14.1 Support and Movement in Humans and Animals
Movement is a key characteristic of living organisms. Humans and vertebrates rely on an internal skeletal system (endoskeleton) integrated with skeletal muscles, tendons, and ligaments to facilitate movement and maintain body posture.
Types of Skeletal Systems in Animals
- Exoskeleton: Hard outer skeleton composed of chitin or calcium carbonate found in invertebrates (e.g., insects, crustaceans). Provides protection, prevents desiccation, and serves as an attachment point for muscles.
- Endoskeleton: Rigid internal framework composed of bone and cartilage found in vertebrates (e.g., mammals, birds, fish). Supports weight, protects internal organs, and enables movement.
- Hydrostatic Skeleton: Fluid-filled body cavity (coelom) under pressure found in soft-bodied invertebrates (e.g., earthworms, jellyfish). Provides support through fluid pressure and shape retention.
14.2 Human Musculoskeletal System
The human skeleton consists of 206 bones divided into the Axial Skeleton and Appendicular Skeleton.
1. Axial Skeleton
- Skull: Cranium protects the brain; facial bones protect sensory organs.
- Vertebral Column: Composed of 33 vertebrae (7 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral fused into sacrum, 4 Coccygeal fused into coccyx). Protects the spinal cord and supports body weight.
- Rib Cage & Sternum: Protects the heart and lungs, plays a role in breathing mechanisms.
2. Appendicular Skeleton
- Pectoral Girdle: Scapula and clavicle; connects upper limbs to the axial skeleton.
- Pelvic Girdle: Hip bones (ilium, ischium, pubis); supports body weight and connects lower limbs.
- Limbs: Upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges) and lower limbs (femur, tibia, fibula, patella, tarsals, metatarsals, phalanges).
Tissues Involved in Movement
- Skeletal Muscles: Voluntary muscle tissue arranged in antagonistic pairs. Attached to bones to generate force via contraction.
- Tendons: Tough, non-elastic connective tissue that attaches muscle to bone. Transmits mechanical force to move bones.
- Ligaments: Tough, elastic connective tissue that connects bone to bone. Holds joints together and prevents dislocation.
- Cartilage: Smooth, flexible tissue covering bone ends at joints. Reduces friction and acts as a shock absorber.
- Synovial Fluid: Lubricating fluid secreted by the synovial membrane inside joint cavities to reduce friction.
14.3 Movement and Locomotion
Movement of body parts relies on the action of antagonistic muscle pairs (when one muscle contracts, the other relaxes).
1. Human Arm Movement (Bending and Straightening)
- Bending (Flexion): Biceps contracts (flexor muscle) while triceps relaxes (extensor muscle). Tendon pulls the radius bone upward, bending the arm at the elbow joint.
- Straightening (Extension): Triceps contracts while biceps relaxes. Tendon pulls the ulna bone downward, extending the forearm.
2. Human Leg Movement (Walking Mechanism)
- Calf Muscle (Gastrocnemius): Contracts to lift the heel off the ground.
- Biceps Femoris (Hamstring): Contracts to bend the knee and lift the leg off the ground.
- Quadriceps Femoris: Contracts to extend the knee and straighten the leg forward to land on the ground.
3. Locomotion in Animals
- Earthworm: Uses a hydrostatic skeleton and alternating peristaltic contractions of circular and longitudinal muscles assisted by chaetae (bristles).
- Circular muscles contract + Longitudinal muscles relax: Segment becomes long and thin.
- Longitudinal muscles contract + Circular muscles relax: Segment becomes short and thick.
- Grasshopper: Uses an exoskeleton and powerful antagonistic leg muscles (flexor and extensor muscles) attached inside the hind legs to execute jumps.
- Fish: Uses an endoskeleton and stream-lined body with antagonistic myotome muscle blocks on alternate sides of the vertebral column for undulatory swimming motion.
- Bird: Uses a lightweight hollow skeleton and large antagonistic flight muscles attached to the sternum (pectoralis major pulls wings down; pectoralis minor pulls wings up).
14.4 Health Issues Related to the Musculoskeletal System
- Osteoporosis: Degenerative bone condition characterized by loss of bone density and mass, making bones fragile and brittle. Commonly caused by calcium/vitamin D deficiency and declining estrogen levels in postmenopausal women.
- Osteoarthritis: Wear and tear of joint cartilage at bone ends over time, leading to painful friction and inflammation during movement.
- Rickets & Osteomalacia: Softening and weakening of bones caused by severe vitamin D or calcium deficiency (rickets in children leads to bowed legs).
- Scoliosis: Abnormal lateral curvature of the vertebral column into an "S" or "C" shape.
- Gout: Form of arthritis caused by hyperuricemia (excess uric acid crystals depositing in joint cavities, especially the big toe).
- Muscle Cramp: Sudden, involuntary, and painful contraction of skeletal muscle caused by electrolyte imbalance or lactic acid accumulation.