Muscle Physiology PDF

Muscle Physiology Chapter 10 Part II 1 Learning Objectives Explain the relationship between sarcomere length and amount of tension that can be generated by a muscle fiber. What is a twitch? What are the phases of a muscle twitch and which events of muscle con...

Muscle Physiology Chapter 10 Part II 1 Learning Objectives Explain the relationship between sarcomere length and amount of tension that can be generated by a muscle fiber. What is a twitch? What are the phases of a muscle twitch and which events of muscle contraction occur during each phase? What is wave summation and how does it allow for increased tension? What is a motor unit? How is the relative size of a motor unit related to the degree of control of a muscle? Are all skeletal muscles completely relaxed at the same time? What is muscle tone and why is it important? Define and distinguish between isometric and isotonic contractions (including both concentric and eccentric). Define agonist, antagonist, synergist, and fixator and what their functions are. What processes produce ATP for muscle contractions, how much ATP is produced by each process, and what duration of activity can each process sustain? What causes muscle fatigue? What is the recovery period. Compare and contrast the features of different types of muscle fibers. How do muscles change in response to training or disuse? Which team will exert more tension on the rope? Why? A B https://commons.wikimedia.org/wiki/ File:Irish_600kg_euro_chap_2009_(cropped).JPG 3 Generation of Tension by Muscle Fibers Amount of tension produced by a single muscle fiber can vary Number of pivoting cross bridges is determined by: 1. Degree of overlap between thick and thin filaments when the contraction begins (=resting length) 2. Frequency of stimulation Length-Tension Relationships by the motor neuron Greatest tension is produced 4 within a narrow range of  Generation of Tension by Muscle Fibers Twitch Single stimulus-contraction-relaxation sequence in a muscle fiber Very short (milliseconds) so only useful in a lab Divided into 3 phases: Latent period - time for action potential to propagate across sarcolemma & Ca++ to be released from SR Contraction period - repeated crossbridge cycles generate tension Tension produced during twitch varies as a result of: Relaxation period - calcium ion levels 1. Timing and frequency of stimulation reduced in cytosol by SR pumps; tension diminishes 2. Length of fiber at rest 5 3. Type of muscle fiber Generation of Tension by Muscle Fibers Tension can be increased with more frequent stimulation as a result of wave summation Ca++ levels remain high and allow the contraction to continue because the action potentials are so frequent that the SR 6 cannot pump the Ca++ Tension produced by Skeletal Muscles (the whole thing; not just a muscle fiber) Determined by: Total number of muscle fibers stimulated Total tension produced by the individual muscle fibers Muscles contain thousands of muscle fibers, but not all are activated at once Motor neurons innervate a subset of muscle fibers Motor unit = all muscle fibers controlled by a single motor neuron Vary in size from a single motor neuron controlling just a few muscle fibers to controlling thousands (determines how precise control over the muscle is) All of the muscle fibers within a single motor unit are the same type 7 Tension produced by Skeletal Muscles (the whole thing; not just a muscle fiber) Motor units are intermingled throughout the muscle, and not all are stimulated at once There are always some active units in skeletal muscle, but which ones are active vs. relaxed alternates Results in muscle tone (resting tension) Heightened muscle tones accelerates recruitment during voluntary contractions, makes skeletal muscles more well defined and increases resting metabolic rate Recruitment increases the number of active motor units and proceeds from the smallest motor units to the largest During sustained contractions motor units are activated on a rotating basis to allow periods of resting and recovery to replenish energy reserves 8 Muscle fibers of different motor units are intermingled, so the The tension applied to the tendon remains relatively forces applied to the tendon remain roughly balanced regardless constant, even though individual motor units cycle of which motor units are stimulated. between contraction and relaxation. 9  Two broad categories that differ in whether the length of the muscle or the tension exerted by the muscle changes Types of Isotonic contractions: tension generated is constant, but muscle Muscle length changes: Contraction Concentric contractions - constant tension while muscle shortens; force generated is s: Isotonic greater than external force (muscle tension exceeds the & Isometric load) Eccentric contractions - constant tension but muscle lengthens; external force is greater than force generated by muscle (tension is less than the load, but you can still control the speed of movement) Isometric contractions - muscle length (of whole muscle) remains unchanged because tension does not exceed the load  Groups of muscles work together to perform movement Functional Agonists (prime movers) – Groups of provide most force for given muscle action Muscles Antagonists – usually on (Terms opposite side of bones and joint where they meet; opposite action of agonist; from allows for modulation and control of agonist movement chapter 9) Synergists – aid agonists by supplying supplemental force, minimizing unwanted movement; help stabilize joints; provide for more efficient movement Fixators – also provide stabilizing force that anchors bone; provides movement efficiency and protection from injury due to unnecessary movements  Muscle metabolism: Reactions to produce ATP Skeletal muscle requires ATP to: 1. Power Na+ / K+ pumps that maintain ion gradients to allow for electrical signaling 2. Release myosin heads from actin active sites (binding of ATP to myosin) and re-cock the myosin head for another power stroke (hydrolysis of ATP) 3. Pump calcium back into the sarcoplasmic reticulum during relaxation Image from: https://www.nature.com/scitable/content/an-atp- molecule-14747828/ Muscles cannot get ATP from blood or other tissues ATP present in muscle cells is only sufficient to power contraction for a few seconds Required ATP is generated through several mechanisms that may occur simultaneously in muscle tissues during contraction depending on resources and needs of the muscle cells 1. Immediate cytosolic reactions using creatine phosphate 2. Glycolytic catabolism in cytosol (anaerobic) 3. Oxidative catabolism in mitochondria (aerobic)  ATP from creatine phosphate provides energy for ~15 seconds Creatine Resting muscle fibers produce more ATP than needed for Phosphate: resting metabolism Immediate energy Extra ATP is used to synthesize creatine phosphate, which sources is more stable than ATP When needed, creatine kinase can transfer P from CP back to ADP to generate new ATP 13 Glycolytic Catabolism Anaerobic process that occurs in the cytosol and results in a net gain of 2 ATP Glucose from the bloodstream + breakdown of glycogen within muscle fibers is broken down into two molecules of pyruvic acid via glycolysis Normally pyruvic enters mitochondria for aerobic respiration (which requires oxygen) If not enough oxygen is present (as occurs during heavy exercise) pyruvic acid is converted to lactic acid Rapid production of lactic acid can result in accumulation in muscle fibers and bloodstream where it can alter pH and lead to muscle fatigue and possibly soreness Oxidative Catabolism (Aerobic Cellular Respiration) When oxygen levels are sufficient pyruvic acid (from glycolysis) enters mitochondria and undergoes a series of reactions that produce ATP, carbon dioxide, water, and heat Slower process than glycolysis, but yields more ATP (30+) Sources of oxygen for muscle tissue include: oxygen diffusing into muscle fibers from the blood and oxygen released from myoglobin within the muscle fibers  Inability of muscle to maintain a given level of intensity during activity Several factors contribute (though precise mechanisms of fatigue are not completely understood) Central fatigue: feelings of tiredness; wanting to stop the activity (may be a protective Muscle strategy to prevent damage to muscles) Fatigue Depletion of metabolites necessary for ATP replenishment (glucose, glycogen, creatine phosphate) Decreased availability of oxygen to muscle fibers Buildup of lactic acid, ADP and other chemicals that interfere with processes within the cell Environmental conditions: extreme heat  sweating  electrolyte disturbances  Time it takes the body & muscles to return to the pre-exercise state Can take minutes to days depending on the intensity of the activity Several things must happen to allow the Recovery body to return to homeostasis during the recovery period Period Heat dissipation Restoration of intra- and extracellular ion concentrations Correction of blood pH Persisting increased rate of ventilation = Excess Postexercise Oxygen Consumption (EPOC) Increased oxygen (over normal resting oxygen consumption) that is taken into the body after exercise 17 Types of Muscle Fibers ATPase at pH 4.6. (a) Normal distribution of Type I (dark) and Type IIa (lightest), Type IIb and IIc (intermediate) fibers (total magnification, × 400. Photo courtesy of Dr. Stephen Coons. Image from: https://www.researchgate.net/figure/ATPase- at-pH-46-a-Normal-distribution-of-Type-I-dark- and-Type-IIa-lightest-Type_fig1_228628116 Preview of things to come in chapter 13: Interspersed within the regular contractile muscle fibers are special sensory structures called muscle spindles that monitor the degree of stretching in a muscle Golgi tendon organs (in the tendons) monitor the degree of tension generated by the muscle  Majority of mature skeletal muscle fiber nuclei are amitotic (do not undergo mitosis) Changes Satellite cells (small population of unspecialized cells) do retain mitotic ability; can Caused help repair injured skeletal muscle Therefore, changes in response to training are within muscle fibers; do typically not By involve changes in number of muscle fibers Physica Precise change that occurs depends on type of training endurance vs. resistance training l Trainin g Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved