Human Anatomy Unit 2 MICROSCOPIC SKELETAL MUSCLE ANATOMY In Anatomy Today Skeletal Muscle Tissue • Characteristics – Striated – Multinucleate – Voluntary (somatic) • Anatomy – Muscle • Myofiber • Myofibril • Myofilament Muscle Microanatomy • Multinucleate – Develop from the fusion of multiple myoblasts – Each myoblast has its own nuclues • Myofiber – Muscle fiber – Muscle cell • Myofibril – Tubes of contractile proteins • Myofilament – Contractile proteins Muscle Fiber • Sarcolemma – Membrane of muscle fiber – Electrically excitable – T-tubules • Sarcoplasm – Muscle cytoplasm • Sarcoplasmic reticulum – Modified smooth ER – Lateral sacs – Stores calcium (Ca2+) • Functional unit of skeletal muscle is the sarcomere Sarcomere Structure • Z line – Defines the sarcomere • A band – Length of thick filament • H band – Space between thin filaments within a sarcomere • I band – Space between thick filaments between sarcomeres • Thick filament • Thin filament • Elastic filament Thin Filament • 2 strands of actin molecules – Has binding sites for myosin • Tropomyosin – Covers binding sites on actin for myosin – Resting fiber • Troponin – Holds tropomyosin in place – Binds to calcium (Ca2+) Thick and Elastic Filament • Thick Filament – Myosin – Tail – Myosin head • Myosin Head – 2 binding sites • ADP + pi • Actin • Elastic filament – Titin – Achors myosin in place within sarcomere Muscle Contraction • Sliding filament theory • Myosin heads bind to actin – 6 thin filaments: 1 thick filament • Heads change conformation and pull the actin filaments across myosin molecule – Discontinuous pulling • Sarcomere shortens – Thick and thin filaments do not shorten! • All or none response Neuromuscular Junction • Somatic motor neuron innervates muscle fiber – Synaptic bulb – Vesicles – Acetylcholine (ACh) • Motor end plate – Invagination of sarcolemma – Receptors for ACh • 1:1 ratio Motor Unit • A single motor neuron and all of the muscle fibers it innervates • A somatic motor neuron has collateral branches that innervate multiple muscle fibers • When a motor neuron sends a signal to contract – All fibers in the motor unit will contract • A muscle will be comprised of multiple motor units – Ratio of motor units:muscle Skeletal Muscle Neuromuscular Junction Cardiac Muscle Smooth Muscle.