Excitation Contraction Coupling in SMOOTH Muscles || Its DIFFERENT than in Skeletal Muscle

In this video, we will talk about  excitation-contraction coupling, in smooth muscles. Let's get started. This is a schematic diagram  of smooth muscle fiber. Calcium ions play a crucial role, in  the contraction of smooth muscles. And most of the stimuli that cause  smooth muscle contraction, do so by increasing calcium entry into the cell. So our discussion starts with the entry of calcium into the smooth muscle fiber. There are two sources of this calcium. Extracellular fluid, and sarcoplasmi

c reticulum. Extracellular fluid is the main source. But there are some interesting things to  know about sarcoplasmic reticulum as well. The sarcoplasmic reticulum in smooth muscles is  not as well developed, as in skeletal muscles. It lies near the cell membrane. Close to them, the cell membrane forms  small invaginations, called caveolae. These caveolae contain calcium channels. When action potential reaches the caveolae, these channels open, and allow diffusion  of extracellular calcium into

the cell. Some of this calcium goes to Ryanodine  Receptor calcium channels located on sarcoplasmic reticulum, and opens them. Their opening allows diffusion of sarcoplasmic  reticulum calcium, into the cytoplasm. So this is how calcium enters the cytoplasm. This process in smooth muscles, is analogous to what happens at  triad junctions in skeletal muscles. There also, the entry of extracellular  calcium from T tubules, increases calcium entry from closely located  sarcoplasmic endoplasmic ret

iculum. Now moving towards the contraction  part… the entry of calcium, increases the calcium concentration inside the cell. Inside the cell, the calcium binds with a protein called calmodulin. The activity of this enzyme is modulated by calcium. Hence the name cal, modulin. And its complex with calcium is  called calcium-calmodulin complex. Next, we have an enzyme called  myosin light chain kinase. The calcium-calmodulin complex  activates this enzyme. Now it's time to bring in the contractile 

machinery(): the actin, and the myosin filaments. The myosin molecule has a regulatory  light chain in the neck region. It regulates the activity of the myosin head. Did you notice any connections in the names here? This chain is light chain, and this  enzyme, is light chain kinase. So as per what kinases do, this kinase  phosphorylates the regulatory light chain. This phosphorylation increases  the ATPase activity of myosin. The ATPase activity, is a part of  the myosin cross-bridge cycle. So

when this activity is present,  the cross-bridge cycle can occur. The cycle keeps happening as  long as this regulatory light chain is in a phosphorylated state. This produces muscle contraction. So this is how the excitation leads to  muscle contraction, in smooth muscles. Now let's talk about how  this process is terminated, to cause relaxation of the smooth muscle. First, the closure of calcium channels prevents  further entry of calcium into the cell. Also, the calcium pumps on the cell  mem

brane, pumps calcium out of the cell. And pumps on the sarcoplasmic reticulum,  pumps calcium back into the reticulum. So cytosolic calcium concentration falls. So calcium dissociates from Calmodulin, and the myosin light chain kinase  also returns to the inactive state. To remove the phosphate from the  regulatory light chain, we have a special enzyme called myosin phosphatase. It breaks phosphate off from the light chain. In a non-phosphorylated state, the  ATPase activity of the head is lost.

So myosin cross-bridge cycle also stops. So this is how the entire process is reversed. Now let's have a quick summary. The calcium for smooth muscle contraction, comes from extracellular fluid, as  well as sarcoplasmic reticulum. Inside the cell, the calcium binds with  calmodulin, and then this calcium-calmodulin complex activates myosin light chain kinase. This enzyme phosphorylates the regulatory light chain of myosin, which restore the  ATPase activity of the myosin head. This allows the m

yosin cross-bridge  cycle, producing muscle contraction. To reverse the process, the calcium  is pumped back into the extracellular fluid, and sarcoplasmic reticulum. This inactivates myosin light chain kinase. And the phosphate from the regulatory light  chain, is removed by myosin phosphatase. That's it for this video.