Clicker questions • Clicker channel: 70 • Response Ware Session ID: psio201Which of the following has ATPase activity during cross bridge cycling? 1. Actin 2. Troponin 3. Myosin 4. Tropomyosin 1. 2. 3. 4.3%4%87%5%When muscle action potentials travel down T-tubules, they A. trigger attachment of myosin to titin. B. trigger detachment of tropomyosin from troponin. C. trigger release of Ca2+ from the SR. D. trigger movement of Na+ out of the myofibrils. A. B. C. D.0%1%97%2%During relaxation, calcium 1. is actively transported out of the sarcoplasmic reticulum (SR) 2. is actively transported into the SR 3. passively diffuses out of the SR 4. passively diffuses into the SR 1. 2. 3. 4.2%5%5%88%Lecture 20: Muscle Mechanics Office hours: Tuesday and Thursday 1-2pm in Gittings 108 or by appointment Claudia Stanescu, Ph.D.1. List the three phases of a muscle twitch Objectives: pp. 311-315, sec. 10.5) 2. Compare and contrast the characteristics of a muscle twitch and tetanic contraction. Describe the development of tetanus in terms of temporal summation 3. Define a motor unit and discuss it in the context of spatial summation 4. Describe the 'size principle' in the context of development of increased tension during a voluntary muscle contraction 5. Describe isometric and isotonic contractions 6. Describe the strategies used to increase force in the muscle including increasing motor unit recruitment, frequency of activation and the length of muscle at the beginning of a contraction.Muscle Mechanics 1. The Muscle Twitch contraction arising from a single electrical stimulus e.g., an action potential 3 Periods 1. Latent 2. Contraction 3. Relaxation1. Latent Period •2 msec or less •Action potential sweeps over the sarcolemma •Calcium ions released from the sarcoplasmic reticulum •No change in tension2. Contraction Period •10 - 100 msec •Calcium binds to troponin •Myosin-binding sites on actin are exposed •Crossbridges form and swivel •Peak tension develops3. Relaxation Period •10-100 msec •Calcium ions transported back into the sarcoplasmic reticulum •Myosin binding sites on actin are covered by tropomyosin •Myosin heads detach from actin •Tension decreasesTimeTensioneyegastroc.soleusThe Time Course of a Twitch varies for different muscles (‘fast twitch’ vs. ‘slow twitch’) Fast twitch Slow twitchMuscle Mechanics 2. Tetanus (or tetanic contraction) ‘summation’ over time (Temporal Summation) of individual twitches Tetanus (temporal summation) is a mechanism for increasing tensionTension Production and the Motor Unit Motor Unit: a motor neuron and all the muscle fibers (cells) it innervatesMuscles typically contain many motor units by activating (‘recruiting’) increasing numbers of motor units, tension can be increased... ‘Spatial Summation’Motor Units and the ‘Size Principle’ The first motor units recruited are small (innervate few fibers) the result is fine, carefully controlled increase in tension As more force is required, larger motor units are recruitedSmall, SO Medium, FFR Large, FF SO = Slow, OxidativeSmall, SO Intermediate, FOG Large, FF FOG = Intermediate, oxidative-glycolytic (fatigue resistant)Small, SO Medium, FOG Large, FG FG = fast, glycolytic (fatiguable)81 85 80 83 87 88 96 707580859095100ClassAverageSI Average Non-SI SI=1 SI=2-3 SI=4-5 SI=6-7Average Grade on Exam 2 Exam 2 Grade Average vs SI AttendanceClicker questions • Clicker channel: 70 • Response Ware Session ID: psio201The muscle twitch takes place in response to a series of action potentials and can vary in duration based on the strength of these electrical signals. A. True B. False TrueFalse94%6%The period during a muscle twitch when the action potential is travelling across the sarcolemma leading to calcium release from the SR and tension is zero is called the A. Latent period B. Contraction period C. Relaxation period A. B. C.6%1%93%Increasing the frequency of action potentials that stimulate a muscle can increase force production. This is called A. Size principle B. Temporal summation C. Spatial summation D. Motor unit recruitment A. B. C. D.2%10%18%71%smallest motor units largest motor units % of motor units recruited add fast motor units add intermediate % maximal force 25 50 75 100 20 60 100 Stand Walk Run Jump Sprint Size Principle motor units are recruited in order from small to largeSize Principle motor units are recruited in order from small to large Slow Intermediate FastIsometric vs. Isotonic Contractions 1. Isometric - ‘same length’ muscle generates tension but does not shorten every contraction begins isometrically…. 2. Isotonic - ‘same tension’ after sufficient tension is generated to move the ‘load’ the muscle changes length Two types: 1) concentric; 2) eccentricSilverthorn, Human Physiology: An Integrated Approach, 6th ed., page 422 Isometric vs. Isotonic ContractionsSilverthorn, Human Physiology: An Integrated Approach, 6th ed., page 422 Isometric vs. Isotonic ContractionsIsotonic contractions Concentric vs. Eccentric contractions 1. When developed tension is sufficient to move the load, the muscle shortens: concentric contraction Effort > Load 2. As tension decreases and the load exceeds tension, the muscle lengthens: eccentric contraction Load > Effort Note: All muscle actions are based on the concentric contraction of a muscleStrategies to increase force: 1) Increase motor unit recruitment = spatial summation 2) Increase frequency of stimulation = temporal summation 3) Optimal muscle fiber length at start of contraction (see length-tension relationship) Nervous SystemLength-Tension Relationship Optimal overlap = more crossbridgesLength-Tension Relationship • Tension generated is proportional to the number of cross bridges formed • Peak tension is developed at an intermediate sarcomere lengthAccording to the length-tension relationship muscle tension is proportional to muscle length. A. True B. False TrueFalse 97%3%Suggested Learning Activities • Draw the a graph of the twitch in a slow twitch fiber, label the three phases and explain each one. How would this graph differ in a fast twitch fiber? • Draw the length tension relationship including a diagram of the sarcomere