Backch 41
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Action and Support: The Muscles and Skeleton
Introduction
The musculoskeletal system provides the framework and mechanisms for movement, support, and protection in animals.
How Do Muscles Contract?
Types of Vertebrate Muscle
Skeletal muscle: Moves the skeleton, also called striated (striped). Most are attached to bones by tendons and are under voluntary control.
Cardiac muscle: Found only in the heart, striated, but involuntary and branched.
Smooth muscle: Found in walls of hollow organs and blood vessels, not striated, involuntary.
Structure of Skeletal Muscle
Skeletal muscles are encased in connective tissue sheaths that merge into tendons attaching muscle to bone.
Muscle fibers (cells) are grouped into bundles, each surrounded by connective tissue.
Muscle fibers contain multiple nuclei and are packed with myofibrils.
Muscle Fiber Anatomy
Each muscle fiber contains many cylindrical myofibrils, surrounded by the sarcoplasmic reticulum (SR), which stores calcium ions essential for contraction.
The plasma membrane forms T tubules that tunnel into the fiber, facilitating signal transmission.
Myofibrils and Sarcomeres
Myofibrils are composed of repeating units called sarcomeres, the functional units of muscle contraction.
Sarcomeres are aligned end to end and separated by Z discs.
Thick and Thin Filaments
Thin filaments: Composed mainly of actin, with regulatory proteins troponin and tropomyosin.
Thick filaments: Made of myosin, with heads that bind to actin and generate force.
Sliding Filament Mechanism
Muscle contraction occurs when myosin heads bind to actin, pulling thin filaments toward the center of the sarcomere.
This process shortens the sarcomere and contracts the muscle fiber.
ATP is required for myosin head movement and detachment from actin.
Equation: ATP hydrolysis by myosin:
Muscle Fiber Types: Fast-Twitch vs. Slow-Twitch
Slow-twitch fibers: Contract slowly, resist fatigue, rich in mitochondria, myoglobin, and capillaries; rely on aerobic respiration.
Fast-twitch fibers: Contract rapidly and powerfully, fatigue quickly, rely more on glycolysis, have fewer mitochondria and less myoglobin.
Neuromuscular Control of Contraction
Motor neurons stimulate muscle fibers at neuromuscular junctions by releasing acetylcholine, triggering an action potential in the muscle fiber.
The action potential travels along T tubules, causing the SR to release Ca2+ ions, which enable contraction by exposing myosin-binding sites on actin.
Relaxation occurs when Ca2+ is pumped back into the SR, allowing tropomyosin to block binding sites.
Motor Units and Force Generation
A motor unit consists of a motor neuron and all the muscle fibers it innervates.
Small motor units allow fine control (e.g., eye muscles); large units generate powerful movements (e.g., thigh muscles).
The force of contraction depends on the number and frequency of active motor units.
Cardiac and Smooth Muscle
Cardiac muscle: Striated, branched, single nucleus per cell, connected by intercalated discs for coordinated contraction; can contract without nervous input.
Smooth muscle: Not striated, single nucleus, found in walls of hollow organs and blood vessels; contracts slowly and involuntarily, regulated by stretch, hormones, and autonomic nervous system.
How Do Muscles and Skeletons Work Together to Provide Movement?
Types of Skeletons
Hydrostatic skeletons: Fluid-filled sacs or tubes (e.g., worms, cnidarians).
Exoskeletons: Rigid external skeletons (e.g., arthropods); movement occurs at joints, muscles attach internally.
Endoskeletons: Internal skeletons (e.g., vertebrates); movement at joints, muscles attach externally.
Antagonistic Muscles
Muscles work in pairs called antagonistic muscles (e.g., biceps and triceps) to move joints in opposite directions.
Muscles can only contract (pull), not actively lengthen.
Functions of the Vertebrate Skeleton
Provides support and protection for internal organs (e.g., skull, rib cage, vertebral column).
Enables locomotion by serving as a framework for muscle attachment.
Red bone marrow produces blood cells; bones store calcium and phosphorus.
Middle ear bones transmit sound vibrations.
Axial and Appendicular Skeleton
Axial skeleton: Skull, vertebral column, rib cage.
Appendicular skeleton: Pectoral and pelvic girdles, limbs.
Girdles link appendages to the body and provide muscle attachment sites.
Components of the Skeleton: Cartilage, Ligaments, and Bone
Cartilage: Flexible, composed of chondrocytes in a collagen and glycoprotein matrix; covers joint surfaces, forms parts of the nose, ears, and respiratory tract, and acts as a shock absorber.
Ligaments: Connect bone to bone at joints; composed of parallel collagen fibers for strength.
Bone: Rigid, composed of collagen matrix mineralized with calcium and phosphate; provides structure and support.
Bone Structure and Types
Spongy bone: Porous, lightweight, rich in blood vessels, found inside bones.
Compact bone: Dense, strong, forms the outer shell; organized into osteons with central canals for nerves and blood vessels.
Bone marrow: Red marrow (blood cell formation) in children, replaced by yellow (fatty) marrow in adults.
Bone Cells and Remodeling
Osteoblasts: Bone-forming cells.
Osteocytes: Mature bone cells, coordinate bone remodeling.
Osteoclasts: Bone-dissolving cells, break down bone matrix.
Bone remodeling: Continuous process of bone resorption and formation, allowing adaptation and repair.
Bone Growth and Repair
During development, cartilage is gradually replaced by bone (ossification).
Bone repair after fracture involves blood clot formation, callus formation (cartilage and bone), and remodeling by osteoblasts and osteoclasts.
Effects of Exercise on Bones and Connective Tissues
Exercise increases the strength and thickness of bones, tendons, and ligaments by stimulating collagen production and bone formation.
Disuse or weightlessness leads to bone loss.
Joints and Movement
Joints are points where two bones meet; movement occurs at movable joints.
Cartilage covers joint surfaces for smooth movement; ligaments hold bones together; tendons attach muscles to bones.
Origin: Fixed muscle attachment; Insertion: Movable attachment.
Flexor muscles bend joints; extensor muscles straighten them.
Types of Joints
Hinge joints: Allow movement in one plane (e.g., elbow, knee, fingers).
Ball-and-socket joints: Allow movement in multiple directions (e.g., hip, shoulder).
