Muscles Involved in
Mastication
Chewing, or mastication, is one of the important functions carried on in
the mouth, and a number of muscles are involved either directly or indirectly
in this activity. However, the four muscles that are primarily responsible for
the forceful chewing movements of the mandible are classified by most authors
as the “muscles of mastication.” These are the masseter, temporalis, lateral
pterygoid, and medial pterygoid muscles.
The masseter muscle is a thick,
quadrangular muscle that is readily palpable on the side of the jaw. It is
described as having a superficial and a deep part, which can be rather easily
separated on the posterior aspect of the muscle but are blended together
anteriorly. The superficial part arises from the inferior border of the
anterior two thirds of the zygomatic arch (zygomatic process of maxilla,
zygomatic bone, and zygomatic process of temporal bone) and runs medially and a
little posteriorly to insert on the lateral surface of the lower part of the
ramus of the mandible. The area of insertion continues to the inferior border
of the mandible. The deep portion of the masseter muscle arises from the
inner surface of the whole length of the zygomatic arch and runs almost
vertically inferiorly to insert on the lateral surface of the coronoid process
and upper part of the ramus of the mandible. The deepest fibers frequently
blend with the adjacent portion of the temporalis muscle. The masseter muscle
is supplied by a masseteric branch from the mandibular division of the
trigeminal nerve, which reaches the deep surface of the muscle by passing
through the mandibular notch.
The temporalis muscle, spread
out broadly on the lateral side of the skull, is a thin
sheet, except where its fibers converge toward the tendon of insertion. It
arises from the whole temporal fossa (the extensive area between the inferior
temporal line and the infratemporal crest) and from the inner surface of the temporal
fascia that covers the muscle. The temporalis muscle inserts by means of a
thick tendon that passes medial to the zygomatic arch and attaches to the apex
and deep surface of the coronoid process of the mandible and the anterior
border of the ramus almost as far as the last molar tooth, with some of the
fibers frequently becoming continuous with the buccinator muscle. Two or three
deep temporal branches of the mandibular nerve enter the deep surface of the
temporalis muscle.
The lateral pterygoid muscle is
somewhat conical in shape and runs horizontally in the infratemporal fossa. It
arises as the fusion of superior and inferior heads.
The superior head attaches to the
infratemporal surface of the greater wing of the sphenoid bone, and the
inferior head attaches to the lateral surface of the lateral pterygoid plate.
The two heads join and form a tendon of insertion that ends on the front of the
condylar neck of the mandible and on the anterior aspect of the capsule and
articular disk of the temporomandibular joint. A lateral pterygoid nerve from
the mandibular branch of the trigeminal enters the deep surface of this muscle.
The medial pterygoid muscle, located
medial to the ramus of the mandible, is thick and quadrangular. Its main origin
is from the medial surface of the lateral pterygoid plate and from the
pyramidal process of the palatine bone between the two pterygoid plates. A
small slip of muscle originates from the tuberosity of the maxilla and the
adjacent surface of the pyramidal process of the palatine bone. The medial
pterygoid muscle inserts on the medial surface of the ramus of the mandible
between the mylohyoid groove and the angle. The medial pterygoid nerve from the
mandibular runs along the medial side of the muscle to enter it.
The muscles of mastication all pass
across the temporomandibular joint, and they are the major muscles producing
the movements allowed at this joint. Elevation of the mandible is brought about
by the masseter, temporalis, and medial pterygoid. They are able to bring the
lower teeth powerfully up against the upper teeth. They also are acting against
gravity in most positions of the head in keeping the mouth closed. If they are
relaxed, the weight of the jaw can gap the mouth to a small degree. The muscle
of mastication that actively opens the mouth is the lateral pterygoid. It does
this by pulling the articular disk and condyle of the mandible anteriorly.
Other muscles that help in opening the mouth against resistance are the
suprahyoid, infrahyoid, and platysma muscles. Protrusion of the jaw is brought
about primarily by bilateral contraction of the lateral pterygoid, because in
this movement, also, the articular disk and condyle of the mandible are brought
anteriorly. The superficial portion of the masseter and the medial pterygoid
can give some minor aid in protrusion. Retraction of the mandible is
accomplished mostly by the posterior part of the temporalis muscle, some of the
fibers of which run nearly horizontally. The digastric and geniohyoid muscles
can contribute to retraction when the hyoid bone is anchored.
All the muscles of mastication are
employed in the act of chewing, because it involves the four movements of the
mandible described above (i.e., elevation, depression, protrusion, and
retraction) and at least one of the muscles of mastication is involved in each
of these movements. For the most part, chewing is done either on one side or
the other, and the condyle of the side on which the chewing is being done
remains more or less in position while the condyle of the other side moves back
and forth, as in protrusion and retraction. This is combined in proper sequence
with slight elevation and depression to bring about the grinding action on the
food.
In order that grinding can be carried
on efficiently, the food must be kept between the teeth by the tongue on one
side and the cheek and lips on the other side. Naturally, the muscular
framework of the cheek and lips is important in accomplishing this. The
framework of the cheek is formed by the buccinator muscle, which takes its
origin from the outer surfaces of the maxilla and mandible in the region of the
molar teeth and between the posterior ends of these lines of attachment from
the pterygomandibular raphe, by means of which it is continuous with the
superior constrictor of the pharynx. From this U-shaped origin, the horizontal
fibers of the muscle run anteriorly, apparently to continue into the orbicularis
oris muscle, with the superiormost and inferiormost fibers going into the
upper and lower lips, respectively, and the intermediate fibers crossing near the
corner of the mouth, so that the superior fibers of this intermediate group go
into the lower lip and the inferior fibers of the intermediate group go into
the upper lip. The buccinator muscle is supplied by the facial nerve. The
framework of the lips is formed by the orbicularis oris muscle. In addition to
the fibers that appear to be the forward prolongations of the buccinator
muscle, fibers come into the orbicularis oris from all of the muscles that
insert in the vicinity. The orbicularis oris muscle is also supplied by the
facial nerve.
