Muscles of the Head and Neck

Facial Expression

The muscles of facial expression are responsible for movements of the face, such as smiling, frowning, and blinking. These muscles are generally attached to the skin and underlying bones of the face.

• Frontalis: Raises eyebrows and wrinkles the forehead.

• Occipitalis: Pulls the scalp posteriorly.

• Orbicularis oculi: Closes the eyelids; involved in blinking and squinting.

• Zygomaticus major and minor: Elevate the corners of the mouth (as in smiling).

• Orbicularis oris: Closes and protrudes the lips (as in kissing).

• Buccinator: Compresses the cheek (as in blowing or sucking).

Mastication

These muscles are involved in chewing and moving the jaw.

• Masseter: Elevates the mandible (closes the jaw).

• Temporalis: Elevates and retracts the mandible.

Moves Head and Neck

Muscles in this group control head and neck movements.

• Sternocleidomastoid: Flexes and laterally rotates the head.

• Trapezius (upper fibers): Elevates, retracts, and rotates the scapula; extends the neck.

• Splenius group: Extends and rotates the head.

Muscles of the Trunk

Vertebral Column

• Erector spinae group: Extends and laterally flexes the vertebral column.

Respiration

• Diaphragm: Primary muscle of inspiration; flattens to increase thoracic cavity volume.

• External intercostals: Elevate ribs during inspiration.

• Internal intercostals: Depress ribs during forced expiration.

Abdominal Wall

• Rectus abdominis: Flexes vertebral column; compresses abdominal contents.

• External oblique: Flexes and rotates vertebral column; compresses abdominal wall.

• Internal oblique: Same as external oblique; located deeper.

• Transversus abdominis: Compresses abdominal contents.

• Linea alba: Fibrous structure running down the midline of the abdomen.

Act on Pectoral Girdle

• Trapezius (again): See above.

• Serratus anterior: Protracts and stabilizes scapula.

Muscles of the Upper Limb

Moves the Arm

• Pectoralis major: Flexes, adducts, and medially rotates the arm.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

• Deltoid: Abducts, flexes, and extends the arm.

• Teres major: Extends, adducts, and medially rotates the arm.

• Supraspinatus: Abducts the arm (part of rotator cuff).

• Infraspinatus: Laterally rotates the arm (rotator cuff).

• Teres minor: Laterally rotates the arm (rotator cuff).

• Subscapularis: Medially rotates the arm (rotator cuff).

Moves the Forearm

• Biceps brachii: Flexes elbow and supinates forearm.

• Brachialis: Flexes elbow.

• Triceps brachii: Extends elbow.

Moves Wrist, Hand, and Digits

• Flexor carpi radialis: Flexes and abducts wrist.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Palmaris longus: Flexes wrist.

• Extensor carpi radialis longus: Extends and abducts wrist.

• Extensor carpi ulnaris: Extends and adducts wrist.

• Extensor digitorum: Extends fingers and wrist.

Muscles of the Lower Limb

Moves the Thigh

• Iliopsoas: Flexes thigh at hip joint.

• Adductor group: Adducts thigh.

• Gluteus maximus: Extends and laterally rotates thigh.

• Gluteus medius: Abducts and medially rotates thigh.

Moves the Leg

• Quadriceps femoris group (Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius): Extends leg at knee.

• Sartorius: Flexes, abducts, and laterally rotates thigh; flexes knee.

• Hamstrings group (Biceps femoris, Semitendinosus, Semimembranosus): Flexes knee and extends thigh.

Moves Ankle, Foot, and Digits

• Tibialis anterior: Dorsiflexes and inverts foot.

• Gastrocnemius: Plantar flexes foot; flexes knee.

• Soleus: Plantar flexes foot.

• Fibularis (peroneus) longus: Plantar flexes and everts foot.

• Flexor digitorum longus: Flexes toes.

• Extensor digitorum longus: Extends toes.

Muscle Microanatomy and Physiology

Muscle Structure and Function

Understanding the microscopic structure of muscle tissue is essential for comprehending how muscles contract and generate force.

• Muscle: An organ composed of muscle tissue, connective tissue, blood vessels, and nerves.

• Muscle cell/fiber: The basic cellular unit of muscle tissue; elongated and multinucleated.

• Myofibril: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Myoglobin: Oxygen-binding protein in muscle cells.

• Myosin: Thick filament protein involved in muscle contraction.

• Perimysium: Connective tissue surrounding bundles of muscle fibers (fascicles).

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcomere: The functional contractile unit of a myofibril, defined by Z-disks.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Striations: Alternating light and dark bands in skeletal and cardiac muscle, due to the arrangement of actin and myosin filaments.

• Z-disks: Boundaries of a sarcomere; anchor thin filaments.

Muscle Naming Conventions

Muscle names often reflect their size, shape, location, action, or number of origins.

• Relative size: e.g., maximus (largest), minimus (smallest), longus (long), brevis (short).

• Shape: e.g., deltoid (triangle), trapezius (trapezoid).

• Location: e.g., temporalis (temporal bone), brachialis (arm).

• Action: e.g., flexor, extensor, adductor.

• Number of origins: e.g., biceps (two), triceps (three), quadriceps (four).

Features of Skeletal Muscles

Skeletal muscles have several key features that distinguish them from other muscle types.

• Origin: The fixed attachment point of the muscle.

• Insertion: The movable attachment point of the muscle.

• Action: The movement produced when the muscle contracts.

• Fascicle: Bundle of muscle fibers within a muscle.

• Epimysium: Connective tissue sheath surrounding the entire muscle.

Table: Comparison of Muscle Features

Basic Muscle Functions

• Movement of body parts

• Maintenance of posture

• Stabilization of joints

• Heat production

Key Terms and Definitions

• Actin: Thin filament protein involved in muscle contraction.

• Myosin: Thick filament protein that interacts with actin to produce contraction.

• Sarcomere: Functional unit of muscle contraction.

• Fascicle: Bundle of muscle fibers.

• Epimysium: Connective tissue covering the entire muscle.

• Perimysium: Connective tissue surrounding fascicles.

• Endomysium: Connective tissue surrounding individual muscle fibers.

Example: Muscle Contraction Equation

The sliding filament theory describes how muscles contract:

Additional info: Students should be able to identify muscles on models, know their origin, insertion, and action, and understand the basic microanatomy and physiology of skeletal muscle tissue.