Question

Describe the important steps in muscle contraction.

Answer 1

During skeletal muscle contraction, the thick filament slides over the thin filament by repeated binding and releases myosin along the filament. This whole process occurs in a sequential manner.

1. Step 1: Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. A 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 by generating an action potential in the sarcolemma.
2. Step 2: The generation of this action potential releases calcium ions from the sarcoplasmic reticulum in the sarcoplasm.
3. Step 3: The increased 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 and this allows myosin heads to attach to this site.
4. Step 4: In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilising energy from ATP hydrolysis. The actin filaments are pulled. As a result, the H-zone decreases. It is at this stage that the contraction of the muscle occurs.
5. Step 5: After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate. ATP molecules bind and detach myosin and the cross bridges are broken.
6. Stage 6: This process of the formation and breaking down of cross bridges continues until there is a drop in the stimulus, which causes an increase in calcium. As a result, the concentration of calcium ions decreases, thereby masking the actin filaments and leading to muscle relaxation.

Answer 2

• Under normal conditions, the sarcomere contains ATP and magnesium ions; Calcium ions are also present in minute quantities. Actin rods remain attached to tropomyosin in a way that they cannot bind to myosin rods.
• When a muscle fibre receives short-hold stimulation through a nerve impulse, Ca⁺ (calcium ions) are released from the endoplasmic reticulum (ER) of the muscle fibre into the sarcomere. These calcium ions combine with tropomyosin, and the actin rods become independent.
• At the same time, energy is released as a result of the decomposition of ATP in water. In the presence of this energy, actin and myosin become active and new bridge bonds are formed. As a result, the actin rods slide over the myosin rods and move towards the centre of the sacmeer.Actin and myosin, together, form actomyosin.
• In this process, the length of the muscle fibre decreases, i.e., it gets contracted. When the stimulation ends, calcium ions are pumped into the interstitial network by active pumping.
• Tropomyosin becomes free, due to which the bridge bonds between actin and myosin are broken. Actin then combines with tropomyosin. The muscle fibre returns to its old length.
• After death, Ca+ cannot go back into the sarcoplasmic reticulum due to a lack of ATP production; Hence, the muscles remain contracted and the body remains stiff.