The Muscular System

Overview of the Muscular System

The muscular system is responsible for producing movement, maintaining posture, and generating heat in the human body. It consists primarily of skeletal muscles, which are under voluntary control, and their associated connective tissues.

• Movement: Skeletal muscles contract to move bones and body parts.

• Posture: Muscles stabilize joints and maintain body positions.

• Heat Production: Muscle contractions generate heat, contributing to body temperature regulation.

• Example: Practicing yoga demonstrates voluntary control of the muscular system.

Muscle Attachments and Interactions

Muscle Attachments

Muscles attach to bones or other structures via specialized connective tissues, enabling force transmission and movement.

• Tendon: A cordlike structure of dense regular connective tissue that attaches muscle to bone.

• Aponeurosis: A thin, flattened sheet of dense irregular connective tissue that attaches muscle to muscle.

Muscle Origins and Insertions

• Origin: The fixed, less movable attachment of a muscle.

• Insertion: The movable end of the muscle that attaches to the bone being pulled.

Functional Roles of Muscles

• Prime Mover (Agonist): The main muscle responsible for a specific movement.

• Antagonist: A muscle that opposes the action of the prime mover.

• Synergist: A muscle that assists the prime mover in performing its action.

• Fixator: A muscle that stabilizes the origin of the prime mover.

• Example: In elbow flexion, the biceps brachii is the prime mover, the triceps brachii is the antagonist, and the brachialis and brachioradialis are synergists.

Muscle Shapes and Fascicle Arrangements

General Muscle Shapes

Skeletal muscles exhibit various shapes, which influence their function and force generation.

• Circular: Fascicles arranged concentrically to form a sphincter (e.g., orbicularis oris).

• Parallel: Fascicles run parallel to the muscle's long axis; often have a central belly; high endurance (e.g., sartorius).

• Convergent: Fascicles converge to a single attachment point (e.g., pectoralis major).

• Pennate: Fascicles attach obliquely to a central tendon, resembling a feather. Subtypes include:

  • Unipennate: Fascicles on one side of the tendon (e.g., extensor digitorum).

  • Bipennate: Fascicles on both sides of the tendon (e.g., rectus femoris).

  • Multipennate: Multiple tendons with fascicles arranged around them (e.g., deltoid).

Functional Implications of Fascicle Arrangement

• Parallel muscles allow greater range of motion but less force.

• Pennate muscles generate more tension but have a shorter range of motion.

Major Muscle Groups and Their Functions

Muscles of Facial Expression

These muscles insert into the skin around the eyes, nose, and mouth, allowing for a wide range of facial expressions by moving the skin rather than bones.

Muscles of the Eyes

• Extrinsic eye muscles: Originate on the skull and insert onto the eyeball, controlling eye movement.

Muscles of Mastication (Jaw Movement)

• Located within the cheek, these muscles originate from the skull and provide leverage for chewing.

Muscles of the Tongue and Swallowing

• Muscles that move the tongue are essential for speech, swallowing, and manipulating food.

• Suprahyoid muscles (above the hyoid bone) and infrahyoid muscles (below the hyoid bone) facilitate swallowing and speech.

Muscles of the Neck and Back

• Move the head, cervical vertebrae, and scapulas.

• Include both superficial and deep muscle layers.

Muscles of the Abdomen

• Rectus abdominis: Medially located, covered by the linea alba.

• External oblique: Outermost layer on the flanks.

• Internal oblique: Middle layer.

• Transversus abdominis: Innermost layer.

• These muscles support the trunk, allow movement, and help maintain posture.

Muscles of Respiration

• Diaphragm: Separates thoracic and abdominal cavities; primary muscle of respiration.

• Intercostal muscles: External, internal, and innermost layers assist in breathing by moving the rib cage.

Muscles of the Pelvic Floor and Perineum

• Support pelvic organs, resist intra-abdominal pressure, and function as sphincters for the urethra, rectum, and vagina.

• Perineal muscles are involved in urination, ejaculation (men), and vaginal contraction (women).

Muscles of the Upper Limb

Muscles That Position the Pectoral Girdle

• Stabilize the scapula and clavicle, providing a base for arm movement.

Muscles That Move the Humerus

• Anterior muscles (e.g., pectoralis major, subscapularis) move the humerus forward.

• Superior muscles (e.g., deltoid) move the humerus upward.

• Inferior muscles (e.g., latissimus dorsi) move the humerus downward.

• Posterior muscles (e.g., infraspinatus) move the humerus backward.

Muscles That Move the Forearm

• Originate in the upper arm and control flexion, extension, pronation, and supination of the forearm.

• Forearm muscles also move the wrists, hands, and fingers.

Intrinsic Muscles of the Hand

• Originate and insert within the hand, allowing fine motor control of the fingers (flexion, extension, abduction, adduction).

Muscles of the Lower Limb

Hip and Thigh Muscles

• Muscles that move the femur generally originate on the pelvic girdle and insert into the femur.

• Anterior femur muscles extend the lower leg and flex the thigh.

• Posterior femur muscles flex the lower leg and extend the thigh.

• Gluteal and thigh muscles also adduct, abduct, and rotate the thigh and lower leg.

Muscles of the Lower Leg

• Anterior compartment: Responsible for dorsiflexion of the foot.

• Posterior compartment: Responsible for plantar flexion of the foot.

• Lateral and medial muscles: Invert, evert, and rotate the foot.

Intrinsic Muscles of the Foot

• Dorsal muscles extend the toes.

• Plantar muscles (three layers) flex the toes and provide strength to support body weight.

Summary Table: Muscle Roles and Examples

Additional info:

• Muscle names often derive from Latin, describing their shape, location, or function (e.g., rectus = straight, deltoid = triangular).

• Superficial muscles are closer to the surface, while deep muscles lie underneath and are often responsible for more subtle or stabilizing actions.