: This is how muscles contract #IndiaNEWS #Education Today Hyderabad: This article is in continuation to the previous articles focusing on Locomotion and Movement. Today, lets discuss the mechanism

This is how muscles contract #IndiaNEWS #Education Today
Hyderabad: This article is in continuation to the previous articles focusing on Locomotion and Movement. Today, lets discuss the mechanism of muscle contraction.
Mechanism of muscle contraction
• Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
Sliding Filament Theory
• Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron.
• A motor neuron along with the muscle fibres connected to it constitutes a motor unit.
• The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate.
• A neural signal reaching this junction releases a neurotransmitter (Acetyl choline), which generates an action potential in the sarcolemma.
• This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
• Increase in Ca level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin.
• Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
• This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i. e. , contraction.
• It is clear from the above steps, that during shortening of the muscle, i. e. , contraction, the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length.
• The myosin, releasing the ADP and P1 goes back to its relaxed state.
• A new ATP binds and the cross-bridge is broken.
• The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding.
• The process continues till the Ca ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments.
• This causes the return of ‘Z’ lines back to their original position, i. e. , relaxation.
• The reaction time of the fibres can vary in different muscles.
• Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.
• Muscle contains a red-coloured oxygen storing pigment called myoglobin.
• Myoglobin content is high in some of the muscles which give a reddish appearance.
• Such muscles are called the red fibres.