Musculoskeletal System

Musculoskeletal System

Introduction

·       The musculoskeletal system is the organ system that provides the human body with support, protection, movement, and posture.

·       It is made up of bones (skeleton), joints, muscles, tendons, ligaments, and cartilage.

·       Bones form the rigid framework (skeleton) of the body.

·       Joints connect bones and allow movement.

·       Muscles contract and generate force to bring about movement, stabilize joints, and maintain posture.

·       Together, this system not only enables locomotion but also plays important roles in hematopoiesis (blood cell formation), mineral storage (calcium & phosphorus), and protection of vital organs.

Skeleton

The skeleton is the bony framework of the body. An adult human skeleton consists of 206 bones.

Functions of the Skeleton

1. Support – Provides a rigid framework to support the body and soft tissues.
2. Protection – Protects delicate organs (skull protects brain, ribs protect heart and lungs, vertebrae protect spinal cord).
3. Movement – Provides levers for muscles to act upon.
4. Mineral Storage – Reservoir for calcium, phosphorus, and other minerals.
5. Blood Cell Production – Red bone marrow produces red blood cells, white blood cells, and platelets.
6. Fat Storage – Yellow marrow stores fats as an energy reserve.

Classification of Bones

1. Long bones – e.g., humerus, femur, tibia.
2. Short bones – e.g., carpals, tarsals.
3. Flat bones – e.g., skull bones, sternum, ribs, scapula.
4. Irregular bones – e.g., vertebrae, hip bone.
5. Sesamoid bones – Small, round bones embedded in tendons, e.g., patella.

Structure of a Long Bone

• Diaphysis (shaft): Long, cylindrical, made of compact bone, encloses medullary cavity with yellow marrow.
• Epiphysis (ends): Expanded ends, made of spongy bone filled with red marrow, covered by articular cartilage.
• Metaphysis: Region between diaphysis and epiphysis; contains growth plate (epiphyseal plate) in children.
• Periosteum: Outer fibrous covering, richly supplied with nerves and blood vessels; essential for bone growth and repair.
• Endosteum: Inner membrane lining medullary cavity.
• Articular cartilage: Hyaline cartilage covering epiphysis; reduces friction and absorbs shock.

Axial and Appendicular Skeleton

Axial Skeleton (80 bones)

Forms the central axis of the body.

• Skull (22 bones): Cranial (8) + Facial (14).
• Hyoid bone (1).
• Auditory ossicles (6): Malleus, incus, stapes.
• Vertebral column (26): Cervical (7), Thoracic (12), Lumbar (5), Sacrum (1), Coccyx (1).
• Thoracic cage (25): Sternum (1) + Ribs (24).

Appendicular Skeleton (126 bones)

Responsible for movement and locomotion.

• Pectoral girdle (4): Clavicle (2), Scapula (2).
• Upper limbs (60): Humerus, radius, ulna, carpals, metacarpals, phalanges.
• Pelvic girdle (2): Hip bones (coxal bones).
• Lower limbs (60): Femur, tibia, fibula, patella, tarsals, metatarsals, phalanges.

Sutures

• Definition: Immovable fibrous joints found between skull bones.
• Types:
• Coronal (between frontal and parietal bones).
• Sagittal (between two parietal bones).
• Lambdoid (between parietal and occipital bones).
• Squamous (between parietal and temporal bones).

Fontanelles

• Soft membranous gaps between cranial bones in infants, allowing skull growth and passage during birth.
• Types:
• Anterior (largest, closes at ~18 months).
• Posterior (closes at ~2–3 months).
• Anterolateral (sphenoidal, closes at ~6 months).
• Posterolateral (mastoid, closes at ~1 year).

Sinuses (Paranasal Sinuses)

• Air-filled cavities within cranial bones, lined by mucous membrane.
• Functions: Lighten skull, resonate voice, humidify air.
• Types: Frontal, Ethmoidal, Maxillary, Sphenoidal.

Ribs

• 12 pairs forming thoracic cage.
• True ribs (1–7): Attach directly to sternum.
• False ribs (8–10): Attach indirectly via costal cartilage.
• Floating ribs (11–12): No anterior attachment.

Vertebral Column

• Central support of the body, protects spinal cord.
• Regions:
• Cervical (7)
• Thoracic (12)
• Lumbar (5)
• Sacral (5 fused)
• Coccygeal (4 fused)

Girdles

• Pectoral Girdle: Clavicle + Scapula; attaches upper limb to trunk.
• Pelvic Girdle: Formed by 2 hip bones (ilium, ischium, pubis fused); supports weight and protects pelvic organs.

Joints

Joints (articulations) are sites where two or more bones meet.

Functions of Joints

• Permit movement.
• Provide stability.
• Absorb shock.

Classification of Joints

1. Structural Classification
• Fibrous joints (immovable): Sutures, syndesmosis, gomphosis.
• Cartilaginous joints (slightly movable): Symphysis (pubic symphysis), synchondrosis (epiphyseal plate).
• Synovial joints (freely movable): Most common type.
2. Functional Classification
• Synarthrosis: Immovable.
• Amphiarthrosis: Slightly movable.
• Diarthrosis: Freely movable (synovial).

Types of Synovial Joints & Movements

• Ball and Socket: Shoulder, hip – movements in all directions.
• Hinge: Elbow, knee – flexion, extension.
• Pivot: Atlas-axis joint – rotation.
• Condyloid: Wrist – flexion, extension, abduction, adduction.
• Saddle: Thumb joint – wide range of motion.
• Gliding/Plane: Intercarpal joints – sliding movement.

Muscles

Human body has ~600 muscles, forming nearly 40–50% of body weight.

Classification of Muscles

1. Skeletal muscle: Voluntary, striated, attached to bones.
2. Cardiac muscle: Involuntary, striated, forms heart wall.
3. Smooth muscle: Involuntary, non-striated, in viscera (stomach, intestines, blood vessels).

Properties of Muscle Tissue

• Excitability: Ability to respond to stimuli.
• Contractility: Ability to shorten and generate force.
• Extensibility: Ability to be stretched.
• Elasticity: Ability to return to original shape.

Structure of Skeletal Muscle

• Muscle fibers: Long cylindrical cells with multiple nuclei.
• Sarcolemma: Plasma membrane of muscle fiber.
• Sarcoplasm: Cytoplasm with myofibrils, mitochondria, glycogen, myoglobin.
• Myofibrils: Contain contractile proteins – actin (thin filament) and myosin (thick filament).
• Sarcomere: Functional unit of muscle, between two Z-lines.
• Bands:
• A-band (dark, myosin + actin overlap).
• I-band (light, actin only).
• H-zone (myosin only).
• M-line (middle of sarcomere).

Types of Skeletal Muscles (Based on Shape)

• Fusiform (biceps brachii)
• Pennate (rectus femoris)
• Circular (orbicularis oculi)
• Flat (external oblique)

Functions of Muscles

• Produce movement.
• Maintain posture and stabilize joints.
• Generate heat (thermogenesis).
• Aid circulation and respiration.

Mechanism of Muscle Contraction (Sliding Filament Theory)

1. Nerve impulse arrives at neuromuscular junction → acetylcholine released → action potential in muscle fiber.
2. Calcium ions released from sarcoplasmic reticulum.
3. Calcium binds to troponin, causing tropomyosin to shift, exposing actin binding sites.
4. Myosin heads attach to actin (cross-bridge formation).
5. Power stroke: Myosin heads pull actin filaments toward center of sarcomere using ATP.
6. ATP binds again → myosin detaches → cycle repeats.
7. When stimulation ceases, calcium reabsorbed, tropomyosin covers actin sites → muscle relaxes.

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