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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👇 Select Your Topic Here 👇

• Topic 1: Human Muscular System
• Topic 2: Mechanism of Muscle Contraction
• Topic 3: Human Skeletal System
• Topic 4: Joints, Disorders of Muscular & Skeletal Systems