1. 1 Energy Sources for Contraction Creatine phosphate – stores energy that quickly converts ADP to ATP 1) Creatine phosphate 2) Cellular respiration  ATP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP P When cellular Creatine ADP ATP P is high Creatine When cellular is low

2. 2 Oxygen Debt Oxygen not available Glycolysis continues Pyruvic acid converted to lactic acid Liver converts lactic acid to glucose Oxygen debt – amount of oxygen needed by liver cells to use the accumulated lactic acid to produce glucose and to restore muscle ATP and creatine phosphate concentrations. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ATP Glucose Glycogen Lactic acid Pyruvic acid Energy to synthesize Energy from Glycolysis and lactic acid formation (in muscle) Synthesis of glucose from lactic acid (in liver) ATP

3. Oxygen Debt Myoglobin – reddish brown pigment in skeletal muscle that can bind and store oxygen for use when demand is high. (Gives muscle its reddish brown color)

4. 4 Muscle Fatigue Inability to contract muscle Commonly caused from: Decreased blood flow Ion imbalances across the sarcolemma Accumulation of lactic acid – decreased pH prevents muscle fibers from responding to stimulation Cramp – sustained, involuntary muscle contraction due to decreased electrolyte concentration in extracellular fluid. (uncontrolled stimulation by motor neuron)

5. 5 9.4: Muscular Responses Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of the muscle fiber. Threshold Stimulus Minimal strength of stimulus required to cause contraction

6. 6 Recording of a Muscle Contraction Recording a Muscle Contraction Twitch = the contractile response of a single muscle fiber Latent period Period of contraction Period of relaxation Refractory period All-or-none response Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Force of contraction Time Latent period Period of contraction Period of relaxation Time of stimulation

8. 8 Length-Tension Relationship Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (b) Overly shortened(c) Overly stretched (a) Optimal length Muscle fiber length Force

9. 9 Summation Process by which the force of individual twitches combine to produce a sustained contraction with an increase in force. This Occurs with increased frequency of stimuli. Can lead to tetanic contractions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Time (c) (b) Force of contraction (a) Force of contraction Force of contraction Myograms: a)Series of twitches b)Summation c)Tetanic contraction

10. 10 Motor Units Motor unit: All the muscle fibers controlled by a single motor neuron As few as four fibers As many as thousands of muscle fibers *The fewer the fibers  the more precise the movement: Eye muscles – less than 10 fibers, produce very slight movements Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Motor neuron of motor unit 2 Motor neuron of motor unit 1 Skeletal muscle fibers Branches of motor neuron axon

11. 11 Recruitment of Motor Units Recruitment - increase in the number of motor units activated Whole muscle composed of many motor units More precise movements are produced with fewer muscle fibers within a motor unit As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated (higher intensity = more motor units activated)

12. 12 Sustained Contractions Smaller motor units (smaller diameter axons) - recruited first Larger motor units (larger diameter axons) - recruited later Produce smooth movements Muscle tone – continuous state of partial contraction - Important for maintaining posture

13. 13 Types of Contractions Isotonic – muscle contracts and changes length Concentric – shortening contraction Eccentric – lengthening contraction Isometric – muscle contracts but does not change length Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Movement (a) Muscle contracts with force greater than resistance and shortens (concentric contraction) (c) Muscle contracts but does not change length (isometric contraction) (b) Muscle contracts with force less than resistance and lengthens (eccentric contraction) No movement 29

14. 14 Fast Twitch and Slow Twitch Muscle Fibers Slow-twitch fibers (Type I) Always oxidative Resistant to fatigue Red fibers Most myoglobin Good blood supply Fast-twitch glycolytic fibers (Type IIa) White fibers (less myoglobin) Poorer blood supply Fewer mitochondria Susceptible to fatigue Twitch faster – have faster ATPase and more extensive SR to store Ca++ than slow- twitch fibers. Fast-twitch fatigue-resistant fibers (Type IIb) Intermediate fibers Oxidative Intermediate amount of myoglobin Pink to red in color Resistant to fatigue