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Muscle tissues
Structure
We know that living organisms can move on their own or can perform other types of movement. Muscle
tissue has a ability to relax and contrast and so bring about movement and mechanical work in various
parts of the body. There are other movements in the body too which are necessary for the survival of the
organism such as the heart beat and the movements of the alimentary canal.
Muscles can be divided into three main groups according to their structure, e.g.:
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Smooth muscle tissue.
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Skeletal muscle tissue.
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Cardiac (heart) muscle tissue.
A whole muscle with many fasciculi
Types of Muscle Tissue
Smooth Muscle Tissue
Smooth muscle tissue is made up of thin-elongated muscle cells, fibres. These fibres are pointed at their
ends and each has a single, large, oval nucleus. Each cell is filled with a specialised cytoplasm, the
sarcoplasm and is surrounded by a thin cell membrane, the sarcolemma. Each cell has many myofibrils
which lie parallel to one another in the direction of the long axis of the cell. They are not arranged in a
definite striped (striated) pattern, as in skeletal muscles - hence the name smooth muscle . Smooth
muscle fibres interlace to form sheets or layers of muscle tissue rather than bundles. Smooth muscle is
involuntary tissue, i.e. it is not controlled by the brain. Smooth muscle forms the muscle layers in the
walls of hollow organs such as the digestive tract (lower part of the oesophagus, stomach and intestines),
the walls of the bladder, the uterus, various ducts of glands and the walls of blood vessels .
Functions of Smooth Muscle Tissue:
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Smooth muscle controls slow, involuntary movements such as the contraction of the smooth muscle
tissue in the walls of the stomach and intestines.
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The muscle of the arteries contracts and relaxes to regulate the blood pressure and the flow of
blood.
Smooth Muscle Tissue
Skeletal Muscle Tissue
Skeletal muscle is the most abundant tissue in the vertebrate body. These muscles are attached to and
bring about the movement of the various bones of the skeleton, hence the name skeletal muscles. The
whole muscle, such as the biceps, is enclosed in a sheath of connective tissue, the epimysium. This
sheath folds inwards into the substance of the muscle to surround a large number of smaller bundles, the
fasciculi. These fasciculi consist of still smaller bundles of elongated, cylindrical muscle cells, the fibres.
Each fibre is a syncytium, i.e. a cell that have many nuclei. The nuclei are oval in shaped and are found
at the periphery of the cell, just beneath the thin, elastic membrane (sarcolemma). The sarcoplasm also
has many alternating light and dark bands, giving the fibre a striped or striated appearance (hence the
name striated muscle). With the aid of an electron microscope it can be seen that each muscle fibre is
made up of many smaller units, the myofibrils. Each myofibril consists of small protein filaments, known
as actin and myosin filaments. The myosin filaments are slightly thicker and make up the dark band (or
A-band). The actin filaments make up the light bands (I-bands) which are situated on either side of the
dark band. The actin filaments are attached to the Z-line. This arrangement of actin and myosin filaments
is known as a sacromere.
During the contraction of skeletal muscle tissue, the actin filaments slide inwards between the myosin
filaments. Mitochondria provide the energy for this to take place. This action causes a shortening of the
sacromeres (Z-lines move closer together), which in turn causes the whole muscle fibre to contract. This
can bring about a shortening of the entire muscle such as the biceps, depending on the number of
muscles fibres that were stimulated. The contraction of skeletal muscle tissue is very quick and forceful.
Functions of Skeletal Muscle Tissue:
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Skeletal muscles function in pairs to bring about the co-ordinated movements of the limbs, trunk,
jaws, eyeballs, etc.
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Skeletal muscles are directly involved in the breathing process.
Skeletal Muscle Tissue
Cardiac (Heart) Muscle Tissue
This is a unique tissue found only in the walls of the heart. Cardiac (Heart) Muscle Tissue shows some of
the characteristics of smooth muscle and some of skeletal muscle tissue. Its fibres , like those of skeletal
muscle, have cross-striations and contain numerous nuclei. However, like smooth muscle tissue, it is
involuntary. Cardiac muscle differ from striated muscle in the following aspects: they are shorter, the
striations are not so obvious, the sarcolemma is thinner and not clearly discernible, there is only one
nucleus present in the centre of each cardiac fibre and adjacent fibres branch but are linked to each other
by so-called muscle bridges. The spaces between different fibres are filled with areolar connective tissue
which contains blood capillaries to supply the tissue with the oxygen and nutrients.
Functions of Cardiac (Heart) Muscle Tissue:
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Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of
the heart.
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It causes the rhythmical beating of the heart, circulating the blood and its contents throughout the
body as a consequence.
Cardiac Muscle Tissue
(Provided by Prof. Li Jicheng)