Chapter 4. Orofacial musculature, mastication and swallowing
Extra-orally, the muscles of mastication move the mandible at the temporomandibular joint while the circumoral muscles of facial expression change the shapes and positions of the lips. In the suprahyoid region, the digastric, mylohyoid and geniohyoid muscles are located in the floor of the mouth. Intra-orally, the soft palate (the movable part of the palate) is raised and elevated by muscles during and after swallowing and the shape and position of the tongue is affected by intrinsic and extrinsic musculature (see pages 52–53). Chewing (mastication) and swallowing (deglutition) are important functions involving the orofacial musculature.
• be able to describe the locations, attachments, functions and innervations of the muscles influencing mandibular movements and movements of the lips, cheeks and floor of the mouth, and the soft palate (for the musculature of the tongue, see pages 52–53)
• understand the physiological mechanisms underlying the processes (and control) of mastication and swallowing.
Muscles of mastication
Although many muscles, both in the head and the neck, are involved in mastication, the ‘muscles of mastication’ is a collective term reserved for the masseter, temporalis, and medial and lateral pterygoid muscles. All the muscles of mastication receive their innervation from the mandibular division of the trigeminal nerve. Closely associated functionally with the muscles of mastication is the digastric muscle. The masseter and temporalis muscles lie on the superficial face, while the lateral and medial pterygoid muscles lie deeper within the infratemporal fossa.
The masseter muscle consists of two overlapping heads:
• The superficial head arises from the zygomatic process of the maxilla and from the anterior two-thirds of the lower border of the zygomatic arch.
• The deep head arises from the deep surface of the zygomatic arch.
Internally, the muscle has many tendinous septa that greatly increase the area for muscle attachment and which provide a multipennate arrangement, thereby increasing its power. The superficial head passes downwards and backwards to insert into the lower half of the lateral surface of the ramus. The deep head, whose posterior fibres are more vertically oriented, inserts into the upper half of the lateral surface of the ramus, particularly over the coronoid process. The muscle elevates the mandible and is primarily active when grinding tough food. Indeed, the muscle exerts considerable power when the mandible is close to the centric occlusal position. On the basis of its fibre orientation, the posterior fibres of the deep head may have some retrusive capability for the mandible.
The temporalis muscle is the largest muscle of mastication. It takes origin from the floor of the temporal fossa of the lateral surface of the skull and from the overlying temporal fascia, and should thus be regarded as a bipennate muscle. The attachment is limited above by the inferior temporal line. From this wide origin, the fibres converge towards their insertion on the apex, the anterior and posterior borders, and the medial surface of the coronoid process. Indeed, the insertion extends down the anterior border of the ramus almost as far as the third molar tooth. The posterior fibres of the muscle pass horizontally forwards while the anterior fibres pass vertically downwards on to the coronoid process. To reach the coronoid process, the muscle runs beneath the zygomatic arch. The anterior (vertical) part elevates the mandible, while the posterior (horizontal) part retracts the protruded mandible. In certain sites, the masseter and temporalis muscles are joined. This is particularly so for the deep fibres of the deep head of the masseter and the overlying temporalis muscle. The functional significance of this ‘zygomatico-mandibular mass’ is unclear.
Both the masseter and the temporalis muscles are innervated by branches of the anterior division of the mandibular nerve. Both receive their blood supply from the maxillary artery (masseteric and deep temporal branches), the superficial temporal artery (transverse facial and middle temporal branches) and, for the masseter muscle, the facial artery.
The lateral pterygoid muscle lies in the roof of the infratemporal fossa and has essentially a horizontal alignment. It has two heads, superior and inferior:
• The superior (upper) head is the smaller and arises from the infratemporal surface of the greater wing of the sphenoid bone.
• The inferior (lower) head forms the bulk of the muscle and takes origin from the lateral surface of the lateral pterygoid plate of the sphenoid bone.
Both heads pass backwards and outwards and appear to merge before their areas of insertion. The fibres of the superior head insert into the capsule and possibly the medial aspect of the anterior border of the intra-articular disc of the TMJ. The fibres of the inferior head of the lateral pterygoid muscle insert into the pterygoid fovea of the mandibular condyle. However, the precise insertions of the muscle are still controversial and have clinical relevance with regard to TMJ disorders, particularly internal derangement, where the disc is displaced, usually in an anteromedial position, and the jaw may become locked. Such conditions may be associated with clicking joints, limited jaw movements and pain, and some have attributed variations in the attachment, and therefore function, of the superior head as an aetiological factor in the disease. Functionally, the superior and inferior heads should be considered as two separate muscles. The inferior head is concerned with mandibular protrusion, depression and lateral excursions. The superior head is activated during mandibular retrusion (providing controlled movements) and during clenching of the mandible.
The medial pterygoid muscle consists of two heads:
• The bulk of the muscle arises as a deep head from the medial surface of the lateral pterygoid plate of the sphenoid bone.
• The smaller superficial head arises from the maxillary tuberosity and the neighbouring part of the palatine bone (pyramidal process).
From these sites of origin, the fibres of the medial pterygoid pass downwards, backwards and laterally to insert into the roughened surface of the medial aspect of the angle of the mandible. Tendinous septa within the muscle increase the surface area for muscle attachment, providing a multipennate arrangement and therefore increasing the power the muscle can exert. The main action of the muscle is to elevate the mandible, but it also assists in lateral and protrusive movements. An accessory medial pterygoid muscle has been described as a separate slip of muscle close to the deep surface of the medial pterygoid. This takes origin from the base of the skull close to the foramen ovale and merges with the deep head of the medial pterygoid. Its function is unknown. The masseter and medial pterygoid muscles together form a muscular sling that supports the mandible on the cranium.
The medial pterygoid muscle is innervated by a branch of the mandibular nerve that arises proximal to the division of the mandibular nerve into anterior and posterior trunks. The lateral pterygoid receives its nerve supply from the anterior trunk. Both muscles receive their blood supply as muscular branches from the maxillary artery.
Because of its functional associations, the digastric muscle is described here, although it usually is not strictly classified as a ‘muscle of mastication’. This muscle is located below the inferior border of the mandible and consists of an anterior and a posterior belly connected by an intermediate tendon:
• The posterior belly arises from the mastoid notch immediately behind the mastoid process of the temporal bone; it passes downwards and forwards towards the hyoid bone, where it becomes the digastric tendon. The digastric muscle passes through the insertion of the stylohyoid muscle and is attached to the greater horn of the hyoid bone by a fibrous loop.
• The anterior belly of the digastric muscle is attached to the digastric fossa on the inferior border of the mandible and runs downwards and backwards to the digastric tendon.
The digastric muscle depresses and retrudes the mandible, and is involved in stabilizing the position of the hyoid bone and in elevation of the hyoid during swallowing.
The anterior belly of the digastric muscle is innervated by the mylohyoid branch of the mandibular division of the trigeminal nerve, the posterior belly by the digastric branch of the facial nerve. This reflects different embryological origins, from first and second Pharyageal (branchial) arch mesenchyme respectively. The anterior belly receives its blood supply from the facial artery, the posterior belly from the posterior auricular and occipital arteries.
Floor of the mouth
The floor of the mouth is the region located between the medial surface of the mandible, the inferior surface of the tongue and the mylohyoid muscles. The mylohyoid muscles are attached to the mylohyoid lines of the mandible and consequently structures above these lines are related to the floor of the mouth, whereas structures below the lines are related to the upper part of the neck. This concept is of considerable clinical importance with respect to the spread of inflammation from infected teeth within the mandible. The two mylohyoid muscles form a muscular diaphragm for the floor of the mouth. Above this diaphragm are found the genioglossus and geniohyoid muscles medially and the hyoglossus laterally. Below the diaphragm lie the digastric and stylohyoid muscles.
The mylohyoid muscle arises from the mylohyoid line on the medial surface of the body of the mandible. Its fibres slope downwards, forwards and inwards. The anterior fibres of the mylohyoid muscle interdigitate with the corresponding fibres on the opposite side to form a median raphe. This raphe is attached above to the chin and below to the hyoid bone. The posterior fibres are inserted on to the anterior surface of the body of the hyoid bone. The muscle raises the floor of the mouth during the early stages of swallowing. It also helps to depress the mandible when the hyoid bone is fixed. The mylohyoid muscle is supplied by the mylohyoid branch of the inferior alveolar branch of the mandibular division of the trigeminal nerve. Its blood supply is derived from the lingual artery (sublingual branch), the maxillary artery (mylohyoid branch of the inferior alveolar artery) and the facial artery (submental branch).
The geniohyoid muscle originates from the inferior genial tubercle (mental spine). It passes backwards and slightly downwards to insert onto the anterior surface of the body of the hyoid bone. The geniohyoid muscle elevates the hyoid bone and is a weak depressor of the mandible. Its innervation is from the first cervical spinal nerve travelling with the hypoglossal nerve. Its blood supply is derived from the lingual artery (sublingual branch).
Muscles of the tongue
The intrinsic and extrinsic muscles of the tongue are described on pages 52–53.
Muscles of facial expression
The muscles of facial expression are characterized by:
• their superficial arrangement in the face
• their activities on the skin (brought about directly by their attachment to the facial integument)
• by their common motor innervation, the facial nerve.
Functionally, the muscles of facial expression are grouped around the orifices of the face (the orbit, nose, ear and mouth) and should be considered primarily as muscles controlling the degree of opening and closing of these apertures; the expressive functions of the muscles have developed secondarily. The muscles of facial expression vary considerably between individuals in terms of size, shape and strength. The superficial muscles around the lips and cheeks may be subdivided into two groups:
• The various parts of the orbicularis oris muscle
• Muscles that are radially arranged from the orbicularis oris muscle.
The fibres of orbicularis oris pass around the lips. The muscle is divided into four parts, each part corresponding to a quadrant of the lips. Its muscle fibres do not gain attachment directly to bone but occupy a central part of the lip. Muscle fibres in the philtrum insert onto the nasal septum. The range of movement produced by orbicularis oris includes lip closure, protrusion and pursing. The radial muscles can be divided into superficial and deep muscles of the upper and lower lips:
• The levator labii superioris, levator labii superioris alaeque nasi and zygomaticus major and minor are superficial muscles of the upper lip.
• The levator anguli oris is a deep muscle of the upper lip.
• The depressor anguli oris is a superficial muscle of the lower lip.
• The depressor labii inferioris and mentalis muscle are deep muscles of the lower lip.
As their names suggest, the levator labii superioris elevates the upper lip, the depressor labii inferioris depresses the lower lip, and the corners of the mouth are raised and lowered by the levator and depressor anguli oris muscles.
Two muscles extend to the corner of the mouth: the risorius and buccinator muscles, risorius lying superficial to the buccinator:
• The risorius muscle stretches the angles of the mouth laterally.
• The buccinator muscle arises from the pterygomandibular raphe and from the buccal side of the maxillary and mandibular alveoli above the molar teeth. Most of its fibres insert into mucous membrane covering the cheek; other fibres intercalate with orbicularis oris in the lips. As the fibres of the buccinator converge towards the angle of the mouth, the central fibres decussate. The main function of the buccinator muscle is to maintain the tension of the cheek against the teeth during mastication.
Muscles of the soft palate
The soft palate is supported by the fibrous palatine aponeurosis whose shape and position is altered by the activity of four pairs of muscles: the tensor veli palatini, the levator veli palatini, the palatoglossus and the palatopharyngeus muscles. In addition, there is the musculus uvulae intrinsic muscle of the soft palate.
Tensor veli palatini
The tensor veli palatini muscle arises from the scaphoid fossa of the sphenoid bone at the root of the pterygoid plates and from the lateral side of the cartilaginous part of the auditory (pharyngotympanic) tube. From its origin, the fibres converge towards the pterygoid hamulus, whence the muscle becomes tendinous, the tendon bending at right angles around the hamulus to become the palatine aponeurosis. The anterior border of the aponeurosis is attached to the posterior border of the hard palate. Medially, it merges with the aponeurosis of the other side. Posteriorly, it becomes indistinct, merging with submucosa at the posterior edge of the soft palate. When the tensor veli palatini muscle contracts, the aponeurosis becomes a taut, horizontal plate of tissue upon which other palatine muscles may act to change its position.
The motor innervation of the tensor veli palatini is derived from the mandibular branch of the trigeminal nerve (via the nerve to the medial pterygoid muscle and the otic ganglion).
Levator veli palatini
The levator veli palatini muscle originates from the base of the skull at the apex of the petrous part of the temporal bone, anterior to the opening of the carotid canal, and from the medial side of the cartilaginous part of the auditory tube. The muscle curves downwards, medially and forwards to enter the palate immediately below the opening of the auditory tube. The levator muscles of the palate form a U-shaped muscular sling. When the palatine aponeurosis is stiffened by the tensor muscles, contraction of the levator muscles produces an upwards and backwards movement of the soft palate. In this way, the nasopharynx is shut off from the oropharynx by the apposition of the soft palate on to the posterior wall of the pharynx.
The palatopharyngeus muscle arises from two heads: one from the posterior border of the hard palate, the other from the upper surface of the palatine aponeurosis. The two heads unite after arching over the lateral edge of the palatine aponeurosis, where the muscle passes downwards beneath the mucous membrane of the lateral wall of the oropharynx as the posterior pillar of the fauces (palatopharyngeal arch). The muscle is inserted into the posterior border of the thyroid cartilage of the larynx. The main action of the palatopharyngeus muscle is to elevate the larynx and pharynx, but it may also arch the relaxed palate and depress the tensed palate.
The palatoglossus muscle arises from the aponeurosis of the soft palate and descends to the tongue in the anterior pillar of the fauces, whence its fibres intercalate with the transverse fibres of the tongue. The action of the palatoglossus is to raise the tongue in order to narrow the transverse diameter of the oropharyngeal isthmus.
The musculus uvulae muscle arises from the posterior nasal spine at the back of the hard palate and from the palatine aponeurosis. It passes backwards and downwards to insert into the mucosa of the uvula. It moves the uvula upwards and laterally, and helps to complete the seal between the soft palate and pharynx in the midline region when the palate is elevated.
With the exception of the tensor veli palatini muscle, the nerve supply to the muscles of the palate is derived from the cranial part of the accessory nerve via the pharyngeal plexus. The arterial supply to the muscles of the soft palate is derived from the facial artery (ascending palatine branch), the ascending pharyngeal artery and the maxillary artery (palatine branches).
Passavant’s muscle is a sphincter-like muscle that encircles the pharynx at the level of the palate, inside the fibres of the superior constrictor muscles. It is formed by fibres arising from the anterior and lateral part of the upper surface of the palatine aponeurosis. Contraction of this muscle forms a ridge (Passavant’s ridge), against which the soft palate is elevated.
The principal role of mastication in human beings is the mechanical breakdown of food placed in the mouth. In doing so it stimulates the secretion of saliva, which in turn assists in the digestive process due to the enzymes present in the saliva, and lubricates and binds the food particles, preparing them for swallowing. Mastication also releases substances from food that dissolve in the saliva and any other fluids taken into the mouth, which in turn contribute to the senses of taste and smell and also play a role in the cephalic phase of gastrointestinal secretions. The amount of mastication that food requires depends on the nature of the substance ingested. Solid substances are subjected to vigorous chewing before they are swallowed, whereas softer substances require less chewing and liquids require no chewing at all and are simply transported to the back of the mouth for swallowing. It has been shown that mastication is necessary for some foods, such as red meats, chicken and vegetables, to be fully absorbed by the rest of the gastrointestinal tract, whereas fish, eggs, rice, bread and cheese do not require to be chewed for complete absorption in the rest of the tract.
Mastication involves the coordinated activities of a number of structures in and around the mouth, primarily the teeth, jaw elevator (closing) and depressor (opening) muscles, temporomandibular joint, tongue, lips, palate and salivary glands. These are collectively called the masticatory apparatus.
Feeding (eating and drinking) is basically a process in which food is ingested and transported along the alimentary tract. For the more solid foods, the process of transportation is interrupted early by the need for mechanical breakdown and mixing by chewing. In the past, all the events that occur from the ingestion of the food to the beginning of the swallow were termed mastication. However, it is now thought that the term should be confined to the process of mechanical reduction of food particles by the act of chewing.
The teeth are the main organ of mastication and are adapted for the functional requirements of the diet. Man is omnivorous (meat and vegetable eater) and consequently the teeth are heterodont in character, in that they have different anatomical forms and functions in different parts of the dental arch. The anterior teeth have sharp edges for grasping, incising and tearing foods, while the posterior teeth are specialized for cutting flesh and grinding fibrous plant material. The teeth in humans are relatively unspecialized in contrast with the specialized dentitions of carnivorous mammals, such as cats and dogs, or herbivorous mammals, such as horses and cattle.
The upper and lower teeth of humans occlude, in that both the maxillary and mandibular teeth meet. Studies of the cusps of posterior teeth in hominids and early man have shown that they are worn down early in life and that the occlusal surfaces are flat and lack any distinctive cuspal features. This suggests that the role of the cusps of human posterior teeth in establishing tooth position and relationships during growth and eruption may be more important than their dietary role.
Mastication in humans involves both vertical and lateral movements of the jaws, like most herbivores (cattle, horses, rabbits and so on) but unlike pure carnivores (cats and dogs) that have only vertical movement of the jaw. Essentially, following breaking of the food using the incisors, the posterior teeth on the side breaking down the food, the so-called working side, are brought into vertical alignment, whilst the posterior teeth on the non-working side may, or may not, be in contact. The food is then crushed and ground by the op/>