Explain mechanism of muscle contraction

Mechanism of muscle contraction :

• Mechanism of muscle contraction is explained by sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
• Muscle contraction is initiated by a signal sent by the central nervous system 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 neuromuscular junction or motor-end plate.
• Neurotransmitter releases here which generates an action potential in sarcolemma.
• These causes release of Ca++   into sarcoplasm. 
• These Ca++ binds with troponin, thereby remove masking of active site.
• Myosin head binds to exposed active site on actin to form a cross bridge, utilizing energy from ATP hydrolysis.
• This pulls the actin filament towards the centre of ‘A’ band.
• ‘Z’ lines also pulled inward thereby causing a shortening of sarcomere i.e. contraction.
• ‘I’ band get reduced, whereas the ‘A’ band retain the length.
• During relaxation, the cross bridge between the actin and myosin break.
• Ca++pumped back to sarcoplasmic cisternae.
• Actin filament slide out of ‘A’ band and length of ‘I’ band increases.  This returns the muscle to its original state.
• Repeated muscle contraction causes accumulation of lactic acid, produced from anaerobic breakdown of glycogen leads to muscle fatigue.
• Muscle contains red coloured oxygen storing pigment called myoglobin.
• Muscle with myoglobin called red muscle fibres, they are also contain large number of mitochondria which can utilize large amount of oxygen stored in them for ATP production also called aerobic muscle.
• Some muscles possess very less quantity of myoglobin and less mitochondrion hence called white fibres. Amount of sarcoplasmic reticulum is high in these muscles. They depend on anaerobic process for energy.