Final Answer:
The outer membrane around the muscle cell or fiber is called the sarcolemma, while the tunnel-like infoldings of the outer membrane form part of the triad called the T-tubules.
Step-by-step explanation:
The sarcolemma is the plasma membrane that surrounds the muscle fiber, serving as a protective barrier for the muscle cell. It plays a crucial role in muscle contraction and communication. T-tubules (transverse tubules) are invaginations or tunnel-like infoldings of the sarcolemma that penetrate deep into the muscle fiber.
Together with the sarcoplasmic reticulum, T-tubules form the triad, a vital structure for transmitting signals during muscle contraction. The triad facilitates the coordination of muscle contractions by allowing the action potential to rapidly reach the interior of the muscle fiber.