muscles and muscle tissue: smooth muscle part c2 prepared by janice meeking, w. rose, and jarvis...
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Muscles and Muscle Tissue: Smooth Muscle
Part C2
Prepared by Janice Meeking, W. Rose, and Jarvis Smith.
Figures from Marieb & Hoehn 8th ed.
Portions copyright Pearson Education
Smooth MuscleFound in walls of most hollow organs (but heart is cardiac
muscle)Often in two layers (longitudinal and circular)
Microanatomy
Spindle-shaped cells: thin and short compared with skeletal muscle fibers
SR: less developed than in skeletal muscle
Pouchlike infoldings (caveolae) of surface membrane sequester Ca2+ (instead of SR)
No T tubules
No sarcomeres or myofibrils
Yes actin & myosin filaments
No tendons; endomysium connects to surrounding tissue
Smooth Muscle InnervationNo tight neuromuscular junction (unlike skeletal muscle
with its sophisticated NMJ)
Autonomic nerve fibers innervate smooth muscle
Varicosities (bulbous swellings) of nerve fibers release neurotransmitters broadly (diffuse junctions)
Smoothmusclecell
Varicosities releasetheir neurotransmittersinto a wide synaptic cleft (a diffuse junction).
Synapticvesicles
Mitochondrion
Autonomicnerve fibersinnervatemost smoothmuscle fibers.
Varicosities
Figure 9.27
Copyright © 2010 Pearson Education, Inc.
Myofilaments in Smooth Muscle
• Thin and thick filaments; have heads along their entire length
• No troponin complex; protein calmodulin binds Ca2+
• Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
• Noncontractile intermediate filaments, anchored to membrane by dense bodies, help preserve cell shape during contraction
Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc. Figure 9.28
Intermediatefilament
Dense bodiesNucleus
Caveolae
(a) Relaxed smooth muscle fiber
(b) Contracted smooth muscle fiber
Dense bodiesNucleus
Gap junctions
Copyright © 2010 Pearson Education, Inc.
Contraction of Smooth Muscle
• Actin & myosin sliding filament mechanism
• Slow, synchronized contractions
• Cells electrically coupled by gap junctions (in some tissues)
• Rate, intensity of contraction regulated by neural and chemical stimuli
• Final trigger is intracellular Ca2+ which comes from (sparse) SR and extracellular space (caveolae)
Copyright © 2010 Pearson Education, Inc.
Role of Calcium Ions in Smooth Muscle
• Calcium binds to and activates calmodulin (a protein)
• activates myosin light chain kinase enzymes (another protein)
• Phosphorylates and activates myosin
• activated heads form cross bridges with actin
Much slower than E-C coupling in skeletal muscle
Figure 9.29
Calcium ions (Ca2+)enter the cytosol fromthe ECF via voltage-dependent or voltage-independent Ca2+
channels, or fromthe scant SR.
ATP
Pi
Pi
Extracellular fluid (ECF)
ADP
Ca2+
Ca2+
Ca2+
Plasma membrane
Sarcoplasmicreticulum
Inactive calmodulin
Inactive kinase
Inactivemyosin molecule
Activated (phosphorylated)myosin molecule
Activated kinase
Activated calmodulin
Cytoplasm
Ca2+ binds to andactivates calmodulin.
Activated calmodulinactivates the myosinlight chain kinaseenzymes.
The activated kinase enzymescatalyze transfer of phosphateto myosin, activating the myosinATPases.
Activated myosin forms crossbridges with actin of the thinfilaments and shortening begins.
Thinfilament
Thickfilament
1
2
3
4
5
E-C coupling in smooth muscle
Next slides show the details
Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 1
Calcium ions (Ca2+)enter the cytosol fromthe ECF via voltage-dependent or voltage-independent Ca2+
channels, or fromthe scant SR.
Extracellular fluid (ECF)
Ca2+
Ca2+
Plasma membrane
Sarcoplasmicreticulum
Cytoplasm
1
Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 2
Ca2+
Inactive calmodulin Activated calmodulin
Ca2+ binds to andactivates calmodulin.
2
Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 3
Inactive kinase Activated kinase
Activated calmodulinactivates the myosinlight chain kinaseenzyme (MLCK).
3
Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 4
ATP
Pi
Pi
ADP
Inactivemyosin molecule
Activated (phosphorylated)myosin molecule
The activated kinase enzymescatalyze transfer of phosphateto myosin (phosphorylation), activating the myosin.
4
Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 5
Activated myosin forms crossbridges with actin of the thinFilaments, and shortening begins.
Thinfilament
Thickfilament
5
Smooth muscle relaxation• Active export of Ca2+ from cytoplasm into SR and
extracellular fluid (caveolae), which causes
• Ca2+ detachment from calmodulin which leads to
• Inactivation of MLCK, which allows
• Dephosphorylation of myosin to “deactivate” it
• Much slower than in skeletal muscle
Regulation of Smooth Muscle Contraction• Neural regulation
• Neurotransmitter binding [Ca2+] in sarcoplasm; either graded (local) potential or action potential
• Response depends on neurotransmitter released and type of receptor molecules
• Hormones and local chemicals regulate contraction• Histamine, excess C02, pH, etc• May either enhance or inhibit Ca2+ entry
Special Features of Smooth Muscle ContractionStress-relaxation response• Stretch causes brief contraction, then muscle adapts to new
length• Retains ability to contract on demand• Stress-relaxation response enables organs such as stomach,
bladder to expand significantly
Length-tension relationship• Can generate contractile force when between half and twice
its resting length (much wider range than skeletal muscle – why?)
Special Features of Smooth Muscle ContractionSingle unit• Cells connected by gap junctions so they all contract
together• Prominent stress relaxation response• Walls of most hollow organs: gut, bladder, uterus…
Multi-unit• Cells not connected by gap junctions• Allows finer control of force• Walls of large arteries, large airways, iris, arrector pili