The structure of the smooth/striated/cardiac muscle tissue with a diagram
1. Smooth muscle:
These muscles are also called non-striated or non straight muscles. The muscle fibres are made up of long narrow spindle shaped cells that are generally shorter than striated muscle cells. They average a length of about 0.2 mm. Each muscle fibre has a single nucleus in the central thick portion. The sarcoplasm consists of a number of parallely arranged myofibrils. These are made up of myosin.
The muscle cells are not bound by a true sarcolemma; the usual plasma membrane however is present. The cytoplasm consists of mitocondria and endoplasmic reticulum but they are not as many in number as in the striated muscles. Smooth muscle fibres may occur individually or in the form of bundles of sheets. The muscle fibres of viscera are usually arranged in the form of sheets made up of many layers of fibres. In villi of the small intestine the muscle fibres occur singly.
In some instances, bundles of smooth muscle fibres get surrounded by connective tissue and are represented by arrector pili which are present at the base of the hairs. It is these which are responsible for the hair to stand erect at certain times.
Smooth muscle fibres occur in the walls of alimentary canal, genital tract, ducts, blood vessels, urinary bladder etc. Smooth muscles undergo slow and rhythmic contraction regulated by the autonomous nervous system. Hence they are called involuntary muscles. The contraction of the muscles may be prolonged and it causes shortening of the organ and increase of the diameter of the lumen.
2. Striated muscles:
Also called striped muscles or voluntary muscles or skeletal muscles they constitute about 80% or more of the soft tissues present in the body. The striated muscle fibres are long, cylindrical, un- branched and have blunt ends. Each fiber may be as long as 40mm. Each muscle fibre is bounded by a conspicuous sarcolemma. The sarcoplasm consists “of many elongated, flattened nuclei. In addition there are a number of glycogen granules and mitochondria.
The most striking feature of the striated muscle fibre is the presence of alternating dark and light transverse bands called stripes or striations. Hence the name striated muscles. The dark bands are called anisotropic or A bands. Each A band has at its middle a somewhat light zone called the H zone or Henson’s line.
The lighter band is isotropic and is called the I band. Each I band has at its centre a dark line called the membrane of Krause or Z line or Z band. The dark and light bands alternate regularly. The portion of the myofibril between two Z lines is called a Sarcomere and it constitutes a contractile unit. Thus each sarcomere consists of A band and half of each adjacent I band. Electron microscopic studies reveal the sarcomere is a bundle of thin myofilaments of two types- primary and secondary.
3. Cardiac Muscles:
The cardiac muscles are confined to the walls of heart only. They have a unique function of contracting rhythmically and continuously. They generate a unique wave excitation as they pass from one fibre to another. The cardiac muscle fibres are made up of short cylindrical fibres joined end to end. These are interconnected by crosslinks forming a network. It is this arrangement that helps in the rhythmic contraction so characteristic of the heart muscles.
The muscle fibres are uninucleate and do not have a sarcolemma. The cardiac muscle fibres are striated but the bands are much paler than those of the striated muscle fibres. The cytoplasm consists of a large number of mitocondira and numerous glycogen granules.
There are some speicalised regions of the membrane at the end of the fibres which bring about cris- cross junction – these areas are called intercalated discs (rather than Z lines of the striated muscles). These intercalated discs help as boosters of the wave action permitting the motion to be transferred from one fibre to the other. The cardiac muscles like skeletal muscles (striated muscles) contract quickly but do not experience fatigue.