Muscle Physiology Chapter 11

Connective Tissue Components Muscle cell = muscle fiber Endomysium – covers muscle fiber Perimysium – binds groups of muscle fibers (fasicles) Epimysium – covers the entire muscle Tendon – fibrous tissue that connects muscle to bone Aponeurosis – broad, flat sheet of connective tissue Fascia – fibrous CT surrounding muscle and tendon

General Function of Muscles Movement –Excitability –Contractility –extensibility Heat Production Posture

Overview of Muscle Cell Muscle cell = muscle fiber Sarcolemma = plasma membrane Sarcoplasm = cytoplasm Sarcoplasmic reticulum (SR) = network of tubules and sacs Multi-nucleated, multiple mitochondrion Bundles of myofibrils extend lengthwise & fill sarcoplasm –Composed of thick and thin myofilaments

Sarcomere Contractile unit of a muscle fiber each myofibril consists of many sarcomeres Z line –Anchors thin filaments –Boundary of sarcomere M line – anchors thick filaments A band: segment of thick & thin filaments I band: segment of thin filaments H zone: where thin and thick filaments will not overlap (only thick)

Sarcomere cont… Elastic filaments – connect thick filaments to Z line T (transverse) tubules – allows impulses traveling along sarcolemma to move deeper within the cell Triad – t tubule sandwiched between sacs of the SR –Allows impulses traveling along a t tubule to stimulate sacs of the SR

Myofilaments Myofibrils – made up of 1000s of thin and thick myofilaments Thin filaments –Actin –Tropomyosin –Troponin Thick filaments –myosin

Muscle Excitation Nerve impulse reaches the end of a motor neuron  releases acetylcholine (Ach) Ach diffuses across the neuromuscular junction and binds with the receptors on the motor endplate

Muscle Contraction Impulses travel along the sarcolemma  t tubules  sacs of SR Ca 2+ is released into the sarcoplasm  binds with troponin on thin myofilaments Tropomyosin shift to expose actin’s active site Energized myosin heads bind with actin’s active site and pulls thin filament towards center of sarcomere –Requires ATP

Muscle Relaxation Nerve impulse is complete  Ca 2+ is pumped back into the sacs of the SR Ca 2+ is stripped from the troponin  tropomyosin covers the actin’s active site Myosin heads can no longer bind with actin  muscle fiber returns to its resting length

Rigor Mortis “stiffness of death” SR releases excess Ca 2+  myosin heads bind with actin’s active sites  contraction of myofilaments Lack of ATP after death causes cross bridges to “stick”

hill.com/sites/ /student_view0/chapter9/

Sliding Filament Theory In fully contracted muscle: H zone disappears I band narrows A band remains the same

Energy for Contractions Hydrolysis (breakdown) of ATP –ATP  ADP (breaking high energy bond btwn 2 nd and 3 rd phosphate groups) ATP binds  myosin head moves to resting position (11-7A) Breakdown of ATP allow myosin head to bind with actin and perform “power stroke” (11-7B-D) ATP binds to return myosin head back to resting position

Alternate Source of Energy ATP must be continually re- synthesized Breakdown of creatine- phosphate (CP) provides energy for ATP re- synthesis Catabolism of food provides energy for ATP and CP synthesis

Oxygen & Glucose O 2 and glucose are the starting materials for cellular respiration (process that makes ATP) During rest oxygen is stored in myoglobin – Supplies muscle fibers with oxygen during period of exercise – High amounts of myoglobin = red fibers = slow twitch fibers – Low levels of myoglobin = white fibers = fast twitch fibers

Aerobic vs. Anaerobic Respiration Aerobic Respiration –Oxygen-requiring process –Produces maximum amount of ATP from one glucose molecule Anaerobic Respiration –Does not require oxygen –Short-term, rapid process to re-synthesize ATP –Produces lactic acid –Burning/soreness in muscles

Heat Production Some energy from catabolic processes is lost as heat Muscle release massive amts of heat Thermoreceptors sense decrease in body temp  hypothalamus integrates information  signal sent to skeletal muscle to contract  shivering  homeostatic balance is maintained

Motor Unit Motor unit = motor neuron + muscle fibers it attaches to Motor neurons can innervate few to 100s of muscle fibers A lower number of muscle fibers within a motor unit = more precise movement –Ex: hand vs abdomen

Isotonic vs Isometric Contractions Isotonic – tension remains the same; length of the muscle changes –Concentric contraction: muscle shortens (contracts) –Eccentric contraction : muscle lengthens Isometric – tension changes; length of the muscle remains the same –Myosin heads unable to move thin filaments –Static tension

Smooth Muscle Contractions Small tapered cell w/ single nuclei No t-tubules; loosely organized SR No sarcomeres –Contract to shorter lengths –Myofilaments crisscross (balled up appearance when contracted) Calcium binds to calmodulin

Smooth Muscle Tissue Types 1. Visceral –Gap junctions connect smooth muscle fibers into sheets –Forms inner muscular layer of hollow structures –Exhibits autorhythmicity Peristalsis, excretion of urine, childbirth, mixing of stomach contents 2. Multiunit –Composed of many single-cell units –Ex: arrector pili muscles, lines blood vessels