Lecture: Muscles and Muscle Tissue 3 Smooth Muscle

Found in the walls of most hollow organs (except heart)

Usually in two layers (longitudinal and circular)

Alternating contraction and relaxations of smooth muscle layers that mix and squeeze sunstances through the lumen and hollow organs (like a propelltion)

• *longitudinal layer contracts--> organs dilates and shortens
• *Circular layer contracts--> organ constricts and elongates

-Spindle shaped fibers: thin and short compared with skeletal muscle fiberts

-Connective tissue: endomysium only

-SR: less developed than in skeletal muscle

-Pouchlike infoldings (caveolae) of sarcolemma sequester Ca²⁺

-No sarcomeres, myofibrils, or T tubules

Autonomic nerve fibers innervate smooth muscle at diffuse junctions

• These variscosities (bulbous swellings) of nerve fibers store and release neurotransmitters
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-Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)

-Thick filaments have heads along their entire length

-No troponin complex; protein calmodulin binds Ca²⁺

-Myofilamets are spirally arranged, causing smooth muscle to contract in a corkscrew manner

-Dense bodies: protiens that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

-Slow, synchronized contractions

-Cells are electrically coupled by gap junctions

-Some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle

-Rate and Intensity if contraction may be modified by neural and chemical stimuli

Sliding filament mechanism

Final Trigger is a raised intracellular Ca²⁺

Ca²⁺ is obtained from the SR and extracellular space

Very energy efficient (slow ATPases)

Myofilaments may maintain a latch state for prolonged contractions

• Relaxation requires:
• *Ca²⁺ detachment from calmodulin
• *Active transport of Ca²⁺ into SR and ECF
• *Dephosphorylation of myosin to reduce myosin ATPase activity

-Ca2+ binds to and activates calmodulin

-Activated calmodulin activates myosin (light chain) kinase

-Activated kinase phosphorylates and activates myosin

• -Cross bridges interact with actin
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• Neural Regulation:
• -Neurotransmitter binding --> raised Ca2+ in sarcoplasm; either graded (local) potential or action potential
• -response depends on neurotransmitter released and type of receptor molecules

• Hormones and Local chemicals:
• -may bind to G protein-linked receptors
• -may either enhance or inhibit Ca²⁺ entry

• Stress-relaxation response:
• -responds to stretch only briefly, then adapts to new length
• -retains ability to contract on demand
• - enables organs such as the stomach and bladder to temporarily store contents

• Length and Tension changes:
• -Can contract when between half anf twice its resting length

• Hyperplasia:
• -Smooth muscle cells can divide and increase their numbers
• Example: estrogen effects on uterus at puberty and during pregnancy

• Single Unit (visceral) smooth muscle:
• -sheets contract rhythmically as a unit (gap junctions)
• -Often exhibit spontaneous action potentials
• -Arranged in opposing sheets and exhibit stress-relaxation response

• Multi-Unit:
• -Located in large airways, large arteries, arrector pili muscles, and iris of eye
• -Gap junctions are rare
• -Arranged in motor units
• -graded contractions occur in response to neural stimuli