Question

Inside a muscle fiber, what triggers sarcomeres to contract?

A. A sudden rise in intracellular Na+
B. A sudden rise in intracellular Ca2+
C. An increased availability of ATP
D. Polarization of the muscle fiber's membrane

Answer 1

Sarcomeres in a muscle fiber contract due to a sudden rise in intracellular Ca2+. This is initiated by excitation-contraction coupling, where an action potential triggers the release of Ca2+ from the sarcoplasmic reticulum, leading to the interaction of actin and myosin resulting in muscle contraction.

Step-by-step explanation:

Inside a muscle fiber, sarcomeres contract in response to a sudden rise in intracellular Ca2+. This process is known as excitation-contraction coupling. When a muscle fiber receives a signal, namely the neurotransmitter Acetylcholine (ACh) from a motor neuron, it triggers depolarization of the sarcolemma membrane as sodium ions (Na+) enter. This depolarization spreads, leading to an action potential that travels through the T-tubules, stimulating the release of Ca2+ from the sarcoplasmic reticulum (SR). The presence of Ca2+ in the sarcoplasm initiates the contraction by allowing the myosin heads to attach to the actin-binding sites, which is facilitated by the movement of troponin and tropomyosin complex, ultimately leading to the sliding of the actin filaments by the myosin filament and the shortening of the muscle fiber.