Low level potentiation of the brain stem laryngeal reflex.

Trains of electrical stimuli were applied to the superior laryngeal nerve (SLN) innervating the upper respiratory tract while evoked potentials were recorded from the recurrent nerve (RN) which innervates the intrinsic laryngeal muscles. Responses of this brain stem reflex (central delay 3.5-5.0 msec) were compared to the post-tetanic potentiation (PTP) of the polysynaptic ventral root response (L6-S1) with trains of stimuli applied to the dorsal root of adult cats. Stimulation of the SLN with a train of pulses varying from 10-45/sec evokes an initial depression of the polysynaptic potential from the RN followed by a low but sustained increase in the amplitude of the integrated signal (1.2-1.6x) lasting 45-130 msec and lacking the early maximum amplitude as seen in the PTP of the polysynaptic spinal reflex. Both the early and late components of the superior laryngeal-recurrent nerve reflex (SLN-RN) demonstrate potentiation whereas the shortest latency components of the spinal reflex account for most of the PTP. Systemic administration of strychnine sulfate (dosage: 0.03-0.45 mg/kg) does not alter the normalized PTP of the SLN-RN reflex despite enhancement of both early and late components of the reflex following control stimuli at 1/sec.