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.