THE MASTICATORY APPARATUS

THE MASTICATORY APPARATUSThe masticatory (chewing) apparatus is defined here as comprising the temporomandibular joint (TMJ), the muscles of mastication, the mouth, the teeth, the tongue and the salivary glands.

THE TEMPOROMANDIBULAR JOINT (TMJ)

The TMJ is a modified hinge joint allowing a gliding motion. It is formed by the articulation of the head of the condyloid process of the mandibular ramus with the mandibular fossa and articular tubercle of the temporal bone. The joint contains a disc which separates the head from the temporal bone and divides the joint into upper and lower synovial cavities. The joint capsule is thickened laterally to form the temporomandibular ligament. Two extrinsic ligaments (stylomandibular and sphenomandibular) help to stabilize the joint.

MUSCLES OF MASTICATION

These are the muscles we use to chew.

MUSCLES OF MASTICATION

MuscleOriginInsertionActionsInnervation

Masseter M.

inferior border and medial aspect of zygomatic arch

fibers course infero-posteriorly to insert into the lateral aspect of mandibular ramus, especially the anglepowerful elevator of mandible, also contributes to a side to side or grinding actionmasseteric nerve, a branch of the mandibular nerve

Temporalis M.

floor of temporal fossa inferior to inferior temporal line and the deep surface of the temporal fascia

fibers converge from anterior, superior and posterior aspects of the origin to pass deep to zygomatic arch and insert into the coronoid process and the anterior border of the mandibular ramuselevator and retractor of mandibledeep temporal branches of mandibular nerve

MUSCLES OF MASTICATION (cont.)

MuscleOriginInsertionActionsInnervation

Medial Pterygoid M.

medial aspect of lateral pterygoid plate and adjacent parts of the maxillary and palatine bonesfibers course infero-posteriorly to insert into the medial aspect of mandibular ramus, especially the angleworks with masseter to elevate and laterally move mandiblebranches from mandibular nerve

Lateral Pterygoid M.

two-headed muscle originating from the infratemporal surface of the greater wing of the sphenoid and the lateral surface of the lateral pterygoid platefibers converge and course posterolaterally to insert into the neck of the mandible and the disc and capsule of the TMJprotrude mandible when both sides act together and cause a displacement to the contralateral side when acting alone

THE MOUTH OR ORAL CAVITY

The mouth or oral cavity is the beginning of the alimentary canal and is also used for breathing and speech in humans. The mouth is bounded anteriorly and laterally by the checks and lips, superiorly by the soft and hard palate and inferiorly by the floor of the mouth. Posteriorly the mouth is continuous with the oropharynx (the oral subdivision of the pharynx). The transition from oral cavity to oropharynx is marked by the palatoglossal arches. The oropharynx is separated from the nasopharynx by the soft palate. The mouth is subdivided into two sub regions: a vestibule or buccal cavity, located between the teeth and gums and the lips and cheeks, and the mouth proper, inside the teeth and gums. The lips act as a sphincter guarding the entrance to the oral cavity. Together the lips and cheeks work as a unit to keep food from accumulating in the vestibule and force it back between the teeth.

THE TEETH

Humans have 10 primary (milk or deciduous) teeth per jaw that begin to develop before birth. These teeth begin to erupt at 6 to 8 months after birth and continue until they are all erupted at about 20 to 30 months. The deciduous teeth are shed between 6 and 12 years of age and replaced by permanent teeth with the final number being 16 per jaw. The process is usually complete by 18 years of age with the possible exception of the third molars (wisdom teeth) which may not erupt until the early 20's or not at all because of being impacted. The wisdom teeth may have to be removed surgically. The complement of permanent teeth is usually described on the basis of onehalf jaw or quadrant.

Each quadrant will normally have, beginning at the anterior midline, two incisors (medial and lateral), one canine, two premolars (bicuspids) and three molars. Each tooth has a crown (the white part you see and which occludes with the opposite tooth), a neck (between crown and root) and one or more roots which anchor the tooth to the alveolus (tooth socket) of the jawbone via dental (periodontal) ligaments.

THE TONGUE

The tongue (Gr. glossa; L. lingua) is a large, highly mobile and muscular structure which fills most of the oral cavity when at rest with the mouth closed. The tongue is found mostly in the oral cavity with its posteriormost part being in the oropharynx. The tongue is an extremely important organ being used in mastication, swallowing (deglutition), speech and oral cleaning. The oral part of the tongue is highly mobile and is loosely attached to the floor of the mouth by a frenulum. The root of the tongue (pharyngeal part) is attached to the hyoid bone and mandible.

Structure of the TongueThe dorsal surface of the tongue is marked by a Vshaped groove, the sulcus terminalis. The apex of the groove points posteriorly and the arms pass anteriorly and laterally. The sulcus terminalis divides the tongue into an oral part (anterior twothirds) and a pharyngeal part (posterior onethird). This division is not arbitrary but has an underlying developmental basis.

Oral Part of Tongue attached to the floor of the mouth by the frenulum the oral part is marked on its dorsum by a median groove or furrow the mucous membrane of the oral tongue is rough due to the presence of many papillae which are associated with touch organs and taste buds taste buds are concentrated along the sides of the tongue and just anterior to the sulcus terminalis taste buds will be covered more thoroughly in your histology course.

Pharyngeal Part of Tongue the mucous membrane has no papillae has a lumpy looking appearance due to underlying nodules of lymphoid tissue which is collectively designated as the lingual tonsil.

Muscles of the Tongue The median furrow of the tongue marks the presence of an underlying vertical raphe which divides the tongue into two halves and provides attachment for the tongue muscles. Each half of the tongue has four extrinsic and four intrinsic muscles.Extrinsic Tongue Muscles four muscles that are primarily responsible for moving the tongue but can contribute to shape change also. All have -glossus (Gr. glossa, tongue) as part of their names.

EXTRINSIC TONGUE MUSCLES

MuscleOriginInsertionActionsInnervation

Genioglossus M.mental spine on inner aspect of chin - body of hyoid bone

(Mental Spine)entire body of tonguebilateral action of posterior fibers protrudes tongue - of anterior fibers depresses tongue - depression of center forms a trough -unilateral action wags tongue side to sidehypoglossal n. (CN XII)

Styloglossus M.styloid process (anterior border)posterior sides of tongueretrudes tongue and elevates posterior edges to form a trough

Hyoglossus M.body and greater horn of hyoid boneinferolateral sides of tongueretrudes and depresses tongue

Palatoglossus M.soft palateposterolateral tongue blending with intrinsic muscleselevates posterior tongue or depresses soft palate - narrows passage from oral cavity to oral pharynx (the fauces)vagus n. (CN X)

Intrinsic Tongue Muscles the intrinsic tongue muscles comprise four organized bundles of fibers which lie in each of the major planes: longitudinal (superior and inferior), transverse and vertical. They act to change the shape of the tongue. All innervated by the hypoglossal nerve.

Sensory Innervation - the developmental history of the tongue is very complex and this is reflected in a complex innervation involving both general sensory and special sensory fibers and branches of three different cranial nerves.

General Sensory - pain, temperature, tactile, etc. general sensory impulses from the oral part of the tongue (anterior twothirds) are conveyed by the lingual nerve, a branch of the mandibular division of the trigeminal nerve (CN V) general sensory impulses from the pharyngeal part of the tongue (posterior onethird) are conveyed by the lingual branch of the glossopharyngeal nerve (CN IX).

Special Sensory(Taste) taste impulses from the taste buds of the oral part of the tongue, except for the circumvallate papillae, are conveyed by branches of the chorda tympani nerve a branch of the facial nerve (CNVII) the chorda tympani fibers are conveyed (hitchhike on) to the tongue by the lingual nerve taste impulses from the taste buds of the pharyngeal tongue and the circumvallate papillae are conveyed by the lingual branch of the glossopharyngeal nerve (CNIX).

THE SALIVARY GLANDSThe salivary glands include diffuse tissues found within the oral mucosa as well as the three, large paired glands.

The Parotid GlandCovered with the face.

The Submandibular GlandThe submandibular gland is a horseshoeshaped gland with one arm deep to he mylohyoid muscle and the other arm superficial and visible in the submandibular triangle. The two arms meet around the posterior free edge of the mylohyoid. The gland empties via the submandibular duct which opens by one to three orifices in the sublingual papilla found at the base of the lingual frenulum. The chorda tympani nerve (a branch of the facial nerve) supplies parasympathetic secretomotor fibers to the gland by way of the submandibular ganglion. Sympathetic fibers arrive by way of branches of the facial artery.

The Sublingual GlandThe sublingual gland is the smallest of the paired salivary glands. The gland is found between the mandible and the genioglossus muscle deep to the oral mucosa and superior to the mylohyoid muscle. The left and right glands unite to form a horseshoeshaped structure which surrounds the base of the lingual frenulum. Each gland empties by 10 to 12 small ducts onto the floor of the mouth. The nervous supply is as described for the submandibular gland.

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Clinical Note: When no chewing or speaking is going on, the muscles of mastication are very quiet electrically, indicating that the closed mandible is in more or less in a state of balance requiring little muscular effort to keep closed (good advice).Clinical Note: An inability to stick out the tongue straight ahead would indicate a problem with the hypoglossal nerve. problems with the vagus nerve would have much more apparent and important symptoms than moving the tongue