Question: what is the role of the sarcoplasmic reticulum in muscle contraction

title: Sarcoplasmic reticulum
text: The sarcoplasmic reticulum is a network of tubules that extend throughout muscle cells , wrapping around ( but not in direct contact with ) the myofibrils ( contractile units of the cell ) . Cardiac and skeletal muscle cells contain structures called transverse tubules ( T - tubules ) , which are extensions of the cell membrane that travel into the centre of the cell . T - tubules are closely associated with a specific region of the SR , known as the terminal cisternae in skeletal muscle , with a distance of roughly 12 nanometers , separating them . This is the primary site of calcium release . The longitudinal SR are thinner projects , that run between the terminal cisternae / junctional SR , and are the location where ion channels necessary for calcium ion absorption are most abundant . These processes are explained in more detail below and are fundamental for the process of excitation - contraction coupling in skeletal , cardiac and smooth muscle . 

Answer: answer: The sarcoplasmic reticulum is a network of tubules that extend throughout muscle cells. The longitudinal SR are thinner projects, that run between the terminal cisternae / junctional SR, and are the location where ion channels necessary for calcium ion absorption are most abundant.