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Properties of Muscle Fibers OBJECTIVES Types of muscle fibers. Properties of muscle fibers. Functions of muscles. Stimulus and properties of stimuli. Effects of multiple stimuli on muscle fibers. Properties of smooth muscle and cardiac muscle fiber. LECTURE OUTLINE MUSCLE STRUCTURE: MCSUMES SKELETAL SMOOTH CARDIAC STRIPED, STRIATED, SOMATIC, VOLUNTARY. PLAIN, UNSTRIPED, NON-STRIATED, VISCERAL, INVOLUNTARY. STRIATED, BRANCHED, INVOLUNTARY. PROPERTIES OF MUSCLE FIBRES Contractility Ability of a muscle to shorten with force Excitability Capacity of muscle to respond to a stimulus Extensibility Muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity Ability of muscle to recoil to original resting length after stretched Function of Muscles Produce movement Maintain posture Stabilize joints Generate heat Nerve Stimulus to Muscles a) b) c) d) Skeletal muscles must be stimulated by a nerve to contract (motor neuron). Motor unit One neuron. Muscle cells stimulated by that neuron Smooth muscles are innervated by autonomic fibers. Cardiac muscle fibers are regulated by pacemaker activity from the SinoAtrial Node (SAN). TYPES OF STIMULUS Electrical- commonly used in labs Mechanical Thermal Chemical Electro-magnetic QUALITY OF STIMULUS Strength of stimulus- subminimal, minimal (threshold), submaximal, maximal and supramaximal Duration of stimulus Muscle Twitch Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers Phases Lag or latent Contraction Relaxation Contraction of a Skeletal Muscle Muscle fiber contraction follows “all or none” principle. All or None response An individual muscle fiber exhibits contraction of an uniform intensity once their particular threshold has been reached FACTORS AFFECTING CONTRACTION a) Strength of stimuli- summation b) Effect of multiple stimuli: i) beneficial effect ii) summation iii) clonus iv) tetanus v) treppe vi) fatigue c) Effect of temperature Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy FATIGUE Def: it is a decrease in the performance due to continuous and prolonged activity Site of fatigue: CNS is the first site of fatigue even though the muscle itself can undergo fatigue. Nerve is indefatigable Muscle Fatigue and Oxygen Debt When a muscle is fatigued, it is unable to contract The common reason for muscle fatigue is oxygen debt Oxygen must be “repaid” to tissue to remove oxygen debt Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less Multiple Motor Unit Summation A whole muscle contracts with a small or large force depending on number of motor units stimulated to contract Multiple-Wave Summation As frequency of action potentials increase, frequency of contraction increases Action potentials come close enough together so that the muscle does not have time to completely relax between contractions. Treppe or Stair case effect Graded response Occurs in muscle rested for prolonged period Each subsequent contraction is stronger than previous until all equal after few stimuli Types of Muscle Contractions Isotonic contractions Myofilaments are able to slide past each other during contractions The muscle shortens Isometric contractions Tension in the muscles increases The muscle is unable to shorten III. CONDUCTIVITY velocity of action potential conduction across skeletal muscle is 5m/sec in nerves it is up to 120m/sec conduction is along the sarcolemma and moves along the T-tubules Some fibers are contracted even in a relaxed muscle Different fibers contract at different times to provide muscle tone. The process of stimulating various fibers is under involuntary or reflex control Muscle Tone Some fibers are contracted even in a relaxed muscle Different fibers contract at different times to provide muscle tone. The process of stimulating various fibers is under involuntary or reflex control Effects of Aging on Skeletal Muscle muscle mass Increased time for muscle to contract in response to nervous stimuli Reduced stamina Increased recovery time Loss of muscle fibers Decreased density of capillaries in muscle Muscle Atrophy Weakening and shrinking of a muscle May be caused Immobilization Loss of neural stimulation Muscle Hypertrophy Enlargement of a muscle More capillaries More mitochondria Caused by Strenuous exercise Steroid hormones Muscle Tonus Tightness of a muscle Some fibers are always contracted Tetany Sustained contraction of a muscle Result of a rapid succession of nerve impulses Tetanus Refractoriness Rigor Mortis Stiffening of the body beginning 3 to 4 hours after death Deteriorating sarcoplasmic reticulum releases calcium Calcium activates myosin-actin cross-bridging and muscle contracts, but can not relax. Muscle relaxation requires ATP and ATP production is no longer produced after death Fibers remain contracted until myofilaments decay.