Muscle Tissue Overview

Smooth Muscle

Smooth or visceral muscle gets its name because it is not striated and appears smooth. This type of muscle is involuntary and found in blood vessels and many organs such as the intestines, stomach, urinary bladder, and uterus.  Smooth muscle contractions control blood flow by vasoconstriction or vasodilation blood vessels.  This muscle also propels food through the digestive tract and causes uterine contractions. 

The endothelial layer of blood vessels is capable of releasing chemicals to control the lumen size of the blood vessel.  These chemicals respond to various stimuli such as increased temperature or pH changes to regulate the distribution of blood to tissues that have a greater demand.

 Exercise and Smooth Muscle

Exercising skeletal muscles have an increased demand for oxygen. The heart supplies the blood but the smooth muscle of the blood vessels controls the delivery of oxygen to where it is most needed.  The ability to maintain VO2max is affected by the ability of smooth muscle to route blood to its area of demand.  The endothelium secretes substances such as nitric oxide (NO) and ATP to relax the smooth muscle and vasodilate blood vessels in areas supplying skeletal muscle.  It has been shown that the red blood cells themselves release substances which interact with vascular smooth muscle and control their delivery to active tissues (Richardson, 2003).

The ability of smooth muscle to respond to increased demand is also influenced by physical activity and fitness level.  Sedentary individuals have been shown to have a decreased sensitivity of smooth muscle to increased oxygen demand and a reduced responsiveness necessary to increase oxygen delivery to active muscles.  Research has also documented that after an 8 week exercise program, previous sedentary individuals exhibited an increased vascular response in smooth muscle during exercise (Richardson, 2003). The ability of exercise to also increase the function of smooth muscle is an important and interesting development that has only recently been observed.   If smooth muscle in physically fit individuals is better able to adjust to meet increased oxygen requirements of skeletal muscle, perhaps, smooth muscle in those individuals can also compensate for hypoxia in other tissues.  Imagine the implications if exercise can also increase the responsiveness of the smooth muscle of coronary arteries to increased oxygen demand by cardiac muscle.

 

Notice the following:

 

• smooth muscle is long & lacks striations
• smooth muscle has a single nucleus per cell


 

 

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(Created by Wendy M. Rappazzo, Harford Community College 8/2007)