Reticulospinal regulative commands in the cat evoked at the segmental level: the effect of repetitive stimuli

The reticular formation of the brain stem participates in the neuronal activity which is evoked by an unexpected or intense stimulus and results in a generalized (startle) reaction of the organism. If this reaction is evoked from the segmental receptor level, it comprises a spino-reticulo-spinal (i.e. spino-bulbo-spinal--SBS) reflex. Repetition of the stimulus leads to a habituation of the startle reaction. We studied the effect of repetition and the frequency of the stimulus on the SBS reflex response in somatic nerves of chloralose-anaesthetized cats. A series of 60 stimuli with a low stimulus repetition frequency (less than or equal to 0.2 Hz) did not induce any trend in changes in SBS responses. Frequencies of 0.5 and 1 Hz in some cases led to sensitization or depression of SBS discharges, while higher frequencies (2 and 5 Hz) caused a depression and a decrease in the incidence of SBS response in efferent discharges in every case. These changes occurred promptly (in the course of 5-15 stimuli at a given frequency); continued stimulation did not induce a habituation trend in the changes of SBS responses. When a series of stimuli with a frequency causing a change in the functional state of the SBS system ended, SBS responses returned to control values in the course of the first 5 stimuli at the basic stimulation frequency (0.2 Hz), except for a few cases in which a persistence of the change in SBS responses was observed. The findings showed that a protracted repetition of the stimulus did not lead to the disappearance of the reticular formation reactions resulting in reticulospinal regulative commands in the form of SBS activity. The level of this activity is adapted promptly in correlation to the change in the stimulation parameters; with constant parameters, SBS activity is relatively constant.