Download 9.9 Smooth Muscle • Found in walls of most hollow organs, except

3/20/2017
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9.9 Smooth Muscle
• Found in walls of most hollow organs, except heart
– Heart contains cardiac muscle
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Microscopic Structure
• Spindle-shaped fibers
• Only one nucleus, no striations
• Lacks connective tissue sheaths
– Contains endomysium only
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Microscopic Structure (cont.)
• All but smallest blood vessels contain smooth muscle organized into two layers
– Longitudinal layer: fibers run parallel to long axis of organ
• Contraction causes organ to shorten
– Circular layer: fibers run around circumference of organ
• Contraction causes lumen of organ to constrict
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Microscopic Structure (cont.)
• Allows peristalsis: alternating contractions and relaxations of layers
– mix and squeeze substances through lumen of hollow organs
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Figure 9.22 Arrangement of smooth muscle in the walls of hollow organs.
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Microscopic Structure (cont.)
• Autonomic nerve fibers innervate smooth muscle
– Contain varicosities of nerve fibers
– Varicosities store and release neurotransmitters into a wide synaptic cleft called a
diffuse junction
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Figure 9.23 Innervation of smooth muscle.
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Microscopic Structure (cont.)
• Smooth muscle does not contain sarcomeres, myofibrils, or T tubules
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Figure 9.24a Intermediate filaments and dense bodies of smooth muscle fibers
harness the pull generated by myosin cross bridges.
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Microscopic Structure (cont.)
• Smooth muscle also differs from skeletal muscle in following ways:
– Thick filaments are fewer and have myosin heads along entire length
– No troponin complex
• Does contain tropomyosin, but not troponin
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Microscopic Structure (cont.)
– Thick and thin filaments arranged diagonally
• Myofilaments are spirally arranged, causing smooth muscle to contract in
corkscrew manner
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Figure 9.24b Intermediate filaments and dense bodies of smooth muscle fibers
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• Smooth muscle also differs from skeletal muscle in following ways:
– Thick filaments are fewer and have myosin heads along entire length
– No troponin complex
• Does contain tropomyosin, but not troponin
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Microscopic Structure (cont.)
– Thick and thin filaments arranged diagonally
• Myofilaments are spirally arranged, causing smooth muscle to contract in
corkscrew manner
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Figure 9.24b Intermediate filaments and dense bodies of smooth muscle fibers
harness the pull generated by myosin cross bridges.
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Table 9.3-1 Comparison of Skeletal, Cardiac, and Smooth Muscle
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Table 9.3-2 Comparison of Skeletal, Cardiac, and Smooth Muscle (continued)
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Table 9.3-3 Comparison of Skeletal, Cardiac, and Smooth Muscle (continued)
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Table 9.3-4 Comparison of Skeletal, Cardiac, and Smooth Muscle (continued)
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Contraction of Smooth Muscle
• Mechanism of contraction
– Slow, synchronized contractions
– Cells electrically coupled by gap junctions
• Action potentials transmitted from fiber to fiber
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Contraction of Smooth Muscle
• Mechanism of contraction continued…
– Some cells are self-excitatory (depolarize without external stimuli)
• Act as pacemakers for sheets of muscle
• May be modified by neural and chemical stimuli
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Developmental Aspects of Muscle
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All muscle tissues develop from embryonic myoblasts
Multinucleated skeletal muscle cells form by fusion of many myoblasts
Growth factor stimulates clustering of ACh receptors at neuromuscular junctions
Cardiac and smooth muscle myoblasts do not fuse, but develop gap junctions
– Cardiac muscle cells start pumping when embryo is 3 weeks old
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Figure 9.26 Myoblasts fuse to form a multinucleate skeletal muscle fiber.
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Developmental Aspects of Muscle
• Regeneration of muscle:
– Myoblast-like skeletal muscle satellite cells have limited regenerative ability
– Cardiomyocytes can divide at modest rate, but injured heart muscle is mostly
replaced by connective tissue
– Smooth muscle regenerates throughout life
• Cardiac and skeletal muscle can lengthen and thicken in growing child
– In adults, leads to hypertrophy
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Developmental Aspects of Muscle
• Muscular development in infants reflects neuromuscular coordination
– Development occurs head to toe, and proximal to distal
• A baby can lift its head before it is able to walk
• Peak natural neural control occurs by midadolescence
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– Cardiomyocytes can divide at modest rate, but injured heart muscle is mostly
replaced by connective tissue
– Smooth muscle regenerates throughout life
• Cardiac and skeletal muscle can lengthen and thicken in growing child
– In adults, leads to hypertrophy
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Developmental Aspects of Muscle
• Muscular development in infants reflects neuromuscular coordination
– Development occurs head to toe, and proximal to distal
• A baby can lift its head before it is able to walk
• Peak natural neural control occurs by midadolescence
– Athletics and training can continue to improve neuromuscular control
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Developmental Aspects of Muscle
• Difference in muscle mass between sexes:
– Female skeletal muscle makes up 36% of body mass
– Male skeletal muscle makes up 42% of body mass, primarily as a result of
testosterone
• Males have greater ability to enlarge muscle fibers, also because of testosterone
– Body strength per unit muscle mass is the same in both sexes
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Developmental Aspects of Muscle
• Aging muscles:
– 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
(limping) and severe pain in leg muscles
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Clinical – Homeostatic Imbalance 9.4
• Muscular dystrophy: group of inherited muscle-destroying diseases
– Generally appear in childhood
• Muscles enlarge as a result of fat and connective tissue deposits, but then atrophy
and degenerate
• Duchenne muscular dystrophy (DMD) is the most common and severe type
– Caused by defective gene for dystrophin
– Inherited, sex-linked trait, carried by females and expressed in males (1/3600)
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Clinical – Homeostatic Imbalance 9.4
– Dystrophin is a cytoplasmic protein that links the cytoskeleton to the extracellular
matrix, stabilizing the sarcolemma
• Fragile sarcolemma tears during contractions, causing entry of excess Ca2+
– Leads to damaged contractile fibers
• Inflammatory cells accumulate
• Muscle mass declines
• Victims become clumsy and fall frequently
– Usually appears between ages 2 and 7
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Clinical – Homeostatic Imbalance 9.4
– Currently no cure is known
– Prednisone can improve muscle strength and function
– Myoblast transfer therapy has been disappointing
– Coaxing dystrophic muscles to produce more utrophin (protein similar to
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• Muscle mass declines
• Victims become clumsy and fall frequently
– Usually appears between ages 2 and 7
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Clinical – Homeostatic Imbalance 9.4
– Currently no cure is known
– Prednisone can improve muscle strength and function
– Myoblast transfer therapy has been disappointing
– Coaxing dystrophic muscles to produce more utrophin (protein similar to
dystrophin) has been successful in mice
– Viral gene therapy and infusion of stem cells with correct dystrophin genes show
promise
• Patients usually die of respiratory failure in their early 20s
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