>>In this video, you will learn about excitation–contraction coupling for smooth muscle, but to do that
you will need to know a little more anatomy of the smooth muscle. So the first thing to
notice about smooth muscle is the actin and myosin. Are those filaments oriented horizontally
or diagonally? Diagonally, right? So you have a diagonal orientation of actin and myosin.
So actin and myosin are present in smooth muscle. In fact, all types have these proteins,
but in smooth muscle they are oriented on a diagonal. The other thing is that these
filaments are connected by dense bodies; these anchor the thin and thick filaments. When
smooth muscle contracts, (so here we are comparing Figure A before contraction and Figure B after
contraction) when it contracts smooth muscle bulges, and that’s because your actin and
myosin are diagonally orientated. Things are very different than skeletal, aren’t they?
In smooth muscle, there is no troponin, no T-tubules, and the sarcoplasmic reticulum
(or the endoplasmic reticulum) is poorly developed, and tropomyosin does not block the myosin
binding sites. Hence, the myosin binding sites are always exposed because tropomyosin does
not cover the myosin binding sites. Okay, so hopefully you are thinking, “Well, then
what’s going to prevent a crossbridge from randomly forming?” Let’s talk about that.
So here we have calcium. In smooth muscle, calcium binds to calmodulin, and that activates
myosin light-chain kinase: MLCK. What does kinase always do? It phosphorylates. So when
myosin light-chain kinase is activated, it will phosphorylate the myosin head. This will
then increase myosin ATPase activity, which allows for a crossbridge to form. So in order
for a crossbridge to form, the myosin head must be phosphorylated. In terms of a calcium
source, the smooth muscle endoplasmic reticulum is poorly developed. So some calcium comes
from the ER, but most of the calcium comes from the extracellular fluid. So let’s start
putting all this together. Here we go, these are the steps in excitation–contraction
coupling for smooth muscle. We are going to start off with calcium influx—calcium
influx from the extracellular fluid and the endoplasmic reticulum into the cytosol of
the smooth muscle cell. Once calcium moves on in, calcium binds to calmodulin. This is
labeled step two. That then causes myosin light-chain kinase to be activated in step
three. Then the myosin head is phosphorylated, which increases myosin ATPase activity, which
then allows for the formation of a crossbridge. All right, so way back in the intercellular
communication module, you learned about a second messenger that binds to the endoplasmic
reticulum causing the calcium channel to open. Which second messenger am I referring to?
The answer is…IP3. So that concludes excitation–contraction coupling in smooth muscle. We will have
another video to discuss relaxation of smooth muscle.