Question

In Week 2, we saw how cells of similar size, shape, and function form tissues. As a tissue, muscle is very diverse. Let's kick off our study of muscles by discussing the subtypes of muscle tissue, beginning with smooth muscle. Where do we find this muscle tissue, how does it contract, and what are its defining characteristics? In what ways are smooth muscles working in your body right now?

Here's the muscle discussion! I bet you will learn something new about muscles this week. First we will tackle smooth muscles. Where are they? How do they work? What do they do?

Answer 1

Smooth muscles are one of the 3 muscle types found in our body - the other being skeletal and cardiac.

They are fusiform spindle shaped muscle cells called myocytes which contract as one unit. They may be 2-500um in length and have high contractility as well as elasticity.

Smooth muscles are found in the linings of the internal organs of the body including the blood vessels.

They are involuntary muscles ( not under our control) and maybe stimulated by nerves, hormones and various other chemicals. Some smooth muscles like those found in the GI tract have pace maker interstitial cells which cause rhythmic contractions of the muscles which is required for the peristaltic movement.

It's function varies from the peristaltic movement in the digestive system to changing the lumen diameter of various anatomical structures like blood vessel, sphincters, bronchioles, etc. They also are know to have secretory functions, eg. Secretion of rain in response to blood pressure.

Their mechNism of contraction is the same as in other types of muscles. Actin and myosin are 2 major protein involved in this. The myosin head interact with actin to form a cross bridge in presence of calcium. This is followed by ATP hydrolysis which causes tilting and release of the myosin head to bind to another location on the actin (10-15um away) resulting in the actin molecules to move closer causing the muscles to contract. The smooth muscle differ from the skeketal and cardiac muscle in the way these actin myosin units are arranged.