Describe the important steps in muscle contraction.

The process of muscle contraction involves various steps.
* Excitation of muscle : A neurotransmitter acetyl choline is released and it generates action potential in sarcolemma. Sarcoplasmic reticulum releases Ca⁺⁺ ions.
* Formation of cross bridges : The head of myosin binds to active sites on actin filaments and form cross bridges.
* Power Stroke : Myosin filaments pull the actin filaments towards the centre of A band. It is called power stroke.
* Recovery Stroke : A new ATP molecule binds to the head of myosin.
* The cross bridges break and myosin head hydrolyses ATP. This cycle is repeated.

Steps of Muscle Contraction and Relaxation

Step 1: Muscle contraction begins when a signal travels through the axon and reaches the neuromuscular junction, also known as the motor end plate. This junction connects a neuron with the sarcolemma of the muscle fiber. The signal causes the release of a neurotransmitter called acetylcholine into the synaptic cleft, triggering an action potential in the sarcolemma.

Step 2: The action potential stimulates the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.

Step 3: The increase in calcium ion concentration activates the actin sites. Calcium binds to troponin on actin filaments, causing the removal of tropomyosin, which normally blocks the actin sites. This exposes the active sites on actin, allowing myosin heads to attach.

Step 4: The myosin heads bind to the exposed actin sites, forming cross-bridges. This process uses energy from ATP hydrolysis. The myosin pulls the actin filaments, reducing the H-zone, which results in muscle contraction.

Step 5: After contraction, the myosin head releases ADP and inorganic phosphate, and new ATP molecules bind to myosin. This breaks the cross-bridges, allowing myosin to detach from actin.

Step 6: The cycle of cross-bridge formation and detachment continues as long as the stimulus persists. When the stimulus decreases, the calcium ion concentration drops. This causes the actin filaments to become masked again, leading to muscle relaxation.