Mastication, TMJ & Swallowing.docx

Learning Objectives: Describe the infratemporal fossa and list its main contents List the muscles of mastication, describe their attachments, actions and innervation Describe the distribution of the sensory and motor branches of the mandibular division of the trigeminal nerve Describe the course and...

Learning Objectives: Describe the infratemporal fossa and list its main contents List the muscles of mastication, describe their attachments, actions and innervation Describe the distribution of the sensory and motor branches of the mandibular division of the trigeminal nerve Describe the course and distribution of the chorda tympani Describe the arrangement of the nerve supply to the teeth and supporting structures and relate this information to the distribution of analgesia after local nerve block for dental procedures Describe the functional anatomy of the TMJ in terms of its bony articular surfaces, capsular attachments, ligaments and articular disc Mastication The process of mastication is possible through movement of the lower jaw This particular movement is possible due to the synovial joint between the mandible and the temporal bone Movement at this joint allows the incisors to function in biting, or the molars to grind the ingested food The muscles of the tongue act to place or keep the food between the molars Infratemporal Fossa It is the space found posterior (behind the maxilla and deep to both the zygomatic arch and ramus of the mandible This fossa is of relevance to the muscles of mastication as it allows the passage of the temporalis and pterygoid muscles It also allows the passage of maxillary artery and pterygoid venous plexus Neurovasculature of muscles of mastication The muscles of mastication receive blood supply from the maxillary artery, the larger of the two terminal branches from the external carotid This artery subdivides into mandibular, pterygoid and pterygopalatine The equivalent drainage is called the pterygoid venous plexus Innervation is via the anterior trunk of the mandibular nerve – the same nerve responsible for innervation of both the musculature and mucosa of the oral cavity or cheeks The pterygoid plexus of veins is the equivalent to the maxillary artery This plexus lies between the temporalis and pterygoid muscles and anastomoses with the facial vein plus the cavernous sinus The facial vein then drains into the internal jugular Muscles of Mastication 4 major masticatory muscles: Masseter Temporalis Medial Pterygoid Lateral Pterygoid Also, a number of minor masticatory muscles, e.g. superior and inferior hyoid muscles Masseter Muscle Quadrilateral in shape Originates from zygomatic bone/arch and inserts on anterior border and angle of ramus of mandible Its superficial fibres run obliquely, in contrast to the deep fibres that are almost vertical in orientation Its actions on mandible include: Elevation Uses both masseter muscles, enabling teeth to be clenched Retrusion Uses superficial fibres of both masseter muscle, retruding a protruded mandible Ipsilateral excursion A single masseter can move the mandible to the same side Temporalis Muscle Fan-shaped muscle Originates from side of the skull and inserts at the coronoid process of the mandible Muscle arises from 2 heads: Temporal bone Temporal fascia Temporalis muscle action on mandible: Resting tonus This muscle maintains a normal mandibular rest position when the subject is in upright position Elevation E.g. biting uses the fibres of both sides Retrusion Involves posterior horizontal fibres of both sides retruding a protruded mandible Ipsilateral excursion Using only one side, the temporalis is capable of pulling the mandible to same side Medial and Lateral Pterygoid Muscles The medial and lateral pterygoid muscles occupy the infratemporal fossa To be seen a portion of the mandible has to be removed The medial pterygoid is a mirror of the masseter muscle on the inner surface of the mandible Medial Pterygoid muscle Actions on Mandible Elevation Acting together, the medial pterygoid plus masseter are powerful elevators Protrusion Right and left muscles aid in protruding the jaw Contralateral excursion Medial pterygoid from one side moves jaw to the opposite side Lateral Pterygoid muscles Actions on mandible: Protrusion Both sides together are prime protractors. Horizontal fibres pull mandible forwards onto articular disc Depression Both sides together not only pull the mandible forwards but depress when combined with the hyoid muscles Contralateral excursion One side moves mandible in opposite direction Hyoid Muscles The supra- and infrahyoid muscles are primarily involved in swallowing and always contract bilaterally The infrahyoid muscles act to depress the hyoid bone and larynx during swallowing The suprahyoid muscles (form the floor of the mouth) generally act to raise the hyoid bone and the larynx when the mandible is stabilised. But can also depress the mandible when acting in conjunction with infrahyoid and lateral pterygoid muscles The incisors and canines take on small portions of solid foods into the oral cavity This food is drenched with saliva from the glands that not only moistens but also contains ptyalin that breaks down the carbohydrates At the same time food is broken down by mechanical action by the molars and premolars, as the mandible is opened and closed through muscle action, e.g. buccinators This is assisted by the tongue that moves food towards the molars, while the cheeks stop it being forced into the vestibule Lips, in turn, reduce the likelihood of the food falling out of the mouth. This is achieved by contracting the orbicularis oris muscle Finally, bilateral contraction of the palatoglossus muscles narrows the oropharyngeal isthmus with the posterior part of the tongue filling the remainder of the gap. This allows breaking to continue while chewing with the mouth closed and prevent food entering the airway Bolus Formation and Swallowing Bolus Formation: Once sufficiently broken up the tongue gathers the food into a bolus and pushes it towards the oral pharynx by raising against the hard palate. At the same time the palatoglossus muscles relax to open the oropharyngeal isthmus Swallowing: (the following takes about 2 seconds, from the oropharynx to the stomach) As the bolus reaches the oropharynx the soft palate is tensed and raised by the levator muscles At the same time the superior constrictor contracts to close the nasopharynx from the oropharynx and the suprahyoid muscles contract to raise and bring the larynx forward, particularly geniohyoid and the anterior belly of digastric that sit under the tongue, forcing the epiglottis over the laryngeal opening The epiglottis acts to deflect any food and drink into the piriform fossa The middle and inferior constrictors then contract to push the bolus towards the oesophagus whereby, through peristalsis, the bolus is pushed towards the stomach The aryepiglottic and transverse arytenoid muscles contract reducing the size of the laryngeal inlet. The vocal and vestibular folds also close. Finally, once the bolus is through the muscles relax, the hyoid bone and larynx return to their usual position and breathing can resume