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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
Copyright © 2010 Pearson Education, Inc. Figure 9.13a Spinal cord Motor neuron cell body Muscle Nerve Motor unit 1 Motor unit 2 Muscle fibers Motor neuron axon Axon terminals at neuromuscular junctions 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.
Copyright © 2010 Pearson Education, Inc. Motor Units Small motor units in muscles that control fine movements (fingers, eyes) Large motor units in large weight-bearing muscles (thighs, hips)
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. Isometric Isotonic
Copyright © 2010 Pearson Education, Inc. Figure 9.18a
Copyright © 2010 Pearson Education, Inc. Figure 9.18b
Copyright © 2010 Pearson Education, Inc. 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?
Copyright © 2010 Pearson Education, Inc. Where does the body get the energy needed for contraction?
Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is the only source 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
Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is regenerated by: 1.Direct phosphorylation of ADP by creatine phosphate (CP) 2. Anaerobic pathway (glycolysis) 3. Aerobic respiration
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. Figure 9.19a Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP (a) Direct phosphorylation Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Creatine kinase ADPCP Creatine ATP
Copyright © 2010 Pearson Education, Inc. Anaerobic Pathway Glycolysis & 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
Copyright © 2010 Pearson Education, Inc. Figure 9.19b Energy source: glucose Glycolysis and lactic acid formation (b) Anaerobic pathway Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol Pyruvic acid Released to blood net gain 2 Lactic acid O2O2 O2O2 ATP
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. Figure 9.19c Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism (c) Aerobic pathway Aerobic cellular respiration Oxygen use: Required Products: 32 ATP per glucose, CO 2, H 2 O Duration of energy provision: Hours Glucose (from glycogen breakdown or delivered from blood) 32 O2O2 O2O2 H2OH2O CO 2 Pyruvic acid Fatty acids Amino acids Aerobic respiration in mitochondria Aerobic respiration in mitochondria ATP net gain per glucose
Copyright © 2010 Pearson Education, Inc. Figure 9.20 Short-duration exercise Prolonged-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.
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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
Copyright © 2010 Pearson Education, Inc. 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?
Copyright © 2010 Pearson Education, Inc. Table 9.3.
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