In the documentation you will find that hsolve is able to 'take over' synaptic channels. This is correct regarding the numerical computations that the synaptic channels perform. However, besides these numerical computations, the synaptic channels also perform event buffering. Event buffering is the management of synaptic events during simulation time: (1) receiving incoming events, (2) sorting events in order of firing time and (3) scheduling events by adding the weight of the firing synapse to the activation value of the channel at the appropriate simulated time step (the activation is the neuro-transmitter concentration in the synaptic cleft during a single time step).