The sarcolemma (or myolemma) (from the Greek σαρκο-, flesh and -λέμμα, cortex) is the cytoplasmic membrane of muscle fibers (cells). It is a semipermeable and lipid membrane, just like the other membranes of other eukaryotic cells. However, the continuity of the membrane in the muscle fiber extends in the form of trabeculae to the interior of the cell through the sarcoplasm. These invaginations of tubular channels with their ramifications are known as T-tubules. This developed system of cisterns in association with the smooth endoplasmic reticulum, known as sarcoplasmic reticulum, contributes to the propagation of the electrical potential that produces the contraction of the muscular fiber, which allows the excitability of the muscle. The composition of a T-tubule plus two sarcoplasmic reticulum terminal cisternae is called the triad (located at the junction of the AI bands of the skeletal muscle fiber), whereas the composition of a T-tubule plus a sarcoplasmic reticulum terminal cisterna is called dyad (located in the Z discs of the cardiac muscle fibers, in which there is only one per sarcomere). Its activation allows the entry of calcium ions, which is the sign of activation of muscle contraction.
Formerly the term sarcolemma was used to refer to a thick "membrane" layer believed to be the cytoplasmic boundary of the muscle cell. Today it is known that this thick sarcolemma consists of the plasma membrane of the cell, its outer lamina and the surrounding reticular lamina.
It is necessary to emphasize that some parts of the muscular cells change their names, a clear example is the Mitochondria, which is called Sarcoma; or even we can highlight the endoplasmic reticulum which happens to acquire the name of. Sarcoplasmatic rículo
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