What are excitatory postsynaptic potentials (EPSPs)?

What are excitatory postsynaptic potentials (EPSPs)?

• A. Depolarizations that increase the likelihood of an action potential.
• B. Hyperpolarizations that decrease the likelihood of an action potential.
• C. Subthreshold changes with no effect.
• D. Inhibitory signals that hyperpolarize.

Answer: (A)

Excitatory postsynaptic potentials are brief depolarizations of the postsynaptic membrane that bring the cell closer to the threshold for firing an action potential. When an excitatory synapse is activated, receptors (often AMPA-type) open cation channels, allowing positive ions like Na+ (and sometimes Ca2+) to enter. This influx reduces the negative resting potential, creating a depolarizing flow. If enough EPSPs arrive quickly from one or multiple synapses, their effects add up in time (temporal summation) and space (spatial summation) to push the membrane potential past threshold, triggering an action potential. Even if a single EPSP doesn’t reach threshold, it still moves the potential toward it, unlike inhibitory postsynaptic potentials which hyperpolarize and move the membrane away from threshold. IPSPs, generated by GABAergic or glycinergic inputs, open Cl− or K+ channels to make the inside more negative, lowering the likelihood of an action potential. In short, EPSPs are depolarizing events that increase the chance of firing, whereas IPSPs do the opposite.

Excitatory postsynaptic potentials are brief depolarizations of the postsynaptic membrane that bring the cell closer to the threshold for firing an action potential. When an excitatory synapse is activated, receptors (often AMPA-type) open cation channels, allowing positive ions like Na+ (and sometimes Ca2+) to enter. This influx reduces the negative resting potential, creating a depolarizing flow. If enough EPSPs arrive quickly from one or multiple synapses, their effects add up in time (temporal summation) and space (spatial summation) to push the membrane potential past threshold, triggering an action potential. Even if a single EPSP doesn’t reach threshold, it still moves the potential toward it, unlike inhibitory postsynaptic potentials which hyperpolarize and move the membrane away from threshold. IPSPs, generated by GABAergic or glycinergic inputs, open Cl− or K+ channels to make the inside more negative, lowering the likelihood of an action potential. In short, EPSPs are depolarizing events that increase the chance of firing, whereas IPSPs do the opposite.