Smooth Muscle Diagram

Smooth muscles are involuntary and non-striated muscles. These muscles are found in almost all organs in the form of bundles or sheaths. They form the major contractile tissues of various organs.

The smooth muscle fibres are present in the following structures –

• Wall of organs like the stomach, oesophagus and intestine
• Ducts of digestive glands
• Alveolar ducts, bronchial tube and trachea of the respiratory tract
• Urethra, ureter and urinary bladder in the excretory system
• Wall of blood vessels in the circulatory system
• Arrector pilorum of skin
• Genital ducts, uterus, mammary glands, scrotum and prostate gland in the reproductive system
• Iris and ciliary body of the eye

Here, let us learn more about the structure of smooth muscles with the help of a diagram.

Labelled Diagram – Smooth Muscles

Smooth Muscles – Description

• Smooth muscle fibres are made of elongated or fusiform cells. These fibres are typically very small, measuring 50 to 200 microns in length and 2 to 5 microns in diameter.
• Their nucleus is single and elongated, and it is centrally placed. Normally, two or more nucleoli are present in the nucleus.
• Well-defined sarcomere and myofibrils are absent in smooth muscles. Thus, the alternate light and dark bands are absent. This provides a non-striated appearance to the smooth muscles.
• Contractile proteins like myosin, actin and tropomyosin are present in the smooth muscle fibre. Troponin is absent in it. The thick filaments (formed by myosin) and thin filaments (formed by tropomyosin and actin) are not orderly arranged as in skeletal muscles.
• There are special structures called dense bodies to which the tropomyosin and actin molecules of the thin filaments adhere. These dense bodies are scattered all over the sarcoplasm in the network of intermediate filaments formed by desmin (protein).
• The anchoring of intermediate filaments, dense bodies and thin filaments aids in the contraction of smooth muscles when sliding occurs between thin and thick filaments.
• These dense bodies are not arranged in a straight fashion, thus giving the muscle fibre a corkscrew-like twist during contraction.
• Also, the neighbouring smooth muscle fibres adhere to the dense bodies, which help to transmit contraction from one cell to another.

Types of Smooth Muscles

The two types of smooth muscle fibres are – visceral or single-unit and multiunit muscle fibres. The single unit comprises muscle fibres with gap junctions, whereas the multiunit comprises smooth muscle fibres without interconnecting gap junctions. Thus in the visceral or single unit, the spread of action potential is rapid and also shows a synchronised contraction as a single unit.

The control of smooth muscles can be by both humoral and nervous factors. The humoral factors include neurotransmitters and hormones. These muscles are supplied by both parasympathetic and sympathetic nerves. However, these nerves are not responsible for the initiation of any activity by the smooth muscles.

Functions

• In the cardiovascular system, the smooth muscles around the blood vessels help regulate the flow of blood and blood pressure.
• The contraction and relaxation of smooth muscle fibres in the respiratory passage aid in the regulation of air.
• Likewise, the smooth muscles in the digestive tract help in the easy movement of food and absorption.
• In the excretory system, they are present in the urinary bladder and help in the voiding of urine.
• The smooth muscle fibres also aid in the movement of sperm in the reproductive tract.

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