David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu C – Il corpo umano Musculoskeletal System

How do muscles contract? What roles do skeletal systems play in movement? Musculoskeletal System

Muscles and skeletons are the effectors that produce movement. Three types of vertebrate muscle: skeletal - voluntary movement, breathing; cardiac - beating of heart; smooth - involuntary, movement of internal organs. Musculoskeletal System - How do muscles contract?

Skeletal muscle (striated): multinucleate cells are called muscle fibers; form from fusion of embryonic myoblasts; one muscle consists of many muscle fibers bundled together by connective tissue.

Musculoskeletal System - How do muscles contract? The structure of skeletal muscle (part 1)

Musculoskeletal System - How do muscles contract? Contractile proteins: actin - thin filaments; myosin - thick filaments. Each muscle fiber has several myofibrils: bundles of actin and myosin filament.

Musculoskeletal System - How do muscles contract? Each myofibril consists of repeating units: sarcomeres. Sarcomere: overlapping actin and myosin filaments. Bundles of myosin filaments are held in place by the protein titin, the largest protein in the body.

Musculoskeletal System - How do muscles contract? The structure of skeletal muscle (part 2)

Musculoskeletal System - How do muscles contract? The structure of skeletal muscle (part 3)

Musculoskeletal System - How do muscles contract? The sliding filament theory of muscle contraction: depends on structure of actin and myosin molecules; myosin heads can bind specific sites on actin molecules to form cross bridges, myosin changes conformation causing actin filament to slide 5–10 nm.

Musculoskeletal System - How do muscles contract? Sliding filaments

Musculoskeletal System - How do muscles contract? Actin and myosin filaments overlap to form myofibrils

Musculoskeletal System - How do muscles contract? Action potentials also travel deep within muscle fiber via T tubules. T tubules (transverse tubules) descend into the sarcoplasm (muscle fiber cytoplasm). T tubules run close to the sarcoplasmic reticulum (ER): a closed compartment that surrounds every myofibril.

Musculoskeletal System - How do muscles contract? T tubules in action

Musculoskeletal System - How do muscles contract? Sarcoplasmic reticulum has Ca 2+ pumps. At rest there is high concentration of Ca 2+ in the sarcoplasmic reticulum. Action potential reaches receptor proteins and opens the Ca 2+ channels, Ca 2+ flows out of sarcoplasmic reticulum and triggers interaction of actin and myosin.

Musculoskeletal System - How do muscles contract? Actin filaments also include tropomyosin and troponin. Troponin has three subunits: one binds actin, one binds myosin, and one binds Ca 2+. At rest, tropomyosin blocks the binding sites on actin.

Musculoskeletal System - How do muscles contract? When Ca 2+ is released, it binds to troponin, which changes conformation. Troponin is bound to tropomyosin— twisting of tropomyosin exposes binding sites on actin. When Ca 2+ pumps remove Ca 2+ from sarcoplasm, contraction stops.

Musculoskeletal System - How do muscles contract? The release of Ca 2+ from the sarcoplasmic reticulum triggers muscle contraction

Musculoskeletal System - How do muscles contract? Cardiac muscle is also striated; cells are smaller than skeletal and have one nucleus. Cardiac muscle cells also branch and interdigitate: can withstand high pressures. Intercalated discs provide mechanical adhesions between cells.

Musculoskeletal System - How do muscles contract? There are three kinds of muscle

Musculoskeletal System - What roles do skeletal systems play in movement? The human skeleton Human skeleton: 206 bones, 2 connective tissue types (cartilage and bone).

Musculoskeletal System - What roles do skeletal systems play in movement? Cartilage cells produce a tough, rubbery extracellular matrix of polysaccharides and protein, mostly collagen. Cartilage is found on bone surfaces in joints, also ears, nose, larynx.

Musculoskeletal System - What roles do skeletal systems play in movement? Bone: extracellular matrix of calcium phosphate. Bone cells: osteoblasts make new bone matrix, when they become enclosed in bone they are called osteocytes; osteoclasts reabsorb bone. Bone is constantly being replaced and remodeled.

Musculoskeletal System - What roles do skeletal systems play in movement? Types of joints

Musculoskeletal System - What roles do skeletal systems play in movement? Joints, ligaments, and tendons

Musculoskeletal System - What roles do skeletal systems play in movement? Bones are a system of levers moved by muscles. Levers have a power arm and a load arm that work around a fulcrum (or pivot point).

Musculoskeletal System - What roles do skeletal systems play in movement? Bones and joints work like systems of levers