Motor Unit: The Nerve-Muscle Functional Unit

1. Motor Unit: The Nerve-Muscle Functional Unit • Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

2. Spinal cord Axon terminals at neuromuscular junctions Motor unit 1 Motor unit 2 Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. Figure 9.13a

3. Motor Units • Small motor units in muscles that control fine movements (fingers, eyes) • Large motor units in large weight-bearing muscles (thighs, hips)

4. Motor Unit • Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle If a muscle has more than one motor unit… • Motor units in a muscle usually contract asynchronously; helps prevent fatigue

5. Muscle Tone • Constant, slightly contracted state of all muscles • Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles • Does not produce movement, but keeps muscles firm, healthy, and ready to respond • Helps maintain posture and stabilize joints

6. 2 Main Categories of Contraction Isotonic Muscle changes in length and moves the load Isometric The load is greater than the tension the muscle is able to develop The muscle neither shortens nor lengthens

7. Isotonic Contractions Isotonic contractions are either concentric or eccentric: • Concentric contractions—the muscle shortens and does work • Eccentric contractions—the muscle generates force as it lengthens

8. Isometric Isotonic

9. Figure 9.18a

10. Figure 9.18b

11. Check Point!!!!!! What is a motor unit? Betty White just signed up for a chin up competition to raise money for a local animal shelter. What type of muscle contractions are occurring in her biceps muscles immediately after she grabs the bar? As her body begins to move upward toward the bar? When her body begins to approach the mat?

12. Where does the body get the energy needed for contraction?

13. Muscle Metabolism: Energy for Contraction • ATP is the onlysource used for muscle contraction • Muscles store only 4–6 seconds worth of ATP at any given time. • ATP must therefore be created as quickly as it is broken down for a contraction to continue

14. Muscle Metabolism: Energy for Contraction ATP is regenerated by: • Direct phosphorylation of ADP by creatine phosphate (CP) 2. Anaerobic pathway (glycolysis) 3. Aerobic respiration

15. Direct Phosphorylation • Fastest form of ATP regeneration • Creatine phosphate (CP) is stored in the muscles • After stored ATP is used, CP combines with ADP to make more ATP Creatine phospate + ADP  Creatine + ATP • Can provide enough energy for 15 seconds of vigorous activity

16. (a) Direct phosphorylation Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP CP ADP Creatine kinase Creatine ATP Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Figure 9.19a

17. Anaerobic PathwayGlycolysis & Lactic Acid Formation • Makes ATP by breaking down glucose • Glucose breaks down into 2 pyruvic acid molecules, releasing energy to make ATP • 2 ATP are produced per glucose molecule • Pyruvic acid is then converted into lactic acid • Produces less ATP than aerobic but it’s FAST! • Can support strenuous activity for about a minute

18. (b) Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol O2 ATP 2 Pyruvic acid net gain O2 Released to blood Lactic acid Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more Figure 9.19b

19. Aerobic Pathway • Produces 95% of ATP during rest and light to moderate exercise • Occurs in the mitochondria and requires Oxygen • Turns fuels (stored glycogen, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids) into ATP • Produces 32 ATP per glucose, CO2, & H20

20. (c) Aerobic pathway Aerobic cellular respiration Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Glucose (from glycogen breakdown or delivered from blood) O2 Pyruvic acid Fatty acids O2 Aerobic respiration in mitochondria Aerobic respiration Amino acids in mitochondria 32 ATP CO2 H2O net gain per glucose Oxygen use: Required Products: 32 ATP per glucose, CO2, H2O Duration of energy provision: Hours Figure 9.19c

21. Prolonged-duration exercise Short-duration exercise ATP stored in muscles is used first. ATP is formed from creatine Phosphate and ADP. Glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP. ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway. This pathway uses oxygen released from myoglobin or delivered in the blood by hemoglobin. When it ends, the oxygen deficit is paid back. Figure 9.20

22. Muscle Fatigue • Physiological inability to contract even with a neural impulse • Occurs when: • ATP production fails to keep pace with ATP usage • When ATP use exceeds its production • Total lack of ATP occurs rarely, results in states of continuous contraction (contracture) Example: Writer’s Cramp

23. Oxygen Deficit Exercise uses oxygen stores in the muscle Oxygen demand rises so extra Oxygen must be taken in by the body to restore normal levels of Oxygen Vigorous exercise  heavy breathing  restores balance

24. Heat Production During Muscle Activity • Only about 40% of the energy released in muscle activity is useful as work • Remaining energy (60%) given off as heat • Dangerous heat levels are prevented by radiation of heat from the skin and sweating • Conversely, shivering will increase body temperature when cold

25. Critical Thinking!!! When Eric returned from jogging, he was breathing heavily, sweating profusely, and complained that his legs ached and felt weak. His wife poured him a sports drink and urged him to take it easy until he could “catch his breath.” On the basis of what you have learned about muscle energy metabolism, respond to the following questions: Why is Eric breathing heavily? What ATP-generating pathway have his working muscles been using that leads to such breathlessness? What metabolic pathway might account for his sore muscles and his feeling of muscle weakness?