repeated stimulus causes the calcium channels in the presynaptic membrane to become less responsive, fewer calcium channels open and fewer calcium ions move into the pre-synaptic neurone when the pre-synaptic membrane is depolarised by an action potential, fewer vesicles move and fuse with the pre-synaptic membrane and less neurotransmitter is released into the synaptic cleft, as less neurotransmitter is available to bind to the receptors on the postsynaptic membrane, fewer fewer cation channels open and fewer sodium ions enter the postsynaptic neurone, with less depolarisation, the post-synaptic excitatory potential is not high enough to trigger an action potential - there is no response