16 parotid bed

16 parotid bed
Lama El Banna

1. Parotid Bed

2. The parotid bed is an irregular space located between the ramus of the mandible, the external acoustic meatus and the mastoid and styloid processes, the digastric muscles, and sternocleidomastoid muscle.

3. The medial extent of this bed is bordered by the posterior belly of the digastric muscle and the muscles attached to the styloid process . Inferiorly, the bed is bounded by the superoanterior border of the sternocleidomastoid muscle . This somewhat irregularly-shaped area houses the parotid gland, which is molded into this space, thus assisting in filling out the contour of the jaw/neck/ear junction. Representatives of two cranial nerves—the facial nerve and trigeminal nerve—pass through the substance of the parotid gland to reach their destinations in and about the head and neck. Similarly , the external carotid artery and some of its branches course through the gland, as do some of the tributaries forming the external jugular vein.

4. The parotid gland is the largest of the three major salivary glands (parotid, submandibular, and sublingual glands) and is enclosed within a capsule that is part of the deep cervical fascia. It is located on the lateral aspect of the face and in the parotid bed. Because the gland is molded into an irregular space, it is also irregular in shape. The superficial aspect of the gland extends superiorly over the masseter muscle to the zygomatic arch, where an accessory portion of the gland may be detached from the main substance. PAROTID GLAND

5. Inferiorly , it is mostly confined to the region between the mastoid process , the sternocleidomastoid muscle , and the angle of the mandible , where it extends over the posterior aspect of the masseter muscle . Medially , the gland extends into the deeper portions of the parotid bed to the styloid process and its attached musculature . Here, a wedge-shaped portion of the gland may intervene between the medial and lateral pterygoid muscles for a short distance. Often, glandular lobes extend into other spaces adjacent to the parotid bed. One such lobe passes between the ramus of the mandible and the medial pterygoid muscle (above that muscle's insertion), whereas other lobes pass between the external auditory meatus and the temporomandibular joint (TMJ) and between the external carotid artery and the superior constrictor muscle of the pharynx.

6. Exiting the anterior aspect of the superficial portion of the gland is the parotid duct ( Stenson duct) , which passes anteriorly, superficial to the masseter muscle, to dive medially into the buccal fat pad; it pierces the buccinator muscle on its way to the oral vestibule. It delivers the parotid salivary secretions at the opening of the parotid papilla located opposite the second maxillary molar .

7. Because the parotid gland is irregular in shape, possessing many fingerlike projections radiating in several directions from the parotid bed, the gland associates with or engulfs many of the structures passing through this region. Structures associated with the superficial aspect of the gland include branches of the great auricular nerve originating from the cervical plexus that provide sensory innervation to the region, and small lymph nodes that drain the superficial area.

8. Structures associated with the deep aspect of the gland, provided it sends projections medial to the styloid process, include the external and internal carotid arteries , the internal jugular vein , and both the vagus and glossopharyngeal nerves. Several structures pass through the gland. The external carotid artery enters the substance of the gland, and it is there that several of its branches arise, including the posterior auricular , maxillary , and superficial temporal arteries . The retromandibular vein , as well as the veins uniting to form it, also pass through the gland.

9. The facial nerve (cranial nerve VII), exits the stylomastoid foramen and enters the substance of the gland. While in the gland, the facial nerve forms a plexus before exiting the gland to innervate the muscles of facial expression. The auriculotemporal nerve, a branch of the mandibular division of the trigeminal nerve (cranial nerve V), enters the substance of the gland from its deep aspect along the neck of the mandible and emerges from the gland just inferior to the root of the zygomatic arch. While within the gland, it communicates with the facial nerve and distributes fibers to the gland.

10. The structures entering the parotid gland exit from its posterior, superior, inferior, and anterior surfaces. The posterior auricular artery exits from the posterior aspect of the gland. The superficial temporal artery and vein , auriculotemporal nerve , and temporal branches of the facial nerve may be observed at the superior margin of the gland. Inferiorly , the retromandibular vein exits the parotid gland just before joining the posterior auricular vein to form the external jugular vein. Emanating from the entire facial margin of the gland are the terminal branches of the facial nerve, grouped into five major branches: the temporal, zygomatic , buccal , mandibular, and cervical branches.

11. The posterior auricular artery , arising from the external carotid artery within the substance of the parotid gland , provides branches that vascularize the gland. Additional small glandular branches arising from the superficial temporal and transverse facial arteries also supply the gland. Venous drainage is via the tributaries passing through the gland, and these vessels empty into the external jugular vein . Vascular supply

12. Lymph percolating through lymph nodes located superficially and within the substance of the gland is delivered into the superficial and deep cervical lymph nodes. Lymphatics

13. The parotid gland receives sensory and autonomic innervation. General sensation is provided by branches of the great auricular nerve , from the cervical plexus, as it ramifies over the surface of the gland. The sympathetic component of the autonomic nervous system reaches the gland via postganglionic sympathetic fibers derived from the carotid plexus ; these fibers travel on the external carotid artery and its branches that course through the gland. Sympathetic innervation to the parotid gland mediates vasoconstriction to these vessels . Innervation

14. Parasympathetic innervation is distributed to the gland by the auriculotemporal nerve [a branch of the trigeminal nerve (cranial nerve V)], even though these parasympathetic fibers do not arise within the trigeminal complex. Preganglionic parasympathetic fibers, from the glossopharyngeal nerve (cranial nerve IX), pass from its tympanic branch via the lesser petrosal nerve to the otic ganglion , where they synapse on postganglionic cell bodies. Postganglionic parasympathetic fibers from here then join the auriculotemporal branch of the mandibular division of the trigeminal nerve, to be distributed to the gland effecting secretomotor functions

15. Several arteries either originate in or are in close association with the parotid bed. These are the ascending pharyngeal , occipital , posterior auricular , maxillary , and superficial temporal arteries . CAROTID ARTERIES

16. The facial nerve, cranial nerve VII, exits from the cranial cavity at the stylomastoid foramen located in the temporal bone just posterior to the styloid process. On exiting, it communicates with the glossopharyngea l and vagus nerves and with the great auricular nerve of the cervical plexus. The auriculotemporal nerve from the mandibular division of the trigeminal nerve communicates with the facial nerve after it has entered the substance of the parotid gland. Presumably, this communication provides general sensory fibers from the trigeminal nerve to the facial nerve for distribution to the face. Branches arising from the facial nerve as it passes through this area include the posterior auricular , digastric , stylohyoid , and parotid plexus with its terminals. The posterior auricular nerve arises near the stylomastoid foramen and ascends behind the ear. This branch supplies motor innervation to the auricular and occipital muscles. The digastric and stylohoid branches provide motor innervation to the like-named muscles as each branch arises near that muscle. FACIAL NERVE

17. muscles Although no muscles reside within the parotid bed, a few are closely associated with that space. These include the masseter , the posterior digastric , and the stylohyoid muscles. Ligaments The stylohyoid ligament , formed by a thickened portion of the parotid fascia, helps to separate the parotid and submandibular glands. STRUCTURES DEEP TO THE PAROTID BED

18. Immediately deep to the muscles originating on the styloid process are the last four cranial nerves , the internal carotid artery , and the internal jugular vein . The glossopharyngeal , vagus , and accessory nerves exit the skull through the jugular foramen, whereas the hypoglossal nerve exits the skull via the hypoglossal canal. As these nerves descend to the structures they innervate, they may be observed passing on the lateral surface of the internal carotid artery and internal jugular vein housed, along with the vagus nerve, in the carotid sheath. Within the carotid sheath, the laterally placed internal jugular vein, originating at the jugular foramen, descends to enter the subclavian vein at the root of the neck. The internal carotid artery ascends within the sheath to enter the carotid canal in the petrous portion of the temporal bone, whereas the vagus nerve descends through the neck on its way to the thorax and abdomen. Nerves and Arteries