This article provides an overview of important anatomic and functional anatomy associated with the parotid gland and facial nerve for the practicing otolaryngologist, head and neck surgeon, facial plastic surgeon, and plastic surgeon. The discussion includes the important anatomic relationships and physiology related to the parotid gland and salivary production. A comprehensive description of the path of facial nerve, its branches, and important anatomic landmarks also are provided.
Key points
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This article provides an overview of important anatomic and functional anatomy associated with the parotid gland and facial nerve for the practicing otolaryngologist–head and neck surgeon, facial plastic surgeon and plastic surgeon.
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The discussion includes the important anatomic relationships and physiology related to the parotid gland and salivary production.
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A comprehensive description of the path of the facial nerve, its branches, and important anatomic landmarks also are provided.
Introduction
The parotid gland and facial nerve have a unique anatomic and functional relationship. The parotid gland is the largest of 3 paired major salivary glands in the head and neck. The major function of the parotid and other salivary glands is to secrete saliva, which plays a significant role in lubrication, digestion, immunity, and the overall maintenance of homeostasis within the human body. The facial nerve (CN VII) originates in the brainstem and travels through the temporal bone before exiting the stylomastoid foramen. The extratemporal branches of the facial nerve are located within the body of the parotid gland and divide it into superficial and deep lobes before innervating the muscles of facial expression ( Fig. 1 ). A thorough understanding of the anatomy of the parotid gland and facial nerve is essential for safe management of related pathology.
The parotid gland
Anatomy
The paired parotid glands are the largest of the major salivary glands. They are each located in the preauricular region and span from the masseter to the posterior surface of the mandible. The gland is divided into superficial and deep lobes by the facial nerve. The superficial lobe is defined as the part of the gland lateral to the nerve and overlies the lateral surface of the masseter muscle. The deep lobe is located medial to the facial nerve and lies between the mastoid process of the temporal bone and the mandibular ramus with deep margins resting in the prestyloid compartment of the parapharyngeal space (PPS).
Most benign neoplasms are found within the superficial lobe and can be removed with a superficial parotidectomy. A tumor of the deep lobe may go unnoticed, as it does not displace the overlying superficial lobe until it extends laterally and causes displacement of the overlying superficial lobe. These deep lobe tumors lie within the PPS and typically grow into a dumbbell shape, because their growth is directed through the stylomandibular tunnel.
The superior boundary of the parotid gland is the zygomatic arch. Inferiorly, the tail of the parotid gland extends down to the sternocleidomastoid muscle (SCM). The tail of the parotid gland extends posteriorly over the superior border of the SCM toward the mastoid tip and the deep lobe lies within the PPS.
Accessory parotid tissue is present in approximately 20% of the population. It is generally found approximately 6 mm anterior to the main parotid gland and is usually adjacent to the parotid duct (Stensen duct) as it passes over the masseter. Multiple accessory glands may be present. Accessory glandular tissue is histologically distinct from parotid tissue in that it may contain mucinous acinar cells in addition to the serous acinar cells generally found in the parotid gland.
The parotid fascia, or parotidomasseteric fascia, forms a dense inelastic capsule over the parotid gland and deeply covers the masseter muscle. This facia should not be confused with the superficial musculoaponeurotic system (SMAS), which is continuous with the platysma inferiorly, and the superficial temporal fascia superiorly. Rather, the parotid fascia is a continuation of the deep cervical fascia as it travels superiorly. Once it reaches the parotid gland, this fascia splits into superficial and deep layers to encase the parotid gland. The superficial fascia is thicker and extends superiorly from the masseter and SCM to the zygomatic arch, where it attaches to the root of the zygoma. The thinner, deep layer extends to the stylomandibular ligament, which is an important surgical landmark when considering the resection of deep lobe tumors.
Within close proximity to the parotid gland are 2 nerves that deserve mentioning. The great auricular nerve, a branch of the cervical plexus, runs parallel to the external jugular vein along the lateral surface of the SCM toward the tail of the parotid gland. It then divides into an anterior and posterior branch to provide sensation to posterior portion of the pinna and the lobule. If injured during parotidectomy, it can result in long-term sensory loss. It may also serve as a suitable nerve graft and can be easily harvested for facial nerve grafting when needed for reanimation purposes. The auriculotemporal nerve is a branch of the mandibular nerve, the third inferior subdivision of the trigeminal nerve (V3). After exiting the foramen ovale, the nerve travels superiorly to innervate the skin and scalp immediately anterior to the ear. It runs parallel to the superficial temporal vessels and anterior to the external auditory canal.
Ductal Organization
Ductal organization of the parotid gland can be divided into 2 parts: proximal and distal. Proximally, when traveling from the Stensen duct toward the terminal acini, a treelike branching pattern develops and the ducts become progressively smaller, with more numerous branches. Distally, the Stensen duct exits the anterior border of the parotid gland and travels 1 cm inferior and parallel to the zygoma in an anterior direction across the masseter muscle. It then turns and pierces the buccinator muscle to enter the oral cavity opposite the second upper molar.
The main excretory ducts of the parotid gland lead into the striated ducts, the intercalated ducts, and the terminal acini ( Fig. 2 ). There is rich adipose tissue present in the parotid parenchyma, with a ratio of adipose-to-glandular tissue of 1:1. Sebaceous elements are uncommon, but may be found in the parotid gland and are believed to explain the sebaceous differentiation that may be seen in some salivary tumors.
Vascular Supply and Lymphatic Drainage
The external carotid artery (ECA) provides arterial blood supply to the parotid gland. From the carotid bifurcation, the ECA travels superiorly and parallel to the mandible before going medial to the posterior belly of the digastric muscle. Once the artery is medial to the parotid gland it divides into its 2 terminal branches, the superficial temporal (STA) and maxillary arteries (MA). The STA runs superiorly from the superior portion of the parotid gland to the scalp within the pretragal region. The MA exits the medial portion of the parotid to supply the infratemporal fossa and the pterygopalatine fossa. Controlling the MA is required when performing a radical parotidectomy, especially when marginal or segmental mandibulectomy is also performed. The transverse facial artery is a branch off the STA and runs anteriorly between the zygoma and parotid duct to supply the parotid gland, parotid duct, and the masseter muscle.
Venous outflow occurs through the retromandibular vein, which is formed by the maxillary and superficial temporal veins. The retromandibular vein travels through the parotid gland just deep to the facial nerve to join the external jugular vein and may have extremely variable anatomy. For instance, it may bifurcate into an anterior and posterior branch. The anterior branch can join the posterior facial vein to form the common facial vein. The posterior facial vein lies immediately deep to the marginal mandibular branch of the facial nerve and is therefore often used as a landmark for identification of the nerve branch. The posterior branch of the retromandibular vein may also combine with the postauricular vein above the SCM and then drain into the external jugular vein.
There is a high density of lymph nodes within and around the parotid gland. The parotid is the only salivary gland with 2 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.
Autonomic Innervation
The glossopharyngeal nerve (CN IX) provides innervation required for secretion of saliva to the parotid gland. CN IX carries preganglionic parasympathetic fibers from the medulla (inferior salivatory nucleus) through the jugular foramen ( Fig. 3 ). Distal to the inferior ganglion, a small branch of CN IX, the Jacobsen nerve, reenters the skull through the inferior tympanic canaliculus to form the tympanic plexus within the middle ear. The preganglionic fibers then become the lesser petrosal nerve and travel into the middle cranial fossa. After exiting from the foramen ovale, they synapse in the otic ganglion with postganglionic parasympathetic fibers. These fibers then exit the otic ganglion beneath the mandibular nerve and join the auriculotemporal nerve in the infratemporal fossa. These fibers will innervate the parotid gland for the secretion of saliva.
Within the gland, the neurotransmitter acetylcholine (ACh) binds muscarinic receptors to stimulate both acinar activity and ductal transport. This leads to vasodilation of the glands and contraction of the myoepithelial cells. Production of inositol trisphosphate leads to increased calcium concentrations within the cell, which significantly increase salivary volume secretion by second messenger activity. Acetylcholinesterases, which inhibit the breakdown of ACh, may be released and allow for continued secretion of saliva. Atropine, the muscarinic antagonist, decreases salivation by competing with ACh for the salivary receptor site.
The neurotransmitter norepinephrine, mediates the effects of the sympathetic nervous system via postganglionic sympathetic fibers that innervate salivary glands, sweat glands, and cutaneous blood vessels. These fibers travel through the external carotid plexus from the superior cervical ganglion via the thoracic spinal nerves. Binding of norepinephrine to beta-adrenergic receptors results in activation of the adenylate cyclase second messenger system, which then results in formation of 3′,5′-cyclic adenosine monophosphate (cAMP). cAMP leads to phosphorylation of various proteins and activation of different enzymes.
Interestingly, ACh can serve as a neurotransmitter for both postganglionic sympathetic and parasympathetic fibers. This is believed to contribute to “gustatory sweating” (Frey syndrome) in some patients following parotidectomy. Regeneration of parasympathetic fibers to the sweat glands leads to aberrant autonomic reinnervation and patients can then develop sweating and flushing of the skin overlying the parotid region during eating.
Functional Anatomy and Physiology
The purpose of the parotid and other salivary glands is to produce saliva. Saliva has several very important functions related to digestion, immunity, and homeostasis. It is involved in the digestion of carbohydrates and fats, and protects mucosa from the deleterious effects of microbial toxins, noxious stimuli, and minor trauma. The salivary mucins (glycoproteins) also act as lubricants for mastication, swallowing, speech, and taste. Secretory immunoglobulin A, enzymes (lysozyme, peroxidase, alpha-amylase, and lactoferrin), and ions, such as thiocyanate and hydrogen, are found within saliva and also contribute to its antibacterial activity.
Saliva is 99.5% water with the remainder is composed of proteins and electrolytes. Its specific gravity is 1.002 to 1.012. Salivary pH ranges from 5.6 to 7.0 (average 6.7) and varies directly with the blood pH. One to 1.5 L of saliva are produced daily from all salivary glands and the parotid glands contribute approximately 45% (450–675 mL) of the total secretions. During the resting state, one-fourth of saliva is produced by the parotid gland and most (two-thirds) is from the submandibular gland. However, during stimulation (presence of food in the mouth, mastication, and nausea), the relative amounts are reversed and two-thirds of secretion comes from the parotid gland.
The basic secretory unit of the parotid gland consists of an acinus, a secretory duct, and a collecting duct (see Fig. 2 ). Acinar cells are extremely polarized and are bounded by a plasma membrane with 2 distinct domains, a basolateral domain and an apical domain, that are separated by tight junctions that link adjacent cells. The parotid gland consists of serous acini that contain pyramidal-shaped cells with round basal nuclei surrounding the lumen and secretory granules at the apex. Each acinus is surrounded by a layer of myoepithelial cells, which in turn is bordered by a distinct basement membrane layer. Myoepithelial cells are elongated or star-shaped nonsecreting cells with long-branching processes that surround the acinus and proximal ducts. They have been found to possess adenosine triphosphate (ATP) activity, have intercellular gap junctions, and contain myofilaments. These properties are believed to provide myoepithelial cells with contractile function that assists in expelling secretions.
The intercalated ducts lie next to the acinus. They are hollow structures lined by a single layer of small cuboidal cells. The intercalated ducts continue as intralobular striated ducts and together form the secretory duct. Striated ducts are lined by columnar cells with a brush border composed of microvilli on their luminal surface. Striated ductal cells are rich in mitochondria. They are thought to be involved with the transport of ions and water. Excretory ducts extend from the striated ducts and are lined by 2 layers of epithelium, a layer of flat cells surrounding the ductal lumen, and an outer layer of columnar cells.
Throughout the secretory unit, active transport processes occur that alter the composition of saliva into a complex mixture of electrolytes and macromolecules ( Table 1 ). All fluid is produced in the acinus and most protein secretion also occurs here. The fluid is derived from the highly permeable local vascular bed in the form of an isotonic solution and is secreted into the acinar lumen before traveling through the ductal system before emptying into the mouth. Unlike the water-permeable cells of the acinus, ductal cells are impermeable to water. Most of the sodium and chloride in the primary secretion is reabsorbed in the duct, and a small amount of potassium and bicarbonate is secreted. In addition, some proteins are added to the salivary fluid as it traverses the secretory duct. By the time the saliva enters the mouth, it has generally been rendered hypotonic. The electrolyte composition of saliva, however, can be influenced by salivary flow rates. The reabsorption of salivary sodium and chloride is directly related to these rates, with decreased reabsorption and increased salivary concentrations of electrolytes with increasing salivary flow rates but potassium reabsorption is independent of flow rates.
