Answer
The sarcoplasmic reticulum (SR) is the smooth endoplasmic reticulum (SER) of muscle cells ( myocytes/myofibers). Each segment of the SR forms a network around each myofibril.
the SR is the source of calcium in skeletal muscle fibers.
Work Step by Step
Organelles of the a myofiber: The following are important parts ( organelles of a muscle fiber:
Sarcoplasmic reticulum( SR). The SR comprises a system of longitudinal tubes which face the
myofibrils, and terminal cisternae which form junctions with transverse tubules
Terminal cisternae (TC). These are dilated sacs at the ends of SR tubes ; they cross myofibers from
side to side
Transverse tubules ( T-tubules) are formed by deep invasions of the sarcolemma into the interior of
a myocyte. T-tubules effectively increase the surface area of muscle fibers.
Triads. These are periodic grouping of two terminal cisternae and one T-tubule with the T -tubule
positioned in the middle ( terminal cisterna-T-tubule-terminal cisterna).. The triads pass from one
myofibril to the next.
Sarcomere: A sarcomere is the contractile unit of a myofibril; it is made up of thick and thin
myofilaments
Functions;
The sarcolemma stores calcium ions (Ca++) and releases them when the muscle fiber is stimulated by a nerve stimulus to contract. The stimulus for contraction travels along the SR which is connected with T-tubules. The T-tubules take the signal for release of Ca++ from adjacent terminal cisternae deep into every contractile unit (sarcomere) of every myofibril. The T-tubules "inform" the SR when to release the burst of Ca++ ions which ultimately results in muscle contraction.
