Duty-cycled medium access protocols allow for long lasting au- tonomous networks by periodically putting nodes to sleep. However, this life expectancy improvement comes at the cost of a lesser network capacity and a poor adaptability to bursty traffic loads. Indeed, existing contention algorithms do not provide efficient algorithms to dynamically elect multiple senders per wake-up periods. In this paper, the medium is divided in several logical chan- nels (eg. obtained by a time/frequency division of the communication medium) and we propose to allocate them dynamically among senders. For this purpose, we propose a joint contention/scheduling algorithm, named Extended Slot Se- lection (ESS), that schedules multiple sender/receiver pairs to available logical channels.