Structural Organisation in Animals — BIOLOGY STD 11 Science — Question

1. Striated Muscles: The striated or skeletal muscles form about 40% of total body weight. These muscles are attached and bring about the movement of the various bones of the skeleton, so are called skeletal muscles. The striated muscles give shape to the body and also release heat during contraction. These muscles have huge supply of nerves and blood vessels. Each striated muscle is a long, narrow, cylindrical, unbranched cell.

The striated muscle fibres are multinucleated or syncytial in nature. The cytoplasm (sarcoplasm) of each fibre has a large number of myofibrils (actin and myosin myofibrils) which are tightly packed. Each myofibril shows dark and light bands of stripes alternating with each other. Hence, they are called as striped muscle fibres.

2. Non-Striated (Smooth) Muscle: The non-striated muscles are found in the posterior part of oesophagus, stomach, intestine, lungs, urinogenital tract, urinary bladder, blood vessels, iris, ciliary body of eye, dermis of skin, etc.

The non-striated muscle consists of long, narrow, spindle-shaped fibres that are generally shorter than the striated muscle fibres. Their size may range from 20 um (small blood vessels)-500 um (in pregnant uterus).

Each non-striated muscle fibre contains a single oval nucleus in its thick middle part. In the cytoplasm, the myofibrils are arranged longitudinally. They are composed of myosin. There is no sarcolemma, however, the fibre is enclosed by the plasma membrane.

The smooth muscles help in the peristalsis which occurs in the tubular viscera. The autonomic nervous system controls these muscles. Hence, they are not under the control of animal's will. 

3. Cardiac Muscles: The cardiac muscles are contractile tissues present only in the heart and in the wall of large veins which enter the heart. The cardiac muscle fibres show the characters of both unstriped and striped muscle fibres. Each fibre is a long and cylindrical structure which has a definite sarcolemma. The fibres are uninucleate and the nuclei lie near the centre.

The myofibrils have transverse faint dark and light bands which alternate with each other.