Muscles of Mastication Overview

Muscles of Mastication

• Four muscle pairs are involved in chewing: masseter, temporalis, medial pterygoid, and lateral pterygoid.
• All are innervated by the mandibular division of the cranial nerve V.
• The masseter muscle attaches to the zygomatic bone and mandible, acting as the prime mover for jaw closure.
• The temporalis muscle connects to the temporal bone and mandible, responsible for elevating and retracting the mandible (closing the jaw). It also helps maintain the jaw's closed position at rest.
• Both the medial and lateral pterygoid muscles have two heads, attaching to the sphenoid bone, palatine bone, and maxilla for the medial pterygoid, and the mandible for both.
• They contribute to additional jaw movements, such as side-to-side grinding motions.
• The buccinator muscle, spanning the mandible and maxilla to the orbicularis oris, compresses the cheek, aiding in keeping food between the teeth during chewing.

Neck and Vertebral Column Muscles: Head and Trunk Movements

• The head is moved by muscles originating from the axial skeleton.
• The sternocleidomastoid muscle, with two heads, attaches to the manubrium, clavicle, and mastoid process of the temporal bone. It lies deep to the platysma and serves as the prime mover for head flexion when both sides contract.
• When one side contracts, it results in head rotation to the opposite side and head tilting towards the same side.
• The scalenes group (consisting of the anterior, middle, and posterior scalenes), found lateral and deeper than the platysma and sternocleidomastoid, elevate the first two pairs of ribs, play a role in coughing, and flex and rotate the neck.
• The splenius muscle, located superficially, attaches to the vertebrae and mastoid process of the temporal bone, acting as a "bandage muscle." If both sides contract, the head extends, and if one side contracts, the head rotates and tilts to the same side.

Erector Spinae Muscles

• The erector spinae muscle group is the primary mover for back extension.
• It consists of three columns of muscles: iliocostalis, longissimus, and spinalis.
• They resist bending forward and act as extensors for returning to an erect position.
• The semispinalis muscle is deeper and composite, extending from the thoracic region to the head. It contributes to extending the vertebral column and head, rotating the head, and works synergistically with the sternocleidomastoid muscles.

Quadratus Lumborum Muscle

• The quadratus lumborum muscle connects to the iliac crest and lumbar vertebrae.
• Individual muscles of this group flex the vertebral column laterally, while acting together they extend the lumbar spine, fix the 12th rib, and maintain upright posture.

Muscles of Respiration: External and Internal Intercostals, Diaphragm

• The external intercostals, consisting of 11 pairs, originate on the rib above and run obliquely.
• They pull ribs upward and outward to elevate the rib cage during inspiration.
• The scalene muscles fix the first rib.
• The internal intercostals, also 11 pairs, originate on the rib below and run perpendicular to the external intercostals.
• They pull ribs downward and inward to depress the rib cage during expiration. The 12th rib is fixed by the quadratus lumborum.
• The diaphragm, dome-shaped when relaxed, is the primary mover of inspiration.

Muscles of the Abdominal Wall

• The rectus abdominis, located medially and superficially, spans from the pubis to the rib cage.
• It participates in lumbar rotation, fixing and depressing the ribs, and stabilizing the pelvis during walking.
• The external oblique, the largest and most superficial lateral muscle, works with the internal oblique to increase abdominal pressure and flex the vertebral column.
• Individual external obliques contribute to rotation.
• The internal oblique is deeper but shares similar actions with the external obliques.
• The transversus abdominis, the deepest layer, compresses the abdominal contents.

Muscles of the Pelvic Floor and Perineum

• Levator ani attaches to the pubis and inner coccyx, supporting the visceral organs of the pelvis.
• It forms a supportive sling at the anorectal junction and vagina, resisting downward pressure increases in the intra-abdominal cavity during activities like coughing or vomiting.
• The coccygeus muscle, spanning from the spine of the ischium to the sacrum and coccyx, supports pelvic viscera and the coccyx, pulling it forward after childbirth.
• The urogenital diaphragm supports pelvic organs and constricts the urethra.
• The levator ani and coccygeus muscles together form the pelvic diaphragm.

Superficial Muscles of the Thorax and Arm Movements

• The pectoralis major muscle, originating from the sternum and rib cartilages and attaching to the humerus, is the prime mover for arm adduction (with the latissimus dorsi) and medial rotation.
• It assists in arm flexion.
• The latissimus dorsi, a broad, flat back muscle, connects to the vertebrae and humerus. It acts as the prime mover for arm extension, an arm adductor, rotates the arm at the shoulder, and plays a role in rowing and swimming.
• The deltoid muscle, a thick, rounded muscle frequently used for intramuscular injections, attaches to the clavicle, scapula, and humerus.
• It is the prime mover for arm abduction, with all fibers contributing to the action.
• Some fibers are responsible for lateral and medial rotations, flexion or extension depending on which fibers are activated. It also swings the arm during walking.

Scapula Muscles

• The trapezius muscle, superficial and spanning from the occipital bone and vertebrae to the scapula, stabilizes, raises, retracts, and rotates the scapula.
• The rhomboids (major and minor), deep to the trapezius, aid in scapular movements, such as squaring the shoulders and rotation when lowering the arm.
• These muscles, along with subscapularis, contribute to the rotator cuff muscles.

Muscle Attachments and Arrangement

• Most muscles cross joints and are attached to bones in at least two places.
• The origin is the less movable attachment, while the insertion is the movable attachment.
• Most attachments are indirect, involving either a tendon or an aponeurosis.
• Skeletal muscles have fascicles, which are bundles of muscle fibers.
• Fascicle arrangements include parallel, pennate (unipennate, bipennate, multipennate), convergent, and circular.

Interactions of Skeletal Muscles

• Muscles can work together or in opposition.
• Three functional groups of muscles are recognized: prime movers, antagonists, and synergists.
• Functions can change depending on the movement.
• Prime movers are the main muscles responsible for a specific movement.
• Antagonists oppose the prime mover, preventing overshoot or controlling the rate of movement.
• Synergists assist the prime mover in several ways: promoting the same movement, reducing undesirable movements, and acting as fixators to stabilize joints for other muscle actions.

Naming Skeletal Muscles

• Muscles can be named based on location, shape, relative size, direction of muscle fibers, number of origins, and actions.

This quiz explores the key muscles involved in mastication, including the masseter, temporalis, and pterygoids. You'll learn about their attachments, functions, and innervation. Test your understanding of how these muscles contribute to the chewing process and jaw movements.