1. PowerPoint ® Lecture Slides prepared by Janice Meeking, Mount Royal College C H A P T E R Copyright © 2010 Pearson Education, Inc. 9 Muscles and Muscle Tissue: Part A

2. Copyright © 2010 Pearson Education, Inc. Review Principles of Muscle Mechanics 1.Same principles apply to contraction of a single fiber and a whole muscle 2.Contraction produces tension, the force exerted on the load or object to be moved 3.Contraction does not always shorten a muscle: Isometric contraction: no shortening; muscle tension increases but does not exceed the load Isotonic contraction: muscle shortens because muscle tension exceeds the load 4.Force and duration of contraction vary in response to stimuli of different frequencies and intensities

3. Copyright © 2010 Pearson Education, Inc. Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies Small motor units in muscles that control fine movements (fingers, eyes) Large motor units in large weight-bearing muscles (thighs, hips) Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle

4. Copyright © 2010 Pearson Education, Inc. Muscle Twitch Response of a muscle to a single, brief threshold stimulus Three phases of a twitch: Latent period: events of excitation-contraction coupling Period of contraction: cross bridge formation; tension increases Period of relaxation: Ca 2+ reentry into the SR; tension declines to zero

5. Copyright © 2010 Pearson Education, Inc. Muscle Twitch Comparisons Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles Latent period Extraocular muscle (lateral rectus) Gastrocnemius Soleus Single stimulus (b) Comparison of the relative duration of twitch responses of three muscles

6. Copyright © 2010 Pearson Education, Inc. Graded Muscle Responses Responses are graded by: 1.Changing the frequency of stimulation 2.Changing the strength of the stimulus

7. Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Frequency A single stimulus results in a single contractile response — a muscle twitch Contraction Relaxation Stimulus Single stimulussingle twitch A single stimulus is delivered. The muscle contracts and relaxes

8. Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Frequency Increase frequency of stimulus (muscle does not have time to completely relax between stimuli) Ca 2+ release stimulates further contraction  temporal (wave) summation Further increase in stimulus frequency  unfused (incomplete) tetanus If stimuli are given quickly enough, fused (complete) tetany results Rarely happens in the body under normal conditions – mostly in lab conditions

9. Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Strength Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs Muscle contracts stronger as stimulus strength is increased above threshold Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action

10. Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Strength Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases Motor unit 1 Recruited (small fibers) Motor unit 2 recruited (medium fibers) Motor unit 3 recruited (large fibers)

11. Copyright © 2010 Pearson Education, Inc. Isotonic Contractions Muscle changes in length and moves the load Isotonic contractions are either concentric or eccentric: Concentric contractions — the muscle shortens and does work For example, concentric contraction is used to lift a glass from a table Eccentric contractions — the muscle contracts as it lengthens Example – someone pulls your arm straight while at the same time you try to keep the arm locked in one position

12. Copyright © 2010 Pearson Education, Inc. Isometric Contractions The load is greater than the tension the muscle is able to develop Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens

13. Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is the only source used directly for contractile activities Available stores of ATP are used in 4–6 seconds ATP is regenerated by: Direct phosphorylation of ADP by creatine phosphate (CP) Anaerobic pathway (glycolysis) Aerobic respiration

14. Copyright © 2010 Pearson Education, Inc. Anaerobic Pathway At 70% of maximum contractile activity: Bulging muscles compress blood vessels Oxygen delivery is impaired Pyruvic acid is converted into lactic acid Lactic acid: Diffuses into the bloodstream Used as fuel by the liver, kidneys, and heart Converted back into pyruvic acid by the liver

15. Copyright © 2010 Pearson Education, Inc. Aerobic Pathway Produces 95% of ATP during rest and light to moderate exercise Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

16. Copyright © 2010 Pearson Education, Inc. Muscle Fatigue Physiological inability to contract or sustain the expected power output It is a reversible condition Fatigue can potentially occur at any of the point involves in muscle contraction – from the brain to the muscle fibers or in any of the systems that are responsible to supply oxygen to the muscles Total lack of ATP occurs rarely during states of continuous contraction

17. Copyright © 2010 Pearson Education, Inc. Muscle Fatigue Central fatigue mechanisms – arise from the CNS Includes subjective feeling of tiredness and desire to cease activity Suggested reasons include low pH, failure to produce enough ACh Peripheral fatigue mechanisms – anywhere between the neuromuscular junction and the muscle Lack of glycogen Ionic imbalances (K +, Ca 2+, P i ) interfere with E-C coupling

18. Copyright © 2010 Pearson Education, Inc. Muscle Fatigue An endurance exercise training can delay onset of fatigue by increasing oxidative capacity: Increased number of mitochondria Increased level of oxidative enzymes Increased number of capillary beds to muscle

19. Copyright © 2010 Pearson Education, Inc. Force of Muscle Contraction The force of contraction is affected by: Number of muscle fibers stimulated (recruitment) Relative size of the fibers — hypertrophy of cells increases strength Frequency of stimulation —  frequency allows time for more effective transfer of tension to noncontractile components Length-tension relationship — muscles contract most strongly when muscle fibers are 80–120% of their normal resting length

20. Copyright © 2010 Pearson Education, Inc. Length-tension relationship The force of muscle contraction depends on the length of the sarcomeres before the contraction begins On the molecular level, the length reflects the overlapping between thin and thick filaments The tension a muscle fiber can generate is directly proportional to the number of crossbridges formed between the filament

21. Copyright © 2010 Pearson Education, Inc. Velocity and Duration of Contraction Influenced by: 1.Muscle fiber type 2.Load 3.Recruitment

22. Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type: Functional Characteristics Speed of contraction – determined by speed in which ATPases split ATP The two types of fibers are slow and fast ATP-forming pathways Oxidative fibers – use aerobic pathways Glycolytic fibers – use anaerobic glycolysis These two criteria define three categories Slow oxidative fibers contract slowly, have slow acting myosin ATPases, and are fatigue resistant Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued

23. Copyright © 2010 Pearson Education, Inc. Remodeling muscles to match function Muscles are continually being remodeled to match functions required The muscle can alter: The diameter Length Strength Vascular supply Types of fibers (slightly)

24. Copyright © 2010 Pearson Education, Inc. Muscle diameter changes Hypertrophy – increased total mass of muscle Result of increased number of the filaments in each fiber The enzyme systems also increase (mainly enzymes for glycolysis) If the muscles are not used for many weeks, the rate of decay of contractile units is more rapid results in atrophy (decrease in mass)

25. Copyright © 2010 Pearson Education, Inc. Adjustment of muscle length It is a type of hypertrophy in which muscles are stretched to greater than normal length That causes the addition of new sarcomeres at the end of the fibers

26. Copyright © 2010 Pearson Education, Inc. Effects of Exercise - Aerobic exercise Aerobic exercise is physical exercise that intends to improve the oxygen system As a result, aerobic (endurance) exercise: Leads to increased: Muscle capillaries Number of mitochondria Myoglobin synthesis Results in greater endurance, strength, and resistance to fatigue May convert fast glycolytic fibers into fast oxidative fibers Examples: running, swimming, cycling

27. Copyright © 2010 Pearson Education, Inc. Effects of Exercise - Aerobic exercise Among the recognized benefits of doing regular aerobic exercise are: Strengthening the muscles involved in respiration, to facilitate the flow of air in and out of the lungs Strengthening and enlarging the heart muscle, to improve its pumping efficiency and reduce the resting heart rate, known as aerobic conditioning Strengthening muscles throughout the body Improving circulation efficiency and reducing blood pressure

28. Copyright © 2010 Pearson Education, Inc. Effects of Resistance Exercise (anaerobic) The use of resistance to muscular contraction to build the strength and size of skeletal muscles. Involves contractions of fast fibers Exercise that uses glycolysis and the reserve of ATP as a source of energy (no oxygen use). It depends on: The amount of ATP available The amount of glycogen The ability of the muscle to tolerate the lactic acid (decreased pH interferes with enzyme activity) The energy during the first 10-20 seconds is from ATP reserves Typically the onset of muscle fatigue occurs within 2 minutes of the start of maximal activity

29. Copyright © 2010 Pearson Education, Inc. Effects of Resistance Exercise (anaerobic) Resistance exercise results in: Muscle hypertrophy (due to increase in fiber size) Increased mitochondria, myofilaments, glycogen stores, and connective tissue Examples – weight lifting, Machines that offer resistance