The striated muscle fiber is a syncytium. The fiber, as a whole, is surrounded by a membrane known as the sarcolemma. The sarcolemma has specialized invaginations that enter the interior of the fiber at right angles to the sarcolemma.

These are called transverse tubules (T-tubules). The T-tubules connect with the extracellular space and allow interstitial fluid to flow in and through the striated muscle fiber.

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The fiber is filled with a type of intracellular fluid called sarcoplasm.

SARCO = flesh

Within the sarcoplasm is a tubular system called the sarcoplasmic reticulum that stores calcium, which is necessary for the muscle activation and contraction.

Myofilaments are found in the sarcoplasm.
MYO = muscle
FIL = thread

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Myofilaments are long complexes of protein molecules, either actin or myosin. Thus, there are two main types of myofilaments: actin and myosin. The myosin filaments are thicker and have appendages known as myosin "bridges."

The myosin filaments are surrounded by the thinner actin filaments. Great numbers of well-developed mitochondria (the "powerhouse" elements of cells) are found in striated muscle fibers.