What is 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.
FAQs on Smooth Muscle
1. What is smooth muscle and what are its main functions?
Smooth muscle is a type of involuntary, non-striated muscle found in the walls of our internal organs. Unlike the muscles in our arms, it appears smooth under a microscope because its protein filaments are not arranged in a striped pattern. Its main job is to manage bodily functions we don't consciously control, like pushing food through the digestive system or regulating blood pressure by adjusting the width of blood vessels.
2. Where can we find smooth muscles in the human body? Give some examples.
Smooth muscles are primarily located in the walls of hollow internal organs. Common examples in the body include:
- The stomach and intestines, where they contract to digest and move food along.
- The walls of blood vessels, helping to control blood flow and pressure.
- The urinary bladder, which they allow to stretch and contract.
- The airways of the lungs, known as bronchioles.
- The uterus in the female reproductive system.
3. What is the main difference between smooth muscle and skeletal muscle?
The key difference lies in their control and structure. Skeletal muscles, like your bicep, are under voluntary control, meaning you decide when to move them. They also have a striped or 'striated' appearance. In contrast, smooth muscles work automatically without you thinking about them (involuntary control) and have a smooth, non-striated appearance.
4. How does the contraction of smooth muscle work?
Smooth muscle contracts when its thin filaments (actin) slide past its thick filaments (myosin), powered by energy from ATP. This process is triggered by calcium ions. Because these filaments are arranged in a crisscross pattern, the cell contracts in a corkscrew-like manner, which is very effective for squeezing hollow organs like the stomach or bladder.
5. Why are smooth muscles also called involuntary muscles?
They are called involuntary muscles because their contractions are not under our conscious control. These muscle actions are automatically regulated by the autonomic nervous system, which manages essential bodily functions like digestion and blood circulation without us having to actively think about them.
6. Why doesn't smooth muscle get tired like the muscles in our arms and legs?
Smooth muscle is built for endurance, not speed. It contracts slowly and uses energy (ATP) far more efficiently than skeletal muscle. This allows it to sustain contractions for very long periods without fatiguing, which is essential for organs like the intestines or blood vessels that must work continuously.
7. How does the unique structure of smooth muscle cells help them perform their function?
The spindle shape and the net-like arrangement of filaments in smooth muscle cells are key to their function. This structure allows them to stretch significantly without being damaged and to contract in a way that squeezes the organ from all sides. This is perfect for organs like the bladder or stomach that need to change volume dramatically.
8. Besides striations, what are some other key differences between smooth, skeletal, and cardiac muscles?
Beyond the striped appearance, there are several other important differences:
- Control: Skeletal muscle is voluntary, while smooth and cardiac muscles are involuntary.
- Cell Shape: Smooth muscle cells are spindle-shaped with one nucleus. Skeletal muscle cells are long cylinders with many nuclei. Cardiac cells are branched and interconnected.
- Location: Skeletal muscles are attached to bones, smooth muscles line internal organs, and cardiac muscle is found exclusively in the heart.
