[Direct observation of the inhibitory action of lingual mechanoreceptor afferents in the frog].

In previous researches it has been demonstrated that the frog's tongue mechanoreceptors afferences exercise a presynaptic inhibition on the motor reflex activity evoked by themselves at the bulbar level. Since the indication on this effect were only indirectly obtained this present investigation is now devoted to obtain on the same subject also more direct observations. Therefore it has been examined the behavior of glossopharyngeal-hypoglossal reflex response after conditioning stimulation in different experimental conditions. In intact preparation it has been tested that the crossed inhibition between the mucosal and papillary tongue afferences. This conditioning activation of mucosal afference obtains persistent inhibition of the small reflex response evoked by electrical stimulation of single fungiform papillae. As expected this effect is no evident when the hypoglossal nerve was axotomized. In these preparations however the papillary test response is enhanced and becomes maximal after conditioning stimulation. Also the glossopharyngeal-hypoglossal reflex is depressed after the conditioning stimulation, but the inhibitory influence remains present as much after axotomization of glossopharyngeal nerve. The results show directly the inhibitory influence of frog lingual mechanoreceptors innervation on the bulbar reflex activity and also its correlation with presynaptic properties of the afferences themselves. In fact the facilitating effects observed after axotomy in the reflex response clearly indicate the ability of presynaptic inhibition to control the spread of excitation in the bulbar neuron circuitry. In this context the presynaptic inhibition appears an essential property of lingual mechanoreceptors innervation in the frog.