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#### Input to Synaptic Channels

We want to set the neurotransmitter concentration that influences the synaptic channels for a short period of time, say just a single time step. The neurotransmitter concentration is stored in the `activation` field of the synaptic channels. However, if you try to find that field, `hsolve` gives an error message:

        genesis > echo {findsolvefield /main /main/main/basket activation}
        ** Error -  unknown or unavailable field activation for basket.

This means that we cannot set that field during simulation, so we have to use messages created before the `SETUP` call instead. A small trick is to set the coordinates of `neutral` elements, then have messages transport the coordinate values to the activation field of the synaptic channels.

        genesis > // point 1
        genesis > create neutral /messengers
        genesis > create neutral /messengers/n1
        genesis > setfield /messengers/n1 x 0.0 y 0.0 z 0.0
        genesis > addmsg /messengers/n1 /main/soma/basket ACTIVATION x
        genesis > addmsg /messengers/n1 /main/main[0-4]/basket ACTIVATION y
        genesis > addmsg /messengers/n1 /main/main[5-8]/basket ACTIVATION z

To activate the messages, we set the coordinate values for the neutral element for a single step of the simulation. `Hsolve` automatically fetches the new values and uses them to calculate the new conductance of the synaptic channel:

        genesis > // point 2
        genesis > step 0.2 -time
        genesis > setfield /messengers/n1 x 1.0 y 1.0 z 1.0
        genesis > step
        genesis > setfield /messengers/n1 x 0.0 y 0.0 z 0.0
        genesis > step 0.2 -time
        genesis > setfield /messengers/n1 x 1.0 y 1.0 z 1.0
        genesis > step
        genesis > setfield /messengers/n1 x 0.0 y 0.0 z 0.0
        genesis > step 0.2 -time

NOTE: Of course `hsolve` does store the `activation` field, but in an optimized and recalculated form. That is why it is not accessible with `findsolvefield`.

NOTE: The Purkinje cell tutorial that comes with the Genesis source code contains a Purkinje cell model with 4000 compartments that is simulated with `hsolve`[3]. Besides being a good example of graphical output using `hsolve`, various 'experiments' are implemented with the `findsolvefield` command and the `script_out` object. If you want to dig into the code of the tutorial, the `gctrace` and `gftrace` commands will prove useful.

Next: Networks of Cells Up: Setup of Experiments Previous: Current Injection Contents