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Chapter 8 Muscle Physiology Sections

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1 Chapter 8 Muscle Physiology Sections 8.4-8.8

2 8.5 Skeletal Muscle Metabolism and Fiber Types
Phosphagens ATP is required for muscle contraction, but storage of ATP is limited Creatine phosphate (vertebrates) and arginine phosphate (nonvertebrates) are the first energy storehouse tapped at the onset of contractile activity Phosphogens contain a high-energy phosphate group that can be quickly donated to ADP creatine kinase Creatine phosphate + ADP <——> creatine + ATP Vertebrate muscle contains 5x as much creatine phosphate as ATP 2

3 8.5 Skeletal Muscle Metabolism and Fiber Types
Oxidative phosphorylation Takes place in muscle mitochondria Requires oxygen Fueled by fatty acids or glucose Rich yield (~30 ATP per glucose) Multistep pathway requires more time Used during light to moderate (aerobic) activity Myoglobin stores oxygen in muscle fibers 3

4 8.5 Skeletal Muscle Metabolism and Fiber Types
Glycolysis Takes place in muscle cytoplasm Can form ATP in the absence of oxygen Fueled by glucose Insects also use trehalose, a nonreducing sugar Low yield (2 ATP per glucose) Proceeds more rapidly than oxid-phos Used during high-intensity (anaerobic) activity Produces lactate and accompanying acidosis 4

5 8.5 Skeletal Muscle Metabolism and Fiber Types

6 Ca2+ pump of sarcoplasmic reticulum
Biceps contracts Biceps relaxes Blood glucose Liver glycogen Muscle fiber During contraction Contraction Relaxation Muscle glycogen Glucose Myosin ATPase Ca2+ pump of sarcoplasmic reticulum Blood During rest (Main source when O2 not present) Glycolysis (Immediate source) Lactate Pyruvate (Main source when O2 present) FIGURE Metabolic pathways producing ATP used during muscle contraction and relaxation. During muscle contraction, ATP is split by myosin ATPase to power cross-bridge stroking. Also, a fresh ATP must bind to myosin to let the cross bridge detach from actin at the end of a power stroke before another cycle can begin. During relaxation, ATP is needed to run the Ca2+ pump that transports Ca2+ back into the lateral sacs of the sarcoplasmic reticulum. The metabolic pathways that supply the ATP needed to accomplish contraction and relaxation are 3a. transfer of a high-energy phosphate from creatine phosphate to ADP (immediate source); 3b.oxidative phosphorylation (the main source when O2 is present), fueled by glucose derived from muscle glycogen stores or by glucose and fatty acids delivered by the blood; and 3c.glycolysis (the main source when O2 is not present). Pyruvate, the end product of glycolysis, is converted to lactate when lack of O2 prevents the pyruvate from being further processed by the oxidative phosphorylation pathway. During rest Fatty acids Oxidative phosphorylation Creatine Phosphate Protein Creatine Amino acids Creatine kinase Rare During contraction Fat stores Figure 8-21 p360 6

7 8.5 Skeletal Muscle Metabolism and Fiber Types
Fatigue Decreased contractile response of exercising muscle to stimulation Causes of fatigue Local increase in ADP and Pi Accumulation of lactate Accumulation of extracellular K+ Depletion of glycogen energy reserves Central fatigue involves a decrease in CNS stimulation of motor neurons 7

8 8.5 Skeletal Muscle Metabolism and Fiber Types
Oxygen deficit An animal must continue to breathe deeply and rapidly after exhaustive activity. Oxygen is needed for recovery of energy systems through oxidative phosphorylation Replenishment of creatine phosphate Conversion of lactate into pyruvic acid and pyruvic acid into glucose Replenishment of glycogen stores 8

9 8.5 Skeletal Muscle Metabolism and Fiber Types
Skeletal muscle fiber types Slow-oxidative fibers (Type I) msec to peak tension Lower myosin-ATPase activity High resistance to fatigue Fast-oxidative fibers (Type IIa) msec to peak tension Higher myosin-ATPase activity Intermediate resistance to fatigue Fast-glycolytic fibers (Type IIb, IId, or IIx) Similar to fast-oxidative fibers in speed and myosin-ATPase activity Low resistance to fatigue 9

10 8.5 Skeletal Muscle Metabolism and Fiber Types
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11 8.5 Skeletal Muscle Metabolism and Fiber Types
Adaptation of muscle fibers Skeletal muscle has a high degree of plasticity Regular endurance activities improve oxidative capacity Increase in number of mitochondria Increase in number of capillaries Regular high-intensity activity stimulates hypertrophy (increased diameter) of fast-glycolytic fibers Increased synthesis of myosin and actin filaments Increased muscle strength 11

12 8.5 Skeletal Muscle Metabolism and Fiber Types
Adaptation of muscle fibers Hormones influence muscle size and strength Testosterone and growth hormone/IGF-I promote synthesis of myosin and actin filaments Myostatin is a negative regulator of muscle growth Interconversion between fast-glycolytic and fast-oxidative fibers takes place with specific forms of regular exercise Unused muscle loses mass and strength (disuse atrophy) When muscle is damaged, limited repair is possible due to ability to form new myoblasts 12

13 High-force and high-frequency operation are both needed for flight
8.6 Adaptations for Flight: Continuous High Power at High Contraction Frequencies High-force and high-frequency operation are both needed for flight Increased body temperature allows more rapid ATP synthesis and increases activity of Ca2+ pumps Birds have higher body temperature than mammals Insects must warm up before flight Mitochondrial structure is altered in birds and insects for higher O2 consumption Synchronous muscle contractions power flight muscle of hummingbirds and large insects 13

14 Asynchronous contractions as adaptations to high-frequency flight
8.6 Adaptations for Flight: Continuous High Power at High Contraction Frequencies Asynchronous contractions as adaptations to high-frequency flight Occurs in most insects A single Ca2+ pulse maintains muscle in an activated state for successive cycles Flight muscles are attached to the walls of the thorax rather than to the wings Contraction is triggered by stretch and deactivated by shortening in the presence of elevated myoplasmic Ca2+ Reduction of Ca2+ cycling reduces ATP demand 14

15 8.6 Adaptations for Flight: Continuous High Power at High Contraction Frequencies
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16 Base of wing Wing hinge Cuticle
FIGURE Mechanics of flight in the housefly. (a) The exoskeleton of insects relying on asynchronous flight muscles is hinged. Each wing is moved through an elastic, hingelike structure as a result of changes in the shape of the thorax. (b) Flight muscles do not attach directly to the hinges or to the wings but rather pull on the sides of the thorax to which they are attached. The two sets of flight muscles are arranged perpendicular to each other. Upward movement of the wings occurs when the vertical dorsoventral muscles contract and pull the flexible dorsal roof downward, flicking the wings up. The downstroke occurs when the longitudinal muscles contract, distorting the roof upward and forcing the wings down. Only the up or down positions are stable, and because of the elasticity of the hinge, much of the energy used to power muscle wing movement is recycled into the subsequent stroke. Smaller muscles acting directly on the hinge allow the wings to move at an angle, resulting in forward movement, so that a complete wing cycle actually involves the wingtip performing a figure-eight pattern. Cuticle Figure 8-22a p367 16

17 Longitudinal muscles relaxed
Wing Pivot point Dorsal–ventral muscles contracted Longitudinal muscles contracted FIGURE Mechanics of flight in the housefly. (a) The exoskeleton of insects relying on asynchronous flight muscles is hinged. Each wing is moved through an elastic, hingelike structure as a result of changes in the shape of the thorax. (b) Flight muscles do not attach directly to the hinges or to the wings but rather pull on the sides of the thorax to which they are attached. The two sets of flight muscles are arranged perpendicular to each other. Upward movement of the wings occurs when the vertical dorsoventral muscles contract and pull the flexible dorsal roof downward, flicking the wings up. The downstroke occurs when the longitudinal muscles contract, distorting the roof upward and forcing the wings down. Only the up or down positions are stable, and because of the elasticity of the hinge, much of the energy used to power muscle wing movement is recycled into the subsequent stroke. Smaller muscles acting directly on the hinge allow the wings to move at an angle, resulting in forward movement, so that a complete wing cycle actually involves the wingtip performing a figure-eight pattern. Dorsal–ventral muscles relaxed Figure 8-22b p367 17

18 8.7 Control of Motor Movement
Motor inputs controlling motor neuron output Afferent neurons Spinal reflexes are important for maintaining posture and basic protective movements Primary motor cortex Fibers of pyramidal cells descend directly to motor neurons (corticospinal motor system) Fine, discrete movements of hands and fingers Brain stem Part of multineuronal (extrapyramidal) motor system Regulation of overall body posture involving involuntary movements of trunk and limbs 18

19 8.7 Control of Motor Movement
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20 ANIMATION: Nervous system and muscle contraction
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21 8.7 Control of Motor Movement
Muscle proprioreceptors monitor changes in muscle length and tension Muscle length is monitored by muscle spindles Bundles of specialized intrafusal fibers lying within spindle-shaped connective tissue capsules Changes in muscle tension are detected by Golgi tendon organs Endings of afferent fibers entwined within bundles of connective tissue fibers in the tendon Frequency of firing is directly related to tension developed in the muscle Afferent information reaches the level of conscious awareness of muscle tension 21

22 8.7 Control of Motor Movement
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23 Contractile end portions of intrafusal fiber Afferent neuron axons
Capsule Alpha motor neuron axon Intrafusal (spindle) muscle fibers Gamma motor neuron axon Contractile end portions of intrafusal fiber Afferent neuron axons Noncontractile central portion of intrafusal fiber Two types of afferent sensory endings that serve as stretch receptors in muscle spindle FIGURE Muscle receptors. (a) A muscle spindle consists of a collection of specialized intrafusal fibers that lie within a connective tissue capsule parallel to the ordinary extrafusal skeletal muscle fibers. The muscle spindle is innervated by its own gamma motor neuron and is supplied by two types of afferent sensory terminals, the primary (annulospiral) endings and the secondary (flower-spray) endings, both of which are activated by stretch. (b) The Golgi tendon organ is entwined with the collagen fibers in a tendon and monitors changes in muscle tension transmitted to the tendon. Extrafusal (“ordinary”) muscle fibers Figure 8-24a p371 23

24 Skeletal muscle Afferent fiber Golgi tendon organ Collagen Tendon Bone
FIGURE Muscle receptors. (a) A muscle spindle consists of a collection of specialized intrafusal fibers that lie within a connective tissue capsule parallel to the ordinary extrafusal skeletal muscle fibers. The muscle spindle is innervated by its own gamma motor neuron and is supplied by two types of affrent sensory terminals, the primary (annulospiral) endings and the secondary (flower-spray) endings, both of which are activated by stretch. (b) The Golgi tendon organ is entwined with the collagen fibers in a tendon and monitors changes in muscle tension transmitted to the tendon. Bone (b) Golgi tendon organ Figure 8-24b p371 24

25 8.7 Control of Motor Movement
Muscle spindles play a key role in stretch reflexes (e.g. patellar tendon or knee-jerk reflex). When a muscle is passively stretched, intrafusal fibers in muscle spindles increase firing of afferent neurons Afferent neurons directly synapse on alpha motor neurons in the spinal cord, resulting in contraction of the muscle that was stretched Gamma motor neurons initiate contraction of muscular end regions of intrafusal fibers to adjust tension in muscle spindles The primary purpose of the stretch reflex is to resist the tendency for passive stretch of extensor muscles by gravity (maintains upright posture). 25

26 8.7 Control of Motor Movement
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27 coactivating alpha and gamma motor neurons Afferent input from
 Descending pathways coactivating alpha and gamma motor neurons Afferent input from sensory endings of muscle spindle fiber Stretch reflex pathway Alpha motor neuron output to regular skeletal muscle fiber Extrafusal skeletal muscle fiber Spinal cord Intrafusal muscle spindle fiber FIGURE Muscle spindle function. Gamma motor neuron output to contractile end portions of spindle fiber (a) Pathways involved in monosynaptic stretch reflex and coactivation of alpha and gamma motor neurons Figure 8-25a p372 27

28 8.7 Control of Motor Movement
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29 Extensor muscle of knee (quadriceps femoris) Muscle spindle
Patellar tendon Alpha motor neuron FIGURE Patellar tendon reflex (a stretch reflex) in a human leg. Tapping the patellar tendon with a rubber mallet stretches the muscle spindles in the quadriceps femoris muscle. The resultant monosynaptic stretch reflex results in contraction of this extensor muscle, causing the characteristic knee-jerk response. Figure 8-26 p373 29

30 ANIMATION: Stretch reflex
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31 8.8 Smooth Muscle and Cardiac Muscle
Mostly in walls of hollow organs and tubes Fibers are smaller than skeletal muscle fibers and spindle-shaped, with a single nucleus Fibers are arranged in sheets Three types of filaments Thick myosin filaments Thin actin filaments anchored at dense bodies Intermediate filaments form a scaffold for dense bodies Diagonal arrangement of filaments -- no striations 31

32 8.8 Smooth Muscle and Cardiac Muscle
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33 Smooth muscle cells Nucleus
FIGURE Microscopic view of smooth muscle cells. (a) Note the spindle shape and single, centrally located nucleus. (b) Note the presence of dense bodies and lack of banding. Figure 8-27a p376 33

34 Smooth muscle cells Dense bodies
FIGURE Microscopic view of smooth muscle cells. (a) Note the spindle shape and single, centrally located nucleus. (b) Note the presence of dense bodies and lack of banding. Figure 8-27b p376 34

35 8.8 Smooth Muscle and Cardiac Muscle
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36 One relaxed contractile unit extending from side to side
Dense body Bundle of thick and thin filaments One relaxed contractile unit extending from side to side One contracted contractile unit Plasma membrane Thin filament Thick filament Thin filament FIGURE Arrangement of thick and thin filaments in a smooth muscle cell in relaxed and contracted states. Thick filament (a) Relaxed smooth muscle cell (b) Contracted smooth muscle cell Figure 8-28 p377 36

37 Bundle of thick and thin filaments
Dense body Bundle of thick and thin filaments One relaxed contractile unit extending from side to side Plasma membrane Thick filament Thin filament FIGURE Arrangement of thick and thin filaments in a smooth muscle cell in relaxed and contracted states. Thin filament Thick filament (a) Relaxed smooth muscle cell Figure 8-28a p377 37

38 One contracted contractile unit
FIGURE Arrangement of thick and thin filaments in a smooth muscle cell in relaxed and contracted states. (b) Contracted smooth muscle cell Figure 8-28b p377 38

39 8.8 Smooth Muscle and Cardiac Muscle
Mechanism of smooth muscle contraction During excitation, cytosolic Ca2+ is increased Ca2+ binds with calmodulin Ca2+-calmodulin complex binds to and activates myosin light chain kinase (MLC kinase) MLC kinase phosphorylates myosin light chains Allows myosin heads to interact with actin and cross-bridge cycling begins 39

40 8.8 Smooth Muscle and Cardiac Muscle
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41 8.8 Smooth Muscle and Cardiac Muscle
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42 Series of biochemical events
Smooth muscle Skeletal muscle Muscle excitation Muscle excitation Rise in cytosolic Ca2+ (mostly from extracellular fluid) Rise in cytosolic Ca2+ (entirely from intracellular sarcoplasmic reticulum) Series of biochemical events Physical repositioning of troponin and tropomyosin Phosphorylation of myosin cross bridges in thick filament Uncovering of cross-bridge binding sites on actin in thin filament FIGURE Comparison of the role of calcium in bringing about contraction in smooth muscle and skeletal muscle. Binding of actin and myosin at cross bridges Binding of actin and myosin at cross bridges P i Contraction Contraction Figure 8-30 p378 42

43 8.8 Smooth Muscle and Cardiac Muscle
Classification of smooth muscle Phasic vs. tonic Phasic smooth muscle contracts in bursts triggered by action potentials that cause increased cytosolic Ca2+ Tonic smooth muscle is partially contracted at all times; varies its contraction according to cytosolic Ca2+ level 43

44 8.8 Smooth Muscle and Cardiac Muscle
Multiunit vs. single-unit smooth muscle Multiple units must be separately stimulated by nerves to contract Contractile activity is neurogenic and phasic Can be initiated by the autonomic nervous system Single-unit muscle fibers are self-excitable and contract as a single unit Gap junctions electrically link neighboring cells (functional syncytium) Contractile activity is myogenic and may be phasic (pacemaker potentials) or tonic (slow-wave potentials) Modified by the autonomic nervous system 44

45 8.8 Smooth Muscle and Cardiac Muscle
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46 FIGURE 8-31 Self-generated electrical activity in smooth muscle
FIGURE Self-generated electrical activity in smooth muscle. (a) With pacemaker potentials, the membrane gradually depolarizes to threshold on a regular periodic basis without any nervous stimulation. These regular depolarizations cyclically trigger self-induced action potentials. (b) In slow-wave potentials, the membrane gradually undergoes self-induced hyperpolarizing and depolarizing swings in potential. A burst of action potentials occurs if a depolarizing swing brings the membrane to threshold. Figure 8-31a p379 46

47 FIGURE 8-31 Self-generated electrical activity in smooth muscle
FIGURE Self-generated electrical activity in smooth muscle. (a) With pacemaker potentials, the membrane gradually depolarizes to threshold on a regular periodic basis without any nervous stimulation. These regular depolarizations cyclically trigger self-induced action potentials. (b) In slow-wave potentials, the membrane gradually undergoes self-induced hyperpolarizing and depolarizing swings in potential. A burst of action potentials occurs if a depolarizing swing brings the membrane to threshold. Figure 8-31b p379 47

48 8.8 Smooth Muscle and Cardiac Muscle
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49 Mitochondrion Vesicle containing neurotransmitter Axon of
postganglionic autonomic neuron Varicosity Neuro- transmitter Varicosities FIGURE Innervation of smooth muscle by autonomic postganglionic nerve terminals. Smooth muscle cell Figure 8-32 p380 49

50 8.8 Smooth Muscle and Cardiac Muscle
Smooth muscle contracts more slowly and uses less energy than skeletal muscle. Lower myosin ATPase activity results in slower contraction Slower rate of Ca2+ removal results in slower relaxation Latch state (vertebrates) or catch state (nonvertebrates) maintains tension for long periods with very low ATP consumption 50

51 8.8 Smooth Muscle and Cardiac Muscle
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52 8.8 Smooth Muscle and Cardiac Muscle
Found only in the heart Cells form a branching network Similarity to skeletal muscle Striated Length-tension relationship Abundance of mitochondria and myoglobin T tubules and sarcoplasmic reticulum Similarity to smooth muscle Self-excitation Interconnected by gap junctions Innervated by the autonomic nervous system 52


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