The Submandibular Gland
The submandibular glands are bilateral salivary glands located in the face. Their mixed serous and mucous secretions are important for the lubrication of food during mastication to enable effective swallowing and aid digestion.
In this article, the location, vasculature and innervation of the submandibular glands will be discussed, and the relevant clinical correlations will be investigated.
The submandibular gland is located within the anterior part of the submandibular triangle. The boundaries of this triangle are:
- Superiorly: Inferior body of the mandible.
- Anteriorly: Anterior belly of the digastric muscle.
- Posteriorly: Posterior belly of the digastric muscle.
Structurally, the submandibular glands are a pair of elongate, flattened hooks which have two sets of arms; superficial and deep. The positioning of these arms is in relation to the mylohyoid muscle, which the gland hooks around.
- Superficial arm – comprises the greater portion of the gland and lies partially inferior to the posterior half of the mandible, within an impression on its medial aspect (the submandibular fossa). It is situated outside the boundaries of the oral cavity.
- Deep arm – hooks around the posterior margin of mylohyoid through a triangular aperture to enter the oral cavity proper. It lies on the lateral surface of the hyoglossus, lateral to the root of the tongue.
Secretions from the submandibular glands travel into the oral cavity via the submandibular duct (Wharton’s duct). This is approximately 5cm in length and emerges anteromedially from the deep arm of the gland between the mylohyoid, hypoglossus and genioglossus muscles. The duct ascends on its course to open as 1-3 orifices on a small sublingual papilla (caruncle) at the base of the lingual frenulum bilaterally.
Relationship with Nerves
Both the submandibular gland and duct share an intimate anatomical relationship with three principal nerves; the lingual nerve, hypoglossal nerve and facial nerve (marginal mandibular branch). The courses of these nerves are briefly outlined:
- Lingual nerve: Beginning lateral to the submandibular duct, this nerve courses anteromedially by looping beneath the duct and then terminating as several medial branches. The terminal branches ascend on the external and superior surface of hypoglossus to provide general somatic afferent innervation to the mucus membrane of the anterior two-thirds of the tongue.
- Hypoglossal nerve: Lies deep to the submandibular gland and runs superficial to hyoglossus and deep to digastric muscle.
- Facial nerve (marginal mandibular branch): Exits the anterior-inferior portion of the parotid gland at the angle of the jaw and traverses the margin of the mandible in the plane between platysma and the investing layer of deep cervical fascia curving down inferior to the submandibular gland.
Clinical Relevance: Submandibular Gland Excision
Submandibular gland excision is a common surgical procedure indicated for conditions such as submandibular gland neoplasia or recurrent calculi.
As discussed previously, the gland and its duct lie in close proximity to three principal nerves, so the surgeon must be acutely aware of this regional anatomy. Consequences of the three nerve injuries are:
- Lingual nerve: Immediate post-operative ipsilateral parathesia and loss of taste from the anterior two-thirds of the tongue, which is rarely permanent.
- Hypoglossal nerve: Ipsilateral paresis or paralysis of the intrinsic muscles of the tongue leading to dysarthria and deviation of tongue to side of the lesion. This nerve is rarely injured in this procedure to an extent to produce noticeable disability.
- Facial nerve (marginal mandibular branch): Ipsilateral paresis or paralysis of the muscles supplying the lower lip and chin, including depressor labii inferioris, which characteristically presents as drooping of the lower lip. This is usually temporary, lasting for 6-12 weeks.
Blood supply is via the submental arteries which arise from the facial artery; a branch of the external carotid artery.
Venous drainage is through the submental veins which drain into the facial vein and then the internal jugular vein.
The submandibular glands receive autonomic innervation through parasympathetic and sympathetic fibres, which directly and indirectly regulate salivary secretions respectively.
Parasympathetic innervation originates from the superior salivatory nucleus through pre-synaptic fibres, which travel via the chorda tympani branch of the facial nerve (CNVII). The chorda tympani then unifies with the lingual branch of the mandibular nerve (CNViii) before synapsing at the submandibular ganglion and suspending it by two nerve filaments.
Post-ganglionic innervation consists of secretomotor fibres which directly induce the gland to produce secretions, and vasodilator fibres which accompany arteries to increase blood supply to the gland. Increased parasympathetic drive promotes saliva secretion.
Sympathetic innervation originates from the superior cervical ganglion, where post-synaptic vasoconstrictive fibres travel as a plexus on the internal and external carotid arteries, facial artery and finally the submental arteries to enter each gland. Increased sympathetic drive reduces glandular bloodflow through vasoconstriction and decreases the volume of salivary secretions, resulting in a more mucus and enzyme-rich saliva.
Clinical Relevance: Salivary Duct Calculi
A calculus or sialolith is a calcified deposit which can block the lumen of a duct. The submandibular duct is the most susceptible to calculi out of all the salivary ducts; accounting for approximately 80% of cases. This is thought to be due to the
- Torturous length of the duct (5cm)
- Ascending secretory pathway
- Nature of salivary secretion
The submandibular glands and the patency of the ducts can be examined by direct injection of a contrast medium. This is known as a sialogram, which is a special type of radiograph. A calculus may present radiographically as a stricture or complete occlusion of the duct.