Response characteristics of lamb trigeminal neurons to stimulation of the oral cavity and epiglottis with different sensory modalities

A region of the trigeminal complex located at the border of the subnucleus interpolaris and subnucleus caudalis receives not only trigeminal nerve inputs from the face, tongue and palate, but also afferent terminations from other nerves which innervate the oral cavity and upper airway. To increase our understanding of the types of sensory information relayed to this region of the trigeminal nucleus, we investigated the response characteristics of single neurons to stimulation of the tongue, palate and epiglottis. Receptive field size and location of 83 trigeminal neurons were mapped, and responses to mechanical, thermal and chemical stimuli were recorded. About 90% of the neurons had one receptive field and no convergence between the oral cavity and epiglottis was observed. Furthermore, only about 15% of the trigeminal neurons responded to more than one stimulus modality. A moving mechanical stimulus elicited responses in over 90% of the cells, and 84% responded to moving and punctate mechanical stimuli. These mechanosensitive neurons generally exhibited rapidly adapting responses. Thermal and chemical stimuli were relatively ineffective. Cooling a receptor surface most often produced excitation, and warning inhibition. Responses to chemical stimuli were only observed for salts at high concentrations. These results suggest that, like oral cavity information relayed by the trigeminal nerve, afferent terminations in the trigeminal nucleus from other nerves subserving the oral cavity and upper airway function to relay mechanical sensory information. This information may be important in the elicitation of mechanically evoked oral and upper airway reflexes, or may provide a pathway to the thalamus for qualitative and quantitative sensory information about these areas.