Muscle tissues contract and relax to provide movements of limbs and internal organs and also locomotion of the organism. They are composed of predominate cells that are specialized to shorten in length by contraction. These are mesodermal in origin. Muscle tissue is made up basically of cells called myocytes. All these cells are elongated in one direction, they are often referred to as muscle fibres (myofibrils).
Functions of Muscle Tissue
- Muscle tissue is responsible for the movement of the body parts.
- Muscle tissue is also responsible for the locomotion of the organism.
- The beating of heart, production of sound, respiratory movements for breathing are all brought out with the help of muscle tissue.
- Muscle tissue helps in maintaining body equilibrium.
- Muscle tissue provides support to the skeletal system.
- Muscle tissue also controls facial expression and gesture.
Types of Muscles
On the basis of their structure, locomotion and functions, muscles can be classified into the following three types.
Striated muscles are also known as skeletal muscles. They are voluntary and striped. When examined under a microscope, fibres of these muscles show prominent transverse striations. Therefore these muscles are called striated muscles. These muscles are made to contract according to our will under the conscious control of the nervous system, they are also said to be voluntary muscle.
Structure of Striated Muscles
Striated muscles are made up of long, cylindrical fibres. The length of the fibres is highly variable. The diameter of fibres varies from 10 to 60µm. Each fibre is a syncytium with hundreds of nuclei. The cytoplasm is called sarcoplasm. Sarcoplasm contains numerous longitudinal fibrils said to be myofibrils.
The most striking feature of striated muscle fibre is the presence of transverse striations on it. These striations appear as alternate dark and light bands. These dark and light bands are called A-bands and I-bands. The dark line running across the middle of each I-band which is called I-band which is called Z-band. The centre of each A-band is transversed by a lighter band called H-band. A dark line runs through the centre of H-band, and this band is said to be M-band.
Non-Striated muscles are present mainly in relation to viscera, but they are not associated with bones. These muscles are present in hollow tubes of gastrointestinal viscera. They are also known as visceral muscles. As these muscles do not have transverse striations, they are also known as unstriped or smooth muscles. These muscles cannot be directly controlled by us according to our will so they are called involuntary muscles.
Structure of Non-Striated Muscles
Non-Striated muscles are made up of long, spindle-shaped cells with a broad central part and tapering ends. The nucleus is oval or elongated and it lies in the central part of the cell. The length of cells varies from 15µm to 500µm. The sarcoplasm of the cell is indistinct due to the presence of myofibrils. Myofibrils are composed of proteins actin and myosin. The sarcoplasm also contains mitochondria, Golgi bodies, granular endoplasmic reticulum and free ribosomes. The muscles are usually arranged in bundles.
These muscles occur exclusively in the heart. They resemble smooth muscles in being involuntary, and with striated muscles in the presence of transverse striations. Cardiac muscles have rhythmic contractibility the rate of which can be modified by autonomic nerves that supply them.
Structure of Cardiac Muscles
Cardiac muscles are short, thick, cylindrical and branches to form a network. A myocyte is about 80µm long and 15µm broad. The nucleus is located at the centre. The cardiac myocyte contains abundant sarcoplasm. There are relatively few myofibrils. At places, myofibrils merge with each other. The myofibrils and striations of the cardiac muscle are not distinct.