Suppose a particular neurotransmitter causes an ipsp in postsynaptic cell x and an epsp in postsynaptic cell y. A likely explanation is that

This situation states that the cells has two different receptors on them.

Explanation:

The particular neurotransmitter can have different receptors on them. An (EPSN) excitatory postsynaptic potential causes change in the membrane potential of the postsynaptic cell which tends to open the channels for the positively charged ions inside the cell.

This leads to the activation of the ligand sensitive channels.

In case of the inhibitory postsynaptic potential ( IPSP) makes the neurons less like to generate any action potential.

So, for the same neurotransmitter, one cell X shows excitatory response and other cell y shows inhibitory response.

you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.

not sure if this what   you'r looking for would be percent composition