Muscle Physiology Connective Tissue Components Muscle cell muscle

Muscle Physiology

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 1000 s 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 myosin’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 myosin muscle fiber returns to its resting length

http: //www. sci. sdsu. edu/movies/actin_myosin_gif. html

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”

http: //highered. mcgrawhill. com/sites/0072507470/student_view 0/chapter 9/

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 -7 A) • Breakdown of ATP allow myosin head to bind with actin and perform “power stroke” (11 -7 B-D) • ATP binds to return myosin head back to resting position

Alternate Source of Energy • ATP must be continually resynthesized • Breakdown of creatinephosphate (CP) provides energy for ATP resynthesis • 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 100 s of muscle fibers • A lower number of muscle fibers within a motor unit = more precise movement – Ex: hand vs abdomen

http: //natchem. files. wordpress. com/2009/11/motor-unit-lg. jpg

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

http: //www. cytochemistry. net/microanatomy/muscle/smooth 1. jpg

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