Muscle Anatomy

Key Muscle Structures

Understanding muscle anatomy requires familiarity with the specialized structures and terminology that describe muscle organization and function. These terms are foundational for studying both microscopic and macroscopic muscle features.

• Endomysium: A thin connective tissue layer that surrounds each individual muscle fiber, providing structural support and carrying capillaries and nerves.

• Sarcolemma: The specialized cell membrane of a muscle fiber, responsible for conducting electrical impulses.

• Sarcomere: The basic contractile unit of striated muscle, defined by the area between two Z discs. It contains thick and thin filaments arranged in a precise pattern.

• Synaptic Cleft: The small gap between the neuron and muscle fiber at the neuromuscular junction, where neurotransmitters are released to stimulate muscle contraction.

• T Tubule: Invaginations of the sarcolemma that help transmit action potentials deep into the muscle fiber.

• Thick Filament: Composed primarily of the protein myosin, responsible for force generation during contraction.

• Thin Filament: Composed mainly of actin, along with regulatory proteins troponin and tropomyosin.

• Z disc: Defines the boundaries of a sarcomere and anchors thin filaments.

• M Line: The center of the sarcomere, where thick filaments are anchored.

Example: During muscle contraction, the sarcomere shortens as thick and thin filaments slide past each other, a process described by the sliding filament theory.

Major Muscles of the Human Body

Facial Muscles

Facial muscles are responsible for expressions, movements of the mouth, and other functions such as chewing and speaking.

• Buccinator: Compresses the cheek, aiding in chewing.

• Depressor anguli oris: Lowers the corners of the mouth.

• Depressor labii inferioris: Depresses the lower lip.

• Levator labii superioris: Elevates the upper lip.

• Mentalis: Protrudes the lower lip.

• Epicranius (frontal belly): Raises eyebrows and wrinkles forehead.

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes lips.

• Platysma: Tenses skin of the neck.

• Zygomaticus major: Elevates the corners of the mouth (smiling).

• Mandible: Not a muscle, but the jaw bone; muscles attach here for mastication.

• Digastric: Depresses the mandible and elevates the hyoid bone.

• Masseter: Elevates the mandible (chewing).

• Temporalis: Elevates and retracts the mandible.

Head, Neck, and Spine Muscles

These muscles support head movement, neck stability, and spinal posture.

• Anterior, Middle, & Posterior Scalene: Flex and rotate the neck; assist in breathing.

• Sternocleidomastoid: Rotates and flexes the head.

• Splenius capitis: Extends and rotates the head.

• Trapezius: Moves the scapula and supports the arm.

• Levator scapulae: Elevates the scapula.

• Quadratus lumborum: Stabilizes the pelvis and lumbar spine.

Breathing / Abdominopelvic Muscles

These muscles are essential for respiration and movements of the trunk.

• Diaphragm: Main muscle of respiration; contracts to enlarge thoracic cavity.

• External intercostal: Elevates ribs during inhalation.

• Internal intercostal: Depresses ribs during exhalation.

• External oblique: Compresses abdomen; flexes and rotates trunk.

• Internal oblique: Compresses abdomen; flexes and rotates trunk.

• Transversus abdominis: Compresses abdominal contents.

• Rectus abdominis: Flexes vertebral column; "six-pack" muscle.

Upper Appendage Muscles

These muscles control movement of the arms, shoulders, and hands.

• Deltoid: Abducts the arm.

• Supraspinatus: Abducts the arm; part of rotator cuff.

• Infraspinatus: Laterally rotates the arm.

• Teres minor: Laterally rotates the arm.

• Teres major: Adducts and medially rotates the arm.

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

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

• Subscapularis: Medially rotates the arm.

• Biceps brachii: Flexes elbow and supinates forearm.

• Brachialis: Flexes elbow.

• Brachioradialis: Flexes forearm at elbow.

• Extensor carpi radialis: Extends and abducts wrist.

• Extensor carpi ulnaris: Extends and adducts wrist.

• Extensor digitorum: Extends fingers.

• Flexor carpi radialis: Flexes and abducts wrist.

• Flexor carpi ulnaris: Flexes and adducts wrist.

• Flexor digitorum: Flexes fingers.

• Pronator teres: Pronates forearm.

• Triceps brachii: Extends elbow.

Lower Appendage Muscles

These muscles are responsible for movement and stability of the legs and hips.

• Adductor longus: Adducts thigh.

• Adductor magnus: Adducts and extends thigh.

• Gracilis: Adducts thigh and flexes leg.

• Gluteus maximus: Extends and laterally rotates thigh.

• Gluteus medius: Abducts and medially rotates thigh.

• Iliacus: Flexes thigh.

• Psoas major: Flexes thigh.

• Biceps femoris: Flexes knee and extends thigh.

• Semimembranosus: Flexes knee and extends thigh.

• Semitendinosus: Flexes knee and extends thigh.

• Rectus femoris: Extends knee and flexes thigh.

• Vastus lateralis & medialis: Extend knee.

• Sartorius: Flexes, abducts, and laterally rotates thigh.

• Tensor fascia latae: Abducts thigh.

• Fibularis: Everts foot.

• Gastrocnemius: Plantar flexes foot and flexes knee.

• Soleus: Plantar flexes foot.

• Tibialis anterior: Dorsiflexes and inverts foot.

Summary Table: Major Muscle Groups and Functions

Additional info:

• Some muscle names and groupings were inferred based on standard anatomical terminology and context.

• Functions and examples were expanded for clarity and completeness.