Muscular Contraction

Characteristics of Muscles Muscle fibres are elongated Contraction of muscles is due to the movement of microfilaments All muscles share some terminology: Prefix myo refers to muscle Prefix mys refers to muscle Prefix sarco refers to flesh

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue

Connective Tissue Wrappings of Skeletal Muscle Endomysium = around single muscle fibre Perimysium = around a fascicle (bundle) of fibres

Connective Tissue Wrappings of Skeletal Muscle Epimysium = covers the entire skeletal muscle Fascia = on the outside of the epimysium

Gross Structure of Skeletal Muscle Skeletal muscles are encased by epimysium: fascia of fibrous connective tissue. A bundle of cylindrical muscle fibres is surrounded by the perimysium. Endomysium surrounds each muscle fibre. Sarcolemma beneath endomysium is a thin, elastic membrane. epi: on; peri: around; endo: inner

Microscopic Anatomy of Skeletal Muscle Cells are multinucleate Nuclei are just beneath the sarcolemma

Microscopic Anatomy of Skeletal Muscle Sarcolemma – specialized plasma membrane Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum (network of membranous tubules within the cytoplasm )

Sarcoplasm (cytosol) is the fluid portion in muscle fibres. Contains contractile proteins, nuclei, mitochondria, sarcoplasmic reticulum.

Microscopic Anatomy of Skeletal Muscle Myofibril Bundles of myofilaments Myofibrils are aligned to give distinct bands I band = light band A band = dark band

Muscle Structure Each myofibril is made up of parallel filaments. There are 2 kinds of filament called thick & thin filaments. These 2 filaments are linked at intervals called cross bridges, which stick out from the thick filaments

Microscopic Anatomy of Skeletal Muscle Sarcomere Contractile unit of a muscle fibre

Microscopic Anatomy of Skeletal Muscle Organization of the sarcomere: Thick filaments = myosin filaments Composed of the protein myosin Has ATPase enzymes

Microscopic Anatomy of Skeletal Muscle Organization of the sarcomere Thin filaments = actin filaments Composed of the protein actin

Distinguished area between Z lines. Thicker filaments confined to A band, a lighter middle region called the H zone. Thinner filaments arise in middle region of I band, at the Z line. During contraction, neither thick nor thin filaments change in length. They slide.

Microscopic Anatomy of Skeletal Muscle Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap

Microscopic Anatomy of Skeletal Muscle At rest, there is a bare zone that lacks actin filaments Sarcoplasmic reticulum (SR) – for storage of calcium

Sarcomere H zone A band I band