Structure and Function of Smooth Muscle with Diagram and Examples
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 Anatomy and Physiology
1. What is smooth muscle?
Smooth muscle is a type of involuntary, non-striated muscle tissue found in the walls of hollow organs and blood vessels. It contracts automatically without conscious control and helps move substances through the body. Key features include:
- Spindle-shaped (fusiform) cells
- A single, centrally located nucleus
- No visible striations under a microscope
- Slow, sustained contractions
2. Where is smooth muscle found in the body?
Smooth muscle is found in the walls of hollow organs and tubular structures throughout the body. Major locations include:
- Digestive tract (stomach, intestines)
- Blood vessels (arteries and veins)
- Respiratory tract (bronchi)
- Urinary bladder
- Uterus
- Iris of the eye
3. What is the function of smooth muscle?
The main function of smooth muscle is to produce involuntary movements that regulate the flow of substances inside the body. Its key functions include:
- Moving food through the digestive tract by peristalsis
- Controlling blood pressure by constricting or dilating blood vessels
- Regulating airflow in the lungs
- Expelling urine from the bladder
- Contracting the uterus during childbirth
4. How is smooth muscle different from skeletal and cardiac muscle?
Smooth muscle differs from skeletal and cardiac muscle in structure, control, and location. The main differences are:
- Control: Smooth muscle is involuntary; skeletal muscle is voluntary; cardiac muscle is involuntary.
- Striations: Smooth muscle is non-striated, while skeletal and cardiac muscles are striated.
- Cell shape: Smooth muscle cells are spindle-shaped; skeletal cells are long and cylindrical; cardiac cells are branched.
- Location: Smooth muscle lines hollow organs; skeletal muscle attaches to bones; cardiac muscle is found only in the heart.
5. Why is smooth muscle called non-striated muscle?
Smooth muscle is called non-striated muscle because it lacks the visible light and dark bands seen in striated muscles under a microscope. In smooth muscle:
- Actin and myosin filaments are arranged irregularly
- There are no organized sarcomeres
- The cells appear smooth and uniform
6. How does smooth muscle contract?
Smooth muscle contracts through a sliding interaction between actin and myosin filaments triggered by calcium ions. The process occurs in steps:
- Calcium ions (Ca²⁺) enter the cytoplasm.
- Calcium binds to calmodulin.
- The calcium–calmodulin complex activates myosin light-chain kinase (MLCK).
- MLCK phosphorylates myosin, enabling cross-bridge formation with actin.
- Filaments slide past each other, causing contraction.
7. What controls smooth muscle activity?
Smooth muscle activity is controlled mainly by the autonomic nervous system, hormones, and local factors. Regulation occurs through:
- Sympathetic and parasympathetic nerves
- Hormones such as adrenaline and oxytocin
- Stretch and chemical signals within tissues
8. What is peristalsis in smooth muscle?
Peristalsis is a wave-like contraction of smooth muscle that moves food or liquid through tubular organs. It works by:
- Sequential contraction behind the food bolus
- Relaxation of muscle ahead of the bolus
- Coordinated movement along the digestive tract
9. What are the types of smooth muscle?
Smooth muscle is classified into single-unit and multiunit types based on how the cells function together. The two types are:
- Single-unit (visceral) smooth muscle: Cells are connected by gap junctions and contract as a unit; found in the intestines and uterus.
- Multiunit smooth muscle: Cells act independently and are individually innervated; found in the iris and large airways.
10. Can smooth muscle regenerate?
Yes, smooth muscle has a relatively good ability to regenerate compared to other muscle types. It can repair itself through:
- Division of existing smooth muscle cells (mitosis)
- Activation of local progenitor cells
