SATHEE: Exemplar Problems

Biology Problems

Exemplar Problems

Question:

Describe the important steps in muscle contraction.

Answer:

Muscle contraction is a complex process that involves a series of steps. Here are the important steps in muscle contraction:

Nerve Signal: Muscle contraction begins when a motor neuron (nerve cell) sends an electrical signal, called an action potential, to the muscle fibers it innervates. This action potential travels down the motor neuron’s axon and reaches the neuromuscular junction.
Neuromuscular Junction: The neuromuscular junction is the point of contact between the motor neuron and the muscle fiber. When the action potential reaches the neuromuscular junction, it stimulates the release of a neurotransmitter called acetylcholine (ACh) from the motor neuron’s axon terminals.
ACh Binding: Acetylcholine diffuses across the synaptic cleft (the tiny gap between the motor neuron and muscle fiber) and binds to receptors on the muscle fiber’s membrane (sarcolemma). This binding triggers a change in the sarcolemma’s permeability to ions, allowing sodium ions (Na+) to enter the muscle fiber.
Action Potential Propagation: The influx of sodium ions into the muscle fiber causes a local depolarization of the sarcolemma, creating an action potential that spreads across the muscle cell’s surface and deep into the muscle fiber through T-tubules (transverse tubules).
Calcium Release: The action potential traveling through the T-tubules stimulates the sarcoplasmic reticulum (a membrane system within muscle cells) to release calcium ions (Ca2+) into the sarcoplasm (muscle cell’s cytoplasm).
Troponin-Tropomyosin Complex: Calcium ions bind to the regulatory protein troponin, causing a conformational change in the troponin-tropomyosin complex. Tropomyosin shifts its position, uncovering the active binding sites on actin, a thin filament in the muscle.
Cross-Bridge Formation: Myosin heads, found on thick filaments within the muscle fiber, bind to the exposed active sites on actin, forming cross-bridges between the two types of filaments.
Power Stroke: Myosin heads undergo a conformational change, using energy from ATP hydrolysis, which results in the myosin heads pulling the actin filaments toward the center of the sarcomere. This shortens the sarcomere and generates muscle force.
Release of ADP and Pi: After the power stroke, the myosin heads release adenosine diphosphate (ADP) and inorganic phosphate (Pi), but they remain bound to actin.
ATP Binding: ATP binds to myosin heads, causing them to detach from actin.
Myosin Reset: The energy from ATP hydrolysis is used to “reset” the myosin heads to their original, low-energy position.
Repeat Cycle: Steps 7 to 11 are repeated as long as there is a supply of ATP and calcium ions. The repeated cycling of cross-bridge formation, power strokes, and detachment causes the actin filaments to slide over the myosin filaments, shortening the sarcomeres and leading to muscle contraction.
Relaxation: When the nerve signal stops, acetylcholine is broken down, calcium ions are actively transported back into the sarcoplasmic reticulum, and the troponin-tropomyosin complex returns to its original position, blocking the active sites on actin. This process leads to muscle relaxation.

These steps represent the fundamental events in muscle contraction, which are repeated many times per second to generate muscle force and movement. The regulation of calcium ions, ATP availability, and nerve signals plays a critical role in controlling the timing and intensity of muscle contractions.

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Engineering Preparation

Medical Preparation

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Important Resources

Other Resources

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Mentorship

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