Pharynx Flashcards
(12 cards)
What is the pharynx?
The pharynx is a 12–14 cm long musculomembranous tube shaped like an inverted cone. It extends from the cranial base to the lower border of the cricoid cartilage (the level of the sixth or seventh cervical vertebra), where it becomes continuous with the oesophagus. The width of the pharynx varies constantly because it is dependent on muscle tone, especially of the constrictors; at rest, the pharyngo-oesophageal junction is closed as a result of tonic closure of the upper oesophageal sphincter, and during sleep, muscle tone is low and the dimensions of the pharynx are markedly decreased (which may give rise to snoring and sleep apnoea). The pharynx is limited above by the posterior part of the body of the sphenoid and the basilar part of the occipital bone, and it is continuous with the oesophagus below. Behind, it is separated from the cervical part of the vertebral column and the prevertebral fascia, which covers longus colli and longus capitis, by loose connective tissue in the retropharyngeal space above and the retrovisceral space below. The muscles of the pharynx are three circular constrictors and three longitudinal elevators.
Boundaries of the nasopharynx
The nasopharynx lies above the soft palate and behind the posterior nares, which allow free respiratory passage between the nasal cavities and the nasopharynx. The nasal septum separates the two posterior nares, each of which measures approximately 25 mm vertically and 12 mm transversely. Just within these openings lie the
posterior ends of the inferior and middle nasal conchael/turbinates. The nasopharynx has a roof, a posterior wall, two lateral walls and a floor. These are rigid (except for the floor, which can be raised or lowered by the soft palate), and the cavity of the nasopharynx is therefore never obliterated by muscle action, unlike the cavities of the oro- and laryngopharynx. The nasal and oral parts of the pharynx.
Boundaries of the oropharynx
The oropharynx extends from below the soft palate to the upper border of the epiglottis. It opens into the mouth through the oropharyngeal isthmus, demarcated by the palatoglossal arch, and faces the pharyngeal aspect of the tongue. Its lateral wall consists of the palatopharyngeal arch and palatine tonsil. Posteriorly, it is level with the bodies of the second, and upper part of the third, cervical vertebrae.
SOFT PALATE
The soft palate is a mobile flap suspended from the posterior border of the hard palate, sloping down and back between the oral and nasal parts of the pharynx. The boundary between the hard and soft palate is readily palpable and may be distinguished by a change in colour, the soft palate being a darker red with a yellowish
tint. The soft palate is a thick fold of mucosa enclosing an aponeurosis, muscular tissue, vessels, nerves, lymphoid tissue and mucous glands; almost half its thickness is represented by numerous mucous glands that lie between the muscles and the oral surface of the soft palate. The latter is covered by a stratified squamous epithelium, while the nasal surface is covered with a ciliated columnar epithelium. Taste buds are found on the oral aspect of the soft palate. In most individuals, two small pits, the fovea palatini, may be seen, one on each side of the midline; they represent the orifices of ducts from some of the minor mucous glands of the palate. In its usual
relaxed and pendant position, the anterior (oral) surface of the soft palate is concave and has a median raphe. The posterior aspect is convex and continuous with the nasal floor, the anterosuperior border is attached to the posterior margin of the hard palate, and the sides blend with the pharyngeal wall. The inferior border is free and hangs between the mouth and pharynx. A median conical process, the uvula, projects downwards from its posterior border.
Vascular supply of the soft palate
The arterial supply of the soft palate is usually derived from the ascending palatine branch of the facial artery. Sometimes, this is replaced or supplemented by a branch of the ascending pharyngeal artery, which descends forwards between the superior border of the superior constrictor and levator veli palatini, and accompanies the latter to the soft palate. The veins of the soft palate usually drain to the pterygoid venous plexus.
Innervation of the soft palate
General sensation from most of the soft palate is carried by branches of the lesser palatine nerve (a branch of the maxillary nerve) and from the posterior part of the palate by pharyngeal branches from the glossopharyngeal nerve and from the plexus around the tonsil (formed by tonsillar branches of the glossopharyngeal and lesser palatine nerves). The special sensation of taste from taste buds in the oral surface of the soft palate is carried in the lesser palatine nerve; the taste fibres initially travel in the greater petrosal nerve (a branch of the facial
nerve) and pass through the pterygopalatine ganglion without synapsing. The lesser palatine nerve also carries the secretomotor supply to most of the mucosa of the soft palate, via postganglionic branches from the pterygopalatine ganglion. Postganglionic secretomotor parasympathetic fibres may pass to the posterior parts of the soft palate from the otic ganglion (which receives preganglionic fibres via the lesser petrosal branch of the glossopharyngeal nerve). Postganglionic sympathetic fibres run from the carotid plexus along arterial branches supplying the palate.
PALATINE TONSIL
The right and left palatine tonsils form part of the circumpharyngeal lymphoid ring. Each tonsil is an ovoid mass of lymphoid tissue situated in the lateral wall of the oropharynx. Size varies according to age, individuality and pathological status (tonsils may be hypertrophied and/or inflamed). It is therefore difficult to define the normal appearance of the palatine tonsil. For the first 5 or 6 years of life, the tonsils increase rapidly in size. They usually reach a maximum at puberty, when they average 20–25 mm in vertical, and 10–15 mm in transverse, diameters, and they project conspicuously into the oropharynx. Tonsillar involution begins at puberty, when the reactive lymphoid tissue begins to atrophy, and by old age only a little tonsillar lymphoid tissue remains.
Tonsillar lymphoid tissue
There are four lymphoid compartments in the palatine tonsils. Lymphoid follicles, many with germinal
centres, are arranged in rows roughly parallel to neighbouring connective tissue septa. Their size and cellular content varies in proportion to the immunological activity of the tonsil. The mantle zones of the follicles, each with closely packed small lymphocytes, form a dense cap, always situated on the side of the follicle nearest to the mucosal surface. These cells are the products of B-lymphocyte proliferation within the germinal centres. Extrafollicular, or T-lymphocyte, areas contain a specialized microvasculature including high endothelial venules (HEVs), through which circulating lymphocytes enter the tonsillar parenchyma. The lymphoid tissue of the reticulated crypt epithelium contains predominantly IgG- and IgA-producing B lymphocytes (including some mature plasma cells), T lymphocytes and antigen-presenting cells. There are numerous capillary loops in this subsurface region.
Vascular supply and lymphatic drainage to palatine tonsil
The arterial blood supply to the palatine tonsil is derived from branches of the external carotid artery. Three arteries enter the tonsil at its lower pole. The largest is the tonsillar artery, which is a branch of the facial, or sometimes the ascending palatine, artery. It ascends between medial pterygoid and styloglossus, perforates the superior constrictor at the upper border of styloglossus, and ramifies in the tonsil and posterior lingual musculature. The other arteries found at the lower pole are the dorsal lingual branches of the lingual artery,
which enter anteriorly, and a branch from the ascending palatine artery, which enters posteriorly to supply the lower part of the palatine tonsil. The upper pole of the tonsil also receives branches from the ascending
pharyngeal artery, which enter the tonsil posteriorly, and from the descending palatine artery and its branches, the greater and lesser palatine arteries. The capillaries rejoin to form venules, many with high endothelia, and the veins return within the septal tissues to the hemicapsule as tributaries of the pharyngeal drainage. The tonsillar artery and its venae comitantes often lie within the palatoglossal fold, and may haemorrhage if this fold is damaged during surgery. Unlike lymph nodes, the palatine tonsils do not possess afferent lymphatics or lymph sinuses. Instead, dense plexuses of fine lymphatic vessels surround each follicle and form efferent lymphatics, which pass towards the hemicapsule, pierce the superior constrictor, and drain to the upper deep cervical lymph nodes directly (especially the jugulodigastric nodes) or indirectly through the retropharyngeal lymph nodes. The jugulodigastric nodes are typically enlarged in tonsillitis, when they project beyond the anterior border of sternocleidomastoid and are palpable superficially 1–2 cm below the angle of the mandible; when enlarged, they represent the most common swelling in the neck.
Innervation of the palatine tonsils
The tonsillar region is innervated by tonsillar branches of the maxillary and glossopharyngeal nerves. The fibres from the maxillary nerve pass through, but do not synapse in, the pterygopalatine ganglion; they are distributed through the lesser palatine nerves and form a plexus (the circulus tonsillaris) around the tonsil together with
the tonsillar branches of the glossopharyngeal nerve. Nerve fibres from this plexus are also distributed to the soft palate and the region of the oropharyngeal isthmus. The tympanic branch of the glossopharyngeal nerve supplies the mucous membrane lining the tympanic cavity. Infection, malignancy and postoperative inflammation of the tonsil and tonsillar fossa may therefore be accompanied by pain referred to the ear.
Tonsillectomy
Surgical removal of the pharyngeal tonsils is commonly performed to prevent recurrent acute tonsillitis or to treat airway obstruction by hypertrophied or inflamed palatine tonsils. Occasionally, the tonsil may be removed to treat an acute peritonsillar abscess, which is a collection of pus between the superior constrictor and the tonsillar hemicapsule. Many methods have been employed, the most common being dissection in the plane of the fibrous hemicapsule, followed by ligation or electrocautery to the vessels divided during the dissection. The nerve
supply to the tonsil is so diffuse that tonsillectomy under local anaesthesia is performed successfully by local infiltration rather than by blocking the main nerves. Surgical access to the glossopharyngeal nerve may be achieved by separating the fibres of superior constrictor.
SPREAD OF INFECTION
Infection that spreads into the parapharyngeal space will produce pain and trismus. There may be swelling in the oropharynx that extends up to the uvula, displacing it to the contralateral side, and dysphagia. Posterior spread from the parapharyngeal space into the retropharyngeal space will produce bulging of the posterior pharyngeal wall, dyspnoea and nuchal rigidity. Involvement of the carotid sheath may produce symptoms caused by thrombosis of the internal jugular vein and cranial nerve symptoms involving the glossopharyngeal, vagus,
accessory and hypoglossal nerves. If the infection continues to spread unchecked, mediastinitis will ensue. A virulent infection in the retropharyngeal space may spread through the prevertebral fascia into the underlying danger space; infection in this tissue space may descend into the thorax and even below the diaphragm, and results in chest pain, severe dyspnoea and retrosternal discomfort.
Pharyngeal infection from mucosa-associated lymph tissues such as the palatine tonsil, or as a result of a penetrating injury (e.g. from an ingested foreign body), may result in the spread of infection into the tissue spaces of the neck adjacent to the pharynx. This is an extremely serious situation because there is potential for rapid spread throughout the neck and, more dangerously, to the superior mediastinum, to cause overwhelming life-threatening infection.
