Human Anatomy Lesson 17
TMJ, Temporal and Infratemporal Fossae
The temporal fossa and infratemporal fossa are two spaces connected to one another on either side of the zygomatic arch. The temporalis muscle is attached to the frontal, sphenoid, parietal, and temporal bones and forms the roof of the temporal fossa. It passes deep to the zygomatic arch to insert onto the coronoid process of the mandible. The infratemporal fossa lies deep to the mandibular ramus and masseter muscle, and contains two of the four muscles of mastication (the medial and lateral pterygoid muscles) in addition to nerves, arteries, veins, and other structures passing from inside the cranial cavity to the head and neck. The temperomandibular joint (TMJ) forms the posterior boundary of the infratemporal fossa.
Recent statistics suggest that as many as 20% of Americans suffer from some form of temporomandibular joint disease (TMD), and that 85% of TMD is acquired during one's lifetime, as opposed to being a genetic disorder. This book is the primary reference source for investigation of TMJ disorders.
Learning Objectives - By the end of this lesson, you should be able to ...
- Describe the structure and function of the temperomandibular joint
- List the muscles of mastication, and describe their origins, attachments, and innervation
- List and describe the structures that run through the temporal fossa
- List and describe the structures that run through the infratemporal fossa
- List and describe the structures that run through the pterygopalatine fossa on their way from the cranial cavity to the infratemporal fossa
The TMJ is a synovial joint, but it is not a ball-and-socket joint like the hip joint. The condyle of the mandible rotates in the mandibular fossa (also called the glenoid fossa) of the temporal bone during the early part of the jaw opening cycle, but then it glides, or translates, forward over the articular tubercle and onto the pre-glenoid plane after the jaw opens past about 15 degrees. The condyle is covered by a double layer of fibrocartilage - the lower part invests the condyle and allows rotation, whereas the upper part attaches to the mandibular condyle and translates forward during the second half of the jaw opening cycle.
- Joint capsule: synovial membrane lines the nonarticular surfaces of the upper and lower compartments and is attached to the margins of the articular disc. The joint capsule itself is enclosed by a fibrous membrane and surrounded by 3 ligaments: (1) lateral ligament, which runs diagonally from the zygomatic process to the posterior border of the mandibular condyle; (2) sphenomandibular ligament, which runs from the sphenoid bone to the lingula on the medial side of the mandibular ramus; (3) stylomandibular ligament, which runs from the styloid process to the angle of the mandible.
- Mandibular movements: opening and closing of the jaw during chewing is a combination of five movements - depression, elevation, protrusion, retraction, and mediolateral (side-to-side) movement. Depression of the mandible is largely due to gravity, but it is initiated by the digastric, geniohyoid, and mylohyoid muscles. We will talk about these muscles in a later lesson. Opening of the mouth beyond 15 degrees requires a greater contribution of these muscles, in addition to the lateral pterygoid muscles. Elevation of the mandible is accomplished mostly by the masseter, temporalis, and medial pterygoid muscles (see below). Protraction of the mandible is accomplished largely by the lateral pterygoid muscle, which pulls the mandible forward over the articular tubercle onto the pre-glenoid plane. Retraction of the mandible is accomplished by the geniohyoid, digastric, temporalis, and masseter muscles.
Muscles of Mastication
As noted above, there are four muscles of mastication - one, the temporalis muscle, forms the roof of the temporal fossa, and the masseter muscle is on the outer border of the mandibular ramus. The medial and lateral pterygoid muscles are in the infratemporal fossa. All four of the muscles of mastication are innervated by branches from the mandibular division of the trigeminal nerve (CN V3).
- Masseter muscle: the masseter is one of the main muscles that elevates the mandible to close the jaw. It originates on the external surface of the zygomatic arch and the zygomatic bone as far anteriorly as the zygomaticomaxillary suture, and inserts onto the lateral surface of the mandibular ramus, running in a slight posteroinferior direction. It is innervated by the masseteric nerve and its blood supply is from the masseteric artery.
- Temporalis muscle: the temporalis is another of the main muscles that elevates the mandible to close the jaw. It originates on the lateral aspect of the cranial vault, and spans the frontal, sphenoid, parietal, and temporal bones, sitting in the temporal fossa, and runs deep to the zygomatic arch to insert onto the coronoid process of the mandible. The superior and inferior temporal lines mark the attachment sites of the temporalis fascia and muscle, respectively. Its anterior fibers run in a vertical orientation to insert onto the coronoid process, and they are the stronger elevators of the mandible, whereas the posterior fibers run in a diagonal or even anteroposterior direction to insert onto the coronoid process and weakly elevate the mandible and retract it. The temporalis is innervated by deep temporal nerves and its blood supply is from deep temporal arteries and the middle temporal artery.
- Medial pterygoid muscle: this muscle is, in many ways, the mirror image of the masseter muscle - it runs diagonally in a posteroinferior direction on the deep border of the mandible, inserts up and down the mandibular ramus, and functions to elevate the mandible. It originates on the medial surface of the lateral pterygoid process, the adjacent surface of the palatine bone, and the maxillary tuberosity, and is innervated by the nerve to medial pterygoid. Please do not be confused by the fact that the medial pterygoid muscle arises from the medial surface of the lateral pterygoid plate, NOT the medial pterygoid plate.
- Lateral pterygoid muscle: this muscle protrudes the mandible and moves it from side to side. It originates from the inferior surface of the greater wing of the sphenoid, infratemporal crest, and lateral surface of the lateral pterygoid process, and runs in a horizontal direction to insert onto the neck of the mandible anterior to the condyle and the TMJ capsule. The lateral pterygoid is innervated by nerve to lateral pterygoid.
Contents of the Temporal Fossa
As noted above, the borders of the temporal fossa are defined by the extent of the temporalis muscle. The temporalis fascia attaches to the superior temporal line, supramastoid crest of the temporal bone, zygomatic arch, and posterior borders of the postorbital space to provide the lateral border for the temporal fossa. Besides the temporalis muscle, the temporal fossa contains the following structures:
- Deep temporal nerves (V3): run in a superior direction from the infratemporal fossa around the greater wing of the sphenoid deep to the temporalis muscle. As noted above, these nerves provide innervation to the temporalis muscle.
- Zygomaticotemporal nerve (V2): branch of the zygomatic nerve, which is itself a branch of the maxillary nerve (CN V2). The zygomatic nerve branches from the maxillary nerve in the pterygopalatine fossa, passes into the orbit, and enters the temporal fossa through the zygomaticotemporal foramen on the posterior surface of the zygomatic bone. It then pierces the temporalis muscle and fascia to provide sensory innervation to skin around the temple.
- Middle temporal artery: runs from its origin at the superficial temporal artery superior to the zygomatic arch anterior to the EAM, penetrates the temporalis fascia, and runs under the temporalis muscle on the deep border of the temporal fossa. It supplies the temporalis muscle.
Branches of the Mandibular Nerve
The mandibular division of the trigeminal nerve, a.k.a. the mandibular nerve, runs through the foramen ovale into the infratemporal fossa, at which point it splits into several named branches - a small meningeal branch and nerve to medial pterygoid, which itself gives off branch to tensor tympani and branch to tensor veli palatini. It then separates into a small anterior trunk and large posterior trunk. The anterior trunk gives off the masseteric nerve, deep temporal nerves, and nerve to lateral pterygoid, all of which provide motor innervation to their named muscles, and the buccal nerve, which primarily provides sensory innervation to skin, oral mucosa, and gingiva associated with the lower teeth. The posterior trunk gives off the auricolotemporal nerve, inferior alveolar nerve (which itself gives off the nerve to mylohyoid and terminates by splitting into the incisive nerve and mental nerve), and the lingual nerve (which gives off the chorda tympani). Descriptions follow:
- Soon after exiting the foramen ovale, the mandibular nerve gives off a small meningeal branch, which immediately re-enters the cranial cavity with the middle meningeal artery through the foramen spinosum, and supplies sensory innervation to dura in the MCF and mastoid air cells.
- The second branch before the split into anterior and posterior trunks is the nerve to medial pterygoid, which provides motor innervation to the medial pterygoid muscle, and which itself gives off the following two small branches:
- Branch to tensor tympani, which provides motor innervation to tensor tympani, which pulls the malleus (one of the three bones in the middle ear) medially in order to dampen the impact of sound vibrations on the tympanic membrane.
- Branch to tensor veli palatini provides motor innervation to the tensor veli palatini muscle, which tensens and elevates the soft palate, thereby preventing food and liquid from entering the nasopharynx during swallowing.
- The first branch off the small anterior trunk is the masseteric nerve, which passes through the mandibular notch to provide motor innervation to the masseter muscle.
- The second set of branches off the anterior trunk includes the two deep temporal nerves, which curve around the infratemporal crest to provide motor innervation to the temporalis muscle.
- Commonly, the third branch off the anterior trunk is the nerve to lateral pterygoid, which provides motor innervation to the lateral pterygoid muscles.
- Finally, the anterior trunk terminates as the buccal nerve, which passes between upper and lower heads of the lateral pterygoid muscle and takes a 90 degree turn, descending anterior to the insertion of the temporalis muscle to enter the cheek lateral to the buccinator muscle. It supplies skin, oral mucosa, and buccal gingivae of lower molars.
- Posteriorly, the posterior trunk gives off the auriculotemporal nerve, which splits around the middle meningeal artery, passing between the sphenomandibular ligament and condyle before curving around the posterior edge of the condyle and taking a 90 degree turn to pass superiorly between the ear and condyle deep to the parotid gland. It provides sensory innervation to the skin around the temple, EAM, external ear, tympanic membrane, and TMJ, and carries postganglionic parasympathetic fibers from CN IX to the parotid gland.
- The posterior trunk continues inferiorly as two nerves: posteriorly, the inferior alveolar nerve and anteriorly, the lingual nerve. The inferior alveolar nerve runs inferiorly on the lateral surface of the medial pterygoid muscle between the sphenomandibular ligament and mandibular ramus to enter the mandibular canal through the mandibular foramen. It supplies branches to the three molar teeth, P4, and associated gingivae before dividing into the:
- Incisive nerve, which provides sensory innervation to P3, the incisor, canine teeth, and related gingivae, and
- the mental nerve, which supplies the lower lip and chin before exiting the mandible through the mental foramen.
- The anterior terminal branch of the posterior root is the lingual nerve, which provides sensory innervation from the anterior 2/3 of the tongue, oral mucosa on the floor of the oral cavity, and lingual gingivae associated with the lower teeth
- The lingual nerve gives off the chorda tympani, which provides special taste sensory innervation to the anterior 2/3 of the tongue and parasympathetic innervation to the salivary glands below the oral fissure
- we won't have time to cover this in spring 2017 -
© 2014 Robert McCarthy