Final answer:
When fifty presynaptic neurons release excitatory neurotransmitters, the resultant EPSPs summate. If the summation crosses the threshold, it triggers an action potential in the postsynaptic neuron. The net effect depends on both EPSPs and any concurrent IPSPs.
Step-by-step explanation:
When fifty presynaptic neurons release a burst of excitatory neurotransmitter, each generates an excitatory postsynaptic potential (EPSP) in a postsynaptic neuron. The process by which these EPSPs combine is known as summation. If these EPSPs are temporally close to each other, they can summate and may cause the postsynaptic neuron to reach the threshold of excitation needed to fire an action potential.
This occurs at the axon hillock. However, if there is concurrent release of inhibitory neurotransmitter from other presynaptic neurons, generating inhibitory postsynaptic potentials (IPSPs), they can counteract the effect of the EPSPs. The collective influence of EPSPs and IPSPs determines whether the neuron will depolarize enough to initiate an action potential. Summation is crucial in the nervous system as it helps prevent random noise from triggering inappropriate responses.