Describe the mechanism of muscle contraction with a diagram.

(N/A) The 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.
This theory was postulated by $A$.$F$. Huxley and $J$. Jensen.
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.
$A$ neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of $Ca^{++}$ (calcium ions) into the sarcoplasm.
Increase in $Ca^{++}$ level leads to the binding of $Ca^{++}$ with a subunit of troponin on actin filaments,and thereby removes the masking of active sites for myosin.
Utilising the energy from $ATP$ hydrolysis,the myosin head now binds to exposed active sites on actin to form a cross-bridge.
This pulls the attached actin filaments towards the centre of the '$A$' 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 (contraction),the '$I$' bands get reduced,whereas the '$A$' band retains its length.
The myosin,releasing the $ADP$ and $P_i$,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.