Final answer:
Action potentials in neurons are initiated when enough EPSPs cause voltage-gated Na+ channels at the axon hillock to open, leading to a rapid depolarization and the 'firing' of the neuron.
Step-by-step explanation:
When the neuron receives enough excitatory input (EPSPs), voltage-gated Na+ channels on the axon hillock open and initiate an action potential. This depolarization occurs when Na+ channels in the axon hillock open, allowing positive ions to enter the cell, which leads to a change in the membrane potential from a resting state towards a more positive charge. As a result, the neuron depolarizes to a membrane potential of about +40 mV, which is the all-or-nothing event of an action potential. After depolarization, to reset its membrane voltage back to the resting potential, the Na+ channels close, beginning the refractory period where no further action potentials can be produced. During this time, voltage-gated K+ channels open to allow K+ to exit the cell, returning the membrane potential to a negative value, and eventually back to resting potential as K+ ions diffuse out through leakage channels.