An in vitro model for studying neuromuscular transmission in the mouse pharynx

The muscles of the pharynx are controlled by networks of neurons under the control of specific regions in the brain stem, which have been fairly well studied. However, the transmission between these neurons and the pharyngeal muscles, at the motor end plates, is less well understood. Therefore, an in vitro model for the study of neuromuscular transmission in the pharyngeal muscle of the mouse was developed. Ring preparations from the inferior constrictor and the cricopharyngeus muscles were isolated and mounted for isometric force recording at physiologic temperature. Preparations from the diaphragm and the soleus muscles were examined in parallel. The muscles were stimulated at supramaximal voltage with short tetani at 100 Hz. Following direct stimulation of the muscle fibers, using a longer pulse duration, the rate of force development of the pharyngeal muscles was similar to that of the diaphragm and faster than that of the soleus muscle. By varying the duration of the stimulation pulses, conditions where the nerve-mediated activation contributed to a major extent of the contractile responses were identified. Gallamine completely inhibited the nerve-mediated responses. In separate experiments the dose dependence of gallamine inhibition was examined, showing similar sensitivity in the inferior pharyngeal constrictor compared to the diaphragm and soleus muscles. We conclude that reproducible contractile responses with an identifiable nerve-induced component can be obtained from the mouse inferior pharyngeal constrictor. The pharyngeal muscles have contractile characteristics similar to those of the faster diaphragm. The sensitivity to the neuromuscular blocking agent gallamine of the inferior pharyngeal constrictor was in the same concentration range as that of the diaphragm and soleus muscles.