David Sadava David M Hillis H Craig Heller

David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia. blu Anatomia e fisiologia dei viventi S

Musculoskeletal Systems

How Do Muscles Contract? Three types of vertebrate muscle: • Skeletal—voluntary movement, breathing, maintaining posture • Cardiac—beating of heart • Smooth—involuntary, movement of internal organs

How Do Muscles Contract? Skeletal muscle (striated): • Cells are called muscle fibers—large and multinucleate • One muscle consists of many muscle fibers bundled together by connective tissue

How Do Muscles Contract? Contractile proteins: Actin—thin filaments Myosin—thick filaments Each muscle fiber has many myofibrils—bundles of actin and myosin filaments. Each myofibril consists of sarcomeres—repeating units of overlapping actin and myosin filaments. Each sarcomere is bounded by Z lines which anchor the actin.

How Do Muscles Contract? Other parts of a sarcomere: • A band in center—contains myosin • H zone and I band—no overlap of actin and myosin in relaxed muscle • M band within H zone— contains proteins that anchor myosin filaments

The Structure of Skeletal Muscle

Sliding Filaments When a muscle contracts, sarcomeres shorten and band pattern changes.

How Do Muscles Contract? The sliding filament model of muscle contraction depends on structure of actin and myosin molecules. • Myosin: two polypeptide chains coiled together, ending in a globular head A myosin filament is many molecules in parallel with heads projecting sideways. • Actin filament: actin monomers in a long, twisted molecule Tropomyosin twists around actin with troponin attached at intervals.

Actin and Myosin Filaments Overlap to Form Myofibrils Contraction of the sarcomere involves many cycles of interaction between actin and myosin molecules.

How Do Muscles Contract? Muscle cells are excitable—the plasma membranes can conduct action potentials. Contraction is initiated by action potentials from a motor neuron at the neuromuscular junction. Motor unit—one motor neuron and all the muscle fibers it synapses with.

How Do Muscles Contract? At the neuromuscular junction, acetylcholine is the transmitter. It binds to receptors in the postsynaptic membrane, ion channels in the motor end plate open, Na+ flows in and the motor end plate is depolarized. Depolarization spreads; when threshold is reached, the muscle fiber membrane fires an action potential.

How Do Muscles Contract? Action potentials in muscle fiber also travel deep within the cell. The plasma membrane is continuous with T tubules that run through the sarcoplasm (muscle fiber cytoplasm). T tubules run close to the sarcoplasmic reticulum (muscle fiber ER) that surrounds every myofibril.

How Do Muscles Contract? An action potential spreads through the T tubules; when it reaches the receptor proteins they change conformation. This opens Ca 2+ channels and Ca 2+ flows out of the sarcoplasmic reticulum.

How Do Muscles Contract? Ca 2+ binds to troponin on the actin filaments—this twists the tropomyosin so that actin binding sites are exposed. When Ca 2+ pumps in the SR remove Ca 2+ from sarcoplasm, contraction stops.

How Do Muscles Contract? Smooth muscle—in most internal organs; under autonomic nervous system control. Smooth muscle cells are arranged in sheets and have electrical contact via gap junctions. Action potential in one cell can spread to all others in the sheet. The plasma membrane of smooth muscle cells is sensitive to being stretched.

How Do Muscles Contract? Cardiac muscle is also striated—cells are smaller than skeletal muscle and have one nucleus. This cells also branch and interdigitate; are resistant to tearing and can withstand high pressures. Intercalated discs provide mechanical adhesions between cells and allow cytoplasmic continuity and electrical coupling.

How Do Skeletal Systems and Muscles Work Together? The human skeleton has 206 bones. The axial skeleton includes skull, vertebral column, sternum, and ribs. Appendicular skeleton includes pectoral and pelvic girdles, and bones of the arms, legs, hands, and feet.

The Human Endoskeleton

How Do Skeletal Systems and Muscles Work Together? Vertebrate endoskeleton consists of two kinds of connective tissue: • Cartilage is found on bone surfaces in joints; also in ears, nose, and larynx. • Bone has an extracellular matrix of insoluble calcium phosphate crystals. It is the reservoir of calcium for the body, and is in equilibrium with soluble calcium in the extracellular fluids.

How Do Skeletal Systems and Muscles Work Together? Bone is constantly being replaced and remodeled by living bone cells. Osteoblasts make new bone matrix. When they become enclosed in bone they are called osteocytes. Osteoclasts are cells that reabsorb bone.

How Do Skeletal Systems and Muscles Work Together? Two types of bone development: Membranous bone forms on a scaffold of connective tissue (e. g. , outer bones of skull). Cartilage bone is first cartilaginous, then ossifies or hardens (e. g. , limb bones). Growth can occur throughout the ossification process.

How Do Skeletal Systems and Muscles Work Together? Bone structure: • Compact—solid and hard • Cancellous—with numerous cavities, appears spongy, lightweight but strong. Most bones have both types.

How Do Skeletal Systems and Muscles Work Together? Compact bone in mammals is called Haversian bone— the structural units are Haversian systems. Each system has concentric bony cylinders with osteocytes in between. Center canal has blood vessels and nerves. Adjacent systems are separated by glue lines.

How Do Skeletal Systems and Muscles Work Together? Joints are where two or more bones come together. Different types of joints allow motion in different directions.

How Do Skeletal Systems and Muscles Work Together? Muscles can exert force in only one direction—they create movement by working in antagonistic pairs. • Flexor—the muscle that bends or flexes the joint • Extensor—the muscle that straightens or extends the joint.

How Do Skeletal Systems and Muscles Work Together? • Ligaments are bands of connective tissue that hold bones together at joints • Tendons are connective tissue straps that join muscle to bone. Bones are a system of levers moved by muscles.

What Are the Different Types of Skeletal System? Skeletal systems are the rigid supports against which muscles can pull. Three types of skeletal systems: • Hydrostatic • Exoskeletons • Endoskeletons

What Are the Different Types of Skeletal System? Hydrostatic skeletons consist of fluid enclosed in a body cavity surrounded by muscle. When muscles oriented in one direction contract, the fluid-filled body cavity bulges out in the opposite direction. Examples: cnidarians, annelids, and other invertebrates.

What Are the Different Types of Skeletal System? An exoskeleton consists of a hardened outer surface to which muscles attach. Contractions of the muscles cause segments of the exoskeleton to move. Examples: mollusks, arthropods. The complex arthropod exoskeleton (or cuticle) covers all outer surfaces and appendages. It is made of chitin secreted by cells just below the exoskeleton. For growth to occur, the exoskeleton must be shed (molting).

What Are the Different Types of Skeletal System? The endoskeleton of vertebrates is an internal scaffold. An advantage is that growth can occur without shedding the skeleton.

Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved