Question 3:Describe the important steps in muscle contraction.

Answer:
During
skeletal muscle contraction, the thick filament slides over the thin
filament by a repeated binding and releases 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.
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 sarcolemma.
Step 2:
The generation of this action potential releases calcium ions from
the sarcoplasmic reticulum in the sarcoplasm.
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.
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. As a result, the H-zone reduces. It is at
this stage that the contraction of the muscle occurs.
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.
Stage 6:
This process of 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.