**1. Introduction**

Dysphagia (from the Greek words *dys,* difficulty, and *phagein,* to eat) is a congenital or acquired swallowing disorder that has structural and functional causes that promote a delay or difficult in the passage of food and liquids from the oral cavity to stomach. Remarka‐ bly, dysphagia is an underestimated neuromuscular disorder, although its consequences frequently are associated with high rates of morbidity and mortality. Estimates of orophar‐ yngeal dysphagia prevalence vary broadly (ranging from 10% to 80%) according to screening methods used and especially the type of study population [1-3]. Dysphagia exhibits a multifactorial etiology, with partipation of exogenous and endogenous factors. The most common causes of dysphagia are divided into the categories of iatrogenic (such as patients with previous history of intubation, tracheostomy, or nasogastric feeding tubes, or a history of infection or metaboic disorders), medications (such as polypharmacy, depressors of the central nervous system, anticholinergics, sympathomimetics, and diuretic drugs) neurological diseases (such as stroke, dementia, amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, extrapyramidal disorders), neuromuscular (such as myasthenia gravis and inflammatory myopathies), or structural obstruction (such as Zenker's diverticulum, oropharyngeal tumors, and factors that causes extrinsic compres‐ sion of the upper aerodigestive tract), as well as other causes [4-7]. Clinically, dysphagia might be classified into three major types: oropharyngeal dysphagia, esophageal dyspha‐ gia, and functional dysphagia. Oropharyngeal dysphagia is the inability to initiate the act

of swallowing, whereas esophageal dysphagia is the perception of difficulty of passing solids or liquids from the throat to the stomach. Functional dysphagia refers to a condi‐ tion in which some patients complain of dysphagia but do not have an organic cause for a swallowing disorder. The most common symptoms of oropharyngeal dysphagia include difficulty in manipulating food, problems with saliva production, and difficulty in chewing the food and swallowing the bolus (a soft mass of chewed food mixed with saliva at the point of swallowing), and an associated impaired quality of life. Frequently, patients with oropharyngeal dysphagia exhibit a series of complications such as nasal regurgitation, coughing, suffocating, gurgle or wet voice after swallowing, unexplained weight loss, anxiety, depression, low-tract respiratory infections and, it's most serious complication, aspiration pneumonia. Problems with social isolation and poor quality of life are a common feature of individuals with dysphagia. Notably, the occurrence of dysphagia is associated with high mortality rate [8].

The different parts of the oral cavity and oropharynx are made up of several cell types and tissues (nerves, fibrovascular, cartilaginous, lining and salivary glandular epithelia, and smooth and striated muscles) along with mineralized tissues (enamel and dentin of the teeth and bones) [8, 9]. Notably, there is an intimate relationship between dysphagia and anatomical, functional, and regulation disturbances of oral cavity and oropharynx components related to physiological salivation, chewing, and swallowing. Salivation depends of the anatomical and functional integrities of the minor and major salivary glands. The saliva lubricates the oral cavity and oropharynx, and an adequate salivary flow assists the initial digestive process by reducing the bolus size of food, begins the enzymatic digestion of some types of carbohydrates, and provides moisture and lubrication of the food particles in order to facilitate the swallowing mechanism, i.e., the movement of the bolus from oropharynx to esophagus [10]. Chewing and swallowing are likely complex and well-coordinated motor programs, combined together as a sequence. During chewing, the food particles are reduced in size and consistency. Chewing is highly dependent of an efficient participation of the teeth, a mineralized tissue whose occlusal surfaces are frequently used for cut off, rip, knead, and grind food during feeding. Moreover, the masticatory muscles have a pivotal role in establishing the muscle strength necessary for the implementation of chewing activity in order to manipulate and grind the food [11-13]. During swallowing two essential and vital functions must be executed: bolus transport and airway protection. After adequate bolus preparation, it needs to be swallowed through an involuntary transport process from the oral cavity and pharynx to the esophagus without allowing the entry of food particles or liquid in the respiratory trac [14, 15]. Together, salivation, chewing, and swallowing, therefore, plays a critical role in alimentary events, allowing food to be initially processed, formed into a bolus, and subsequently transported in the digestive system. Individuals with health problems related to these mechanisms often present with complaints of oropharyngeal dysphagia.

In this present chapter, we will highlight a series of morphological and physiological aspects related to the oral cavity and oropharynx. Moreover, we will discuss the physiopathological aspectos of the salivation, chewing, and swallowing mechanisms in order to allow to health professionals to obtain essential knowledge for management of oropharyngeal dysphagia.
