Muskelzytosol

Muskelzytosol, or muscle cytosol, is the cytosolic fraction of a skeletal muscle cell. It is the intracellular fluid within a muscle fiber, surrounding the myofibrils and other organelles, and bounded by the sarcolemma. As in other cells, it provides the aqueous environment where soluble enzymes, metabolites, and signaling molecules carry out most cytosolic reactions.

The Muskelzytosol contains water, inorganic ions (such as potassium and magnesium, with calcium kept at low resting levels and transiently elevated during contraction), and small molecules including glucose, glucose-6-phosphate, ATP, ADP, and inorganic phosphate. It also houses energy carriers like phosphocreatine, glycogen particles, glycolytic enzymes, lactate, and various buffering systems. Mitochondria lie within the muscle cell and contribute to energy production, but they are separate organelles located in the cytosol rather than being part of it.

Functions in muscle physiology include hosting glycolysis, which provides rapid ATP production during activity, and acting as a reservoir for energy carriers. The cytosol also contains enzymes involved in the metabolism of sugars and lactate and participates in signaling pathways that regulate contraction, metabolism, and adaptation. During contraction, Ca2+ is released from the sarcoplasmic reticulum into the cytosol to trigger cross-bridge cycling, and is then pumped back to the SR to restore relaxation.

Relation to contraction: the contractile machinery (sarcomeres) is organized within the cytosol of the muscle fiber, and the cytosolic metabolic state directly influences force production and endurance. Compared with non-muscle cells, muscle cytosol exhibits high metabolic flux and close spatial coupling to mitochondria and the SR to support rapid energy turnover.