Question

Describe the important steps in muscle contraction.

Answer 1

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.

Answer 2

Muscle Contraction Method:

Step 1 – Depolarization of Sarcolemma

The central nervous system (CNS) sends signals through a motor neuron that reaches the neuromuscular joint or the motor end plate.

The interaction between a fiber sarcolemma and a motor neuron is called a neuromuscular junction.

The release of a neurotransmitter called acetylcholine is created when the signal reaches the neuromuscular system. The strength of the action in the sarcolemma is set by this release.

Step 2 – The release of calcium ions

The sarcolemma transfers the action force to the sarcoplasmic reticulum to release calcium ions from the sarcoplasm

Step 3 – Occurrence of conformational changes in Actin filaments

Calcium ions are released, which tend to bind troponin and tropomyosin found in active fibers

This interaction results in a modification of the three-dimensional structure of the actin-troponin-tropomyosin complex. The active site of myosin found in actin filament is exposed.

Step 4 – Activation of Myosin heads

The release of calcium ions triggers the activation of myosin heads which in turn causes the release of energy in the form of ATP.

ATP Hydrolysis releases the energy that binds myosin heads to the active sites found in actin fibers which is why it forms a crossing bridge.

Step 5 – Actin Filaments slides over Myosin

Myosin head rotates due to the construction of a crossing bridge, pulling the actin fibers in the center of the A-band which is H zone.

The z-line attached to the actin strands also pulls inwards

The contraction of the sarcomere occurs when the actin filaments are drawn in different directions..

During the cutting process, the I-band decreases and the A-band maintains its length which causes the muscles to tighten.