Copyright © 2010 Pearson Education, Inc. THE MUSCULAR SYSTEM (PHYSIOLOGY) CHAPTER # 9(b)

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

Copyright © 2010 Pearson Education, Inc. Review Principles of Muscle Mechanics 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

Copyright © 2010 Pearson Education, Inc. Review Principles of Muscle Mechanics 4.Force and duration of contraction vary in response to stimuli of different frequencies and intensities

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 Unit 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 Motor units in a muscle usually contract asynchronously; helps prevent fatigue

Copyright © 2010 Pearson Education, Inc. Muscle Twitch Response of a muscle to a single, brief threshold stimulus Simplest contraction observable in the lab (recorded as a myogram)

Copyright © 2010 Pearson Education, Inc. Muscle Twitch 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

Copyright © 2010 Pearson Education, Inc. Figure 9.14a Latent period Single stimulus Period of contraction Period of relaxation (a) Myogram showing the three phases of an isometric twitch

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

Copyright © 2010 Pearson Education, Inc. Figure 9.14b Latent period Extraocular muscle (lateral rectus) Gastrocnemius Soleus Single stimulus (b) Comparison of the relative duration of twitch responses of three muscles

Copyright © 2010 Pearson Education, Inc. Graded Muscle Responses Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: 1.Changing the frequency of stimulation 2.Changing the strength of the stimulus

Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Frequency A single stimulus results in a single contractile response—a muscle twitch

Copyright © 2010 Pearson Education, Inc. Figure 9.15a Contraction Relaxation Stimulus Single stimulussingle twitch A single stimulus is delivered. The muscle contracts and relaxes

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

Copyright © 2010 Pearson Education, Inc. Figure 9.15b Stimuli Partial relaxation Low stimulation frequency unfused (incomplete) tetanus (b) If another stimulus is applied before the muscle relaxes completely, then more tension results. This is temporal (or wave) summation and results in unfused (or incomplete) tetanus.

Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Frequency If stimuli are given quickly enough, fused (complete) tetany results

Copyright © 2010 Pearson Education, Inc. Figure 9.15c Stimuli High stimulation frequency fused (complete) tetanus (c) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus.

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 more vigorously 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

Copyright © 2010 Pearson Education, Inc. Figure 9.16 Stimulus strength Proportion of motor units excited Strength of muscle contraction Maximal contraction Maximal stimulus Threshold stimulus

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

Copyright © 2010 Pearson Education, Inc. Figure 9.17 Motor unit 1 Recruited (small fibers) Motor unit 2 recruited (medium fibers) Motor unit 3 recruited (large fibers)

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 Keeps muscles firm, healthy, and ready to respond

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 Eccentric contractions—the muscle contracts as it lengthens

Copyright © 2010 Pearson Education, Inc. Figure 9.18a

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

Copyright © 2010 Pearson Education, Inc. Figure 9.18b

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 depleted in 4–6 seconds

Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is regenerated by: Direct phosphorylation of ADP by creatine phosphate (CP) Anaerobic pathway (glycolysis) Aerobic respiration

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 At 70% of maximum contractile activity: Bulging muscles compress blood vessels Oxygen delivery is impaired Pyruvic acid is converted into lactic acid

Copyright © 2010 Pearson Education, Inc. Anaerobic Pathway Lactic acid: Diffuses into the bloodstream Used as fuel by the liver, kidneys, and heart Converted back into pyruvic acid by the liver

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 Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

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 Occurs when: Ionic imbalances (K +, Ca 2+, P i ) interfere with E- C coupling Prolonged exercise damages the SR and interferes with Ca 2+ regulation and release Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions)

Copyright © 2010 Pearson Education, Inc. Oxygen Deficit Extra O 2 needed after exercise for: Replenishment of Oxygen reserves Glycogen stores ATP and CP reserves Conversion of lactic acid to pyruvic acid, glucose, and glycogen

Copyright © 2010 Pearson Education, Inc. Heat Production During Muscle Activity ~ 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

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

Copyright © 2010 Pearson Education, Inc. Force of Muscle Contraction The force of contraction is affected by: 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

Copyright © 2010 Pearson Education, Inc. Figure 9.21 Large number of muscle fibers activated Contractile force High frequency of stimulation Large muscle fibers Muscle and sarcomere stretched to slightly over 100% of resting length

Copyright © 2010 Pearson Education, Inc. Figure 9.22 Sarcomeres greatly shortened Sarcomeres at resting length Sarcomeres excessively stretched 170% Optimal sarcomere operating length (80%–120% of resting length) 100%75%

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

Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type Classified according to two characteristics: 1.Speed of contraction: slow or fast, according to: Speed at which myosin ATPases split ATP Pattern of electrical activity of the motor neurons

Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type 2.Metabolic pathways for ATP synthesis: Oxidative fibers—use aerobic pathways Glycolytic fibers—use anaerobic glycolysis

Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type Three types: Slow oxidative fibers Fast oxidative fibers Fast glycolytic fibers

Copyright © 2010 Pearson Education, Inc. Table 9.2

Copyright © 2010 Pearson Education, Inc. Figure 9.23 Predominance of fast glycolytic (fatigable) fibers Predominance of slow oxidative (fatigue-resistant) fibers Small load Contractile velocity Contractile duration

Copyright © 2010 Pearson Education, Inc. Figure 9.24 FO FG SO

Copyright © 2010 Pearson Education, Inc. Influence of Load  load   latent period,  contraction, and  duration of contraction

Copyright © 2010 Pearson Education, Inc. Figure 9.25 Stimulus Intermediate load Light load Heavy load (a) The greater the load, the less the muscle shortens and the shorter the duration of contraction (b) The greater the load, the slower the contraction

Copyright © 2010 Pearson Education, Inc. Influence of Recruitment Recruitment  faster contraction and  duration of contraction

Copyright © 2010 Pearson Education, Inc. Effects of Exercise 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

Copyright © 2010 Pearson Education, Inc. Effects of Resistance Exercise Resistance exercise (typically anaerobic) results in: Muscle hypertrophy (due to increase in fiber size) Increased mitochondria, myofilaments, glycogen stores, and connective tissue

Copyright © 2010 Pearson Education, Inc. The Overload Principle Forcing a muscle to work hard promotes increased muscle strength and endurance Muscles adapt to increased demands Muscles must be overloaded to produce further gains