Welcome to MBC Biology! In this post, I'm presenting you concise description of Muscular tissues.
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| Muscles that move humerus bone OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/e/ee/1119_Muscles_that_Move_the_Humerus_b.png |
The muscular tissue is highly contractile. It has elongated cells called muscle fibres. Each fibre contains fine longitudinal fibrils, called the myofibrils. Its cytoplasm is called sarcoplasm. Each muscle fibre is externally covered by a membrane, called sarcolemma. This tissue originates from mesoderm.
Muscles are
supplied with blood vessels. Those blood vessels provide nutrition and take
away metabolic wastes. Each muscle has its own nerve supply. The muscular
tissues are used for movements. Muscles are of three kinds – straited (striped
or skeletal or voluntary), unstraited (unstriped or involuntary
muscles), and cardiac muscles.
I. Straited (striped or skeletal or voluntary muscles) Muscles
The muscles
composed of long and cylindrical fibres with blunt ends are straited muscles.
Each fibre is bounded by an elastic sarcolemma and contains many
acentric nuclei (syncytial nuclei). Each straited muscle has numerous myofibrils.
Myofibrils have alternately arranged dark and light bands.
Dark
band is also called
anisotropic band (A-band). Each dark band is bisected at a midpoint
by a thin line, called Hensen’s band or H-band. Dark band is made
up of myosin protein. It is highly refractive.
Light
band is also called
isotropic band (I-band). Each light band is bisected at a midpoint by a
thin dark line, known as Krause’s membrane or Z-band. It is
formed of actin protein.
In
vertebrates, straited muscles are attached to the skeleton. So, they are called
skeletal muscles. They are found in the body wall, limbs, tongue,
pharynx, beginning of the oesophagus, etc. They are under the conscious or will
control. Hence, they are called voluntary muscles. These muscles are
quickly tired. For their contraction, Ca++ ions are required.
II. Unstraited (unstriped or involuntary) muscles
Unstraited muscles are spindle
shaped with numerous fine longitudinal myofibrils. Each myofibril contains an
oval nucleus at the middle. It does not have sarcolemma. The plasma membrane
externally binds the fibre. These muscles are called smooth and unstraited
or unstriped muscles due to absence of light and dark bands. They are
also called involuntary muscles as they are under control of autonomic
nervous system but not under the control of will. They exhibit slow and
prolonged contraction. They do not get tired.
III. Cardiac muscles
These muscles are found only on the heart wall. Structurally, they have the properties of both the striped and unstriped muscles. These muscles are composed of cylindrical fibres interconnected by oblique bridges to form a continuous contractile network. The intercalated discs, which allow the fibres to interlock for greater strength during contraction, are present in the fibres.
These muscles rhythmically contract throughout the life without getting fatigued. The cardiac muscles are myogenic as the contractions are generated within the muscles. These are supplied with both the central nervous system (vagus nerve) as well as by the automatic nervous system. They are straited but involuntary.
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| Figure 19 Different types of muscular tissues |
