Smooth Muscle Definition

It is a type of a muscle which is non-striated along with a nucleus centrally located and is not under our control, present in human body. Smooth muscles in human body have various roles. The shape of non-striated cells is spindle shape.

Smooth muscle is found stomach, blood-vessels, intestine and bladder. They are also found in the nervous system. Their functions are that help in digestion in gastrointestinal track. It removes toxins from the body, maintains electrolyte balance, blood pressure and oxygen content in tissues. In absence of smooth muscles this function would not take place.

Characteristic of Smooth Muscle

a) Single Nucleus: Smooth muscle being non-striated consist of a single nucleus located centrally in each of the cell. the activities outside and inside are controlled by the nucleus.

b) Shape of Smooth Muscle Cell: The shape of the smooth muscle is spindled shape, i.e filled in the middle and narrowed towards the end. Elasticity of smooth muscle is excellent. It can contract and expand very quick, which makes it suitable to be present in human body parts such as bladder, stomach and etc.

c) Filaments: If smooth muscle are observed under microscope, they appear like red line of smooth muscle which can travel from region to the other by cell membrane. The red filaments are actually actin filaments.

d) Contractility: Unlike the other muscles that can contract on their own, smooth muscle cannot be contracted on their own.

e) Elasticity: Smooth muscles are quite elastic and can stretch and return back to its original length.

f) Sarcomeres Absent: Sarcomere is like a bunch of filaments that has dark and white bands adjacent. However, this sarcomere is absent in skeletal muscle.

g) Tropomyosin Complex Absent: Smooth muscle does not contain tropomyosin complex but has instead Ca Calmodulin complex that carries out the phosphorylation.

Histology and Structure of Smooth Muscle Tissue

Smooth muscles have a length of 20-200µm and a thickness of 3-10µm. In the center of the smooth muscle is the nucleus which is cigar shaped during contraction. Smooth muscles cytoplasm is eosinophilic made of filaments.

Smooth muscle cell originates from mesoderm. Cell membrane has invaginations around itself. The cells remain intact due to the connective tissue beneath and the basal lamina. Muscle cells are spindle in shape. Smooth muscle has thick and thin filaments, but lack sarcomeres thus not forming a striated pattern. When observed under microscope its cytoplasm contains huge amount of actin and myosin.

Actin and Myosin

The muscle contraction proteins are actin and myosin. Actin filaments are spread everywhere and are connected to dense bodies. Myosin filaments are beneath actin filaments. When observed under electron microscope, it appears as black bodies. The smooth muscles are present in various organs such as

a) In stomach and intestine

b) In the lymphatic and blood vessel

c) In the ciliary muscle and eye

d) In the bladder

e) In skin

f) In male and female reproductive tract

g) In respiratory system

h) In urinary system

Types of Smooth Muscle

There are two types of smooth muscle and they are

a) Single Unit Smooth Muscle (Visceral): As the name suggest it functions together as a single unit.

b) Multiunit Smooth Muscle: Due to multiple type they cannot work together and function independently.

a) Single Unit Smooth Muscle (Visceral)

These cells forms the inner lining of various organs in the body. Example in the gastrointestinal tract digestion of food, reproductive system, urinary bladder, circulatory system and also in eyes where it can change its shape and size. These are also found in the blood vessel and are responsible for the contraction and relaxation of blood vessel. They have gap junctions thus, can work collectively.

b) Multiunit Smooth Muscle

These muscles are found in the hair follicles, lungs, pili muscles, eye muscles and they help in lens formation and control the amount of light that can pass. They do not have gap junctions; thus, they work independently.

Smooth Muscle Function

There are various functions of smooth muscle and are:

a) Contraction: When muscles contract, they allow the movement. For example, in the gut smooth muscle allows the food to pass down.

b) Self -working: The smooth muscle function on their own, i.e they are voluntary because they are under the control of autonomic nervous system. Due to the smooth we can breathe according to our will. In presence of CO2, they vasodilates and in presence of oxygen it vasoconstricts.

c) Maintains Homeostasis: The smooth muscles present on the skin hair follicles maintain the temperature automatically when it’s too cold by raising the hair follicles. This maintains body heat.

Smooth Muscle Dysfunction

It is a disorder or disease of smooth muscle which is deadly. It gets triggered due to disorder of smooth muscle throughout the body. It is supposed to be hereditary. The treatment of this disease is anti-smooth muscle antibodies like lupus or hepatitis. It is said to be deadlier because muscle failure will lead to a major problem. Many organs being connected to smooth muscle can halter the functioning of the body.

Smooth Muscle Action Potential

To start a contraction in the smooth muscle cell, membrane potential is required. Although the action potential of smooth muscle cell is slow but stands for a longer duration. For the smooth muscle, sodium channels are responsible, which open quite slowly in smooth muscle cell.