1. Tardigrade
  2. Question
  3. Biology
  4. Assertion: On stimulation, a muscle cell releases calcium ions (Ca2+) from sarcoplasmic reticulum. Reason: By reacting with a protein complex, Ca2+ uncover active sites on the actin filaments.

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Q. Assertion : On stimulation, a muscle cell releases calcium ions $(Ca^{2+})$ from sarcoplasmic reticulum.
Reason : By reacting with a protein complex, $Ca^{2+}$ uncover active sites on the actin filaments.

AIIMSAIIMS 2017Locomotion and Movement

Solution:

A neural signal on reaching the neuromuscular junction releases a neurotransmitter (acetylcholine) which generates an action potential in the sarcolemma.This spreads through the muscle fibre and causes the release of calcium ions from sarcoplasmic reticulum into the sarcoplasm. Increase in $Ca^{2+}$ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby removing the mask of active sites for myosin.