized with urethan-chloralose. Stimulation during the E phase evoked powerful effects on the phrenic nerve discharge comprising (1) a primary response (Prim. R.) during the train; (2) a patterned response (Patt. R.) resembling that of the normal inspiratory (I) phase and lasting 170 to 1,000 ms. The patterned response corresponded to activation of the inspiratory on-switch (E-I switching) and appeared either immediately after the Prim. R. or within a latent period of 100 ms. The primary response was always obtained during the E phase whatever the stimulation intensity (0.1-1.0 mA). The patterned response was a function (a) of the stimulus time: the later the stimulus in the E phase, the longer the duration of the response; (b) of the stimulus intensity: with 1.0 mA current strength the response was obtained throughout the E phase; with weaker stimuli (0.4 to 0.5 mA) the response was always evoked by trains delivered early (0-300 ms) or late (1,000-1,800 ms) in the E phase; while it occurred irregularly to stimulation in the middle (300-1,000 ms) of the E phase. These results indicate that: (a) the system which promotes inspiration is progressively rather than abruptly depressed at the beginning of expiration, followed by a progressive inspiratory-promoting facilitation at the end of expiration; (b) the patterned response, mostly independent of the stimulus parameter, is not stereotyped and displays considerable plasticity.