Structure & Function of Skeletal Muscles: Motor Units to Fibers & Contraction, Study notes of Biological Sciences

Download Structure & Function of Skeletal Muscles: Motor Units to Fibers & Contraction and more Study notes Biological Sciences in PDF only on Docsity! 1 1 The Motor Unit and Muscle Action Lu Chen, Ph.D. MCB, UC Berkeley 2 Three types of muscles Smooth muscle: internal actions such as peristalsis and blood flow. Cardiac muscle: pumping blood. Skeletal muscle: moving bones. A motor unit consists of a motor neuron and the muscle fibers that it innervates. Skeletal muscle fibers are innervated by α-motor neurons. 2 3 The Skeletal Muscle - Organization Muscle Fascicles Muscle Fibers Myofibrils Thick filament (myosin) Thin filament (f-actin, tropomyosin, troponin) 50-100 um 2-6 cm 4 Muscle Unit Ensemble of muscle fibers innnervated by the same motor neuron Motor Unit A muscle unit together with the motor neuron that innervates it. Except during development, each muscle fiber is only innervated by one motor neuron in one place (in mammals). The Sarcomere is the contractile Unit of the Striated Muscle The Skeletal Muscle - Organization 5 9 Excitation – Contraction Coupling II: The cross-bridge cycle Attached (rest) Head rotation and force generation ATP-binding ATP hydrolysis and Recock of myosin head Cross bridge formation 10 Excitation – Contraction Coupling II: The cross-bridge cycle 6 11 Type of contractions 1 - isotonic - tension or force generated by the muscle is greater than the load & the muscle shortens (successfully lift up a bucket or water) 2 - isometric - load is greater than the tension or force generated by the muscle & the muscle does not shorten (tried to lift up a bucket of water but failed) Each single AP of the nerve elicit a single muscle twitch, which contains: Latent period Contraction period Relaxation period 12 1- The frequency of the action potentials fired 2- The muscle fibers’ lengths (which depend on the initial overlap between thick and thin fibers) The force of the contraction of a muscle depends on: Passive force: generated when lengthening the connectin filaments that tether the thick filaments between the z disks Active force: dependent on the degree of overlap of thick and thin filament. 10 g 7 13 Three types of muscle units Type 1 Type 2A Type 2B 40 pps for 0.33 s, repeated every second 13 pps 20 pps 25 pps 14 intermediatelargesmallForce large (but maybe not as large as type IIB) largesmallFiber Size rungallop, jumpposture‘typical usage’ fatigue resistant (though less than type I) easy fatigableFatigue resistantFatigue moderate to highlowhighOxidative capacity intermediatevery littlemanyMitochondria fastfastslow Contraction Speed fast-fatigue resistant (type IIA) fast-fatigable (type IIB)Slow –twitch fibers (type I) Fiber type WhiteRed The MN that innervates fast-twitch muscle fibers usually innervates many large fibers. These MN have relatively large cell bodies and large axon diameters, so they can conduce action potentials fast. (the opposite is true for those MN that innervate the small fibers) Types of Striated Muscle/ Types of Muscle Fibers: