The muscular system is responsible for a large part of our body movements. There are about 700 named muscles attached to our skeletal system. These muscles help us in performing a range of motions, from extremely minute movements which govern the functioning of different organs, to the bigger, more aggressive ones used for locomotion. These muscles make up about half our body weight, and are present almost everywhere in the body- almost every organ makes use of muscles for some function or the other.

There are broadly three variations of muscles found in our body. These muscles are:

• Skeletal muscles- responsible for voluntary movements.
• Cardiac muscles, which are found in the heart, and,
• Smooth muscle fibres –  responsible for the involuntary movements in our body.

Smooth muscle fibres- They are found lining the inside of organs like the stomach. They have a unique cell structure, which is what gives them their characteristic smooth texture. They perform a range of tasks from transporting substances to the regulation of urine flow.

Types of muscles

• Skeletal muscles: These are the most frequently found muscles in our body.  Since they are directly under the control of the central nervous system, these muscles can be consciously controlled by us. These muscles are attached to different bones in our body and work in tandem with the skeletal system to produce different movements. These muscles produce irregular, strong contractions
• Cardiac muscles- These muscles are found in the walls of the heart. Under control of the autonomic nervous system, these muscles ensure the constant and healthy functioning of the heart, so that our organs are regularly supplied with blood. Cardiac muscles experience strong rhythmic movements.
• Smooth muscle fibres: These muscles are found in places like the gastrointestinal tract, the lungs, and the uterus. Smooth muscle fibres are under the control of the autonomic nervous system. This means that the functioning of these muscles is automatically regulated by the body.  These muscles have a smooth texture, as they do not have any stripes or “striations” like the other two types of muscles. The contractions in these muscles are usually slow and rhythmic.

Structure of a smooth muscle

When seen at a cellular level, smooth muscle fibres can be described as involuntary muscles which are non-striated. Understanding the structure of the smooth muscle fibres is seminal to answering the question, “What are smooth muscle fibres?”

Smooth muscles consist of fibres of different thicknesses. These aren’t arranged in groups but are in random order. This is why under a microscope the cells look uniform and smooth. The cytoplasm in a smooth muscle cell contains a large amount of actin and myosin- the main chemicals that are involved in muscle contraction. A smooth muscle cell is about 3 to 10 micrometres thick and 20 to 200 long. It is long and spindle-shaped and the nucleus is located in the centre. 

Smooth muscle fibres form groups in the form of branching bundles. Unlike skeletal muscles, these bundles are not grouped in any order but run in different ways forming a much more complicated structure. This is why smooth muscle fibres have much stronger contractions than skeletal muscles.

Functions of smooth muscles

Smooth muscles are generally found inside hollow organs like the stomach, Blood vessels, uterus, liver, intestines, and pancreas. They serve a variety of purposes throughout the body, including the regulation of most of the organs, and ensuring their smooth functioning.

Some functions of smooth muscles are:

• It helps regulate the secretion of various enzymes by controlling the ducts of the exocrine glands. The smooth muscles contract or relax, making the ducts close or open, which either allows or restricts the flow of enzymes
• Smooth muscles throughout the digestive system help in transporting the digested food through the different tracts and chambers. This is done by forming wave-like contractions in order to push the food forward.
• They are present at the urethra in the form of sphincters and help in the regulation of urine flow.
• Smooth muscles in the uterus facilitate cramps when a baby is being delivered, which helps to push the baby out of the uterus.
• Smooth muscles are present even in our eyes and perform various functions like adjusting the shape of the lens and controlling the size of the pupil

They are present inside blood vessels and take a part in the regulation of blood flow and controlling blood pressure.

Conclusion

Our body is always full of motion. Even when we are sleeping, a thousand things are going on inside our body, and the internals of our body is rich with movement. Perhaps the most obvious of these movements is those facilitated by the muscular system. In this article, we learned about smooth muscle fibres. In the future, this article might serve as a smooth muscle tissue explanation.