Smooth muscle Flashcards
What are the 2 categories of smooth muscle?
- Unitary or visceral connected by low resistance gap junctions and act as a functional syncitium.
- Multiunit smooth muscle, a collection of NOT interconnected cells.
Describe smooth muscle actin and myosin organization.
Mostly disordered with dense bodies (kinda like z lines) and dense plaques (like focal adhesions)
What are 3 types of actin in smooth muscle?
alpha and gamma (contractile) and beta (cytoplasmic)
Unlike skeletal muscle which has bipolar myosin filaments, smooth muscle myosin is _____-polar.
Side-polar, which allows complete cross over of filament on one side.
Why can smooth muscle thin filaments shorten so extensively?
Filaments are quite a bit longer and the lack of z line means filaments aren’t limited by the length of a sarcomere and can overlap extensively.
What ways can Ca2+ enter the smooth muscle cell to trigger contraction?
- Voltage/ligand gated channel
- Ryanodine calcium receptor (+ feedback)
- IP3 gated calcium channel
How is myosin activity regulated in smooth muscle?
Phosphorylation. No troponin/tropomyosin!!
How does the myosin kinase (MLCK) get activated?
4Ca2+ calmodulin binding
How does calcium affect myosin-dephosphorylation (contractility agonist)?
See pic
How does protein kinase G act as a contractility antagonist?
see pic
What are tonic vs phasic smooth muscles?
Tonic expend little energy and can stay contracted for ages (e.g. artery) while phasic consume a lot (e.g. GI contraction/relaxation)
How do tonic muscles maintain contraction?
Latch state: modification of the crossbridge cycle, possibly by dephosphorylation of crossbridges while they are still attached to actin.
What is stretch-activation and how does it work?
When smooth muscle is stretched, it automatically contracts most likely via stretch-activated channels that lead to depolarization.
When SM stretch is insufficient to reach threshold, it relaxes (stress-relaxation). What property is this an example of?
Visco-elasticity
Are multiunit or single unit smooth muscles governed by pacemakers?
Single unit. Multinuit are innervated individually.
Are increased electrical spikes (higher frequency) associated with greater muscle tension in SM?
Yes
What is peristalsis?
The gradual, wavelike contraction typical of smooth muscle
How does tension change in smooth muscle?
The tone/tension can vary and they’re never truly contracted or relaxed.
Smooth muscle can contract down a ____% of their original length.
Major, due to structure of SM.
The velocity of smooth muscle contraction is quite _____, because ATP is split _______ by the myosin isozyme (myosin ATPase).
slow, slower,
What are 6 important properties of smooth muscles?
- High % contraction
- Slow contraction
- Vary tone
- Latch (maintain tone w/out energy expenditure)
- Stretch activation
- Pacemaker
How do SMs vary tone so well?
No troponin, use signal transduction instead
Can smooth muscles achieve the same level of maximum force as skeletal muscle?
Yes, smooth muscles are economical (slow/less ATP) but can be just as strong
Can the MLCK function in the absence of calcium-calmodulin?
No!
How does cAMP affect relax the muscle?
Activates PKA which increases calcium sequestration and thus relaxation.
Does activation of beta receptors increase or decrease smooth muscle contractility?
Decrease
Does activation of alpha receptors increase or decrease smooth muscle contractility?
Increase
What is the action of rho kinase?
Phosphorylates the phosphatase and diminishes its activity. This will lead to longer A-M binding and increase contractility!
How does cGMP affect contractility?
Activates protein kinase G (PKG) which inhibits Rhokinase ultimately leading to LESS contraction.
How does NO affect contractility?
Decreases contractility by activating cGMP.
How does calcium act in the latch mechanism?
Once tension is reached, calcium rises with tension but then falls down to some super basal level but NOT zero.
What part of myosin is unstable in aqueous environments?
Single chain helices (part of the flexible lever arm). Stabilized by phosphorylation
What happens to myosin if not stabilized by phosphorylation?
Myosin heads stick to each other and bind to their own thick filament
When stretch activated channels are activated, what enters the cell?
Positively charged ions causing local depolarization which can open nearby voltage-gated channels
