Please review this pre-lab module regarding osteology of the skull and mandible, dentition, the temporomandibular joint, and mechanics of mastication prior to the lab.
Primary Lab Objectives and Goals:
Identify, transect, and reflect the masseter and temporalis muscles. As you do this, consider their roles in jaw movement. Remove the zygomatic arch and a portion of the ramus of the mandible.
Locate the medial and lateral pterygoid muscles. As you do this, consider their role in jaw movements.
Find the maxillary and mandibular nerves and trace their branches. Identify the maxillary artery and locate its branches.
Identify the submandibular and the sublingual glands. Examine the mylohyoid and geniohyoid muscles.
Remove a portion of the mandible between the attachments of the medial pterygoid and geniohyoid muscles. Examine the branches of the hypoglossal and lingual nerves.
Expose the pterygopalatine and otic ganglia.
Dissect the temporomandibular joint, and consider its movements.
Derivatives of the first pharyngeal arch develop into structures of the chewing apparatus and are innervated by branches of the trigeminal nerve (CN V). The maxillary (CN V2) and mandibular (CN V3) branches of the trigeminal nerve innervate structures of and areas associated with the upper and lower jaws respectively..
The mandibular nerve (CN V3) is the motor nerve to all 8 of the first-arch muscles (these are paired on either side of the head so there are 16 muscles in total):
- One muscle of the palate (the tensor veli palatini muscle) (observed in previous lab)
- One in the middle ear (the tensor tympani muscle)(observed in previous lab)
- Six that attach to the mandible; the four muscles of mastication (the temporalis, masseter, and medial and lateral pterygoid muscles), and the two muscles that open the jaw (the mylohyoid and anterior digastric muscles)
1. THE MASSETER AND TEMPORALIS MUSCLES
- On one side of the head, define the borders of the temporalis and masseter muscles.
NOTE: you may have already completely removed much of the temporalis muscle from the skull when you cleared the side of the head for the removal of the calvarium. If this is the case, identify the temporal lines on the parietal bone (on the calvaria you removed) where it would have attached.
The fascia overlying the temporalis muscle is tough and ligamentous. It serves the important function of supporting the zygomatic arch against the downward pull of the masseter muscle, and also provides an area of attachment for the temporalis muscle. Fibers of the temporalis muscle arise from both the bones of the temporal fossa and the deep surface of the temporalis fascia.
The masseter and temporalis muscles both serve to help close (or adduct) the jaw during chewing. They have different attachments and leverage. The masseter muscle has a large superficial and a smaller deep portion. The fibers of the deep portion are more posterior, shorter, and have a more transverse direction of pull on the mandible. The masseter muscle provides optimal leverage during chewing with the molars. On the other hand, the temporalis muscle is better situated for incision. Both are used during mastication.
- Transect the masseter muscle (both the superficial and the deep portions) midway along its length. Leave the upper half of the masseter muscle attached to the zygomatic arch.
- Observe the cross section of the masseter to appreciate the two portions that make up the muscle.
- Transect the temporalis muscle just above the zygomatic arch and observe the cross section of the muscle.
NOTE: PRIOR TO MAKING THE CUTS INDICATED BELOW (particularly cut #4), PASS A PROBE OR A SCALPEL HANDLE BEHIND THE MANDIBLE. KEEP IT THERE TO PROTECT THE DEEPER STRUCTURES (namely the INFERIOR ALVEOLAR AND LINGUAL NERVES). Use your probe to clear these nerves away from the deep surface of the mandible.
- Use an electric or hand saw to remove the zygomatic arch anterior to the jaw (temporomandibular) joint (arrows 1 and 2 in the figure below).
- Use an electric or hand saw to cut the neck of the mandible (below the mandibular condyle) and the mid-ramus of the mandible (arrows 3 and 4 in the figure below)
- Remove the upper ramus with the coronoid process, and the portions of the attached masseter and temporalis muscles, respectively.
2. MEDIAL AND LATERAL PTERYGOID MUSCLES
- Once the zygomatic arch and the upper part of the ramus are removed, you will be able to look into the infratemporal fossa.
The infratemporal fossa - the space between the mandible and the walls of the pharynx - is largely filled by the medial and lateral pterygoid muscles. These muscles originate from the medial and lateral sides of the lateral pterygoid plate respectively. Both muscles can move the lower jaw from side to side. The medial pterygoid helps elevate (adduct) the mandible when closing of the mouth. The lateral pterygoid protrudes the jaw during opening by pulling the mandibular condyles forward, but is also active during jaw closing (more details on this below).
HINT: It is worth going to the model room or using a skeleton in the lab to examine the medial and lateral pterygoid plates, noting the hamulus on the medial plate.
- Place a finger in front of your ear opening. Open your mouth, and feel the mandibular condyle slide forward and downward in its mandibular fossa. This movement is caused by the contraction of the lateral pterygoid muscle.
- On the cadaver, identify the medial and lateral pterygoid muscles.
- Trace the buccal nerve and artery across the surface of the pterygoid muscles, and down toward the lining of the cheek
- Free the lateral pterygoid muscle from the underlying deep head of the medial pterygoid muscle.
- Transect the lateral pterygoid muscle at both ends, and remove it.
NOTE: be careful when transecting this muscle that you don’t cut any of the structures deep to it. Cut it at one end, carefully reflect it back and clear away anything on the deep surface and then cut the other end.
- Note that between the two pterygoid muscles, run important structures: the mandibular division (CN V3) of the trigeminal nerve, accompanied by the branches of the maxillary artery.
The larger inferior head of the lateral pterygoid muscle originates from the lateral side of the lateral pterygoid plate (hence its name). It runs backward to insert into the neck of the mandible and onto the joint capsule of the temporomandibular joint. The smaller superior head arises from the sphenoid's greater wing, and inserts primarily into the articular disk. The medial pterygoid muscle originates from the medial side of the lateral pterygoid plate. It inserts into the angle of the mandible, like the masseter muscle, but on the medial side. Again, like the masseter muscle (and temporalis muscle), the medial pterygoid muscle acts to close the mouth. Of the four masticatory muscles, only the lateral pterygoid muscle acts to open the mouth.
3. THE MANDIBULAR AND MAXILLARY NERVES, AND MAXILLARY ARTERY
- Identify the mandibular nerve (CN V3) and the maxillary nerve (CN V2).
- Try especially to find the chorda tympani (branch of CN VII) as it joins the lingual nerve (branch of CN V3).
HINT: The chorda tympani exits the middle ear through the petrotympanic fissure which is a small opening in the mandibular fossa. Then it joins the lingual nerve. The dissection can be clarified by clearing away the veins on the surface of the medial pterygoid muscle.
- Identify the maxillary artery.
- Trace as many branches from the maxillary artery as you can.
- Trace its path back to the external carotid artery.
4. FLOOR OF THE ORAL CAVITY
- Identify the submandibular gland.
- Clear away its superficial portion.
- Sever and reflect the anterior digastric muscle, experimenting as you do so with the mobility of the digastric tendon in its fascial "pulley".
The anterior head of the digastric muscle is an important jaw opener (abduction/depression of the mandible). The mylohyoid and geniohyoid stabilize the floor of the mouth and can also help with depression of the mandible.
- Identify the mylohyoid muscle, which is now exposed. Notice how the mylohyoid muscle (along with its companion on the other side) forms a muscular floor for the oral cavity.
- Note the deep portion of the submandibular gland protruding around the back edge of the mylohyoid muscle. Realize that the body of the gland lies deep to the muscle in the oral cavity.
- Reflect the mylohyoid muscle laterally, revealing the rest of the submandibular gland.
- Identify the geniohyoid muscle, and the sublingual salivary gland.
5. THE HYPOGLOSSAL AND LINGUAL NERVES
HINT: You will have already seen the the sublingual region from the inside of the oral cavity in step 4. Below you will see the sublingual region from another view by cutting the mandible in the procedures that follow.
- Make a saw mark on the mandible just in front of the attachment of the medial pterygoid muscle (i.e. just behind the last molar where the angle of the mandible meets the body), and another just behind the geniohyoid muscle attachment (i.e. between the 2nd incisor and the 1st premolar tooth).
- Carefully detach the mucous membrane of the mouth from the inside of the mandible between these two marks.
- Finish sawing through the mandible in both places, and remove the detached piece to expose the floor of the mouth as shown below. You can cut the mylohyoid muscle close to the bone.
HINT: Be sure your saw cuts go through only the mandible and not any deeper structures. You may wish to place a probe or scalpel handle on the inside of the mandible when making the cuts to protect the deeper structures.
- Study the branches of the hypoglossal (CN XII) and lingual nerves (CN V3), lying lateral to the hyoglossus muscle.
- Transect and reflect the hyoglossus muscle to expose the lingual artery.
HINT: The lingual artery usually runs along the same appoximate course as the hypoglossal nerve, except the lingual artery is DEEP to the hyoglossus muscle.
As you dissect, bear in mind that the hypoglossal nerve (CN XII) is the motor nerve to the tongue muscles, while the submandibular and sublingual salivary glands are innervated by parasympathetic fibers. The preganglionic fibers come from the facial nerve (CN VII) and reach the submandibular ganglion via the chorda tympani. They synapse in the submandibular ganglion, then the postganglionic fibers travel via the lingual nerve, a branch of CN V3.
6. THE PTERYGOPALATINE AND OTIC GANGLIA
The maxillary nerve (CN V2) emerges from the braincase by passing through the foramen rotundum and into the pterygopalatine fossa, where it is joined by the maxillary artery. The greater petrosal nerve, a branch of the facial nerve (CN VII), enters the pterygopalatine fossa via the pterygoid canal. Fibers of the greater petrosal nerve synapse in the pterygopalatine ganglion, which is attached to the maxillary nerve inside the pterygopalatine fossa. From there, postganglionic parasympathetic secretomotor fibers pass to the lacrimal, nasal, and palatal glands. From the pterygopalatine fossa, branches of the maxillary nerve pass to the orbit, the nasal fossa, and the palate, as well as the bones and teeth of the upper jaw. These branches of CN V2 accompany branches of the maxillary artery.
Hint as you proceed with the dissection below: The superior alveolar branches of the maxillary nerve (CN V2) and artery are small and hard to dissect, but the infraorbital and palatine branches are fairly easy to display.
- On a skull, find the inferior orbital fissure.
- Trace the course of the infraorbital nerve on the skull - out of the pterygopalatine fossa, through the inferior orbital fissure, along the infraorbital groove (also called "sulcus"), and out onto the face through the infraorbital foramen.
- On the cadaver, examine the eye socket from which you already removed the contents.
- Strip away the periorbita covering the bony walls of the eye socket to display the infraorbital nerve and vessels.
- From within the nasal cavity, break through the thin medial wall of the palatine canal (just behind the middle and inferior conchae).
- Look for the greater and lesser palatine nerves and vessels traversing the canal.
- The pterygopalatine fossa lies at the upper end of the palatine canal. You should try to display the pterygopalatine ganglion lying in the fossa.
- Try and reflect tensor veli palatini muscle to expose the otic ganglion on the MEDIAL side of the mandibular nerve. It is typically located at the point where the lingual and alveolar nerves branch from the mandibular nerve (CN V3).
HINT: Exposing the pterygopalatine ganglion is relatively easy and productive. Exposing the otic ganglion may be more challenging. It may be worth consulting an instructor to discuss dissection of these ganglia.
7. THE TEMPOROMANDIBULAR JOINT
Histologically, the temporomandibular joint is unlike other synovial joints; the articular cartilage is fibrocartilage, and the articular disk is composed of fibrous connective tissue. The disk divides the joint into two separate synovial cavities. Hinge movements occur between the mandibular condyle and the disk, while the disk and condyle slide as a single unit against the skull when the mandible is protracted or rotated. The lateral part of the joint's fibrous capsule is thickened to form a temporomandibular ligament that prevents the mouth from opening too wide. The lateral ligament of the capsule prevents excessive posterior movement of the mandibular condyle.
- Cut through the temporomandibular ligament (removing part of the mandibular condyle if necessary) to display the articular disk, and the inferior and superior cavities of the joint.
- Note that the lateral pterygoid muscles insert onto the articular disk, as well as into the neck of the mandible.
When the lateral pterygoid muscle contracts, it pulls the jaw forward and the articular disk is drawn forward along with it, so the mandibular condyle does not override the disk. The lateral pterygoid muscle is also active during jaw closing, where it contracts eccentrically to guide the mandibular condyles as they slide back along the articular eminence (aka articular tubercle) into the mandibular fossa.
Epistaxis and maxillary artery
Epistaxis is a fancy name for nose bleed. Branches of the facial artery and maxillary artery supply the nasal cavity and nasal septum. Epistaxis in the posterior part of the nasal cavity can be intractable. Occlusion of the sphenopalatine artery or its branches (sometimes by introduction of emboli by interventional radiologists) can reduce blood flow and stop the nose bleed.
TMJ disc displacement
The temporomandibular joint (TMJ) is unusual in having a fibrocartilage disc within the joint space. When the condyle of the mandible becomes distracted from the articular surface of the glenoid fossa of the base of the skull the disc can become displaced and damaged. This can lead to significant pain and audible clicking in the joint.