Sarcoplasmic Reticulum (SR) and the T-System
A Closer Look at the Membranous Communication Channels in Muscles Cells
The sarcoplasmic reticulum (SR) lies over the myofibrils like a net. It is actually smooth endoplasmic reticulum and extends long tubules that connect between and around the myofibrils. Where its tubules meet – where A-Bands and I-Bands come together (A-I junctions), it forms sac-like channel called, terminal cisternae.
The sarcolemma (cell membrane) of the muscle cell also forms hollow transverse tubules (T-tubules) at each A-I junction punching deep inside every muscle cell. So the T-tubules, surrounded by the terminal cisternae on both sides form a triad of sorts.
Thousands of t-tubules surround the myofibrils to form a T-System in the muscle cells.
When a nerve stimulus reaches the sarcolemma (muscle cell membrane), it followed the T system deep into the cell and immediately transmits the message throughout the muscle cell > CONTRACT!
This delivery system is also how all the myofibrils receive oxygen, glucose and other ions needed for smooth muscles contractions. The main job of the SR is to keep calcium (CA+) available when it’s needed for muscle contraction. The SR can store and release calcium as needed.