This report was written by the Japanese Society of Dysphagia Rehabilitation, the Japanese Association of Rehabilitation Nutrition, the Japanese Association on Sarcopenia and Frailty, and the Society of Swallowing and Dysphagia of Japan to consolidate the currently available evidence on the topics of sarcopenia and dysphagia. Histologically, the swallowing muscles are of different embryological origin from somatic muscles, and receive constant input stimulation from the respiratory center. Although the swallowing muscles are striated, their characteristics are different from those of skeletal muscles. The swallowing muscles are inevitably affected by malnutrition and disuse; accumulating evidence is available regarding the influence of malnutrition on the swallowing muscles. Sarcopenic dysphagia is defined as dysphagia caused by sarcopenia of the whole body and swallowing‐related muscles. When sarcopenia does not exist in the entire body, the term “sarcopenic dysphagia” should not be used. Additionally, sarcopenia due to neuromuscular diseases should be excluded; however, aging and secondary sarcopenia after inactivity, malnutrition and disease (wasting disorder and cachexia) are included in sarcopenic dysphagia. The treatment of dysphagia due to sarcopenia requires both dysphagia rehabilitation, such as resistance training of the swallowing muscles and nutritional intervention. However, the fundamental issue of how dysphagia caused by sarcopenia of the swallowing muscles should be diagnosed remains unresolved. Furthermore, whether dysphagia can be caused by primary sarcopenia should be clarified. Additionally, more discussion is required on issues such as the relationship between dysphagia and secondary sarcopenia, as well as the diagnostic criteria and means for diagnosing dysphagia caused by sarcopenia. Geriatr Gerontol Int 2019; 19: 91–97.

Swallowing-related neurons (SRNs) were recorded systematically in the medulla oblongata of urethane-anesthetized cats. The SRNs received orthodromic inputs from the superior laryngeal nerve (SLN) and showed transient changes in their activity synchronous with swallowing. These neurons could be divided into three types. Type I SRNs are sensory-relay neurons from the SLN in the nucleus of the tractus solitarius (NTS), type II are interneurons located diffusely in the parvocellular reticular formation ventral to the NTS, which received oligosynaptic inputs from the SLN, and type III are motoneurons in the nucleus ambiguus. Some type II neurons still showed the swallowing activity after the animals were paralysed, which suggests that they could be involved in the generation of swallowing outputs.

A central pattern generator (CPG) for swallowing in the medulla oblongata generates spatially and temporally coordinated movements of the upper airway and alimentary tract. To reveal the medullary neuronal network of the swallowing CPG, we examined the cytoarchitecture of the swallowing CPG and axonal projections of its individual neurons by extracellular recording and juxtacellular labeling of swallowing-related neurons (SRNs) in the medulla in urethane-anesthetized and paralyzed guinea pigs. Three major types of neuronal discharge patterns were identified during fictive swallowing induced by stimulation of the superior laryngeal nerve: early (burst-like activation during the pharyngeal stage), late (activation after the pharyngeal stage), and inhibited (inhibition during the pharyngeal stage) types. Sixteen neurons were successfully labeled in the nucleus tractus solitarii (NTS) and in the medullary reticular formation (RF). No motoneuron was labeled. The SRNs in the NTS had axons projecting to the NTS, RF, nucleus ambiguus, nucleus hypoglossus, and dorsal motor nucleus of the vagus on the ipsilateral side. Some NTS SRNs projected only within the NTS. The axons of SRNs in the RF projected also to the NTS, RF, motor nuclei on the ipsilateral side, and to the other side RF. These findings show anatomic substrates for the neuronal network of the CPG for swallowing, which consists of complex neuronal connections among SRNs in the NTS, RF, and motor nuclei.

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