Anatomy of the salivary glands

Have you ever smelled something so good that your mouth begins to water? Well you can thank your salivary glands for this mouth-watering sensation. The salivary glands, while often overlooked, are a key part of our digestive system.

There are three main pairs of salivary glands: the parotid, submandibular, and sublingual glands. There are also a few smaller companions called accessory salivary glands, which are sprinkled over the palate, lips, cheeks, tonsils, and tongue.

When it comes to function, salivary glands secrete saliva into the oral cavity. Saliva, as you may know, is a clear, tasteless, and odorless fluid that keeps the mouth’s mucosa hydrated.

Saliva also helps lubricate food while we chew, making swallowing easier, and it also starts the digestion of starch, because it contains an enzyme called amylase.

Saliva also acts as ‘nature’s mouthwash’, since it’s rich in antimicrobial compounds such as hydrogen peroxide to keep our mouths clean. Accessory salivary glands have a similar role, except they tend to secrete less saliva.

The parotid glands are the largest of the three paired salivary glands. Superficially, each parotid gland is triangular in shape, where it sits upon the masseter muscle.

However, most of the parotid gland actually sits in the retromandibular fossa, anteroinferior to the external acoustic meatus, where it is wedged between the the ramus of the mandible and the mastoid process and sternocleidomastoid muscle posteriorly.

Its apex is situated posterior to - or along - the angle of the mandible, while the base is associated with the zygomatic arch.

The parotid gland is surrounded by a protective sheath derived from the investing layer of the deep cervical fascia called the parotid sheath.

Now, a number of structures are closely associated with the parotid gland. First the extracranial portion of the facial nerve passes through the parotid gland and forms the parotid plexus.

Although it is embedded within the gland, the parotid plexus does not innervate it, on its way to the muscles of facial expression.

It also encloses the retromandibular vein, and the external carotid artery. The auriculotemporal nerve and superficial temporal artery can also be found traversing the superior part of the parotid gland.

Another important structure associated with the parotid gland is the parotid duct which is how the saliva produced in the parotid gland gets transported to the oral cavity.

The duct passes horizontally along the parotid gland, where it reaches the anterior border of the masseter and turns medially to pierce the buccinator muscle, entering the oral cavity opposite the second maxillary molar where it releases saliva.

Now, regarding nerve supply, we’ve mentioned that the parotid plexus formed by the facial nerve doesn’t innervate the parotid gland.

Instead, sensory innervation is provided by a branch of the mandibular division of the trigeminal nerve called the auriculotemporal nerve, and the great auricular nerve which is a branch of the cervical plexus.

Like all glands, the parotid gland also needs parasympathetic and sympathetic innervation to ensure adequate saliva secretion.

For the parotid gland, the stimulatory parasympathetic fibers are from the glossopharyngeal nerve. The glossopharyngeal nerve sends preganglionic fibers to the otic ganglion, which is located in the infratemporal fossa, just inferior to the foramen ovale, medial to the mandibular nerve and posterior to the medial pterygoid muscle.

From the otic ganglion, the postganglionic parasympathetic fibers reach the parotid gland by hitching a ride along the fibers of the auriculotemporal nerve.

Sympathetic fibers are supplied by the superior cervical ganglion via the external carotid plexus, whose fibers travel along the external carotid artery to reach the parotid gland. Parasympathetic innervation stimulates saliva production, while sympathetic stimulation will reduce production.

Time for a quick break! Can you recall the structures that pass through the parotid gland?