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Parotid gland

[puh-rot-id]

For the toad wart, see parotoid gland.

The parotid gland is the largest of the salivary glands. It is found wrapped around the mandibular ramus, and it secretes saliva through Stensen's duct into the oral cavity, to facilitate mastication and swallowing.

Anatomy

Location

The parotid gland is found in the subcutaneous tissue of the face, overlying the mandibular ramus and anterior and inferior to the external ear. The gland occupies the parotid fascial space, an area posterior to the mandibular ramus, anterior and inferior to the ear. The gland extends irregularly from the zygomatic arch to the angle of the mandible. This gland is effectively palpated bilaterally. Start anterior to each ear and move to the cheek area and then inferior to the angle of the mandible (Illustrated Head and Neck Anatomy, Fehrenbach and Herring, Elsevier, 2007, p. 170-1).

The facial nerve and its branches pass through the parotid gland, as does the external carotid artery, which gives off its two terminal branches, the maxillary artery and the superficial temporal artery, inside the gland.

Posteriorly, it is related to the posterior belly of the digastric, the stylohyoid, and the sternocleidomastoid.

The parotid gland resembles a three sided pyramid. The apex of the pyramid is directed downwards. the gland has four surfaces: (1) superior (2) superficial (3) anteromedial & (4) posteromedial. The surfaces are separated by three borders: (1) anterior (2) posterior & (3) medial.

Excretory portion

The duct to this gland (also known as Stensen's duct) empties within the buccal cavity (the inside of the cheek) opposite the upper second molar. The parotid papilla is a small elevation of tissue that marks the opening of the parotid duct on the inner surface of the cheek (Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2006, p. 166).

Serous fluid (as opposed to mucous fluid) is produced by the parotid gland.

Innervation

Although the facial nerve (CN VII) runs through this gland, it does not control it.

Secretion of saliva by the parotid gland is controlled by postsynaptic parasympathetic fibres originating in the inferior salivatory nucleus; these leave the brain via the typanic nerve (branch of glossopharyngeal nerve (CN IX), travel through the tympanic plexus (located in the middle ear), and then form the lesser petrosal nerve until reaching the otic ganglion. After synapsing in the Otic ganglion, the postganglionic (postsynaptic) fibers travel as part of the auriculotemporal nerve (a branch of the mandibular nerve (V₃)) to reach the parotid gland.

Sympathetic nerves originating from Superior Cervical Ganglion reach the gland by traveling along the internal carotid artery through the neck and then via smaller blood vessels to the parotid gland.

Parasympathetic stimulation produces a water rich, serous saliva. Sympathetic stimulation leads to the production of a low volume, enzyme-rich saliva. This is done by vasoconstricting to blood supply to the parotid gland reducing the potential for water collection. There is no inhibitory nerve supply to the gland.

Vascularization

Branches of the external carotid artery traverse the glandular tissue and supply the parotid gland with oxygenated blood. The main branch to supply the gland is the transverse facial artery, whereas numerous local veins drain the organ. These veins drain into tributaries of external and internal jugular veins.

The maxillary vein and superficial temporal vein meet to form the retromandibular vein within the parotid gland, but are not responsible for draining it.

Lymphatics mainly comprise pre-auricular lymph nodes.

Pathology

Inflammation of one or both parotid glands is known as parotitis. The most common cause of parotitis is mumps. Widespread vaccination against mumps has markedly reduced the incidence of mumps parotitis. Other infections such as bacterial infections can cause parotitis as may blockage of the duct, whether from salivary duct calculi or external compression. Stones mainly occur within the main confluence of the ducts and within the main parotid duct. The patient usually complains of intense pain when salivating and tends to avoid foods which produce this symptom. In addition the parotid gland may become enlarged upon trying to eat. The pain can be reproduced in clinic via squirting lemon juice into the patient's mouth. Surgery depends upon the situation of the stone, if within the anterior aspect of the duct a simple incision into the buccal mucosa with sphinterotomy may allow removal, however if further posterior within the main duct, complete gland excision may be necessary.

The most common of tumors in the parotid gland are benign and only affect the superficial gland. These include pleomorphic adenoma and adenolymphoma. Their importance is in relation to their anatomical position. The tumorous growth can also change the consistency of the gland and cause facial pain on the involved side since the facial nerve travels through the gland (Illustrated Head and Neck Anatomy, Fehrenbach and Herring, Elsevier, 2007, p. 172). Critically, the relationship of the tumor to the branches of the facial nerve (CN VII) must be defined because resection may damage the nerves, resulting in paralysis of the muscles of facial expression. If the tumor is deep within the gland, the patient should give consent for potential damage of the facial nerve.

Secretion

The parotid gland secretes alpha-amylase which is the first step in the digestion of starches during mastication. It breaks down amylose (straight chanin starch) and amylopectin (branched starch) by hydrolyzing alpha 1,4 bonds.