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A Handbook of Biology
Synaptic vesicles release a neurotransmitter acetylcholine. It
generates an action potential in the sarcolemma that spreads
through the muscle fibre. It causes the release of Ca2+ ions from
sarcoplasmic cisternae into sarcoplasm.
Ca2+ binds with a subunit of troponin on actin filaments and
unmask the active sites for myosin.
Using energy from ATP hydrolysis, myosin head binds to exposed
active sites on the actin to form a cross bridge. This pulls attached
actin filaments on both sides towards the centre of A-band. The
actin filaments partially overlap so that H- zone disappears.
The Z- line attached to actins is also pulled inwards. It causes a
shortening (contraction) of sarcomere.
I-bands get shortened, whereas A-bands retain the length. Myosin
releases ADP and Pi and goes back to its relaxed state. A new ATP
binds and the crossbridge is broken. (Karnataka NEET 2013)
The ATP is again hydrolyzed by the myosin head and the above
process is repeated causing further sliding.
When Ca2+ ions are pumped back to sarcoplasmic cisternae,
actin filaments are again masked. As a result, Z-lines return to their
original position. It results in relaxation.
The reaction time of the fibres varies in different muscles.
Repeated activation of muscles leads to the accumulation of
the lactic acid due to anaerobic breakdown of glycogen causing
muscle fatigue.
Muscles are divided into two types on basis of presence of
myoglobin:
Red muscles
White muscle
Red coloured due to myoglobin
White
coloured
due
to
lesser
myoglobin
More mitochondria
Less mitochondria
Aerobic metabolism
Anaerobic metabolism
Slow & sustained contraction
Fast contraction for short period
Less sarcoplasmic reticulum
More sarcoplasmic reticulum