RESPONSE CHARACTERISTICS OF LAMB PONTINE NEURONS TO STIMULATION OF THE ORAL CAVITY AND EPIGLOTTIS WITH DIFFERENT SENSORY MODALITIES

1. To better understand sensory information processing in pontine neurons that receive afferent fiber terminations from oral cavity and upper airway receptors, we investigated the response characteristics of single neurons to stimulation of the oral cavity and epiglottis with different stimulus modalities. These response characteristics were then compared with previously recorded response properties of neurons located in other brain stem regions that receive oral cavity and upper airway sensory inputs. 2. Receptive field sizes of pontine neurons were mapped, and responses to mechanical, thermal, and chemical stimuli were determined. A total of 47 neurons were isolated and most neurons were located near the dorsomedial border of the rostral trigeminal subnucleus oralis and caudal principal trigeminal nucleus. The likelihood that a particular stimulus modality would elicit a response was somewhat dependent on a neuron's location. Neurons that responded to chemical stimuli were always located outside the trigeminal nucleus, whereas neurons that responded exclusively to mechanical or thermal stimuli were more frequently located in the trigeminal nucleus. Receptive fields were mapped for 45 of the 47 neurons. Forty-three of the neurons had a single ipsilateral receptive field and >80% of the receptive fields were > 100 mm2. The majority of neurons responded to only one of the three stimulus modalities. The remaining neurons were multimodal and the combination of stimulus modalities most frequently observed was mechanical and chemical. 3. Mechanical stimuli were the most effective of the three stimulus modalities, eliciting responses in >65% of the neurons. Neurons that responded to mechanical stimuli were generally rapidly adapting and a moving stimulus was more effective than a punctate stimulus. Mechanosensitive neurons that also responded to chemical stimuli exhibited larger mean response frequencies than mechanosensitive neurons that did not respond to chemical stimuli. Chemical stimuli elicited responses in about half the neurons. A greater percentage of neurons with receptive fields on the epiglottis than neurons with oral cavity receptive fields responded to chemical stimuli. The effectiveness of a chemical stimulus was dependent on a neuron's receptive field. NH4CI was the most effective stimulus for neurons with receptive fields located in the oral cavity, whereas KCl was more effective for neurons with receptive fields on the epiglottis. Thermal stimuli were relatively ineffective whatever the location of a neuron's receptive field. The majority of neurons showed an increase in response frequency to cooling the receptive field and in all thermosensitive neurons the response was restricted to the dynamic phase of thermal stimulation. 4. A comparison of pontine neural response characteristics with previously recorded response characteristics of nucleus tractus solitarius (NTS) and caudal trigeminal nucleus (cSPV) neurons that receive oral cavity and upper airway sensory inputs revealed several differences. Neurons located in the pons were more spontaneously active than either NTS or cSPV neurons and they exhibited higher response frequencies. Furthermore, the number of multimodal neurons in the pons was less than that previously observed in the NTS, but greater than that observed in cSPV. A comparison of the effectiveness of the different stimulus qualities found the following: a smaller percentage of neurons in the pons responded to mechanical stimuli; a similar percentage of pontine and NTS neurons responded to chemical stimuli; and thermal stimuli were equally effective at all three locations. 5. The response characteristics and anatomic connections of neurons located in the region of the pons sampled in the present study suggests that these neurons are important in the integration of sensory information from the upper alimentary tract and the transmission of this information to supramedullary structures.