Skeleton and Movement

Skull, vertebral column, and thoracic cage

The axial skeleton consists of bones along the central axis of the body. Step 1: It includes the skull (22 bones), which protects the brain and forms the face. Step 2: The vertebral column (26 bones in adults) supports the body and protects the spinal cord. Step 3: The thoracic cage includes the sternum and ribs, protecting heart and lungs. Step 4: Arms, legs, and girdles belong to the appendicular skeleton, which attaches to the axial skeleton. The axial skeleton forms the central framework around which the body is built.

Sliding filament theory

The sliding filament theory explains how muscles contract at the molecular level. Step 1: This theory was proposed by H.E. Huxley and A.F. Huxley. Step 2: According to this theory, muscle contraction occurs when actin filaments slide over myosin filaments. Step 3: Myosin heads bind to actin filaments forming cross-bridges, then tilt to pull actin filaments inward. Step 4: This sliding action shortens the sarcomere without changing the length of individual filaments. Lock and key theory relates to enzyme action, fluid mosaic theory to cell membranes, and cell theory to basic cell principles.

Tendons

Tendons are the structures that connect muscles to bones. Step 1: Tendons are made of dense, fibrous connective tissue composed mainly of collagen fibers. Step 2: They are inelastic and very strong, allowing efficient transfer of force from contracting muscle to bone. Step 3: When muscles contract, tendons pull on bones to create movement. Step 4: Ligaments connect bone to bone at joints, cartilage covers bone surfaces at joints, and joints are the meeting points of bones - none of these directly connect muscle to bone like tendons do.

Atlas and axis

The atlas and axis vertebrae work together to allow head movements. Step 1: The atlas (1st cervical vertebra) connects to the skull via occipital condyles, forming the 'yes joint' that allows nodding up and down. Step 2: The axis (2nd cervical vertebra) has an odontoid process that fits into the atlas, forming the 'no joint' for side-to-side head rotation. Step 3: These specialized cervical vertebrae have unique structures different from other vertebrae. Step 4: Thoracic vertebrae connect to ribs, lumbar vertebrae support the lower back, and sacral vertebrae form the sacrum - none of these are