Smooth Muscle Contraction Flashcards
Types if smooth muscle:
Unitary
Multiunit
Unitary smooth muscle:
Sheets of cells which act in unison - syncytium
Where is unitary smooth muscle usually found?
Gut
Multiunit smooth muscle:
Bundle of cells
Electrical isolation of cells
Where is multiunit smooth muscle found?
Finer tissues - vas deferens
How are unitary and multiunit smooth muscle different?
Unitary have gap junctions
What’s obviously different between smooth muscle structure and skeletal muscle?
No visible striations - filaments not aligned by Z discs
How are actin and myosin filaments arranged on smooth muscle?
Diagonally
Attached at dense bodies (a-actinin-rich) throughout sarcoplasm
What other types of filaments does smooth muscle have?
Contractile arrays - fixed by intermediate filaments (vimentin and desmin)
How are contractile arrays anchored?
By dense plaques to sarcolemma
How are smooth muscle cells connected to eachother?
By focal adhesions (adherens junctions)
Thick myosin filaments in smooth muscle compared to skeletal:
Same tertiary structure
Different amino acid sequence
What chains do myosin filaments contain?
2 myosin heavy chains and 2 myosin light chains
How are thin actin filaments in smooth muscle different to skeletal?
Slightly different structure - have two smooth muscle actin isoforms
The two isoforms of smooth muscle actin:
Alpha and gamma
What do smooth muscle thin filaments contain?
Tropomyosin
NO troponin
What do thin filaments have instead of troponin?
2 regulatory proteins - caldesmon and calponin
How are actin sites exposed?
Calcium-calmodulin complex
OR
Phosphorylation of regulatory proteins by calcium-calmodulin dependent protein kinase
Difference in sarcolemma of smooth muscle to skeletal:
No T-tubules
Have Caveolae
Difference in SR with smooth muscle and skeletal:
Not as extensive
What form must myosin be in to bind to actin?
Phosphorylated form - allows cross-bridge cycling
How is myosin phosphorylated?
Myosin light chain kinase (MLCK)
What must happen to myosin to allow relaxation?
Dephosphorylated - no longer able to bind
How is myosin dephosphorylated?
Myosin light chain phosphatase (MLCP)
What initiated smooth muscle contraction?
Calcium from ECF and SR
What happens when calcium enters cytoplasm?
Binds to calmodulin to form calcium-cadmodulin-MLCK complex
What happens to the calcium complex?
Results in phosphorylation of myosin-light chain
Requires ATP
What then happens to phosphorylated myosin?
Binds to actin
Power stroke
What is required to release myosin head from actin?
ATP
What must happen for smooth muscle relaxation?
Decrease in calcium concentration
How is calcium concentration decreased?
Pumped out of cell
OR
Pumped into SR
What does low calcium concentration cause?
Calmodulin releases calcium
Why does low calcium concentration lead to relaxation?
Myosin light-chain phosphatase removes phosphate from MLC
Causes detachment of myosin head from actin filament
3 mechanisms that increase calcium concentration:
Voltage-gated L-type calcium channels
Receptor-operated calcium channels
Store operated calcium channels
Voltage-gated L-type calcium channels:
Result in calcium-induced calcium release
Activates ryanodine receptor on SR
Receptor-operated calcium channels:
Results in IP3 receptor activation on SR
Releases calcium
Store operated calcium channels:
Maintains calcium concentration in SR
What must happen for smooth muscle to maintain high force for long periods?
Formation of latchbridges
Slow cycling rate - minimal ATP use
What is a latch bridge?
Dephosphorylated myosin still attached to actin
When can latchbridges only occur?
High intracellular calcium concentration
What removes calcium?
Calcium ATPase pump (SERCA)
Socdium-calcium exchangers
What refills SR stores of calcium?
Store-operated calcium channels
What senses low calcium concentration in SR?
Stim1 - signals to Orai
Opens store operated calcium channels —-> influx of calcium
WhatWhat regulates smooth muscle contraction?
Spontaneous electrical activity
Stretch
Neurotransmitters
Hormones
Locally induced changes in chemical composition in ECF
How can smooth muscle cells generate action potentials?
By generation of pacemaker potential
What happens if membrane potential drifts up and down in smooth muscle cells?
Slow waves of activity - result in action potential if excitatory stimulus applied
Where are pacemaker cells found?
GI tract
Why do smooth muscle cells have varicosities?
No motor end-plate region
What do varicosities contain?
Neurotransmitters in vesicles
Position of varicosities:
From single axon - located along several muscle cells
What do varicosities originate from?
Postganglionic fibers of sympathetic and parasympathetic neurons
Neurotransmitters and contractile activity:
Can both stimulate or decrease activity
Why may neurotransmitter produce opposing effects in contractile activity?
Different interactions with G-proteins
What can local factors include?
Paracrine signals
Acidity
Oxygen and carbon dioxide levels
What can local factors cause?
Alteration of smooth muscle tension
Induce relaxation
