Although task mapping and scheduling in wired networks of processors has been extensively studied, their application to WSNs remains largely unexplored. Existing algorithms cannot be directly implemented in WSNs due to limited resource availability and shared communication medium. In this work, an application-independent task mapping and scheduling solution in multi-hop WSNs is presented that provides realtime guarantees. Using a novel multi-hop channel model and a communication scheduling algorithm, computation tasks and associated communication events are scheduled simultaneously with a dynamic critical-path scheduling algorithm. Dynamic Voltage Scaling (DVS) mechanism is implemented to further optimize energy consumption. According to the simulation results, the proposed solution outperforms existing mechanisms in terms of guaranteeing application deadlines with minimum energy consumption.