Type | Example |
Simple tubular | Intestinal glands, crypts of Lieberkuhn in ileum. |
Simple coiled tubular | Sweat glands in man |
more...
Contractility and motility (movement) are fundamental properties of protoplasm. That is why, all cells possess potential motility. Contraction for motility in the cells results essentially from the interaction of two contractile proteins, actin and myosin. These tissues are obviously responsible for movements of organs and locomotion of the body in response to stimuli. These develop from embryonic mesoderm except for those of the iris and ciliary body of eyes, which are ectodermal in origin. About 40% to 50% of our body mass is of muscles. The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres. These possess large numbers of myofibrils formed of actin and myosin. Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology. Types of muscle are following –
Striated or striped muscles
Most muscles of body are striated. These generally bring about voluntary movements under conscious control of brain and, hence, called voluntary muscles. Most of these are inserted at both ends upon bones in different parts of the body depend upon these muscles. Hence, these are also called skeletal muscles. Movements of limbs and the body solely depend upon these muscles. Hence these are also called somatic muscles. These are also called phasic type of muscles, because contraction in these is rapid, but brief and fatigue occurs quickly.
Fine structure of striated muscle fibres : Striated muscle fibres shows transverse striation in the form of regular alternate dark A (anisotropic) and light I (isotropic) bands. The ‘A’ band contains about \[120{AA}\] thick and \[1.8\,\,\mu \] long “myosin filaments”. The I band contains about \[60{AA}\] thick and \[1.0\,\,\mu \] long “actin filament” which are twice as many as myosin filaments. Each I band is divided into two equal halves by a thin, fibrous and transverse zig-zag partition, called ‘Z’ band (‘ Z’ disc) or Krause’s membrane. Each segment of a fibril between two adjacent ‘Z’ bands is called a sarcomere. It is \[2.3\,\,\mu \] long in uncontracted mammalian striated fibres. A slender transverse line, the ‘M’ or Hansen’s line is visible in middle of each ‘A’ band. The major, middle region of ‘A’ band is comparatively lighter, but its terminal parts appear darker. The middle lighter region is called ‘H’ zone. Due to the geomatric bonding pattern, the end of each myosin filament is, thus, encircled by the ends of six actin filaments (hexagon), while the end of each actin filaments is encircled by the ends of three myosin filaments (trigon).
Ultrastructure of myofilaments : At the molecular level, each myosin filament is composed of about 500 thread-like myosin molecules. Three different kinds of proteins participate in the composition of actin filaments. The major part of an actin filament is a coiled double helical strand whose each arm is a linear polymer of small and globular molecules (monomers) actin protein. Another coiled double helical, but thiner, strand runs along the whole length of actin strand. Each arm of this strand more...
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