Final answer:
The statement is true; smooth muscle relaxation is initiated when Ca2+ levels drop, but a small concentration remains to maintain muscle tone. Additionally, ATP is necessary for the relaxation process.
Step-by-step explanation:
The statement 'Smooth muscles relax when intracellular Ca2+ levels drop but may not cease contractions' is true. This is because, while smooth muscle contraction is dependent on Ca2+ ions, the process of relaxation involves more than just the reduction of calcium levels. When signaling from the motor neuron ends, the sarcolemma and T-tubules repolarize, and the voltage-gated calcium channels in the sarcoplasmic reticulum (SR) close. This results in Ca2+ ions being pumped back into the SR, causing tropomyosin to cover the binding sites on actin strands and ending the contraction.
However, muscle contraction can also persist even when Ca2+ levels are lowered because a small concentration of calcium remains in the sarcoplasm, maintaining muscle tone. This is essential in some body areas, like around blood vessels. Additionally, ATP is required for both contraction and relaxation processes. Therefore, if a muscle runs out of ATP, it may also cease to contract and become fatigued.