Message Sequence Chart(MSC) is a visual language that illustrates the scenarios of system operations, offering user-friendly, easy to understand behavior descriptions. Furthermore, high-level MSC(HMSC) provides composition mechanisms to deal with complexity of the system. However, for the MSC to be used to describe complex reactive systems, there should be ways to deal with the reactive behaviors that require the system to respond immediately. In this paper, we propose extensions of MSC to describe complex reactive behaviors in a systematic way. We define the formal semantics of the extended features based on process algebra as well as the visual and textual syntax. The extensions allow structured, systematic and succinct description of complex reactive behaviors.