This muscle is found only in the heart wall. The muscle of the heart is called the myocardium. Myocardium has two layers. Epicardium and Endocardium. The outer layer is called epicardium and the inner layer is called endocardium.

Cardiac cells appear long, cylindrical and branched. Its length is 0.8 mm and diameter is 12-15 micrometers.

1. Sarcolemma: It is covered by a membrane called sarcolemma.
2. Sarcoplasm: Cytoplasm of muscle is called sarcoplasm. It contains mitochondria.
3. Nucleus: At the center of every cell is a nucleus. It performs biological functions of cells.
4. Myofibrils: Myofibril fibers run along the length of the cell. Myofibrils have striated spots.
5. Intercalated disc: Two adjacent sarcolemmas fuse to form a disc. One is called an intercalated disc. It is one of the identifying features of heart muscle.

The muscle that forms the wall of the heart is called cardiac muscle. It is a type of involuntary muscle. Between the heart muscles are the connective tissue of the areola. It never gets bored.

Muscles whose actions are not controlled by the will of the animal are called involuntary muscles. They are also called visceral muscles because they are present in the walls of various organs (vessels).

Structure of involuntary muscles
Involuntary muscles are squash-shaped in appearance. That is, it is wide in the middle and narrow at both ends. Its length is 15-200 micrometers and diameter is 8-10 micrometers.
1. Sarcolemma: The sheath of muscle is called sarcolemma. In involuntary muscles the sarcolemma is inconspicuous.
2. Sarcoplasm: Cytoplasm of muscle is called sarcoplasm. Cytoplasm contains mitochondria. Its cytoplasm is called sarcoplasm. Each cell has a nucleus and numerous fibers called myofibrils.
3. Nucleus: Each muscle cell has a nucleus. It performs biological functions of cells.
4. Myofibrils: Myofibril fibers run along the length of the cell. Myofibril fibers are composed of actin and myosin filaments. Actin and myosin filaments together are called sarcomere.

Location of involuntary muscles
Involuntary muscles are located in alimentary canal, blood vessels, trachea, urethra, uterus etc.

Function/importance/role of involuntary muscles
1. The contraction-expansion capacity of this muscle is slow and long-lasting.
2. It regulates the rhythmic activity of the organs.
3. It controls the movement of food material in the process of peristalsis.
4. Controls the contraction and expansion of blood vessels, airways, excretory canals etc.

1. The contraction-expansion capacity of this muscle is slow and long-lasting.
2. It regulates the rhythmic activity of the organs.
3. It controls the movement of food material in the process of peristalsis.
4. Controls the contraction and expansion of blood vessels, airways, excretory canals etc.

Involuntary muscles are located in alimentary canal, blood vessels, trachea, urethra, uterus etc.

Involuntary muscles are squash-shaped in appearance. That is, it is wide in the middle and narrow at both ends. Its length is 15-200 micrometers and diameter is 8-10 micrometers.

1. Sarcolemma: The sheath of muscle is called sarcolemma. In involuntary muscles the sarcolemma is inconspicuous.
2. Sarcoplasm: Cytoplasm of muscle is called sarcoplasm. Cytoplasm contains mitochondria. Its cytoplasm is called sarcoplasm. Each cell has a nucleus and numerous fibers called myofibrils.
3. Nucleus: Each muscle cell has a nucleus. It performs biological functions of cells.
4. Myofibrils: Myofibril fibers run along the length of the cell. Myofibril fibers are composed of actin and myosin filaments. Actin and myosin filaments together are called sarcomere.

Muscles whose actions are not controlled by the will of the animal are called involuntary muscles. They are also called visceral muscles because they are present in the walls of various organs (vessels).

It is a long and cylindrical muscle. Its length is 1-4 cm and diameter is 10-40 micrometers. It is also called muscle fiber because the cells look like fine fibers.

1. Sarcolemma: Muscle tissue is covered by a sheath called sarcolemma.
2. Sarcoplasm: Its cytoplasm is called sarcoplasm. Cytoplasm contains mitochondria. Because these cells require a lot of energy.
3. Nucleus: Multiple nuclei in each muscle cell. It performs biological functions of cells.
4. Myofibrils: Myofibril fibers run along the length of the cell. Myofibril fibers are composed of actin and myosin filaments. Actin and myosin filaments together are called sarcomere. Myofibrils have two types of bands. Anisotropic band (A) and isotropic band (I). Darker bands are called anisotropic or A bands and lighter bands are isotropic or I bands.
5. Transverse lines: Several transverse lines are seen in optional muscles. These lines are called linear muscles or marked muscles.
6. Fasciculus: Muscle fibers are arranged in bundles. Each bundle is called a fasciculus. Each fasciculus is covered by perimycium. Many fasciculi join together to form large clusters. Each large cluster is covered by epimycium.

 

Optional muscle position

Animals have eyes, tongue, pharynx etc.

 

Optional muscle function/importance/role

1. Human movement is through these muscles.
2. Controls body organs through contraction and expansion.
3. Regulates body posture and muscle tension.
4. Regulates body temperature.

1. Human movement is through these muscles.
2. Controls body organs through contraction and expansion.
3. Regulates body posture and muscle tension.
4. Regulates body temperature.