Muscles and Muscle Tissue: Smooth Muscle Part C2 Prepared by Janice Meeking, W. Rose, and Jarvis Smith. Figures from Marieb & Hoehn 8 th ed. Portions copyright Pearson Education

Smooth Muscle Found 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 Ca 2+ (instead of SR) No T tubules No sarcomeres or myofibrils Yes actin & myosin filaments No tendons; endomysium connects to surrounding tissue

Smooth Muscle Innervation No 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)

Smooth muscle cell Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction). Synaptic vesicles Mitochondrion Autonomic nerve fibers innervate most smooth muscle 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 Ca 2+ 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. Figure 9.28 Intermediate filament Dense bodies Nucleus Caveolae (a) Relaxed smooth muscle fiber (b) Contracted smooth muscle fiber Dense bodies Nucleus 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 Ca 2+ 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 (Ca 2+ ) enter the cytosol from the ECF via voltage- dependent or voltage- independent Ca 2+ channels, or from the scant SR. ATP PiPi PiPi Extracellular fluid (ECF) ADP Ca 2+ Plasma membrane Sarcoplasmic reticulum Inactive calmodulin Inactive kinase Inactive myosin molecule Activated (phosphorylated) myosin molecule Activated kinase Activated calmodulin Cytoplasm Ca 2+ binds to and activates calmodulin. Activated calmodulin activates the myosin light chain kinase enzymes. The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases. Activated myosin forms cross bridges with actin of the thin filaments and shortening begins. Thin filament Thick filament E-C coupling in smooth muscle Next slides show the details

Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 1 Calcium ions (Ca 2+ ) enter the cytosol from the ECF via voltage- dependent or voltage- independent Ca 2+ channels, or from the scant SR. Extracellular fluid (ECF) Ca 2+ Plasma membrane Sarcoplasmic reticulum Cytoplasm 1

Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 2 Ca 2+ Inactive calmodulinActivated calmodulin Ca 2+ binds to and activates calmodulin. 2

Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 3 Inactive kinaseActivated kinase Activated calmodulin activates the myosin light chain kinase enzyme (MLCK). 3

Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 4 ATP PiPi PiPi ADP Inactive myosin molecule Activated (phosphorylated) myosin molecule The activated kinase enzymes catalyze transfer of phosphate to myosin (phosphorylation), activating the myosin. 4

Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 5 Activated myosin forms cross bridges with actin of the thin Filaments, and shortening begins. Thin filament Thick filament 5

Smooth muscle relaxation Active export of Ca 2+ from cytoplasm into SR and extracellular fluid (caveolae), which causes Ca 2+ 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   [Ca 2+ ] 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 Ca 2+ entry

Special Features of Smooth Muscle Contraction Stress-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 Contraction Single 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