Parotid Glands

The paired parotid glands are the largest of the major salivary glands and typically weigh 15 to 30 g. Located in the preauricular region and along the posterior surface of the mandible, each parotid gland is divided by the facial nerve into a superficial lobe and a deep lobe. The superficial lobe, overlying the lateral surface of the masseter, is defined as the part of the gland lateral to the facial nerve. The deep lobe is medial to the facial nerve and located between the mastoid process of the temporal bone and the ramus of the mandible. The parotid gland is bounded superiorly by the zygomatic arch. Inferiorly, the tail of the parotid gland abuts the anteromedial margin of the sternocleidomastoid muscle. This tail of the parotid gland extends posteriorly over the superior border of the sternocleidomastoid muscle toward the mastoid tip. The deep lobe of the parotid lies within the parapharyngeal space. The parotid duct (Stensen) secretes a serous saliva into the oral cavity. From the anterior border of the gland, it travels parallel to the zygoma, in an anterior direction across the masseter muscle. It then turns sharply to pierce the buccinator muscle and enters the oral cavity opposite the second upper molar tooth.

The submandibular gland is the second largest major salivary gland and weighs 7 to 16 g. The gland is situated in the submandibular triangle, its superior boundary is formed by the inferior edge of the mandible, and its inferior boundaries are formed by the anterior and posterior bellies of the digastric muscle. The submandibular triangle also contains the submandibular lymph nodes, facial artery and vein, mylohyoid muscle, and the lingual, hypoglossal, and mylohyoid nerves. The marginal mandibular branch of the facial nerve courses along the capsule of the gland laterally. Most of the submandibular gland lies posterolateral to the mylohyoid muscle. Often, smaller, tonguelike projections of the gland follow the duct, as it ascends toward the oral cavity, deep to the mylohyoid muscle ( Fig. 6.18 ).

The submandibular gland has both mucous and serous cells that empty into ductules, which in turn empty into the submandibular duct. The duct exits anteriorly from the sublingual aspect of the gland, coursing deep to the lingual nerve and medial to the sublingual gland. It eventually forms the Wharton duct between the hyoglossus and mylohyoid muscles on the genioglossus muscle. The Wharton duct, the main excretory duct of the submandibular gland, is approximately 4 to 5 cm long, running superior to the hypoglossal nerve while inferior to the lingual nerve. It empties lateral to the lingual frenulum through a papilla in the floor of the mouth behind the lower incisor tooth. The openings for the sublingual gland, or the sublingual caruncles, are located near the midline of the sublingual fold in the ventral tongue.

The sublingual glands, the smallest of the major salivary glands, typically weigh 2 to 4 g. Consisting mainly of mucous acinar cells, they lie as a flat structure in a submucosal plane within the anterior floor of the mouth, superior to the mylohyoid muscle and deep to the sublingual folds opposite the lingual frenulum. Several ducts from the superior portion of the sublingual gland either secrete directly into the floor of mouth or empty into the Bartholin duct, which then drains into the Wharton duct.

Approximately 1,000 minor salivary glands, ranging in size from 1 to 5 mm, line the oral cavity and oropharynx. The greatest number of these glands is in the lips, tongue, buccal mucosa, and palate. They can also be found along the tonsils, supraglottis, and paranasal sinuses. Each gland has a single duct, which secretes directly into the oral cavity; saliva can be serous, mucous, or mixed.

Arteries

The blood supply to the parotid gland is from branches of the external carotid artery. The external carotid courses superiorly from the carotid bifurcation and parallel to the mandible under the posterior belly of the digastric muscle. The artery travels medial to the parotid gland and splits into two terminal branches. The superficial temporal artery runs superiorly from the superior portion of the parotid gland to the scalp within the superior pretragal region. The maxillary artery leaves the medial portion of the parotid to supply the infratemporal and pterygopalatine fossae.

The transverse facial artery branches off the superficial temporal artery and runs anteriorly between the zygoma and parotid duct to supply the parotid gland and duct. The submandibular as well as the sublingual glands are supplied by the submental and sublingual arteries, branches of the lingual and facial arteries. The facial artery is the main arterial blood supply of the submandibular gland.

Veins

The retromandibular vein, formed by the union of the maxillary vein and the superficial temporal vein, runs through the parotid gland just deep to the facial nerve to join the external jugular vein. This vein is often crossed by the facial nerve branches.

Lymphatics

The parotid harbors two nodal layers, both of which drain into the superficial and deep cervical lymph systems. Approximately 90% of the nodes are located in the superficial layer between the glandular tissue and its capsule. The parotid gland, external auditory canal, pinna, scalp, eyelids, and lacrimal glands are all drained by these superficial nodes. The deep layer of nodes drains the gland, external auditory canal, middle ear, nasopharynx, and soft palate.

In contrast to the parotid gland, the lymph nodes draining the submandibular gland are located between the gland and its fascia but are not embedded in the glandular tissue. They lie in close approximation to the facial artery and vein at the superior aspect of the gland and empty into the deep cervical and jugular chains.

Nerves

Salivary flow is regulated predominantly by the autonomic nervous system. Although both sympathetic and parasympathetic stimulation produces saliva, the parasympathetic system is dominant. The glossopharyngeal nerve, cranial nerve (CN) IX, provides visceral secretory innervation to the parotid gland. The nerve carries preganglionic parasympathetic fibers from the inferior salivatory nucleus in the medulla oblongata through the jugular foramen. Distal to the inferior ganglion, the tympanic nerve, a branch of the glossopharyngeal nerve, enters the skull through the inferior tympanic canaliculus and into the middle ear to form the tympanic plexus. The preganglionic fibers course along as the lesser petrosal nerve into the middle cranial fossa and out the foramen ovale to synapse in the otic ganglion. Postganglionic parasympathetic fibers exit the otic ganglion beneath the mandibular nerve to join the auriculotemporal nerve in the infratemporal fossa. These fibers innervate the parotid gland for the secretion of saliva. Postganglionic sympathetic fibers innervate salivary glands, sweat glands, and cutaneous blood vessels through the external carotid plexus from the superior cervical ganglion. Acetylcholine serves as the neurotransmitter for both postganglionic sympathetic and parasympathetic fibers ( Fig. 6.19 ).

The submandibular as well as the sublingual glands are innervated by the secretomotor fibers of the facial nerve (CN VII). Parasympathetic innervation comes from the superior salivatory nucleus in the pons, passes through the nervus intermedius and into the internal auditory canal. The fibers are next carried by the chorda tympani nerve in the mastoid segment of CN VII, which travels through the middle ear and petrotympanic fissure to the infratemporal fossa. The lingual nerve, a branch of the mandibular division of CN V, then carries the presynaptic fibers to the submandibular ganglion. The postsynaptic nerve leaves the ganglion to innervate both the submandibular and sublingual glands to secrete serous (watery) saliva. The sympathetic innervation from the superior cervical ganglion accompanies the lingual artery to the submandibular tissue and stimulates glandular production of mucoid saliva.

Salivary Gland Secretory Unit

The salivary glands are made up of acini and ducts; together these constitute the secretory unit. The acini contain cells that secrete mucus, serum, or both. These cells drain into the intercalated duct, followed by the striated duct, and finally into the excretory duct. Myoepithelial cells surround the acini and intercalated duct and serve to expel secretory products into the ductal system. Basal cells along the salivary gland unit replace damaged elements. The combined salivary glands produce 1 to 1.5 L of saliva per day. About 45% is produced by the parotid gland, 45% by the submandibular glands, and 5% each by the sublingual and minor salivary glands ( Table 6.21 ).

Acute Sialadenitis

Acute sialadenitis is an acute inflammation of a salivary gland. Several viral microorganisms have been identified as causative agents for infections of the salivary glands. The most common virus that causes sialadenitis is the mumps virus (Rubulavirus). Others include coxsackievirus, echovirus, influenza virus, and human immunodeficiency virus (HIV).

Signs and Symptoms

Symptoms of acute sialadenitis include acute painful swelling of the affected salivary gland ( Fig. 6.20 ). At times only one gland is affected initially. There may be swelling of adjacent cervical lymph nodes. The duct orifice may be erythematous. Other symptoms include fever and trismus.

Acute Bacterial Sialadenitis

Acute bacterial sialadenitis is a suppurative infection that most commonly infects the parotid glands in debilitated, dehydrated, or elderly patients. Acute bacterial sialadenitis is thought to be an ascending bacterial infection due to decreased salivary flow. Situations that cause a tendency for acute sialadenitis include diabetes mellitus, immunocompromised state, and poor oral hygiene. Other rarer infectious causes of sialadenitis include tuberculosis, actinomycosis, syphilis, and HIV infection.

Signs and Symptoms

Symptoms include diffuse, painful swelling of the affected gland ( Fig. 6.21 ). The skin over the gland may be warm, red, and tight. The orifice duct of the affected salivary gland may be red, and massage of the gland may express purulent material from the orifice. Trismus may be present.

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