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Copyright © 2010 Pearson Education, Inc. Table 9.3
Copyright © 2010 Pearson Education, Inc. Special Characteristics of Muscle Tissue Excitability (responsiveness or irritability): ability to receive and respond to stimuli Contractility: ability to shorten when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil to resting length
Copyright © 2010 Pearson Education, Inc. Muscle Functions 1.Movement of bones or fluids (e.g., blood) 2.Maintaining posture and body position 3.Stabilizing joints 4.Heat generation (especially skeletal muscle)
Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Each muscle is served by one artery, one nerve, and one or more veins
Copyright © 2010 Pearson Education, Inc. Figure 9.1 Bone Perimysium Endomysium (between individual muscle fibers) Muscle fiber Fascicle (wrapped by perimysium) Epimysium Tendon Epimysium Muscle fiber in middle of a fascicle Blood vessel Perimysium Endomysium Fascicle (a) (b)
Copyright © 2010 Pearson Education, Inc.
Microscopic Anatomy of a Skeletal Muscle Fiber Cylindrical cell 10 to 100 m in diameter, up to 30 cm long Multiple nuclei Many mitochondria Glycogen storage, myoglobin for O 2 storage
Copyright © 2010 Pearson Education, Inc. NucleusLight I bandDark A band Sarcolemma Mitochondrion (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended afrom the cut end of the fiber. Myofibril
Copyright © 2010 Pearson Education, Inc. Figure 9.2c, d I band A band Sarcomere H zone Thin (actin) filament Thick (myosin) filament Z disc M line (c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Z disc M line Sarcomere Thin (actin) filament Thick (myosin) filament Elastic (titin) filaments (d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.
Copyright © 2010 Pearson Education, Inc. Figure 9.3 Flexible hinge region Tail Tropomyosin TroponinActin Myosin head ATP- binding site Heads Active sites for myosin attachment Actin subunits Actin-binding sites Thick filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. Thin filament A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Thin filament Thick filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Longitudinal section of filaments within one sarcomere of a myofibril Portion of a thick filament Portion of a thin filament Myosin molecule Actin subunits
Copyright © 2010 Pearson Education, Inc. Figure 9.5 Myofibril Myofibrils Triad: Tubules of the SR Sarcolemma Mitochondria I band A band H zoneZ disc Part of a skeletal muscle fiber (cell) T tubule Terminal cisternae of the SR (2) M line
Copyright © 2010 Pearson Education, Inc. Figure 9.6 I Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber I A ZZ H IIA ZZ 1 2
Copyright © 2010 Pearson Education, Inc. Figure 9.8 Nucleus Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Ca 2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Fusing synaptic vesicles 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Figure 9.8
Copyright © 2010 Pearson Education, Inc. Figure 9.8 Nucleus Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Ca 2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Junctional folds of sarcolemma Fusing synaptic vesicles ACh Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. Na + K+K+ Ach – Na + K+K+ Degraded ACh Acetyl- cholinesterase Postsynaptic membrane ion channel closed; ions cannot pass. 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. 3 Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 5 ACh binding opens ion channels that allow simultaneous passage of Na + into the muscle fiber and K + out of the muscle fiber. 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 1 Axon terminal of motor neuron Muscle fiber Triad One sarcomere Synaptic cleft Setting the stage Sarcolemma Action potential is generated Terminal cisterna of SR ACh Ca 2+
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 2 Action potential is propagated along the sarcolemma and down the T tubules. Steps in E-C Coupling: Troponin Tropomyosin blocking active sites Myosin Actin Active sites exposed and ready for myosin binding Ca 2+ Terminal cisterna of SR Voltage-sensitive tubule protein T tubule Ca 2+ release channel Myosin cross bridge Ca 2+ Sarcolemma Calcium ions are released. Calcium binds to troponin and removes the blocking action of tropomyosin. Contraction begins The aftermath
Copyright © 2010 Pearson Education, Inc. Figure Actin Cross bridge formation. Cocking of myosin head. The power (working) stroke. Cross bridge detachment. Ca 2+ Myosin cross bridge Thick filament Thin filament ADP Myosin PiPi ATP hydrolysis ATP 24 3 ADP PiPi PiPi
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. 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. 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 Anaerobic pathway Aerobic respiration
Copyright © 2010 Pearson Education, Inc. Anaerobic Pathway At 70% of max contractile activity: Bulging muscles compress blood vessels Oxygen delivery is impaired Build up lactic acid
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. 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
Copyright © 2010 Pearson Education, Inc. Smooth Muscle Found in walls of most hollow organs (except heart) Usually in two layers (longitudinal and circular)
Copyright © 2010 Pearson Education, Inc. Figure 9.26 Small intestine (a) (b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other. Mucosa Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section) Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers)
Copyright © 2010 Pearson Education, Inc. Peristalsis Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs Longitudinal layer contracts; organ dilates and shortens Circular layer contracts; organ constricts and elongates
Copyright © 2010 Pearson Education, Inc. Contraction of Smooth Muscle Very energy efficient (slow ATPases) Myofilaments may maintain for prolonged contractions
Copyright © 2010 Pearson Education, Inc. Special Features of Smooth Muscle Contraction Hyperplasia: Smooth muscle cells can divide and increase their numbers Example: estrogen effects on uterus at puberty and during pregnancy
Copyright © 2010 Pearson Education, Inc. Developmental Aspects With age, connective tissue increases and muscle fibers decrease By age 30, loss of muscle mass (sarcopenia) begins Regular exercise reverses sarcopenia Atherosclerosis may block distal arteries, leading to intermittent claudication and severe pain in leg muscles
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