Sunday, August 9, 2020

Locomotion and Movement - Notes | Class 11 | Part 2: Mechanism of Muscle Contraction

20. LOCOMOTION AND MOVEMENT 

MECHANISM OF MUSCLE CONTRACTION


According to sliding filament theory, contraction of a muscle fibre occurs by the sliding of thin filaments over thick filaments.
The steps are given below:
  • An impulse from the CNS reaches the neuromuscular junction (Motor-end plate) via motor neuron.
Neuromuscular junction is the synapse between a motor neuron and the sarcolemma of the muscle fibre.
A motor neuron + muscle fibres = a motor unit.
  • 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.
  • Ca 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 active sites on the actin to form cross bridge. This pulls actin filaments on both sides towards the centre of A-band. 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 cross-bridge is broken.
  • The ATP is again hydrolyzed by the myosin head and the above processes are 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 causing muscle fatigue. This is due to anaerobic breakdown of glycogen in muscles.

Red muscle fibres and white muscle fibres

Red (Aerobic) 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


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