The effect of a neurotransmitter at a synapse depends on a number of factors, including how much neurotransmitter is released and the action of the neurotransmitter on the post-synaptic neurone.
Some receptors are linked to ion channels that open when neurotransmitter molecules bind to them. This allows ions to flow across the membrane of the post-synaptic neurone, producing an electrical signal, or postsynaptic potential. These post-synaptic potentials may be excitatory (EPSPs) or inhibitory (IPSPs), depending on whether they bring the post-synaptic neurone closer to, or further away from, the threshold required to produce an action potential. A PSP is approximately 1-2mV.
When a neurone receives excitatory and/or inhibitory inputs at the same time it 'adds' the inputs together. If the number of excitatory inputs overall are sufficiently great, it will result in the generation of an action potential in the post-synaptic neurone and the transmission of a nerve impulse. This addition of 'inputs' is called summation.
Spatial summation occurs when two or more separate inputs arrive almost simultaneously from different pre-synaptic neurones. The individual PSPs add together.
Temporal summation occurs when two or more action potentials (nerve impulses) arrive in rapid succession along a single pre-synaptic neurone.