Muscle II PDF

PL1400 and PL1001 Skeletal Muscle Continued Muscle continued Recall the names of the different components of a muscle fiber Outline the relationship between resting length of a muscle and the force it generates when it contracts Describe how the force/strength of contraction of skeletal m...

PL1400 and PL1001 Skeletal Muscle Continued Muscle continued Recall the names of the different components of a muscle fiber Outline the relationship between resting length of a muscle and the force it generates when it contracts Describe how the force/strength of contraction of skeletal muscle can be varied What is the significance of the cardiac muscle action potential for muscle contraction Outline the mechanism of contraction for cardiac muscle Outline the mechanism of contraction for smooth muscle Summarise the key differences between smooth, cardiac and skeletal muscle How to alter the Force/Strength of skeletal muscle contraction? Need to look at 2 things 1. Tetanus/Tetany 2. Recruitment of Motor Units 3 A Simple Muscle Twitch A contraction followed by relaxation 4  Tetanus in Skeletal Muscle Tetanus/Tetany:/ A sustained muscle contraction At high levels of stimulation, temporal summation (more Action Potentials) there is a sustained rise in intracellular [calcium] which allows for a greater force than during a single twitch – 5 Relationship between Muscle Fibers and Motor Units What is a motor Unit? It is an α motor neurone and all the muscle fibres innervated by that neurone See diagram > Motor units can be large or small. How big or small determines how precise the control is on that muscle > important for skilled movements To increase force of contraction smaller MU tend to be recruited first. As more and more force is needed, more MU come into play i.e. are recruited so the increase in force/strength of contraction is graded appropriately During prolonged contraction of a muscle, units rotate > some are contracting while some are not > prolongs the onset of fatigue 6 Effect of Sarcomere Length on Force Generation in a muscle > The Length-Tension Curve As muscle length increases there is an increase in tension developed but there is a limit to this C > Overstretched sarcomere with no overlap between actin and myosin There is an optimal resting length for the sarcomere in order to achieve maximum strength of contraction i.e. > B Finally if the sarcomere is too short the actin filaments collide into each other and tension is also reduced A Therefore, B is optimal In addition (not on graph) velocity of muscle contraction varies inversely with the load on the muscle A B C 7 Main Types of Muscle Contraction Isotonic – Constant tension, muscle length changes > Moving an object through a distance Isometric – Constant length, tension doesn’t exceed the load > Object doesn’t tend to move! Both types are essential for everyday activities 8 Cardiac and Smooth Muscle Learning Outcomes What is the significance of the cardiac muscle action potential for muscle contraction Outline the mechanism of contraction for cardiac muscle Outline the mechanism of contraction for smooth muscle Summarise the key differences between smooth, cardiac and skeletal muscle  Cardiac Muscle Striated muscle distributed across atria on top and ventricles on the bottom Cardiac muscle has its own “built in” pacemaker > sinoatrial node capable of spontaneous depolarization > action potentials Cardiac action potential is much longer than that for skeletal muscle > this stops the heart from going into a state of tetany > Remember the heart is a pump! Once an action potential is generated here it spreads rapidly via coupled cells, Gap Junctions, through the atria and into the ventricles> In other words, the entire muscle behaves as an electrical syncytium where all muscle cells behave like one cell, i.e. contraction is “all or nothing” so No motor units > Contraction is “all or nothing” 11 Overview of Smooth Muscle No sarcomeres No Troponin Poorly developed SR therefore extracellular Ca++ very important together with SR Ca++ Contractions also occur in response to other stimuli e.g hormones and stretch – Calmodulin is Ca++ binding protein, that activates the enzyme Myosin Light Chain Kinase (MLCK) - 12 Smooth Muscle 2 Types Single Unit Muscle: Many fibres contracting as a single unit, hence importance of gap junctions - Multi Unit Muscle: Controlled mainly by nerve signals – Individual muscle fibres may be stimulated by individual nerve fibres – Both types regulated by the autonomic nervous ststem The Mechanism of Smooth Muscle Contraction Basic mechanism of actin/myosin interaction with cross-bridge formation is the same as for the other 2 muscles But Thick filament regulated > via MLCK activation of myosin 14