First of all we can use maximum two transport blocks in one TTI for downlink if we are using spatial multiplexing.
In LTE the scheduler TTI is set to 1ms. The eNodeB scheduler will inform the UE's of allocated radio resources. The eNB schedules the UE's both on the downlink and on the uplink. For each UE scheduled in a TTI the user data will be carried in a Transport Block (TB). The TB is delivered on a transport channel. In LTE the number of channels is decreased compare to UMTS.
For the user plane there isonly one shared transport channel in each direction. The TB sent on the channel, can therefore contain bits from a number of services, multiplexed together.