Human Physiology - Muscle and motility (HL)

Antagonistic Muscle Pairs: Muscles can only exert force by contracting (shortening). Therefore, they work in pairs; while one muscle (agonist) contracts, the other (antagonist) relaxes to return the bone to its original position. Example: Biceps and Triceps.

Structure of Skeletal Muscle: Muscles are composed of bundles of muscle fibers. Each fiber is a single multinucleated cell containing specialized organelles called myofibrils, which are made of repeating units called sarcomeres.

Sarcomere Components: The sarcomere is the functional unit of contraction. It consists of thin actin filaments and thick myosin filaments. It is defined by $Z$-lines at each end, an $A$-band (length of myosin), an $I$-band (actin only), and an $H$-zone (myosin only).

The Role of $Ca^{2+}$ and Regulatory Proteins: In a relaxed state, tropomyosin blocks myosin-binding sites on actin. When an action potential stimulates the sarcoplasmic reticulum, $Ca^{2+}$ ions are released. $Ca^{2+}$ binds to troponin, causing a conformational change that moves tropomyosin away from the binding sites.

The Sliding Filament Theory: Muscle contraction occurs as actin filaments slide over myosin filaments. This is driven by the 'cross-bridge cycle' involving the hydrolysis of $ATP$.

ATP Hydrolysis in Contraction: $ATP$ binds to the myosin head, breaking the cross-bridge. $ATP$ is then hydrolyzed to $ADP$ and inorganic phosphate ($P_i$), 'cocking' the myosin head into a high-energy state for the next power stroke.