Submandibular gland

Structure

The submandibular glands are relatively small, paired, irregular structures each situated in their ipsilateral submandibular fossa. Remember that the submandibular fossas are triangular depressions found bilaterally on the medial surfaces of the body of mandible. Each fossa is located within the submandibular (digastric) triangle on the same side. Respectively, they are formed by the mylohyoid line superiorly, the base of mandible inferiorly and the inferior part of ramus of mandible posteriorly.

On average, the submandibular glands measure about 3 – 4 cm along its long axis and weigh roughly 10 – 15 grams each. Anteriorly, each gland is unevenly divided into a larger superficial and a smaller deep component by the mylohyoid muscle. However, the superficial and deep portions are confluent beyond the posterior border of the mylohyoid muscle. Each gland has its own excretory duct – called submandibular or Wharton’s duct – that excretes the contents of the gland onto the floor of the oral cavity.

From a histological perspective, the submandibular glands are mixed glands, made up of serous and mucous acini, as well as seromucous glands called serous demilunes.

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Innervation

While the anatomy of the submandibular gland is relatively straightforward, understanding the innervation of these glands can be challenging. 

The preganglionic parasympathetic fibers originating from the superior salivatory nucleus travel first through the facial nerve (CN VII) then to chorda tympani. They then join the lingual nerve (branch of mandibular division of trigeminal nerve [CN V3]) and pass through the posterior filament, which then enters the submandibular ganglion. They form synapses with the postganglionic secretomotor fibers, which then leave the ganglion to innervate submandibular (and sublingual) glands.

The vasomotor activity is regulated by the postganglionic sympathetic fibers arising from the plexus of the facial artery (cell bodies located in superior cervical ganglion). These fibers pass through submandibular ganglion without synapsing.

Note that chorda tympani (not the lingual nerve) gives parasympathetic supply to the gland. The lingual nerve only functions as a conduit for these parasympathetic fibers and is not directly responsible for this form of innervation.

Blood supply

The glands are supplied indirectly by the external carotid artery through the branches of the facial and lingual arteries.

Venous drainage follows a similar pattern.

Note also that the lymphatic drainage of the glands occurs by way of the submandibular nodes to the deep group of cervical lymph nodes; specifically jugulo-omohyoid nodes.

Function

The primary role of the submandibular gland is to contribute to the production and secretion of saliva. Saliva is an extracellular fluid that is made up mostly of water along with few enzymes, electrolytes and antimicrobial entities. Saliva may also contain varying amounts of two specific types of proteins that will determine its classification. If the mixture contains mostly ptyalin, which is an alpha-amylase enzyme (breaks down starch), then it is considered a serous secretion. On the other hand, if it contains mostly mucin, which is a lubricating protein, then it is considered a mucous secretion. The submandibular gland produces both serous and mucous secretions and is therefore considered a seromucous gland. In contrast, parotid gland is exclusively serous and the sublingual gland is also seromucous.

Consequently, this gland not only helps to lubricate the tongue, buccal cavity and oropharynx, but it also aids in the early stage of digestion of starches. Additionally, it lubricates the food bolus during mastication to allow for easier deglutition.

Another important role of saliva is the neutralization of bacteria in the buccal cavity. Additionally, saliva helps to wash away bacteria from buccal surfaces thus reducing their destructive effects.