# Electrical Stimulation May Improve the Feeding and Nutritional Status of Children with Dysphagia
## 2 The Swallowing Process  
Swallowing is a complex process. It involves more than 30 muscles in the coordination of the lips, tongue, palate, pharynx, larynx, and esophagus [5; 37]. The cranial nerves play an important role in swallowing, a complex cognitive and sensorimotor process that moves any bolus from the mouth to the stomach [38].  
Four stages of swallowing can be distinguished: oral preparatory, oral transit, pharyngeal, and esophageal [4; 5; 17]. During the oral preparatory stage, food is kept in the front part of the mouth, where it is moistened and mixed with saliva. The process involves various muscles of the orofacial complex, including the facial muscles, masticatory muscles, suprahyoid and infrahyoid muscles, tongue muscles, soft palate muscles, pharyngeal muscles, and their functions [39]. During the preparatory stage, the masticatory musclesplay a key role in grinding and chewing food. The process involves the mixing of food with saliva to facilitate swallowing [40]. Next, as a result, a bolus is formed that is then placed on the tongue [4; 5]. At this stage, temporomandibular muscles are responsible for chewing and jaw movements [18; 41]. The lesser and greater pterygoid muscles allow forward and backward jaw movements. Cheek and tongue muscles aid in the formation of a food bolus and its transit to the throat [5; 38; 42].  
Once a food bolus has been processed, the oral transit stage begins with the tongue raising up against the hard palate while the back of the tongue drops to allow the bolus to move into the back of the mouth (Figure 2). Simultaneously, the soft palate rises up against the back wall, closing off the passageway into the nose and preventing the regurgitation of any food or liquids into the nose [3; 5; 24]. In the transit stage, the bolus is moved to the pharyngeal isthmus. As a result, the soft palate begins to tense. This acts as a barrier, separating the nasal cavity from the oral cavity:4,40 The palatine arches move closer together, which stops food from going back into the mouth. The larynx moves up, which closes the epiglottis and protects the airway from food aspiration [38; 40]. As a result, the only open path for the food bolus is the pharynx.  
The pharyngeal stage is a rapid sequential activity during which the bolus is propelled into the esophagus. The larynx elevates while the epiglottis flips down, covers the trachea, and protects the airways from aspiration. Pharyngeal muscles contract, squeezing the bolus downwards. The upper esophageal sphincter (cricopharyngeal muscle) relaxes and opens to allow food and liquids to pass into the esophagus [4; 40].  
Once the bolus passes the esophageal sphincter, the esophageal stage begins. The sphincter closes back up to protect any regurgitation back up into the throat. The esophageal muscles continue to propel the bolus into the stomach via coordinated muscle contraction and relaxation, known as peristalsis. The mouth and throat reset to prepare for the next swallowing event [41; 42].  
Figure 2: Food bolus position in the transit stage of swallowing.