Describe the function of the structural components associated with major and minor salivary glands.
Explain the function of saliva and the mechanism of production.
Explain how alterations in flow rate may impact the composition of saliva and the physiological significance of a low flow rate.
Explain the neural mediated reflex pathway that controls stimulated salivary secretion.
Describe the types of afferent stimuli involved in eliciting salivation. Include the types of sensory receptors and the afferent path followed.
Describe the efferent outflow path for the major salivary glands. Include both parasympathetic and sympathetic fibers.
Explain how medications may affect the efferent signaling pathway and alter secretion.
Overview of the Salivary Glands
Salivary glands associated with the oral cavity produce a complex, slightly alkaline watery secretion that contains various proteins, ions, and enzymes. Salivary secretion occurs continuously, at low levels, with intermittent increases occurring in response to eating and oral stimulation through autonomic innervation. The composition and continual secretion of saliva function to maintain oral cavity homeostasis. Saliva serves to cleanse and protect the oral mucosa, provide antimicrobial protection, maintain a neutral pH, and preserve tooth integrity. The lubrication and moisture produced by saliva facilitates speaking and mediates digestive functions, including the process of chewing, taste, bolus formation, and swallowing. Salivary gland dysfunction has a significant impact on oral health and quality of life. This chapter discusses the anatomical structure, location, and neural mechanisms that control the secretion of the major and minor salivary glands.
General Development of Salivary Glands
Salivary glands develop concomitantly with the formation of the orofacial complex during the 6 to 12 weeks of embryonic development and arise from the oral epithelium.
Each gland undergoes extensive branching within the underlying connective tissue to form clusters of salivary secretory cells, known as acinar units, and excretory ducts, which open onto the epithelial surface to release saliva into the oral cavity.
Secretory acinar units function to produce saliva and may consist of two types of secretory acinar cells: serous and mucous cells.
Salivary glands may contain just serous cells, only mucous cells, or both. The predominant type of acinar unit present in a gland defines the type of saliva produced. Glands that contain predominantly serous cells produce a thin watery enzyme-rich secretion, whereas glands comprised mainly of mucous cells secrete a thick, viscous, mucin-rich secretion.
The duct system serves to collect and then modify the ionic composition of the saliva produced by the secretory acinar cells. The smaller collecting ducts found within the gland eventually coalesce to form larger excretory ducts that transport saliva to the oral mucosal surface.
Gland Classification
Salivary glands are primarily classified into two groups based on size and location, as major and minor salivary glands. Alternatively, salivary gland classification may be based on the type of saliva produced and include serous glands, mucous glands, and seromucous (mixed) glands ().
Major salivary glands include the parotid, submandibular, and sublingual glands.
The three major salivary glands are large, bilateral, paired structures that reside outside the oral cavity (extraoral) and empty salivary secretions into the oral cavity via long excretory ducts. The excretory ducts open onto either side of the dental arch and serve to saturate the food bolus with saliva during chewing.
The three major salivary glands collectively produce 90% of the total salivary output.
Minor salivary glands, which are named based on their anatomical location within the oral cavity, include labial, buccal, lingual, palatal, and pharyngeal glands.
Fig. 24.1 Location of the three major salivary glands. Right lateral view: mandible and mylohyoid removed. The parotid, submandibular, and sublingual gland represent the major salivary glands and produce 90% of the total salivary output. The major glands are bilateral paired structures, located outside the oral cavity proper. Each gland empties into the oral cavity via a long excretory duct.
Anatomical Overview of Major and Minor Salivary Glands
Parotid Glands
The paired parotid glands, which are the largest of the three major glands consist of serous acinar cells, and produce a thin, low-viscosity, enzyme-rich secretion ( ).
Each parotid gland resides in a triangular space, the parotid fossa, in the preauricular region, just anterior to the ear and along the posterior border of the mandibular ramus.
The facial nerve (cranial nerve [CN] VII), the external carotid artery, and the retromandibular vein also lie within the parotid fossa and should be considered during surgery involving the parotid gland.
The stylomandibular ligament and facial nerve (CN VII) pass through the parotid gland, dividing each gland into a superficial lobe and a deep lobe.
The large excretory duct of the parotid gland, known as Stensen’s (parotid) duct, emerges from the deep lobe along the anterior border of the parotid gland, crosses the masseter muscle, and pierces the buccinator muscle to enter the oral cavity.
The duct opens at the parotid papilla onto the buccal mucosa that lines the vestibule of the mouth in the region opposite the second maxillary molar.
Nerve structures anatomically associated with the parotid gland include the facial nerve (CN VII), greater auricular nerves (C2–C3), and auriculotemporal nerves (CN V3).
The facial nerve (CN VII), which passes through the parotid gland, does not provide innervation to the gland.
The motor (SVE) division of the facial nerve exits the base of the skull and immediately gives rise to three branches that innervate the stylohyoid, posterior digastric, and auricularis muscles.
The facial nerve enters the gland, bifurcates into two main trunks, and then divides into five terminal branches to form the parotid nerve plexus. The five terminal branches of the plexus include the temporal, zygomatic, buccal, marginal mandibular, and cervical nerve branches, which innervate the muscles of facial expression.
The greater auricular nerve, which originates from C2–C3 of the cervical plexus, and the auriculotemporal branch of V3 nerve, both transmit general sensation (general somatic afferent [GSA] fiber) from the region of the skin covering the parotid gland.
Secretomotor (general visceral efferent [GVE] fiber) innervation originates from both parasympathetic and sympathetic divisions. The parotid glands produce approximately 25% of total salivary output under unstimulated (basal) conditions. However, during autonomic stimulation, the parotid secretes 60% of the total salivary flow.
Parasympathetic innervation arises from the inferior salivatory nuclei and travels to the parotid gland via the tympanic branch of CN IX and the lesser petrosal nerve (CN IX). Postganglionic fibers originate from the otic ganglion and accompany the auriculotemporal branch (V3) for secretomotor distribution to the parotid gland.
Sympathetic innervation arises from neurons in the lateral horn of the thoracic spinal (T1–T4) cord and synapse in the superior cervical ganglion. Postganglionic fibers follow the vascular supply.
Fig. 24.2 Left lateral view of the anatomical location of the parotid gland in the parotid fossa. (a) The parotid gland, which consists of a superficial and deep lobe, sits anterior to the ear and along the posterior border of the mandibular ramus. The parotid duct arises from the deep lobe, crosses the face superficial to the masseter muscle, and pierces the buccinator to enter the oral cavity. The parotid duct opens into the buccal vestibule of the mouth opposite the second maxillary molar. The parotid produces a pure serous, watery secretion. (b) Magnified image of the parotid gland demonstrating the position of the facial nerve. The facial nerve passes through the parotid gland, splitting the gland into a superficial lobe and a deep lobe, but does provide innervation to the gland. The facial nerve emerges from the stylomastoid foramen, enters the parotid gland, and divides into five terminal branches, to form the parotid nerve plexus. (c) Schematic of the terminal motor branches of the facial nerve. (a,b: Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. c: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Frey’s syndrome is a relatively rare disorder associated with gustatory sweating, which occurs shortly after eliciting a salivary reflex in response to chewing. It is characterized by unilateral flushing and sweating of the skin in the region supplied by the auriculotemporal nerve. The segmental distribution pattern of innervation includes the frontotemporal region (forehead), check, anterior ear, and parotid region. Frey’s syndrome occurs most often following parotid surgery and is presumably associated with damage to the GSA and GVE fibers associated with the auriculotemporal nerve. It is predicted that following surgery, the GVE parasympathetic fibers join the sympathetic fibers as the auriculotemporal nerve fibers regenerate. The intermingling of fibers results in flushing and sweating during salivation due to sympathetic innervation.
Neoplasms arising in the parotid gland, which may be benign (80%) or malignant (20%), occur in the superficial or deep lobe, respectively. A key consideration for any parotidectomy is the isolation of the facial nerve at the level of the main trunk as it exits the stylomastoid foramen. Benign tumors are most often associated with the superficial lobe and removed through a superficial parotidectomy. Tumors occurring in the deep lobe must be resected from the stylomandibular ligament, which separates the superficial and deep lobes. Deep parotid tumors may push the facial nerve (CN VII) superficially and increase the risk of injury.
Parotitis refers to inflammation of the parotid gland or duct that may be caused by bacterial or viral infections. Other diseases associated with inflammation such as the mumps, tuberculosis, the autoimmune disease, Sjogren’s syndrome, or the human immunodeficiency virus (HIV) may also cause parotitis. Inflammation of the gland leads to decreased salivary flow and, in some cases, scarring and obstruction of the ducts, which allow for bacteria and viruses to infect the secretory portions of the gland. Clinical manifestations include localized ear pain, tenderness anterior to the ear, and difficulty or pain with chewing and swallowing. Neurological complications are rare but may include meningitis (inflammation of meninges), deafness, and facial nerve inflammation (facial neuritis).
Submandibular Glands
The paired submandibular glands, which are smaller than the parotid, are mixed salivary glands containing primarily serous cells with some mucous acinar cells.
Each submandibular gland sits just below the angle of the mandible, in the submandibular (digastric) triangle, an anatomic region formed by the anterior and posterior digastric muscles and the inferior border of the mandible. The submandibular gland creates a C-shaped ring around the posterolateral edge of the mylohyoid muscle to form a superficial lobe and a deep lobe ().
The main excretory duct of the submandibular gland, known as Wharton’s duct, exits the medial side of the gland and passes between the hyoglossus and mylohyoid muscles to enter the floor of the oral cavity.
The submandibular duct exhibits a long, convoluted path in comparison to the ducts of other major glands. The long route may serve as a contributing factor to the formation of salivary calculus (sialolithiasis) and lead to the obstruction of saliva released into the oral cavity.
Each duct empties just lateral to the midline, behind the mandibular incisors at the sublingual papillae (caruncle), which is near the base of the lingual frenulum ().
Nerve structures associated with the submandibular gland include the marginal mandibular branch of VII, the lingual nerve (CN V3), and the hypoglossal nerve (CN XII).
Each of these nerves exhibits a close association with the submandibular gland and Wharton’s duct as the duct passes along the floor of the oral cavity. The nerves may be at risk of injury with the removal of the submandibular gland or surgical intervention to dislodge salivary stones (sialolith).
The lingual nerve, a branch of the mandibular (V3) division of the trigeminal, enters the posterior portion of the oral cavity through the infratemporal fossa and travels with the chorda tympani (VII) along the floor of the oral cavity.
The submandibular parasympathetic ganglion lies on the lateral surface of the hyoglossus muscle, near the deep lobe of the submandibular gland, and attaches to the lingual nerve. The ganglion serves as the synaptic connection for the preganglionic (GVE) fibers of the chorda tympani nerve ().
The lingual nerve crosses the submandibular (Wharton’s) duct passing from the lateral side, inferiorly, and then medial to the submandibular duct, to supply GSA fibers to the oral mucosa and submandibular glands ( and ).
The hypoglossal nerve (CN XII) enters the posterior portion of the oral cavity through the submandibular triangle and runs inferior to Wharton’s duct, the submandibular gland, and the lingual nerve. The hypoglossal nerve does not innervate the submandibular gland; it provides GSE fibers to all intrinsic and extrinsic muscles of the tongue, except for the palatoglossus muscle ().
The submandibular gland, which produces approximately 50% of the total salivary output during resting conditions, receives neural stimulation from both parasympathetic and sympathetic innervation. During autonomic stimulation, salivary secretions constitute only 30% of the total salivary output.
Parasympathetic (GVE) innervation originates from the superior salivatory nuclei and travels to the submandibular gland via the chorda tympani branch of the facial nerve (CN VII). Postganglionic (GVE) fibers arise from the submandibular ganglion and continue to travel with the lingual nerve (V3) for secretomotor distribution to the submandibular and sublingual glands.
Sympathetic innervation arises as preganglionic fibers from the intermediolateral cell column (IMLC) of the spinal cord (T1–T4), which then synapse in the superior cervical ganglion of the sympathetic trunk. Postganglionic fibers follow the vascular supply to reach the gland.
Fig. 24.3 Anatomical location of the submandibular and sublingual glands in the floor of the mouth. Superior view of the floor of the oral cavity, with the tongue removed. The submandibular glands are mixed salivary glands producing both mucous and serous secretions. The glands sit below the angle of the mandible in the floor of the oral cavity. The glands wrap as a C-shaped ring around the posterolateral edge of the mylohyoid to form a superficial lope and a deep lobe. The submandibular duct (Wharton’s duct) emerges from the medial surface of the gland, crosses over the lingual nerve, and passes anteriorly to open on to the sublingual papilla. Note the submandibular duct cross over the lingual nerve. the sublingual glands are predominantly mucous-secreting salivary glands, located anteriorly in the floor of the oral cavity, between the oral mucosa and the mylohyoid muscle. The sublingual gland drains through several smaller excretory ducts that open on the sublingual fold. Alternatively, the sublingual glands may have a main duct that opens into the submandibular duct at the sublingual papillae. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Fig. 24.4 The location of the submandibular parasympathetic ganglion in the submandibular triangle. Right lateral view of the medial surface of the mandible, with the medial pterygoid muscle cut. The ganglion is suspended inferiorly from the lingual nerve and lies on the lateral surface of the hyoglossus muscle, near the deep lobe of the submandibular gland. The preganglionic parasympathetic fibers from the superior salivatory nucleus in the central nervous system (CNS) accompany the nervus intermedius and the chorda tympani nerve (CN VII). The chorda tympani nerve joins the pathway of the lingual nerve in the infratemporal fossa and travels with the lingual nerve to reach the submandibular ganglion. Postganglionic secretomotor fibers pass to the submandibular, sublingual, and minor lingual and labial glands in the anterior floor of the mouth and lower lip. (Modified with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Fig. 24.5 Left lateral view of the anatomical relationship between the submandibular duct, submandibular ganglion, hypoglossal nerve, and lingual nerve, with the submandibular gland and the mylohyoid muscle removed. The lingual nerve and the submandibular ganglion lie on the lateral surface of the hyoglossus muscle, near the deep lobe of the submandibular ganglion. The duct runs superior to the hypoglossal nerve (CN XII) and inferior to the lingual nerve as it passes between the hyoglossus and mylohyoid muscle. The lingual nerve crosses the duct laterally at the anterior border of the hyoglossus muscle, and then loops beneath the submandibular duct, and continues medially to the duct as the nerve passes anteriorly toward the tip of the tongue. The hypoglossal nerve (CN XII) lies inferior and deep to the submandibular gland and passes superficial to the hyoglossus muscle. (Modified with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Salivary gland inflammation, known as sialadenitis, may be associated with pain, tenderness, redness, and swelling in the region of the gland. Bacterial and viral infections of the parotid or submandibular gland are often associated with acute onset of sialadenitis. In cases of infection, fever, malaise, and inflammation of the ductal papillae are often associated with symptoms. Chronic and recurring forms of sialadenitis also exist. Obstruction due to salivary calculi (sialoliths) or stricture in the duct often leads to pain, inflammation, and swelling. Obstructive sialadenitis due to a sialolith is referred to as sialolithiasis and most often occurs in the submandibular duct (Wharton’s duct). The increased incidence of sialoliths in Wharton’s duct is attributed to the length, position, and path of the duct. Patients often complain of tenderness, sudden swelling, and increased pain that is associated with salivation during eating. Dehydration, anticholinergics, and conditions related to xerostomia (dry mouth), including the autoimmune disease, Sjogren’s disease, may increase the incidence of sialolithiasis (stone formation) and possible infection due to decreased salivary flow. Long-term complications of chronic sialadenitis and untreated sialolithiasis can lead to scarring and glandular atrophy.
Based on the anatomical relationship of the submandibular duct and submandibular gland to the vessels and nerves that pass through the submandibular triangle, several structures should be considered during the excision of the submandibular gland, or during the removal of salivary duct stones. The superficially located marginal mandibular and cervical branches of VII, as well as the medially positioned hypoglossal nerve (CN XII), and the lingual nerve (CN V3), should be considered during the excision of the submandibular gland, or during the removal of salivary duct stones. The facial artery passes deep to the gland, whereas the facial vein lies superficially. Among structures at risk, the marginal mandibular branch (VII) is the most likely structure damaged or bruised during submandibular gland removal. However, the hypoglossal and lingual nerves, along with the secretomotor fibers carried by the chorda tympani nerve, are also potentially at risk. During surgery, the submandibular duct should also be identified and dissected to avoid damage to the lingual nerve, or the submandibular ganglion.
Sublingual Glands
The paired sublingual glands (), which are the smallest of the three major salivary glands, lack a definitive connective capsule and lie below the oral mucosa, anteriorly in the region of the floor of the oral cavity.
The sublingual glands are mixed, consisting mainly of mucous acinar cells with only a few serous cells.
The sublingual glands, which contribute only 5 to 10% of the total salivary output during resting conditions, receive neural stimulation from both parasympathetic and sympathetic innervation.
The parasympathetic and sympathetic secretomotor (GVE) fibers that innervate the submandibular gland also provide innervation to the sublingual glands.
The main duct, Bartholin’s duct, opens into the submandibular duct at the sublingual papillae (caruncle).
Several small sublingual ducts, also known as the ducts of Rivinus, may also be present and open along the sublingual fold on the floor of the oral cavity.
Both the sublingual glands and the minor salivary glands exhibit a limited excretory duct system, with striated ducts often absent. The short ducts reduce the occurrence of salivary duct stone formation.
Fig. 24.6 Sublingual gland and sublingual papilla. Anterior view showing the ventral surface of the tongue (superficial and deep views). The paired sublingual glands lie below the oral mucosa in the floor of the oral cavity. The main duct (Bartholin’s duct) empties into sublingual papilla. (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Minor Salivary Glands
Approximately 600 to 1,000 minor salivary glands reside in the submucosal connective tissue within the oral cavity and the oropharynx ().
The glands are nonencapsulated, consisting of small aggregates of mucous and serous acinar cells, which produce approximately 5 to 10% of total salivary output (see and ).
Minor glands secrete saliva continuously through constitutive exocytosis; however, feedback from the parasympathetic and sympathetic system, as well as physiological and pharmacological input, may modify secretory activity.
Secretomotor fibers that innervate the minor glands follow preganglionic fibers of the glossopharyngeal or facial nerves. Postganglionic fibers are distributed through branches of CNs V2 and V3.
Most of the saliva (70%) secreted from the minor glands is a thick, viscous, mucin-rich secretion that is essential for the lubrication and hydration of the oral mucosa in the palatal, oropharyngeal, labial and buccal regions.
Minor glands that contain primarily mucous cells are found in the submucosa of the oropharyngeal isthmus, the posterior hard and soft palates, and the posterior one-third of the tongue.
Minor mixed glands reside in the tip of the anterior tongue and the labial and buccal regions.
Minor glands containing only serous acinar cells are situated anterior to the sulcus terminalis in association with taste buds of the circumvallate and foliate papillae.
The duct system in minor glands is less extensive than the major glands. Small collecting ducts serve to release unmodified, isotonic saliva constitutively onto the mucosal lining of the oral cavity.
Fig. 24.7 (a,b) Schematic depiction of the location of types of minor salivary glands. In addition to three paired glands, 600 to 1,000 minor salivary glands secrete pure mucous, pure serous, or a mixed salivary secretion into the oral cavity. Minor glands only produce 5 to 8% of the total saliva output, but this amount suffices to keep the oral cavity lubricated when the major glands are at rest. (Modified with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
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