Question

Write the structure of skeletal muscle in detail.

Answer 1

Structure Of Skeletal Muscle: 

• Each skeletal muscle is spindle-shaped & has 2 distinct ends viz., Origin & Insertion. The origin & insertion remain attached to the bones with the help of tendon (cord-like structure) or aponeuroses (membrane-like structure). 
• The skeletal muscles of the tongue & upper part of the oesophagus are not attached to the skeleton. These muscles are also called as fatigued muscles or phasic muscles or striated muscles or somatic muscles. 
• Their structural unit is called as muscle fibre. 
• The muscle fibre is multinucleated or syncytial & bounded by sarcolemma. The muscle fibres are found in group & each group is called as fasciculus. The fasciculus is bounded by endomysium. 
• The fasciculi are also found in the group and each group of fasciculi is bounded by perimysium which is made up of connective tissues. 
• The whole skeletal muscle is bounded by an epimysium. 

Fine structure of muscle Fibre:

• The maximum length of the muscle fibre is 30 cm (1 – 30 cm) and the diameter is 0.01 to 0.1 mm. It is bounded by a sarcolemma or plasmalemma or myolemma. Its Sarcoplasm has many mitochondria which are called as acrosomes. The ER found in the sarcoplasm is called as Sarcotubules. 
• The sarcoplasm has three soluble proteins viz., myogenin, myoglobin & my albumin. 
• The sarcoplasm stores glycogen. In addition, it has less quantity of Na, Ca, P& Mg & more quantity of K. 
• The muscle fibre has many myofibrils. The length of the myofibril is similar to the muscle fibre but its diameter is 1 to 3µ. 
• The Myofibril has alternate A – bands (Anisotropic) & I – bands (Isotropic).
• There is a Z – line (zigzag) in the middle of the I – band which is also called as Krau’s membrane. 
• The middle zone of the A-band is called an H – zone which has a central M – line. 
• The distance between two Z – lines is called sarcomere which is a functional unit of the muscle. 
• Each sarcomere includes one complete A – band and 2 halves l – bands on both the sides (12 + A + I/2). 
• The A – band is made up of mainly myosin filaments. The length of the myosin filament is 1.5µ & the diameter is 100Å. 
• The l-band is made up of mainly actin filaments. The length of the actin filament is 2µ & the diameter 50Å. 0.2µ of both the ends of the actin filaments size remain in the A – bands. 
• During the resting phase, the length of the A-band is 1.5µ & of the half I-band is 0.8µ. Hence, the length of the sarcomere is 3.1µ. 
• In the transverse section of the myofibril, through the end of Aband, has 5000 filaments (Both actin & myosin). 
• Each myosin filament is surrounded by 6 actin filaments (Hexagon). Similarly, one actin filament is surrounded by 3 myosin filaments (Trigon) The myosin filaments are provided with cross-links or bridges. 
• The cross-links remain directed towards the actin. They are attached at an angle of 60° & are equally spaced by 60 – 70 Å. 
• The H – zone is without cross-links.