Unit Two: Membrane Physiology, Nerve, and Muscle Chapter 6: Contraction of Skeletal Muscle Guyton and Hall, Textbook of Medical Physiology, 12th edition

Physiological Anatomy of Skeletal Muscle Skeletal Muscle Fiber Sarcolemma (plasma membrane) is a thin membrane enclosing a muscle fiber Myofibrils are composed of actin and myosin Titin filaments keep the myosin and actin filaments in place Sarcoplasm is the intracellular fluid between myofibrils Sarcoplamic reticulum is a specialized endoplasmic reticulum of muscle cells

Fig. 6.1

General Mechanism of Muscle Contraction An action potential travels along a motor nerve to the motor end plate The nerve secretes acetylcholine The AcH binds to sarcolemma and opens gated channels Large amounts of Na+ enter the cell and initiates and AP AP travels along the sarcolemma the same as in a nerve cell AP causes depolarization and triggers release of Ca++ from the sarcoplasmic reticulum Ca++ initiate the contraction cycle After contraction, Ca++ ions are reabsorbed by the sarcoplasmic reticulum

Fig. 6.3 Organization of proteins in a sarcomere

Molecular Mechanism of Muscle Contraction Sliding Filament Mechanism of Muscle Contraction Fig. 6.5 Relaxed and contracted state of a myofibril

Molecular Mechanism (cont.) Molecular Characteristics of the Contractile Filaments Myosin filaments are composed of multiple myosin molecules Fig. 6.6

Molecular Mechanism (cont.) ATPase activity of the myosin hear Actin filaments are composed of actin, tropomyosin and troponin Fig. 6.7 Actin filament

Molecular Mechanism (cont.) d. Tropomyosin molecules-wrapped spirally around the sides of the F-actin helix Troponin and its role in muscle contraction-helps attach the tropomyosin to the actin; strong affinity for calcium during contraction Inhibition of the actin filament by the troponin-tropomyosin complex and activation by calcium ions Interaction between actin and myosin cross bridges h. Chemical events in the motion of the myosin heads

Molecular Mechanism (cont.) Fig. 6.8 “Walk along” mechanism for contraction

Amount of Actin and Myosin Filament Overlap Determines Tension Developed by the Contracting Muscle Fig. 6.9 Length-tension diagram

Effect of Muscle Length on Force of Contraction in Whole Intact Muscle Fig. 6.10 Relation of Muscle Length to Tension

Relation of Velocity of Contraction to Load-contracts rapidly when it contracts against no load; velocity decreases as load increases Fig. 6.11 Relation of Load to Velocity of Contraction

Energetics of Muscle Contraction Work Output-when a muscle contracts against a load it performs work. Energy is transferred from the muscle to the external load to lift an object Sources of Energy for Muscle Contraction Phosphocreatine Glycolysis (uses stored glycogen as energy source Oxidative metabolism

Characteristics of Whole Muscle Contraction Muscle Twitch-demonstrated by eliciting single muscle twitches Isotonic vs Isometric Contraction Isometric-muscle does not shorten during contraction Isotonic-muscle shortens but the tension remains constant during the contraction

Muscle Contraction (cont.) Characteristics of Isometric Twitches From Different Muscles Fig. 6.13

Muscle Contraction (cont.) Fast vs. Slow Muscle Fibers Slow (Type I, Red Muscle) Smaller fibers Innervated by smaller nerve fibers Extensive blood vessel system for oxygenation Increased numbers of mitochondria Large amounts of myoglobin (combines with oxygen and stores it until needed

Muscle Contraction (cont.) b. Fast (Type II, White Muscle) Large fibers for great strength of contraction Extensive sarcoplasmic reticulum for rapid release of calcium Large amounts of glycolytic enzymes for glycolysis Less extensive blood supply (anaerobic) Fewer mitochondria Low levels of myoglobin

Mechanics of Skeletal Muscle Contraction Motor Unit-all the muscle fibers innervated by a single nerve fiber Summation-adding together of individual twitch contractions to increase the intensity of the overall muscle contraction; can happen two ways: (1) increasing the number of motor units, or (2) increasing the frequency of contraction Multiple Fiber Summation Frequency Summation and Tetanus

Mechanics (cont.) Fig. 6.14 Frequency of Summation and Tetanus

Maximum Strength of Contraction The Staircase Effect (Treppe)-when a muscle begins to contract after a long rest, its initial strength may be as little as ½ its strength 10 to 50 muscle twitches later. Thought to be due to a progressive increase of calcium ions in the sarcoplasm. Muscle Tone-when muscles are at rest, a certain degree of tautness remains. Due to a low rate of impulses coming from the spinal cord; also involves the muscle spindles (receptors)

Muscle Fatigue-increases almost in the direct proportion to the depletion of muscle glycogen; also contributing is a loss of ATP, oxygen, decreased blood flow to the muscle Lever Systems of the Body-muscles operate by applying tension to their points of insertion; analysis of the lever systems depends on the point of muscle insertion its distance from the fulcrum of the lever the length of the lever arm the position of the lever