sarcoplasma

Sarcoplasm is the cytoplasm of a muscle cell, especially a skeletal or cardiac muscle fiber. It lies inside the sarcolemma and surrounds the contractile apparatus, which is organized into myofibrils. The saroplasm houses the organelles and cytosolic components needed for muscle metabolism, contraction, and maintenance.

It contains many mitochondria and, in slow-twitch (oxidative) fibers, abundant myoglobin. Glycogen granules and lipid droplets store energy substrates. The sarcoplasm also contains the sarcoplasmic reticulum, a specialized endoplasmic reticulum that regulates intracellular calcium, and transverse tubules that transmit action potentials into the fiber.

During excitation-contraction coupling, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm, triggering actin–myosin cross-bridge cycling and muscle shortening. Calcium is then pumped back into the SR to terminate contraction and allow relaxation. The composition of the saroplasm varies with fiber type and metabolic demands, influencing contraction speed, fatigue resistance, and metabolic pathways.