Skeletal Muscle Refresher PDF

Skeletal Muscle Contraction Refresher Adapted from lecture by Dr. Jeanine Ursitti, PhD Assistant Professor [email protected] Lecture and slides modified from Liron Boyman, Roger A. Bannister and Christopher W. Ward ...

Skeletal Muscle Contraction Refresher Adapted from lecture by Dr. Jeanine Ursitti, PhD Assistant Professor [email protected] Lecture and slides modified from Liron Boyman, Roger A. Bannister and Christopher W. Ward  Lecture Outline Skeletal Muscle Contraction Muscle fiber Excitation- anatomy and Contraction contraction Regulation sarcomeric mechanism (EC) of force ultrastructure coupling Skeletal Muscle: Structure Across Scales Myofib ril Tendon Fascia Muscle T-tubule Sarcoplasm Fascicle ic Reticulum Sarcolemm a (SR) Muscle Fiber/ Muscle Cell  Muscle cell (fiber) anatomy multinuclear sarcolemma is the surface plasma membrane sarcolemma isolates individual fibers each fiber is innervated by a single motoneuron (MN) myofibrils are highly-organized bundles of myofilaments the sarcoplasmic reticulum (SR) surrounds individual myofibrils  Myofilaments: Thick and thin thick filaments are tail-to-tail myosin II multimers with radially protruding heads thin filaments are composed of 2 helical chains of actin monomers and associated regulatory proteins TEM X-section circles: thick filaments dots: thin filaments  Thick filament: myosin II multimers two heavy chains globular heads + rodlike tails the globular heads include: ATPase ©2018 by Cold Spring Harbor Laboratory Press actin-binding site two pairs of light chains essential regulatory  Thin Filament: actin and actin-binding proteins Actin – contains myosin binding site Tropomyosin –obstructs actin-myosin interaction Troponin complex 1. TnC – Ca2+ sensor 2. TnI – binds actin, inhibits actin-myosin interaction 3. TnT – binds tropomyosin Nebulin – limits F-actin length  Sarcomeric ultrastructure thin filaments are anchored at the Z-lines I-bands contain only thin filaments thick filaments align tail-to-tail at the M line in the H-zone thin and thick filaments overlap at the ends of the A-band Sliding filament mechanism relaxed A-bands are stable I bands shrink as actin slides into A-band Z-discs move closer together H-zone shrinks with greater actin-myosin overlap contracted  Lecture Outline Skeletal Muscle Contraction Muscle fiber Excitation- anatomy and Contraction contraction Regulation sarcomeric mechanism (EC) of force ultrastructure coupling  Ca2+ is the trigger for contraction muscle force production is dictated by myoplasmic [Ca2+] resting [Ca2+] = ~100 nM (no detectable force) maximal activation of myofilaments (force) is observed at ~10 mM [Ca2+] source of Ca2+ is the SR  Ca and the Troponin complex regulate 2+ access of myosin heads to actin 4 Ca2+ bind to TnC conformational rearrangement in Tn complex “rolls” tropomyosin deep into actin groove exposes the myosin binding site on actin cross-bridge cycling can now proceed freely  Cross-bridge cycling “slides” actin filaments When tropomyosin is out of the way, cross- bridge cycling can occur repeatedly a) myosin + ATP has low affinity for actin - no binding (1) b) myosin hydrolyzes ATP to (ADP + Pi) causing heads to cock (2) c) myosin (ADP + Pi) has a high affinity for actin – forms cross-bridge (3) d) Pi is kicked off causing the “power stroke” (4) e) ADP is released after the power stroke – retains affinity for actin (5) f) cycle is reset with ATP loading (1)  Isometric tension is maximal at sarcomere lengths with optimal myofilament overlap isometric = constant muscle length Lo = optimal length Lo Lo 14  Isotonic force-velocity relationship isotonic = constant opposing force V0 the bigger the opposing force (i.e., load), the slower contraction becomes isometric at V0 heavy loads stretch sarcomeres and oppose cross-bridge cycling/ filament sliding  Lecture Outline Skeletal Muscle Contraction Muscle fiber Excitation- anatomy and Contraction contraction Regulation sarcomeric mechanism (EC) of force ultrastructure coupling  Propagation of AP into transverse-tubules acetylcholine released from a motor neuron (MN) activates nicotinic receptors at the Neuromuscular Junction (NMJ) active zone elevated membrane potential activates Na+ channels on the sarcolemma an action potential (AP) is propagated on the sarcolemma and into the t-tubules the t-tubules are “tunnels” in which the AP is carried into the core and distal ends of the fiber  Structures underlying EC Coupling the t-tubules radiate into the muscle fiber to ensure fast and even contraction of a single fiber the SR (the Ca2+ store) surrounds a myofibril the t-tubules are flanked by SR terminal cisternae the t-tubules and SR interface at triad junctions  EC coupling occurs at triad junctions the unique structure of triad junctions enables physical coupling between the voltage-sensor (L-type Ca2+ channel; LTCC) and the SR Ca2+ release channel (ryanodine receptor) RyR1-/- TT SR Protasi et al. (2002) C. Franzini-Armstrong Ashcroft (2006) Nature  EC coupling: LTCCs and ryanodine receptors AP depolarization causes conformational changes in LTCC voltage-sensor these conformational changes are communicated to the ryanodine receptor in the SR via physical coupling opening of the ryanodine receptors provides a conduit for Ca2+ flux from SR into the myoplasm Ca2+ binds TnC, tropomyosin moves, etc. KG Beam SR [Ca2+] is ~1 mM  SERCA pumps Ca out of the myoplasm back 2+ into the SR during relaxation myoplasmic [Ca2+] reaches 10 mM+ during contraction SERCA returns [Ca2+] to ~100 nM Ca2+ dissociates from TnC and cross-bridge cycling ends uses ATP to drive pump against SR Ca2+ gradient generates heat Periasamy et al. Diabetes & Metabolism Journal 2017;41(5):327-336.  Lecture Outline Skeletal Muscle Contraction Muscle fiber Excitation- anatomy and Contraction contraction Regulation sarcomeric mechanism (EC) of force ultrastructure coupling Two types of summation regulate total force 1. Temporal: increased single fiber force 2. Spatial: increased # and size of motor units  Single fiber force is increased by increased MN firing frequency myoplasmic [Ca2+] declines to rest between well-spaced twitches – no summation more frequent input causes temporal summation because [Ca2+] is not entirely cleared between stimuli tetanus occurs at high frequencies  Muscle force is proportional to the # of activated motor units a single MN and the muscle fibers that it innervates are called a motor unit motor units fire “all or none” spatial summation-muscle can produce a graded range of force and shortening by recruiting more (or less) motor units  Henneman’s size principle smaller motor neurons serve motor units with fewer fibers (fine tasks) 100 % Maximal Force larger motor neurons serve motor small units with more fibers (running, 50 motor units jumping, etc.) large motor smaller motor neurons recruited units first, then larger motor neurons with 0 0 50 100 stronger stimuli % of Motor Units Summary skeletal muscle anatomy is uniquely purposed for graded function EC coupling in skeletal muscle relies on physical coupling between LTCCs and ryanodine receptors contraction is triggered by Ca2+ release from the SR via ryanodine receptors actin thin filaments “slide” past myosin thick filaments to shorten sarcomeres. Myosin is responsible for converting chemical energy into mechanical energy frequency and spatial summation determine whole muscle force