Answer:
Sliding
filament theory
This theory is
applicable to smooth, cardiac and skeletal muscles. The essential features of
this theory are as follows
(i) During
muscle contraction, thin myofilaments slide inward towards the H-zone.
(ii) The
sarcomere, the basic unit of muscle contraction, shortens, without changing the
length of thin and thick myofilaments.
(ii) The
cross-bridge of the thick myofilaments connect with the portions of actin of
the thin myofilaments. (iii) These cross-bridge move on the surface of the thin
myofilaments, rfisiiltina in the sliding of thin and thick myofilaments over
each other.
(iv) The
length of the thick and thin myofilaments do not change during muscle
contraction.
(v) A muscle
fibre maintains a resting potential under resting conditions just like a nerve fibre.
As soon as a nerve impulse reaches the terminal end of the axon, small sacs called
synaptic vesicles fuse with the axon membrane and release a chemical transmitter.called
acetylcholine.
It diffuses
across the synaptic cleft (the space between the axon membrane and the motor
end plate) and binds to the receptor sites of the motor end plate.
(vi) As soon
as depolarisation of the motor end plate reaches a certain level, it creates an
action potential. After this, an enzyme cholinesterase present along with the
receptor sites for acetylcholine breaks down acetylcholine into acetate and
choline.
A portion of
the choline diffuses back to the axon and is reused to synthesise more acetylcholine
for the transmission of subsequent impulses.
(vii) Calcium
plays a key regulatory role in muscle contraction. The ions bind to
troponin causing a change in its shape and position. This in turn alters the
shape and position of tropomyosin.
This shift
exposes the active sites on the F-actin molecules and myosin cross-bridges are
then able to bind to these active sites.
(viii) The
head of each myosin molecule contains an enzyme myosin ATPase. In the presence
of myosin ATPase, and ions, ATP breaks
down into ADP and inorganic phosphate as Energy
(ix)Energy
from ATP causes energised myosin cross-bridges to bind to actin. The energised
cross-bridge move, causing the thin myofilaments to slide along the thick myofilaments.
This movement is like the movement of the oars of a boat.
(x)As stated
earlier in theory, there is no shortening of thin and thick myofilaments.
However, the
sarcomere shortens, because of the sliding of the thin myofilaments produced by
cross-bridge movements. The H-zone and 1-band shorten, but the width of the
A-band remains constant.
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