Smooth Muscle

Define smooth muscle -  Smooth muscle under microscope shows that there are no cross stripes in the smooth muscles. Smooth muscle is also called Involuntary muscle or unstriated muscle. Smooth muscles are mainly divided into two subgroups: a single unit and a multi-unit smooth muscle. You will have some basic understanding of the appearance referring to the below smooth muscle diagram. 

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Smooth muscle Diagram 

Smooth muscles are found in the hollow organs like the stomach, intestine,  urinary bladder and uterus, and in the walls of the passageways, circulatory system, and in the tract of the respiratory, urinary and reproductive system. They are also found in the eyes which are used to dilute and contract the iris, and in the skin which makes the hair to stand erect in response to cold temperature or fear. 

Smooth Muscle Structure 

The smooth muscle, each cell is spindle-shaped and it is three to ten micrometre thick and 20 - 200 micrometre long. Most of the smooth Muscles are muscles, which means the whole muscle contracts or the whole muscle relaxes, but some of the muscles are a multiunit muscle like the trachea, large elastic arteries and the iris of the eyes. Single unit smooth muscles are commonly found in the lines of the blood vessels (excluding only large elastic arteries), the urinary tract and the digestive tract. 

The term single and multi-unit smooth muscles represent the fact that smooth muscles for the most part are controlled and influenced by a combination of different neural elements. Sometimes it is seen that there is cell to cell communication and activator or inhibitors are produced locally, this leads to a coordinated response in multi-unit smooth muscles. 

The structure of involuntary muscles or smooth muscle is fundamentally different compared to the skeletal muscle and the cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling. Smooth muscles are known well as myocytes having a fusiform shape and like striated muscles can tense and relax. Smooth muscles tend to have greater elasticity and it functions within a larger length-tension curve compared to the striated muscles. The ability to stretch and main its contractibility is important in some of the organs like the intestines and the urinary bladder. 

Molecular Structure of Smooth Muscle 

The volume of the cytoplasm of the smooth muscle cells substantial portion is occupied by the myosin and the actin. Both of them together have the ability to contract through the chain of tensile structures. Myosin is primarily class two in the smooth muscle and actin is the thin filament that takes part in the contractile machinery composed of alpha and gamma actin. 

Characteristics of Smooth Muscle 

The smooth muscle cells consist of long narrow and unbranched spindle-shaped fibres. Each fibre of the smooth muscle cells contains a thick oval nucleus in its thick middle part. The cross striation is absent in the smooth muscles, so their fibres look smooth hence it is named non-striated. The contraction of the smooth muscle is not under the control of the animal, so it is called involuntary muscles.  

Smooth Muscles in Invertebrates 

In invertebrates, Smooth muscle contraction begins with the binding of calcium directly to the myosin and then rapidly cycling cross bridges, they generate force.  The mechanism of the invertebrates smooth muscle is similar to the vertebrate smooth muscle. In the vertebrate smooth muscle, there is low calcium and low energy utilization catchphrase. 

Do You Know?

Why is smooth muscle more efficient compared to skeletal muscle?

Vascular smooth muscle generates comparable force utilizing three hundred times less chemical energy comparing 50 times shorter vascular smooth muscle with the facts skeletal muscle. This difference is because of the mechanical properties of the striated and unstriated muscle, responsible for muscle shortening and force production.