Lecture 2 - Muscle Structure and Function - Part 1 PDF

Skeletal muscle and motor unit structure Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Review basic anatomy and organization of skeletal muscle and motor neuron Understand how intrinsic and extrinsic muscle structure influences the muscle for...

Skeletal muscle and motor unit structure Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Review basic anatomy and organization of skeletal muscle and motor neuron Understand how intrinsic and extrinsic muscle structure influences the muscle force output Describe the structure and function of the motor neuron Explain the neuromuscular junction and its role in excitation-contraction coupling Brief Review of Skeletal Muscle Skeletal Muscle: a highly organized tissue (collection of similar cells) with a unique set of properties. ~40% of body mass. What is it good for? 1. thermogenesis (heat production) 2. protein storage - metabolism 3. transform electrochemical Energy mechanical work to generate force (tension) by contraction, and ultimately to produce movement. Factors that affect Muscle Force/Torque output Fibre type composition Muscle mass Cross-bridge function Passive stiffness Torque generating capacity Muscle geometry intrinsic extrinsic Antagonist activation Agonist activation Activation history Sarcomere (repeating functional unit) Thick and thin filaments Two large polypeptide heavy chains + four smaller light chains Heavy chain Two binding sites on globular head 1 for ATP 1 for actin Light chain Specialized proteins for stability Site of regulation (e.g., muscle potentiation) Polymer (polypeptide of globular actin) Each has binding site for myosin Intertwined helical chain Regulatory proteins troponin and tropomyosin Regulate the exposure of the myosin binding site for crossbridge cycling (in response to calcium) Thick filament structure ~300 myosin filaments arranged Tail portion of heavy chains combine to form the thick filaments Myosin heads pointing I opposite directions Anchored to z-line by protein Titin Sarcomere Functional unit of skeletal muscle Smallest component that can contract in myofibre Thin filaments Polymerized actin filament Regulatory molecules Troponin and tropomyosin In pairs, attached to Z-line Thick filaments Chains of intertwined myosin Two binding sites/myosin head One for actin One for ATP Interaction between thick and thin filaments = crossbridge force generation mechanism Sarcomere A band Centre of sarcomere where thick filaments span Wide dark band in EM images I band Between the ends of A band No overlap with thick filaments Light band in EM images Z line Extremities of sarcomere Anchor for many regulatory proteins (notably thin filaments) ~2.2 µm in length H zone Centre of A band without any thin filament overlap A new model of sliding-filament theory? The 3 filament model Sarcoplasmic reticulum and ttubular system SR enlarged near t-tubule, termed terminal cisternae T-tubules are invaginations of the sarcolemma at regular intervals Physical proximity of t-tubule, SR, and myofibirils….why? Why the close interaction between t-tubular system, sarcoplasmic reticulum, and contractile machinery?? Calcium regulatory sites for contraction! How true is this depiction of skeletal muscle structure? myofibrils are not entirely ‘rod’ like and do not run the entire length of the muscle Muscle fibre orientation within connective tissue network is complex Variations of Muscle Shapes and Architecture Fusiform/ non-pennate unipennate multipennate Summary of Pennation Angle Concept Non-pennated muscle Pennated muscle Greater shortening capacity Larger PCSA = greater overall force capacity (PCSA) Biceps brachii Soleus Tibialis anterior Measure fibre CSA perpendicular to fibre direction Physiological CSA Attempts to measure fibre CSA while accounting for pennation angle Affect of pennation – angle of attachment to tendon Force applied to tendon = total fibre force x cosβ (correction factor) Small loss of force per fibre, but great increase in packing density of fibres and overall greater force potential. Angle of pennation in humans – from 0 to 30° Interactions between pennation angle, fascicle length, and P-CSA Assuming same A-CSA Greater pennation angle will lead to a greater P-CSA (see right figure) Larger P-CSA = more force generating capacity overall Trade off Greater pennation angle, smaller fascicle length! Any given change in muscle length, will have greater ‘relative’ change in fibre length Decreases the working range and shortening velocity Assuming same A-CSA How do you assess pennation angles? Ultrasound is most common Muscle fibre type in humans – heavy chain myosin ATPase Many ways to identify fibre type Heavy chain myosin ATPase is common Humans display I, IIA, and IIX Fibre type has implications for contractile function and fatigability Vmax & Myosin ATPase Human (elbow flexors) mouse (EDL) mouse (soleus) Fibre type classification Should be IIX (humans) Fibre CSA Small Intermediate Large Determinants of Muscle Force Output 1. Structural or intrinsic Structural: Size (mass), Shape (geometry) and Composition (fibre types) Size = Force Muscle force is proportional to cross-sectional area (CSA), or more correctly, physiologic cross-sectional area (PCSA) Shape = compartments, length of fibres, and pennation arrangement Slow twitch/fast twitch fibre type composition affects speed and endurance of muscle contraction 2. Extrinsic – neural control, activation history, antagonist function