Muscles

The Muscular System Overview

How the Body Produces Movement

  • Muscle: Tissues that facilitate movement.

Key Muscles and Their Functions

  • Masseter: Origin: Zygomatic arch; Insertion: Mandible; Action: Elevates mandible.

  • Sternocleidomastoid: Origin: Sternum and clavicle; Insertion: Mastoid of the skull; Action: Flexes neck and turns head.

  • Trapezius: Origin: Occipital bone and thoracic vertebrae; Insertion: Clavicle and acromion of the scapula; Action: Elevates, adducts, depresses, rotates scapula, extends neck.

  • Deltoid: Origin: Clavicle and scapula; Insertion: Humerus; Action: Raises arm.

  • Rectus abdominis: Origin: Pubis; Insertion: Costal cartilage and xiphoid process; Action: Flexes vertebral column.

  • Biceps brachii: Origin: Coracoid process of scapula; Insertion: Radius; Action: Flexes at elbow.

  • Triceps brachii: Origin: Humerus and scapula; Insertion: Ulna; Action: Extends at elbow.

  • Latissimus dorsi: Origin: Lower thoracic vertebrae, ribs, lumbar vertebrae; Insertion: Humerus; Action: Extends, lifts, and rotates shoulder.

  • Rectus femoris: Origin: Ilium; Insertion: Tibia; Action: Extends at knee, flexes at hip.

  • Biceps femoris: Origin: Ischium and femur; Insertion: Tibia and fibula; Action: Flexes at knee and extends/rotates at hip.

  • Tibialis anterior: Origin: Tibia; Insertion: 1st metatarsal; Action: Flexes foot.

  • Gastrocnemius: Origin: Femur; Insertion: Calcaneus; Action: Flexes the ankle, points the foot.

Functions of Skeletal Muscle

  • Movement of the skeleton

  • Posture and body position

  • Support soft tissues

  • Guard entrances and exits

  • Maintain body temperature

Muscle Structure

  • Epimysium: Surrounds entire muscle.

  • Perimysium: Divides muscle into compartments.

  • Muscle fascicle: Bundle of muscle fibers.

  • Endomysium: Surrounds each muscle fiber.

  • Tendon: Attaches muscle to bone.

  • Aponeurosis: Attaches muscle to muscle.

Unique Features of Skeletal Muscle

  • Large size (100 micrometers wide, up to 2 feet long).

  • Multinucleate (hundreds of nuclei).

Muscle Fiber Structures

  • Sarcolemma: Muscle cell membrane.

  • Sarcoplasmic reticulum: Smooth ER around myofibrils.

  • Sarcoplasm: Cytoplasm of muscle cells.

  • Transverse tubules: Carry signals for contraction.

  • Myofibril: Contains thick (myosin) and thin (actin) filaments.

  • Myofilaments: Actin and myosin filaments.

  • Cross-bridge: Connection between thick and thin filaments.

Neuromuscular Junction

  • Synaptic terminal: End of nerve cell.

  • Synaptic cleft: Gap between nerve and muscle.

  • Acetylcholine (ACh): Neurotransmitter for contraction.

  • Acetylcholinesterase (AChE): Breaks down ACh.

  • Motor end plate: ACh receptor area.

  • Action potential: Electrical impulse in muscle.

Muscle Contraction Steps

  1. Action potential reaches synaptic terminal.

  2. Vesicles release ACh into synaptic cleft.

  3. ACh binds to receptors, causing Na+ influx.

  4. AChE breaks down ACh.

Contraction Cycle

  1. Active site exposure from Ca2+ binding to troponin.

  2. Myosin cross-bridge formation.

  3. Myosin head pivots, ADP and phosphate released.

  4. Cross-bridges detach upon ATP binding.

Muscle Tension

  • Twitch: Single contraction-relaxation sequence.

  • Summation of Twitches: Increased tension from successive contractions.

  • Incomplete Tetanus: Partial relaxation between stimuli.

  • Complete Tetanus: No relaxation, maximum tension.

Energy Sources

  • Cellular Respiration: ATP production via glycolysis (anaerobic) and Krebs cycle/electron transport chain (aerobic).

  • Creatine phosphate: Quick ATP supply during activity.

  • Oxygen debt: Recovery process post-exercise.

Types of Skeletal Muscle Fibers

  • Fast fibers: Quick response, large glycogen reserves, fewer mitochondria.

  • Slow fibers: Endurance, rich in mitochondria, extensive capillary network.

Physical Conditioning

  • Anaerobic endurance: Supported by glycolysis and ATP/CP reserves.

  • Aerobic endurance: Supported by mitochondrial activity.

Muscle Types

  • Cardiac Muscle: Involuntary, rhythmic contractions, relies on aerobic respiration, contains intercalated discs.

  • Smooth Muscle: Involuntary, found in organs, capable of sustained contraction.

Diseases of the Muscular System

  • Hernia: Protrusion of an organ through the muscle wall.

  • Tetanus: Bacterial infection causing stiffness.

  • Myasthenia Gravis: Autoimmune disorder affecting muscle strength.

  • Duchenne Muscular Dystrophy: Genetic disorder leading to muscle degeneration.