Question

Explain sliding filament theory of muscle contraction with neat
sketches.

Answer 1

Structure of sarcomere

Muscle fibre has a functional unit called sarcomere. It has a large number of parallelly arranged filaments known as myofibrils having light and dark bands composed of actin and myosin contractile proteins respectively.
The light bands are known as an I-band or isotropic band, whereas dark bands are known as anisotropic or A-band.
In the centre of each I-band, a Z-line is present and an M-line is present in the A-band, the A & I-bands are arranged alternately throughout the myofibrils.
The portion between 2 successive Z-line is known as sarcomere.

Diagram


Sliding filament theory introduction

Sliding filament theory explains the mechanism of muscle
contraction.Using a high power microscope , A. F. Huxley and R. Niedergerke (1954) and H. E. Huxley and J. Hanson (1954) described the phenomena of muscle contraction. In brief, thin filaments slide over thick filaments during muscle contraction( hence the name), in a sequential manner.

Process of contraction

● According to this theory, an action potential is generated at
sarcolemma , when the nerve impulse arrives at the neuromuscular junction.
● It creates an action potential at sarcolemma and triggers the
sarcoplasmic reticulum inside sarcoplasm, to release calcium ions.
● This will further remove the inhibitory masking effect of the
troponin unit from the active site of actin.
● As the active site of actin becomes free, it forms a cross bridge with myosin head, using ATP.
● The cross bridge pulls (power stroke) the attached actin filaments towards the centre of A-band along with Z-line attached to these actin filaments.
● Due to this, H zone is overlapped, the I bands are shortened and the Z line comes in contact with the A bands. Thus the length of the sarcomere decreases. This is the contracted state of the muscle fibre.

Relaxation of the muscles

● When the myosin releases the ADP and Pi, it goes back to its
relaxed state and the cross bridge breaks.
● The process continues until the Ca${}^{++}$ ions are pumped back to the sarcoplasmic cisternae, resulting in the masking of active sites of actin filaments by the troponin inhibitory unit once again.
● The length of the sarcomere returns to its original state.

Images