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

The Feasibility of Home-Based Treatment Using Vibratory Stimulation in Chronic Severe Dysphagia

PURPOSE

Previously, externally placed vibratory laryngeal stimulation increased rates of swallowing in persons with and without dysphagia. This study examined the feasibility of using a vibratory device on the skin over the thyroid cartilage for home-based swallowing rehabilitation in long-standing dysphagia.

METHOD

Only participants with long-standing dysphagia (> 6 months) following cerebrovascular accident or head/neck cancer who had not previously benefited from dysphagia therapy participated. The device had two modes used daily for 90 days. In automatic mode, participants wore the device when awake, which vibrated for 4-8 s every 5 min to trigger a volitional swallow. In manual mode, participants practiced by activating vibration while swallowing rapidly. Study-related adverse events, such as pneumonia, and device-recorded adherence were tracked. Swallowing function on a modified barium swallow study was assessed at baseline and after 3 months of device use. Outcome measures included the Dysphagia Outcome and Severity Scale (DOSS), Penetration-Aspiration Scale (PAS), and swallowing timing measures. Participants' perceptions of the vibratory device and training were obtained.

RESULTS

The intent to treat analysis showed seven of 11 participants completed the study, all with severe chronic dysphagia. Of those seven participants completing the study, two developed respiratory complications (possibly due to pneumonia) that cleared after antibiotic intervention. For prescribed practice trials, adherence was 80% or greater in four of seven participants (57%) whereas prescribed automatic stimulations were met in only two of seven participants (29%). Three participants (43%) had a modest benefit on DOSS. The time to vestibule closure after the bolus passed the ramus was reduced in five participants (71%) on the modified barium swallow study.

CONCLUSION

Overall, the results have indicated that intensive home-based practice with stimulation may provide limited functional benefits in severe chronic dysphagia.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.21498591.

Separate functions for responses to oral temperature in thermo-gustatory and trigeminal neurons

Oral temperature is a component and modifier of taste perception. Both trigeminal (V) and taste-sensitive cells, including those in the nucleus of the solitary tract (NTS), can respond to oral temperature. However, functional associations in thermal sensitivity between V and gustatory neurons are poorly understood. To study this we recorded electrophysiological responses to oral stimulation with cool (9, 15, 25, 32, and 34 °C) and warm (40 and 45 °C) temperatures from medullary V (n = 45) and taste-sensitive NTS (n = 27) neurons in anesthetized mice. Results showed temperatures below 34 °C activated the majority of V neurons but only a minority of NTS units. V neurons displayed larger responses to cooling and responded to temperatures that poorly stimulated NTS cells. Multivariate analyses revealed different temperatures induced larger differences in responses across V compared with NTS neurons, indicating V pathways possess greater capacity to signal temperature. Conversely, responses to temperature in NTS units associated with gustatory tuning. Further analyses identified two types of cooling-sensitive V neurons oriented toward innocuous or noxious cooling. Multivariate analyses indicated the combined response of these cells afforded distinction among a broad range of cool temperatures, suggesting multiple types of V neurons work together to represent oral cooling.

Sensory input pathways and mechanisms in swallowing: a review

Over the past 20 years, research on the physiology of swallowing has confirmed that the oropharyngeal swallowing process can be modulated, both volitionally and in response to different sensory stimuli. In this review we identify what is known regarding the sensory pathways and mechanisms that are now thought to influence swallowing motor control and evoke its response. By synthesizing the current state of research evidence and knowledge, we identify continuing gaps in our knowledge of these mechanisms and pose questions for future research.

Influence of food properties and body position on swallowing-related muscle activity amplitude

This study was designed to examine the effect of different food properties and body positions on electromyographic amplitudes of the anterior tongue (AT) and suprahyoid (SH) muscles during swallowing in normal subjects (six men and three women; 21-30 years old). Each subject was asked to swallow three foods (consisting of 2.0%, 5.7% or 9.1% of a thickening agent dissolved in water) with different textural properties in one of four body positions: upright, inclined at 60 degrees or 30 degrees , or supine. Surface electromyograms (EMG) were recorded from the anterior tongue and SH muscles, and the integrated and cumulated EMG were used to evaluate the amplitude changes. The increases in food hardness and adhesiveness elevated the integrated and cumulated EMGs of both the AT and SH muscles during swallowing in a stepwise manner (P < 0.0001, one-way anova) that was independent of the body position. There was a linear relationship between the concentrations of the thickening agent and the logarithmic values of the food hardness. There was also a parallel relationship between the logarithmic hardness values and the final logarithmic values of the cumulated muscle activity during swallowing. Unlike these simple amplitude responses of the AT and SH muscles to the different food properties, our previous studies have shown that the durational responses of these muscle activities to different food properties are more complicated. Possible neural mechanisms responsible for the amplitude responses observed are discussed.

The neurobiology of swallowing and dysphagia

The neurobiological study of swallowing and its dysfunction, defined as dysphagia, has evolved over two centuries beginning with electrical stimulation applied directly to the central nervous system, and then followed by systematic investigations that have used lesioning, transmagnetic stimulation, magnetoencephalography, and functional magnetic resonance imaging. The field has evolved from mapping the central neural pathway and peripheral nerves, to defining the importance of specific regions of the lower brain stem in terms of interneurons that provide sequential control for multiple muscles in the most complex reflex elicited by the nervous system, the pharyngeal phase of swallowing. The field is now emerging into defining how the higher cortical regions interact with this brain stem control and is providing a broader perspective of how the intact nervous system functions to control the three phases of swallowing (i.e., oral, pharyngeal, and esophageal). Much of the present interest focuses on how to retrain a damaged nervous system using a variety of stimulus techniques, which follow fundamentals in rehabilitation of the nervous system.

Mechanisms of oral sensation

Sensory nerves that supply mechanoreceptors in the mucosal lining of the oral cavity, pharynx, and larynx provide the substrate for a variety of sensations. They are essential for the perception of complex or composite sensory experiences including oral kinesthesia and oral stereognosis. Relevant to the concerns of the oral health care delivery specialist they also contribute to initiation of reflexes and coordination and timing of patterned motor behaviors. The response of oral mechanoreceptors to natural stimuli is determined to a large degree by morphological factors such as the nature of the relationship between nerve ending and certain cellular specializations, their distribution in the mucosa, the diameter of their primary afferent nerve fibers, and the central distribution of these fibers in the brainstem. Because of morphological similarities to certain cutaneous mechanoreceptors, the mucosal lining may be considered as an internal continuation of the large "receptor sheet" for localization and detection of mechanical stimuli. In some regions of the oral, pharyngeal, and laryngeal mucosa, this analogy is appropriate whereas in others, existing data suggest a different role consistent with regionally specific demands (i.e., initiation of protective reflexes).

Fine structure of taste buds located on the lamb epiglottis

BACKGROUND

Taste buds located on the aryepiglottal folds and laryngeal surface of the epiglottis are the principal receptors responsible for the initiation of the laryngeal chemoreflex. In contrast to the wealth of information available concerning the ultrastructure of oral taste buds, little comparable data exists for taste buds located at the entrance to the larynx. Therefore, the present study was designed to investigate the fine structure of taste buds located on the lamb epiglottis.

MATERIALS

Stained thick and semi-serial thin sections from taste buds located on the lamb epiglottis were examined with light and electron microscopy.

RESULTS

Based on morphological criteria, three types of cells could be identified in the taste bud: Type I, Type II, and basal cells. Both Type I and Type II cells extended into the apical taste pore, but there were differences between these two cell types with regard to nuclear profiles, electron density, and the relative density of ribosomes, apical mitochondria, and rough and smooth endoplasmic reticulum. Basal cells did not extend a process into the taste pore. Nerve processes were observed throughout the taste bud. Synapses were observed between both Type I and Type II cells and nerve fibers. These synapses exhibited membrane thickenings and accumulations of clear and dense-cored vesicles of varying proportions in the taste cell cytoplasm adjacent to membrane specializations.

CONCLUSIONS

The taste buds located on the lamb epiglottis share several structural similarities to taste buds located in the oral cavity and other regions of the pharynx and larynx of many mammalian species. The presence of synapses on both Type I and Type II cells of the lamb epiglottal taste bud suggests that both cell types are involved in laryngeal chemoreception.