Involuntary Muscles: Definition, Types, and Examples

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Involuntary Muscle Definition

Muscles which are under our control are called as Voluntary muscle, whereas those which cannot be controlled by ourselves are called as involuntary muscle. These muscles are under the control of autonomic nervous system. Functions such as contraction, expansion and other functions are involuntary. An example is heart which is controlled by involuntary muscle. To perform the involuntary action, autonomic nervous system, stimulates the nerves and other signal transferring molecule to carry out the function. However, in other cases stretching of muscle can also trigger contraction.

Involuntary vs Voluntary Muscles

Involuntary muscles are under our control, whereas voluntary muscles are under our control. Examples of involuntary muscles are smooth muscles and cardiac muscle. These two same muscles are also example of voluntary muscle; however, they differ from each other.

Involuntary muscles are related with organs that are involved in contraction, such as heart, respiratory system and digestive system. Voluntary muscles are associated with the skeletal system. Involuntary muscles are also known by the name non-striated muscle or smooth muscle. Striated or skeletal muscles are also called as Voluntary muscle. Location of involuntary muscle is stomach, intestine, hearts cardiac muscle, urinary bladder and blood vessel.

Voluntary muscle are located in the skeletal system. Voluntary muscle do possess Striations, whereas involuntary muscle lacks striations. Around the in-voluntary muscle there is a thin layer of sarcolemma. Voluntary muscle has a thick layer of sarcolemma.

Intercalated disc is not found in voluntary muscle, whereas involuntary muscle does have intercalated disc. Sarcomeres are present in voluntary muscle. Involuntary muscle lacks sarcomeres. Involuntary muscle are under the possession of autonomic nervous system. Voluntary muscle are under the control of somatic nervous system. Involuntary muscle requires less energy, whereas voluntary muscle requires huge amount of energy.

The movement of thus, voluntary muscle is quite quick and thus they get tired very quick, whereas involuntary have slower movements and work continuously. Voluntary muscles are triggered by the fibers of the muscle, whereas the voluntary muscle get stimuli from outside the nervous system. Example of involuntary muscle are respiratory tract, blood vessel and others. Examples of voluntary muscle are biceps, triceps, pectoral muscle and quadriceps.

Involuntary Muscles Examples

There are two types such as Cardiac and Smooth muscle.

i. Cardiac Involuntary Muscle

These are striated muscle present in the wall of the heart which contract and relax. Cardiomyocytes are single heart muscle. Cardiac muscles are formed when intercalated disc are joined by Cardiomyocytes. These muscles are coated with fibers of collagen. As cardiac muscles get stimulus from within the muscle, they are myogenic.

For the cardiac muscle to contract they obtain electrical stimulus. Thus, action potential gets generated and in the sarcoplasmic reticulum, calcium ions get deposited. Due to huge amount of calcium ions, excitation occurs and muscles get contracted.

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ii. Smooth Involuntary Muscle

These muscles form a lining to the internal organs such as respiratory, urinary, intestine and blood vessel and they are non-striated. An example is ciliary which regulates iris movement by dilating the eye. It consist of thick and thin filaments and are spindle in shape. When they are observed under microscope, they look uniform, hence the name smooth muscle.

For contraction to happen these muscle contain calcium in the sarcoplasmic reticulum, which increases the calcium levels, thus causing excitation and leading to contraction. They also have actin and myosin in the cytoplasm. They are of two type: single unit smooth muscle which contract and relax together, whereas multi-unit smooth muscle contract and expand differently, as they are not electrically coupled. Example digestive system contraction is an example of single unit muscle.

Example of multi-unit muscle is ciliary muscle in the eye. As they lack sarcomeres, thus they are not quite flexible, however gets relaxed when the muscles organ is stretched, organ fills up. Thus, a stress relaxation response gets created and contraction are induced. However, after contraction when it relaxes, the occupied space within the organ does not empty up prior. This mechanism is seen in the bladder, to ensure smooth functioning of the unwanted products. Thus, the elasticity of smooth muscle is of vital importance.

Difference Between Cardiac and Smooth Muscle

Although cardiac and smooth muscle are involuntary, they do are quite different from each other:

a) Smooth muscles are located in the lining of internal organs, whereas cardiac muscle in aorta and heart.

b) Smooth muscles have the ability to regenerate, whereas cardiac muscle cannot.

c) Smooth muscle is directly under the control of autonomic nervous system, while cardiac muscle are through the cardiac pacemaker.

d) Smooth muscles are non-striated, whereas the cardiac muscles are striated present in the heart.

Involuntary Muscle Function

There are various functions of involuntary muscle and they are:

a) Involuntary muscles of heart pumps blood throughout the body.

b) Due to the elasticity of smooth muscle, it can hold up the urine and other excretory products in the bladder.

c) In the exocrine glands, duct as well, involuntary muscle plays an important role.

d) Involuntary muscle contractions in the digestive tract allows the peristaltic movement in the intestine. Thus, allowing its movement and proper mixing of food in the digestive system.

e) In cold conditions or in fear, involuntary muscle preset on the skin called the arector pili are responsible for goosebumps.

f) For the uncontrollable flow of blood and for maintaining blood pressure, smooth muscles present in the arteries contract.

g) To alter the shape of the lenses, ciliary muscles dilate and contract.

h) The contraction and relaxation of the muscles closes the orifice. Ex uterus.

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