Musculoskeletal System - Muscles Types Micro-anatomy Excitation Excitation-Contraction Coupling Contraction Properties of Whole Muscles Red vs. White Meat

Muscle Tissue Types All muscle cells contract and relax. 3 types: Skeletal Connected to bones Ex) biceps Striated Smooth Tapered Ex) stomach “involuntary” Cardiac Only present in the heart Have specialized junctions to propagate contraction.

Micro-Anatomy Muscle fiber = single muscle cell Muscle = many muscle fibers stuck together Tendons = link muscles to bones

Micro-Anatomy Myofibril Sarcomere = smallest unit of contraction Bundle of filaments within a muscle fiber Give the cell a striated appearance. One unit of these repeating fiber striations is called a sarcomere. Sarcomere = smallest unit of contraction

Micro- Anatomy - Sarcomere Z-line: dark zig-zag band marks the end of a sarcomere Actin (thin filaments): looks like two stranded pearls other proteins cover its grooves Myosin (thick filaments) bundled each has a long tail and a double head all pull on actin in the same direction http://www.classzone.com/cz/books/bio_09/resources/htmls/animated_biology/unit9/bio_ch33_1009_ab_muscle.html

Micro- Anatomy Tropomyosin Troponin A protein that covers myosin binding sites on actin. When on, a muscle cannot contract. Troponin regulatory protein bound to tropomyosin When calcium binds to it, it pulls tropomyosin off the actin filaments

Micro-Anatomy T-tubules Sarcoplasmic Reticulum extensions of the cell membrane that move into the cell and surround each myofibril Sarcoplasmic Reticulum A membrane like the ER that stores Calcium and surrounds each myofibril

Excitation of a Muscle Fiber Motor neurons synapse on muscle fibers and “tell” them when to contract. The muscle fiber membrane depolarizes and the action potential spreads over the muscle fiber and into the T-tubules

Excitation-Contraction Coupling Depolarization of T-tubule causes SR to release Calcium

Excitation-Contraction Coupling Ca+2 binds to troponin Troponin moves tropomyosin out of the way. Note: When nervous signal stops, the SR takes up the Ca+2 (uses ATP) When Ca+2 is gone, troponin/tropomyosin complex covers the binding sites again. http://www.youtube.com/watch?v=gJ309LfHQ3M&feature=related

Contraction Called the sliding filament model A cross-bridge forms when a myosin head attached to a binding site on actin. Cyclic Powered by ATP energy ATP split causes myosin to bind to actin. Energy release causes power stroke – myosin bends. Pulls actin filaments towards the center of the sarcomere. New ATP causes myosin to detach. Cycle begins again, a bit closer to the z-line. A single contraction is many power strokes. No ATP = rigor mortis

Contraction (Again)

Sequence of Events Between a Motor Neuron Action Potential & Skeletal Muscle Fiber Contraction

Properties of Whole Muscles Muscle tension mechanical force the muscle exerts on an object opposed by weight or gravity 3 scenarios: Isometric no sarcomere shortening muscle tension = load ex) holding a book out Isotonic sarcomere shortening; lifting a load muscle tension > load ex) lifting the book Lengthening sarcomere lengthens while holding a load muscle tension < load ex) putting down the book Isotonic Lengthening

Properties of Whole Muscles

Properties of Whole Muscles Motor unit: A neuron + all the muscle cells it controls Muscle twitch Electrical stimulus of a motor unit All-or-nothing The greater the force on an object, the more muscle units that are recruited.

Properties of Whole Muscles Treppe Constant stimulation causes muscle to “warm up” and strength of contraction increases. Due to increasing free Ca+2 Temporal Summation: Stimulus is added before complete relaxation of a motor unit. Additional influx of Ca+2 Tetany: Stimulus so common that contractions fuse into continuous contraction. Most common in movements of the body

Red vs. White Meat Red muscle White muscle Slow-twitch Slow to engage but long lasting High blood flow, mitochondria, and myoglobin = high ATP Good for distance sports White muscle Fast-twitch Rapid and powerful Not sustainable Low mitochondria and myoglobin Good for sprinter