Smooth Muscle Contraction Flashcards
A smooth muscle has thick and thin filaments but lacks what structure?
A sarcomere. The thin filaments are anchored to a cytoskeletal specialization called a dense body.
What characterizes smooth muscle contractions?
They are have slow contraction velocity and slow relaxation time, but large contraction (1/3 initial size)
How is the action potential of smooth muscle propagated?
It is Ca dependent instead of Na dependent. The inward depolarizing current is carried out by Ca ions.
What are caveoli? What do they “replace”?
Caveoli are micro domains that are enriched for cell receptors and ion channels. They function similar to the T-tubules in other muscle types. T-tubules are not found in smooth muscle. The caveoli are thus in close proximity to SR or mitochondria.
What is unitary smooth muscle?
It refers to smooth muscle in which millions of cells organized into sheets or bundles contract in a coordinated fashion as a single unit. The smooth muscle is electrically coupled via gap junctions and can be spontaneously active (ie. peristalsis)
What is multiunit smooth muscle?
It is composed of discrete smooth muscle fibers each of which is innervated by a single nerve ending as in skeletal muscle.
Which muscle types utilize two sources of Ca, one from the SR and one from the extracellular fluid?
Cardiac and smooth muscle
Decreasing the influx of Ca with L-type Ca channel blocking drugs is an effective means of reducing contractile activity in what muscle types?
Smooth and cardiac muscle.
In smooth muscle there are multiple ways for Ca to enter a cell, what are they?
Activation of a G protein (Gq), activates phospholipase C leads to formation of IP3 and the release of Ca from SR. There is also some Ca induced Ca release in smooth muscle. Some smooth muscle opens Ca channels in response to stretch.
There is no Troponin C in smooth muscle so how is contraction triggered?
Calcium activation of Myosin Light Chain Kinase (MLCK) results in contraction of smooth muscle. Ca binds to a calmodulin moiety on MLCK resulting in phosphorylation of the Regulatory Light Chain (RLC). The conformation change permits myosin to interact with actin.
How does smooth muscle relax?
MLCP, a phosphatase in the sarcoplasm, dephosphorylates the regulatory light chain of myosin inhibiting the interaction of myosin and actin causing the muscle to relax. Reduction of Ca also results in relaxation.
What are the two categories of smooth muscle contraction?
Electrochemical (involving an AP or stretch) and pharmacomechanical (ligand binding to cell surface receptor). They both have the same downstream effect and operate via increased cytosolic Ca and MLCK
Describe the membrane potential of smooth muscle and the role of specific ions in APs
The resting potential is -50 - -60mV. Depolarization is caused mainly by an inward current of Ca followed by depolarization by an outward current of potassium. Most smooth muscle APs have no Na current.
What is the basal electric rhythm?
It refers to waves of rhythmic depolarization of intestinal smooth muscle cells which originate at a specific point and then are propagated along the length of the GI tract. There is no tension produced with the depolarizations unless stimulatory neurotransmitters are added.
In smooth muscle the cross bridge cycle includes what additional step?
The “latch-bridge” state. Despite drops in Ca and MLC dephospho rylation the muscle maintains tension. This occurs when MLCP dephosphorylation of the myosin light chain happens while the myosin head IS bound to actin. If it is NOT bound then it doesn’t occur.
