Responses of neurons in the lamb nucleus tractus solitarius to stimulation of the caudal oral cavity and epiglottis with different stimulus modalities

Abstract

Receptors located in the posterior oral cavity and on the epiglottis play an important role in the initiation of upper airway reflexes such as swallowing, gagging, coughing and apnea. Peripheral nerves which innervate these receptor areas terminate in the nucleus tractus solitarius (NTS). We have recorded the responses of 61 neurons in the lamb NTS to stimulation of the caudal tongue, palate and epiglottis with mechanical, chemical and thermal stimuli and mapped receptive field location. Although there was some overlap in the areas of the NTS from which neurons with oral cavity and epiglottal receptive fields could be recorded, a significant difference was observed in the mean recording sites of the two groups of neurons. Neurons with oral cavity receptive fields were located more rostral, lateral and ventral in the NTS than neurons with receptive fields on the epiglottis. Little convergence of sensory input onto single cells in the NTS was observed between the oral cavity and the epiglottis. Only one NTS neuron had a receptive field in both of these receptor areas. In contrast, a large number of neurons with oral cavity receptive fields received input from two receptor areas. These neurons had a receptive field on the tongue which was located directly beneath the receptive field on the palate. Mechanical stimuli were the most effective for neurons with either oral cavity or epiglottal receptive fields and thermal stimuli were the least effective. Neurons which responded to mechanical stimuli responded better to a moving stimulus than to a punctate one, and large increases in the strength of a punctate stimulus were required to elicit significant increases in response frequency. Most NTS neurons responded to more than one of the stimulus modalities. However, a significant difference in the mean number of stimulus modalities which elicited responses was observed between neurons with oral cavity and epiglottal receptive fields. The number of multimodal neurons with epiglottal receptive fields was higher than those with oral cavity receptive fields. The multimodal nature of neurons which responded to epiglottal or oral cavity stimulation combined with their location in reflexogenic areas of the NTS suggests that these neurons could be important in the integration of afferent input from the oral cavity and upper airway. If these NTS neurons are involved in the control of oral and upper airway reflexes it would be important for them to respond to as many of the stimulus cues as possible and the majority of these neurons do just that.

Sucking and swallowing rates after palatal anesthesia: an electromyographic study in infant pigs

“…The motor program for the swallow originates in and around the nucleus ambiguus (NA) in the brain stem, which is also part of the CPG for swallowing, and is influenced by both the NTS and descending cortical input. Trigeminal afferents synapse in the trigeminal sensory nucleus in the brain stem; however, it is known that it also sends information to the NTS, which means it could also influence swallowing centers within the NTS or NA (Capra 1995;Sweazey and Bradley 1989). Alternatively, trigeminal sensation could influence the NA by altering descending cortical input.…”

Effects of Mechanical, Cold, Gustatory, and Combined Stimulation to the Human Anterior Faucial Pillars

“…Fujiu et al [38] provided mechanical stimulation to the anterior faucial pillar at a rate of 8 taps/s and a force of 0.4 N before and after application of topical anesthesia and found that the latency of glossopharyngeal-evoked potentials did not change, thus providing evidence for the importance of deep receptors in conveying mechanical sensation. Sweazey and Bradley [20] measured mean response frequencies of oral cavity and epiglottic neurons in the lamb nucleus tractus solitarius while increasing the strength of the mechanical stimuli (2±7 g) and found relatively small increases in response frequency with increases in stimulus strengths for both moving and punctate stimuli. Stimulation at 1 g, however, was less eective in eliciting neuronal responses.…”

“…Much of what we know about chemosensitive regions important for evoking the swallow response has come from animal research [19,20,34,35]. Similar research in human subjects is sparse [23,36].…”

“…Much of the animal research has applied electrical stimulation to the peripheral nerves and regions of the CNS while measuring muscular responses [15±18]. Others have applied more common stimuli, such as mechanical (touch), chemical (taste), and thermal (cold/warm), to the oropharyngeal mucosa while monitoring muscular and neuronal responses in decerebrated animals [19,20]. More recently, electromagnetic stimulation of the cortex and peripheral nerves in healthy human subjects has shown that cortical swallowing pathways can be modulated by cranial nerve aerent stimulation [21,22].…”

Occlusion of the upper airway does not augment the cardiovascular response to arousal from sleep in humans

“…In animal models, sites within the medulla receiving afferent information from the mechanoreceptors are also known to be involved in the regulation of sympathetic outflow to the heart and blood vessels, and involved with areas associated with the alerting response. Specifically, peripheral nerves from the UA terminate in the nucleus tractus solitarius, an area acknowledged to mediate cardiovascular reflexes (11,35,36,47). We therefore reasoned that stimulation of the UA mechanoreceptors could interact centrally with the arousalrelated sympathetic outflow to augment the cardiovascular response.…”