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Controlling the Actin - Myosin Interaction By Ca++ ions
Muscle contraction is initiated by nerve impulse arriving at the neuromuscular junction. All the ibres
innervated by a single motor neuron are a “motor unit” and contract simultaneously in response
to the action potential ired by the motor neurons. The sarcolemma of muscle ibre penetrates
deep into the cell to form hollow elongated tube, the transverse tubule, T-tubule the lumen of
which is continuous with the extracellular luid. The thousands of T - tubules of each muscle cell are
collectively called T-system. It extends and encircles the myoibril at the level of Z-line or A and I -
junction. The T-tubule and the terminal portion of the adjacent envelope of sarcoplasmic reticulum
form triads at regular intervals along the length of the ibril. The nerve impulse is carried through
the T-tubule to the adjacent sarcoplasmic reticulum (SR). The calcium gates of the SR open releasing
calcium into the cytosol, thus binding calcium ion to troponin molecules of the thin ilament. The
binding sites are exposed and cross bridges with myosin can form, and contraction occurs.
All or None Response : The contraction of each muscle ibre is based on “all or none” principle
i.e. all of its ibrils participate in contraction. The degree of contraction depends upon the number
of muscle ibers that participate in contraction.
Sliding Filament Model
When muscle ibre contracts, the thin and thick ilaments undergo shifting. The I-band reduces in
length and Z-line gets closer.
Huxley and A. F. Huxley and their colleagues suggested a hypothesis in 1954 to explain all events
in muscle contraction, this is called “Sliding ilament model” of muscle contraction. According to
this theory, the thin ilaments slide past the thick one so that actin and myosin ilaments overlap to
greater degree. Thus the Z-line is brought close together, I-band shortens, the H zone disappears.
In this process of contraction, the cross bridges of thick ilament become attached to binding sites
on the actin ilament. The cross bridges then contract to pull the actin ilament towards the center
of the sarcomere.
How the bridges are controlled
When the muscle is at rest, the tropomyosin is disposed in such a way that it covers the sites on the
actin chain where the head of myosin becomes attached. When the muscle is required to contract,
calcium ions bind with the troponin molecule and cause them to move slightly. This has the efect
of displacing the tropomyosin and exposing the binding sites for the myosin head. Once the myosin
head has become attached to the actin ilament, ATP is hydrolysed and the bridge goes to its cycle.
This ATP is provided by the large number of mitochondria present in each muscle cell.
From the above account it is revealed that ATP is needed to break the link between the myosin and
the actin. After death, the amount of ATP in the body falls. Under these circumstances the bridges
can not be broken and so they remain irmly bound. This results in the body becoming stif, a
condition known as rigor mortis.