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Smooth Muscle is an involuntary unstriped muscle commonly found in the walls of hollow organs and blood vessels.
- It assists with digestion and nutrient absorption in the stomach and intestines.
- It is found throughout the urinary system, where it aids in maintaining electrolyte balance and helps the body get rid of pollutants.
- It is a component of arteries and veins and is essential for the control of blood pressure and tissue oxygenation.
- The ability of the smooth muscle to contract and be regulated involuntarily is what makes it different from skeletal muscles.
- Nervous System uses smooth muscle to closely regulate a number of subsystems of the body without the consciousness of the human.
Read More: NCERT Solutions For Class 11 Biology Structural Organisation In Animals
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Key Terms: Smooth Muscle, Involuntary Muscles, Blood Pressure, Skeletal Muscles, Cardiac Muscles, Urinary System
What is Smooth Muscle?
Smooth Muscles are one of the three major types of muscular tissue along with Skeletal Muscles and Cardiac Muscles.
- They are also referred to as Unstriated or Involuntary Muscles.
- The two main subgroups of smooth muscles are single-unit and multi-unit smooth muscles.
- Smooth muscles are found in the walls of the passageways, the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems.
- They are also found in hollow organs like the stomach, intestine, urine bladder, and uterus.
- They can also be found in the skin, which causes the hair to stand up straight in response to cold temperatures or fear, and in the eyes, which are used to diffuse and constrict the iris.
Smooth Muscles
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Importance of Smooth Muscles
Smooth Muscles regulate the vascular system, which distributes blood throughout the body.
- They contract to control blood pressure and other cardiovascular activities.
- Smooth muscles maintain and control the blood pressure and flow of oxygen throughout the body.
- They assist in the digestion of food through peristalsis, the periodic wavelike contraction of the muscles.
- They exist throughout the urinary system, where they help to get rid of toxins and work in electrolyte balance.
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Smooth Muscle Structure
The structure of smooth muscles can be described as follows:
- Each spindle-shaped smooth muscle cell measures three to ten micrometers in thickness and 20 to 200 micrometers in length.
- Most smooth muscles are multiunit muscles, such as the trachea, big elastic arteries, and the iris of the eyes.
- The walls of blood vessels, the digestive system, the urinary tract, and all except the largest elastic arteries, all include single-unit smooth muscles.
- In terms of form, function, regulation of contraction, and excitation-contraction coupling, smooth muscles have fundamentally distinct properties from skeletal muscle and cardiac muscle.
- Like striated muscles, smooth muscles are well-known as myocytes with a fusiform structure and the ability to contract and release.
- In comparison to striated muscles, smooth muscles act along a bigger length-tension curve and tend to be more elastic.
- Some organs, including the intestines and the urine bladder, depend on their capacity to expand while maintaining their ability to contract.
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Molecular Structure of Smooth Muscles
- Myosin and Actin occupy a significant amount of the smooth muscle cells' cytoplasm in terms of volume.
- They can both contract together through the network of tensile structures.
- In smooth muscles, myosin is largely class two, while actin is the thin filament that participates in the contractile apparatus made up of alpha and gamma actin.
Types of Smooth Muscles
Smooth Muscles can be classified into two major types:
- Single-Unit Smooth Muscle: It is a different kind of smooth muscle that can be found in numerous organs, including the intestines and stomach. It is present both inside and outside the tissues of these organs.
- Multi-Unit Smooth Muscle: It is a type of muscle that is present in the blood vessel walls. It is made up of two or more separate units (cells) that cooperate to contract, which narrows the vessel.
Single-Unit and Multi-Unit Smooth Muscles
Characteristics of Smooth Muscles
The characteristics of smooth muscles are as follows:
- Smooth muscle cells are made up of unbranched, long, narrow fibers that resemble spindles.
- The thick central portion of each smooth muscle cell fiber houses an oval, thick nucleus.
- The fibers of smooth muscles appear smooth because there is no cross striation, thus the term "non-striated."
- Because the animals cannot regulate smooth muscle contraction, these muscles are known as involuntary muscles.
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Smooth Muscles in Invertebrates
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- Smooth Muscles in Invertebrates contract by calcium binding to myosin first, which causes them to cycle quickly across bridges and produce force.
- The smooth muscle of invertebrates and vertebrates both function in a similar manner.
- There is minimal calcium and low energy utilization in the smooth muscle of vertebrates.
Diseases Related to Smooth Muscles
The ability of smooth muscles to function can be impacted by specific diseases.
- A genetic disorder known as Multisystemic Smooth Muscle Dysfunction Syndrome (MSMDS) prevents an embryo from forming adequate smooth muscles for the digestive system.
- Anti-smooth muscle antibodies can also be brought on by several autoimmune diseases, including cirrhosis, lupus, and hepatitis (ASMA).
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Things to Remember
- Smooth muscle is a kind of tissue that is present in the walls of hollow organs like the stomach, uterus, and intestines.
- Smooth muscle is present in the walls of arteries and veins in the cardiovascular system.
- The urinary, pulmonary, and reproductive tracts all contain smooth muscles.
- Smooth muscle can be found in the eyes, where it alters the shape and size of the lens and iris.
- Smooth muscle cell is 20–200 m in length and 3–10 m thick.
- Its cells are stronger, more durable, and use less energy than skeletal muscle cells even though they contract more slowly.
- Single-Unit Smooth Muscle and Multi-Unit Smooth Muscle are the two types of smooth muscles.
Previous Years’ Questions
- Smooth muscles are… (NEET - 2016)
- Largest smooth muscle present in… (UP CPMT - 2002)
- Intercalated discs are characteristic of muscles found in… (JIPMER - 2012)
- During muscle contraction in humans, the… (NEET - 2013)
Sample Questions
Ques. What are Smooth Muscles? (2 Marks)
Ans. Smooth Muscles are a form of non-striated muscles that are present in the walls of hollow organs and blood vessels. Though smooth muscles are present all over the body, they are particularly prevalent in the digestive system, in the walls of blood vessels and arteries, and around other organs like the eyes.
Ques. What types of Smooth Muscles are there in Human Body? State their sanctions. (3 Marks)
Ans. Smooth muscles are divided into two categories namely Single-Unit Smooth Muscle and Multi-Unit Smooth Muscle.
- Several organs, including the stomach and intestines, include single-unit smooth muscles.
- It is present both inside and outside the tissues of these organs.
- Blood artery walls include multi-unit smooth muscle, a form of smooth muscle.
- It is made up of two or more distinct cells that cooperate to contract, which narrows the vessel.
The functions of smooth muscles are as follows:
- Numerous components of the digestive system, the circulatory system, and even the hairs on the arm are lined with smooth muscles.
- Smooth muscles in the circulatory system are essential for maintaining and regulating blood pressure and the passage of oxygen throughout the body.
Ques. Why do smooth muscles outperform skeletal muscles in terms of efficiency? (2 Marks)
Ans. In comparison to skeletal muscle, which is 50 times longer, vascular smooth muscle creates the same amount of force while using 300 times less chemical energy. The mechanical characteristics of striated and unstriated muscles, which are in charge of muscle shortening and force output, account for this discrepancy.
Ques. What Makes Smooth Muscle Unique? (2 Marks)
Ans. Smooth muscles do not have striations like skeletal muscles and cardiac muscles because actin and myosin are not regularly arranged throughout the length of the cell. Instead, filament arrangements have a less structured form and take the form of a spiral.
Ques. Describe the structure of Smooth Muscles. How many smooth muscles are there in the human body? (3 Marks)
Ans. Smooth Muscle Cells have a single central nucleus, are spindle-shaped, elastic, and lack striae. Since smooth muscle cells are organized in sheets, they can contract at the same time. They do, however, include sarcomas and other typical cell organelles, albeit in smaller quantities.
Over 650 identified skeletal muscles are thought to be present in the body. The body can have billions of smooth muscle cells since other types of muscle tissue, including smooth muscle, normally develops at the cellular level.
Ques. How Does Smooth Muscle Contract? (1 Mark)
Ans. Myosin and actin filaments moving over one another is what causes the smooth muscle to contract. ATP hydrolysis supplies the necessary energy for this process to take place.
Ques. How Do Involuntary Muscles Work? (2 Marks)
Ans. Involuntary Muscles are often referred to as non-striated or unstriated muscles. These muscles are referred to as involuntary muscles since they contract without the animal's consent or awareness. The autonomic nervous system manages these muscles.
Ques. Describe the shape of Smooth Muscles. (3 Marks)
Ans. Smooth Muscles, also known as involuntary muscles, are muscles that do not exhibit cross-stitching when magnified at a microscopic scale. It is made up of slender, spindle-shaped cells with a lone, central nucleus.
The structure of Smooth Muscles is as follows:
Ques. Which Smooth Muscle is the most common? (2 Marks)
Ans. The walls of hollow organs are formed by a type of smooth muscle known as unitary smooth muscle, also known as visceral muscle, which is the most frequently seen in the human body. Single-unit smooth muscles contract slowly and steadily, enabling substances to flow through the body, including food in the digestive tract.
Ques. Make a comparative chart of three kinds of muscles in the human body. (5 Marks)
Ans. Human Body consists of three major types of muscular tissue namely Skeletal Muscles, Cardiac Muscles, and Smooth Muscles.
| Features | Skeletal Muscles | Smooth Muscles | Cardiac Muscles |
| Movement | Voluntary Muscle | Involuntary Muscle | Involuntary Muscle |
| Location | Attached to the skin and bones. | The lining of walls of internal organs in the body. | Present only in the heart. |
| Cells | Long, multinucleated, cylindrical cells all over the muscle. | Uni-nucleated, single cells with tapering ends. | Straight uni-nucleated cells with branched chains. |
| Muscle Type | Myofibrils are arranged in an order to form striated muscles. | Varying lengths of myofibrils form unstriated muscles. | Orderly arrangement of myofibrils forms striated muscle. |
| Controlled By | Nervous System | Nervous and Endocrine Systems and other chemicals physical stretching. | Central Nervous System, Endocrine System, and different chemicals secreted by the body. |
| Stimulation | Non-Self-Stimulating, but stimulated by motor neurons of the nervous system. | Self-Stimulating Tissue | Self-Stimulating Tissue |
| Energy Requirement | High Energy | Low Energy | Intermediate Energy |
| Contraction Speed | Fast and Non-rhythmic Contraction | Slow, Rhythmic in Some Organs | Both Intermediate Speed and Rhythmic Contraction |
| Fatigue Creation | Fatigues Very Easily | Does not Fatigue Generally | Never Fatigue |
| Strength | Increases with stretching distance | Stress-relaxation response is seen | Increases with stretching distance |
| Function | Movement and locomotion of the body. | Maintain the flow of food and fluids through the internal organs in the body. | Helps in pumping blood flow into and out of the heart of the body. |
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