Question

Explain “Excitation-Contraction Coupling” by describing the chain of events that occurs from the Action Potential approaching the terminal end of the motor neuron through the production of a concentric contraction in the skeletal muscle

Answer 1

EXCITATION - CONTRACTION-COUPLING:

excitation of muscle - muscle contraction is initiated by a signal sent by the central nervous system via motor neuron.

A neural signal reaching the neuromuscular junction releases a neurotransmitter (acetylcholine) which generates a action potential in the muscle fibre.

When the action potential spreads to the triad system through the T tubules, the sarcoplasmic reticulum release calcium ions in the muscle fibers.

CONTRACTION-COUPLING:

Mechanism of muscle contraction is explained by the 'sliding filament theory'.

The process by which shortening of the contractile elements in the muscle is brought about is the sliding of actin filament over the thick (myosin) filaments.

The sliding of filaments is brought about by the interaction between the cross-bridges of myosin and actin molecules. The force of contraction is developed by the cross bridges in the overlap region between actin and myosin.

Formation of cross bridge-

Increase in the calcium level leads to the binding of calcium ions to the subunit of troponin-c of the troponin of the thin filaments. This makes troponin and tropomyosin complex to move away from the active sites of the actin molecules.Now the active sites of actin is exposed to the heads of the myosin.

Utilising the energy released from the hydrolysis of ATP, the myosin head now binds to the exposed active sites on the actin molecules to form a cross bridge.

Power stroke - The cross bridge pulls the attached actin filaments towards the centre of the A band. The Z lines attached to these actin filaments are also pulled inwards from both the slides thereby causing shortening of the sarcomere i.e contraction.

During the shortening of the muscle, the I bands get reduced in size/length, whereas the A bands retain their size/length. As the thin filaments are pulled deep into A band making the H bands narrow, the muscle shows the effect - contraction.