Describe the important steps in muscle contraction.

(N/A) During skeletal muscle contraction,the thick filament slides over the thin filament by a repeated binding and release of myosin along the filament. This whole process occurs in a sequential manner.
Step $1$: Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. The neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result,Acetylcholine (a neurotransmitter) is released into the synaptic cleft,generating an action potential in the sarcolemma.
Step $2$: The generation of this action potential releases calcium ions $(Ca^{2+})$ from the sarcoplasmic reticulum into the sarcoplasm.
Step $3$: The increased level of calcium ions in the sarcoplasm leads to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin wrapped around actin filaments. Hence,active actin sites are exposed,allowing myosin heads to attach to these sites.
Step $4$: In this stage,the myosin head attaches to the exposed site of actin and forms cross-bridges by utilizing energy from $ATP$ hydrolysis. The actin filaments are pulled toward the center of the $A$-band. As a result,the $H$-zone reduces. This is the stage where the actual contraction of the muscle occurs.
Step $5$: After muscle contraction,the myosin head pulls the actin filament and releases $ADP$ along with inorganic phosphate. New $ATP$ molecules bind to the myosin head,causing it to detach and the cross-bridges to break.
Step $6$: This process of formation and breaking down of cross-bridges continues until the stimulus stops. When the stimulus ceases,calcium ions are pumped back into the sarcoplasmic reticulum. As a result,the concentration of calcium ions in the sarcoplasm decreases,thereby masking the actin filaments again and leading to muscle relaxation.