Explanation
This module describes the structural/anatomical features needed to understand dysphagia rehabilitation and the application of such knowledge to clinical practice.
Explanation
Positional relationship between the organs involved in eating and swallowing
Food taken into the oral cavity is transferred to the pharynx and then into the esophagus. The pharynx also serves as an air passage and is connected to the nasal cavity and larynx. During swallowing, this connecting passage is closed to prevent the entry of food. The oral cavity, the opening of which is the oral fissure formed by the upper and lower lips, is separated from the pharynx by the oropharyngeal isthmus, which is a narrow space bound by the velum palatinum, palatoglossal arch, palatopharyngeal arch, and tongue base. The pharynx is a space surrounded by skeletal muscles and extends from the skull base down to the 6th cervical spine. It is divided, from top to bottom, into the upper pharynx (nasal pharynx, nasopharynx), middle pharynx (oral pharynx, oropharynx), and lower pharynx (laryngeal pharynx, laryngopharynx). The boundary of the upper and middle pharynx is at the height of the hard palate or the elevated soft palate, while the boundary of the middle and lower pharynx is at the height of the epiglottis. The larynx is the entrance to the trachea from the pharynx and is located anterior to the lower pharynx at the height of the 4-6th cervical spine. The larynx is surrounded by cartilage and maintains a luminal structure. Located posterior to the larynx is the esophagus, which wall is composed of muscle layers and which is normally collapsed.
Explanation
The oral cavity is bound by the lips anteriorly, the cheeks laterally, the palate superiorly, and the floor of the mouth formed by the tongue and hypoglottis inferiorly. The anterior part of the palate, referred to as the hard palate, is supported from inside by bone; the posterior part, referred to as the soft palate, is supported from inside by the palatal muscles and is elevated posterosuperiorly to close off the connection to the nasopharynx during swallowing. The dentition consists of the front teeth (incisors and canines) that are used for biting off and the molar teeth (molars and premolars) that are used for grinding. In adults, both the maxillary and mandibular dentition consists of, moving out from the center in a lateral direction, the central incisor (No. 1), lateral incisor (No. 2), canine (No. 3), first premolar (No. 4), second premolar (No. 5), first molar (No. 6), second molar (No. 7), and third molar (No. 8), totaling 32 teeth. The canine is usually called the incisor in animals. The third molar is also known as the wisdom tooth. (The primary teeth, including the primary central incisor, primary canine, first primary molar, and second primary molar, totaling 20 teeth, are replaced by the permanent teeth as the jaws grow between around age 6 to 12 years.) The groove between the teeth and the cheek and lips is referred to as the oral vestibule, while the wider space behind the dentition is referred to as the proper oral cavity. The boundary between the proper oral cavity and the pharynx is the oral isthmus. The upper wall of the oral isthmus is formed by the velum palatinum (whose middle part, the palatine uvula, protrudes inferiorly), which is located posterior to the soft palate and lateral wall formed by the palatoglossal arch, and the palatopharyngeal arch extending inferolaterally from the velum palatinum.
Explanation
The tongue can be divided into two parts: the body occupying the anterior two-thirds and the base occupying the posterior one-third that is attached to the floor of the mouth. The tip of the tongue is referred to as the apex and the upper surface of the tongue as the dorsum. The body and base parts are separated by a V-shape groove known as the terminal sulcus. A recess is formed at the vertex of the V-shape terminal sulcus and is called the foramen cecum of the tongue. The tongue's mucosal surface is covered by an abundance of small projections, the lingual papillae. There are 4 types of lingual papillae: filiform, fungiform, foliate, and circumvallate papillae. Except for the filiform type, all the other papillae have taste buds, which serve as taste receptors. Taste buds are also found in the soft palate, palatine uvula, and pharynx but are predominantly distributed in the lingual papillae. The anterior two-thirds of the tongue is innervated by the facial and trigeminal nerves for taste and perception, respectively; the posterior one-third is innervated by the glossopharyngeal nerve for both taste and perception. In the epiglottis and pharynx, both taste information and perception are transmitted by the vagus nerve. Tongue movement is controlled by the hypoglossal nerve.
Explanation
The tongue contains 8 muscles. The 4 extrinsic muscles, comprising the genioglossus, hyoglossus, styloglossus, and palatoglossus muscles, originate from bone outside the tongue and end in the tongue, and they are involved in the tongue's major movements. The 4 intrinsic muscles, comprising the superior longitudinal, inferior longitudinal, transverse, and vertical muscles, originate and end in the tongue, and they change the shape of the tongue. Most of the extrinsic and intrinsic muscles are innervated by the hypoglossal nerve, except for the palatoglossus muscle, which is innervated by the pharyngeal plexus composed of the vagus and glossopharyngeal nerves.
Explanation
Masticatory muscles
There are 4 muscles involved in mastication: the masseter, temporal, medial pterygoid, and lateral pterygoid muscles. The masseter muscle, which arises from the zygomatic arch and inserts on the lateral aspect of the mandibular angle, lifts the mandible, and occludes the teeth. The temporal muscle, which arises from the temporal fossa of the temporal region and inserts on the muscular process in the anterior part of the mandibular ramus, lifts the mandible and pulls it backward. The medial pterygoid muscle originates from the pterygoid process of the sphenoid bone, extends inferiorly, and inserts on the medial aspect of the mandibular angle. This muscle lifts the mandible. The lateral pterygoid muscle arises from the greater wing and pterygoid process of the sphenoid bone, extends posteriorly, and inserts on the anterior aspect of the mandibular condyle. Unilateral functioning of this muscle results in mandibular movement toward the body; bilateral functioning results in anterior movement of the mandible. All the masticatory muscles are innervated by the mandibular nerve (the 3rd branch of the trigeminal nerve).
Explanation
Saliva is deeply involved in eating and swallowing by moistening the oral cavity, being mixed with food to facilitate mastication, killing microorganisms, and digesting food. Salivary glands can be classified into the major salivary glands comprised of the parotid, submandibular, and sublingual glands and the minor salivary glands comprised of the labial, buccal, molar, and lingual glands. The major glands secrete saliva produced by secretory cells through thick ducts; the minor glands secrete saliva from submucosal secretory cells through numerous thin ducts. The parotid gland is the largest salivary gland and is located anteroinferior to the ear. Its duct, the parotid duct, is 5 to 6 cm long and opens on the parotid papilla, which is opposite the maxillary second molar, into the oral vestibule. The submandibular gland is located below the mylohyoid muscle and between the mandible and the digastric muscle. The submandibular duct opens on the sublingual papilla. The sublingual gland is located below the mucosa on the floor of the mouth and above the mylohyoid muscle. The major sublingual duct merges with the submandibular duct and opens on the sublingual papilla; the minor sublingual ducts are more abundant and have their openings along the sublingual fold. There two types of saliva: serous saliva rich in amylase, a starch-digesting enzyme, and mucous saliva rich in mucopolysaccharides and glycoproteins that smooths the mucosal surface. The parotid gland secretes serous saliva in response to stimulation by the parasympathetic glossopharyngeal nerve (lesser petrosal nerve). The submandibular and sublingual glands secrete a mixture of serous and mucous saliva when stimulated by the parasympathetic facial nerve (chorda tympani nerve).
Explanation
The naopharynx opens anteriorly into the nasal cavity. The oropharynx is connected to the oral cavity. The nasopharynx is where the opening of the eustachian tube connecting the pharynx and tympanum (pharyngeal opening of eustachian tube) is located. The hypopharynx is attached to the thyroid cartilage anteriorly, and the pharyngeal wall wraps around both sides of the larynx, forming a deep recess referred to as the piriform sinus (piriform fossa). A wedge-shaped gap formed between the epiglottis, which serves as a lid to shut off the larynx from the anterior side during swallowing, and the tongue base is known as the vallecula.
Explanation
The lateral and posterior sides of the pharynx are surrounded by layers of muscles. The lateral and posterior pharyngeal walls are formed by the superior, middle, and inferior pharyngeal constrictor muscles. The superior pharyngeal constrictor muscle arises from the pterygoid process of the sphenoid bone, soft palate, tongue base, and mandible; the middle muscle arises from the greater and lesser horns of the hyoid bone; and the inferior muscle arises from the thyroid and cricoid laryngeal cartilages. All these muscles insert into the pharyngeal raphe in the middle of the pharyngeal posterior wall. The lower fibers of the superior pharyngeal constrictor muscle insert into the tongue base and constitute the glossopharyngeal muscle. During swallowing, this muscle pulls the hyoid bone backward while causing the pharyngeal posterior wall to bulge anteriorly at the level of the tongue base. The lower fibers of the inferior pharyngeal constrictor muscle extend from the posterolateral portion of the cricoid cartilage on one side to the corresponding portion on the other side and are often referred to as the cricopharyngeal muscle. The cricopharyngeal muscle is normally contracted to prevent inhaled air from entering the esophagus and relaxes during the passage of food. This muscle is also known as the upper esophageal sphincter or pharyngoesophageal constrictor. Except for the stylopharyngeus muscle innervated by the glossopharyngeal nerve, all other pharyngeal muscles are innervated by the vagus nerve.
Explanation
Inner pharyngeal muscle layers and palatal muscles The inner layers of the pharynx are composed of the salpingopharyngeus, stylopharyngeus, and palatopharyngeus muscles, which course longitudinally and pull up the pharynx and larynx during swallowing. The salpingopharyngeus and palatopharyngeus muscles are innervated by the pharyngeal plexus composed of the glossopharyngeal and vagus nerves; the stylopharyngeus muscle is innervated by the glossopharyngeal nerve. The palatopharyngeus muscle functions in conjunction with the palatoglossus muscle to pull down the soft palate Velopharyngeal closure is achieved when the palatine velum is tensed and elevated by the tensor veli palatini and levator veli palatini muscles and the palatine uvula is constricted by the uvulae muscle. The tensor veli palatini muscle is innervated by the mandibular (trigeminal) nerve; the levator veli palatini and uvulae muscles are innervated by the pharyngeal plexus.
Explanation
The larynx is a tubular organ about 5 cm long located at the upper end of the trachea. As the entry point for air formed in the digestive tract, the larynx also serves as a vocal organ and prevents the entry of food into the airway during swallowing. Its lumen, the laryngeal cavity, is bound by 6 laryngeal cartilages, which are connected to each other by joints and ligaments and to which many laryngeal muscles insert. The entrance to the larynx is referred to as the laryngeal inlet and is located on the anterior wall of the lower pharynx. At the uppermost portion of the larynx is the epiglottis, which arises from the anterior wall at the uppermost part of the larynx and the posterior aspect of the thyroid cartilage, is anchored by the thyroepiglottic ligament, and extends upward. The anterior and posterior aspects of the epiglottis are referred to as the lingual and pharyngeal surfaces, respectively. The epiglottis is composed of epiglottic cartilage. The lower one-third of the anterior aspect of the epiglottis is located posterior to the tongue base and is connected to the hyoid bone via the hyo-epiglottic ligament.
Explanation
A wedge-shaped gap formed by the epiglottis and the tongue base is called the vallecula. The right and left aryepiglottic folds extend from the lateral margins of the epiglottis to the apex of the arytenoid cartilage, forming the laryngeal inlet. Behind the aryepiglottic folds are pairs of the cuneiform tubercle, composed of cuneiform cartilage, and the corniculate tubercle, composed of corniculate cartilage, on both sides. The space between the bilateral corniculate tubercles is referred to as the interarytenoid notch. At the lower end of the aryepiglottic folds, the vestibular ligament and muscles form folds, which are known as the false vocal cords (also the vestibular folds). The false vocal cords are located superiolateral and parallel to the vocal cords. The funnel-shape space between the laryngeal inlet and false vocal cords is referred to as the laryngeal vestibule. The vocal cords, which are composed of the vocal cord muscle and thyroarytenoid muscle, are located below and parallel to the false vocal cords and extend from the thyroid notch anteriorly to the vocal process of the arytenoid cartilage posteriorly. The space between the bilateral vocal cords is the rima glottidis, and the combination of the vocal cords and the rima glottidis is referred to as the glottis. The space between the false and true vocal cords is the laryngeal ventricle. The space below the vocal cords is the infraglottic cavity and leads to the tracheal lumen. Vocal cord movement is controlled by the recurrent nerve, which is a branch of the vagus nerve.
Explanation
The hyoid bone is a U-shaped bone located between the mandible and pharynx. This bone has no joint with other bones. It is attached anteriorly to the mandible via the anterior belly of the digastric muscle, the mylohyoid muscle, and the geniohyoid muscle and posteriorly to the posterolateral wall of the temporal bone (the mastoid and styloid processes) via the posterior belly of the digastric muscle and stylohyoid muscle, hanging like a hammock with anterior and posterior bands. The lower aspect of the hyoid bone is connected to the larynx via the thyrohyoid muscle and thyrohyoid ligament; thus, the anterosuperior lifting of the bone results in the lifting of the larynx in the same direction. The hyoid muscles can be divided into the suprahyoid muscles located above the hyoid bone (the digastric, styloglossus, mylohyoid, and geniohyoid muscles) and the infrahyoid muscles located below the bone (the sternohyoid, omohyoid, sternothyroid, and thyrohyoid muscles).
Explanation
The esophagus begins at the lower margin of the laryngeal cricoid cartilage at its upper end, passes through the esophageal hiatus of the diaphragm into the abdominal cavity, and then leads to the stomach. The part of the esophagus between its beginning and the upper sternal margin is referred to as the cervical esophagus; the part between the upper sternal margin and the diaphragm is the thoracic esophagus; and the part between the diaphragm and the esophagogastric junction is the abdominal esophagus. The esophagus has 3 physiological narrowings: the first narrowing at the beginning of the esophagus, the second at the tracheal bifurcation, and the third at its penetration through the diaphragm. The first narrowing is at the upper end of the esophagus corresponding to the level of the 6th cervical spine and about 15 cm from the incisor; the second narrowing is at the level of the tracheal bifurcation corresponding to the 4th to 5th thoracic spine and about 25 cm from the incisor; and the third narrowing is at the esophageal hiatus of the diaphragm corresponding to the level of the 11th thoracic spine and about 40 cm from the incisor. The esophagus is innervated by the sympathetic nerve, which is involved in vasomotion, and the vagus nerve, which is involved in muscle motion and glandular secretion. There are 2 muscle layers: inner circular and external longitudinal. Food is transported by peristaltic movements of these muscle layers.
Recommended readings
- Saitoh E and Mukai M: Dysphagia Rehabilitation, 2nd Edition, Ishiyaku Pub. Inc.
- Ide Y and Koide K: Supplement to Research in Prosthodontics - Fundamental functional anatomy for chairside jaw function examinations, Ishiyaku Pub. Inc.
- Jeri A Logemann: Evaluation and Treatment of Swallowing Disorders, Ishiyaku Pub. Inc.
- Ogawa T et al.: Anatomy Part 3, Kanehara Publisher
