Final answer:
The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.
It stores calcium ions needed for muscle contractions. Invaginations of the muscle cell membrane form T-tubules that contain extracellular fluid.
Step-by-step explanation:
The sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum found in muscle cells.
It is responsible for storing calcium ions (Ca²+) that are necessary for triggering muscle contractions.
The SR is composed of invaginations of the muscle cell membrane, known as T-tubules, which contain extracellular fluid.