5 Postsynaptic Potentials

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Postsynaptic potentials are changes in the membrane potential of a neuron in response to a stimulus, enabling communication between cells. These changes, classified as excitatory or inhibitory, depend on the type of ion channels that open

Postsynaptic Potentials

Postsynaptic potentials are changes in membrane potential that move the cell away from its resting state. For our purposes, postsynaptic potentials are measured in the dendrites and cell bodies. Ion channels that are opened by a stimulus allow brief ion flow across the membrane. A stimulus can range from neurotransmitters released by a presynaptic neuron, changes in the extracellular environment like exposure to heat or cold, interactions with sensory stimuli like light or odors, or other chemical or mechanical events. The change in membrane potential in response to the stimulus will depend on which ion channels are opened by the stimulus.

 

Animation 5.1. A stimulus acting on the postsynaptic membrane can cause ion channels open, allowing specific ions to flow across the membrane. This movement of charge changes the membrane potential of the postsynaptic cell, creating a postsynaptic potential. ‘Postsynaptic Ion Flow’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Incoming signals can cause either an excitatory response or an inhibitory response in the neuron

Excitatory Postsynaptic Potentials (EPSPs)

An excitatory postsynaptic potential (EPSP) occurs when sodium channels open in response to a stimulus. The electrochemical gradient drives sodium to rush into the cell. When sodium brings its positive charge into the cell, the cell’s membrane potential becomes more positive, or depolarizes. This change is called a depolarization because the cell’s membrane potential is moving toward 0 mV, and the membrane is becoming less polarized. At 0 mV, there is no potential or polarization across the membrane, so moving toward 0 would be a decrease in potential. This depolarization increases the likelihood a neuron will be able to fire an action potential, which makes this ion flow excitatory. Therefore, an EPSP is an excitatory change in the membrane potential of a postsynaptic neuron.

A postsynaptic potential is typically brief, with ion channels closing quickly after the stimulus occurs. If there is not another stimulus, the cell will return to the resting membrane potential.

 

Animation 5.2. An excitatory postsynaptic potential occurs when positive ions, such as sodium, flow into the cell, making the membrane potential more positive (depolarization). This drives the membrane potential closer to threshold, increasing the likelihood of an action potential. ‘EPSP’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Inhibitory Postsynaptic Potentials (IPSPs)

An inhibitory postsynaptic potential, or IPSP, on the other hand, is caused by the opening of chloride channels. The equilibrium potential of chloride is -65 mV, so if the neuron is at rest at -60 mV, when chloride channels open, the electrochemical gradients drive chloride to flow into the cell. Chloride brings its negative charge into the cell, causing the cell’s membrane potential to become more negative, or hyperpolarize. This change is called a hyperpolarization because the cell’s membrane potential is moving away from 0 mV, and the membrane is becoming more polarized. An IPSP decreases the likelihood a neuron will be able to fire an action potential, which makes this ion flow inhibitory. Therefore, an IPSP is an inhibitory change in the membrane potential of a postsynaptic neuron.

Like an EPSP, an IPSP is also typically brief, and the membrane potential will return to rest if no additional stimulation occurs.

 

Animation 5.3. When a stimulus opens chloride channels, and the resting membrane potential is more positive than chloride’s equilibrium potential of -65 mV, chloride rushes into the cell. This causes an inhibitory hyperpolarization called an inhibitory postsynaptic potential (IPSP). After the stimulus, the ion channels close, and the membrane potential returns to rest. ‘IPSP’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The Resting Membrane Potential is Critical

The direction of ion movement depends on the membrane potential of the cell

In the previous example, the resting membrane potential of that cell was -60 mV, so chloride moved into the cell. If the resting membrane potential was instead equal to chloride’s equilibrium potential of -65 mV, then chloride would be at equilibrium and move into and out of the cell, and there would be no net movement of the ion. Even though this would lead to no change in membrane potential, the opening of chloride channels continues to be inhibitory. Increased chloride conductance would make it more difficult for the cell to depolarize and to fire an action potential.

 

Animation 5.4. If the cell’s resting membrane potential equals chloride’s equilibrium potential of -65 mV, opening chloride channels produces no net ion movement and no change in membrane potential. Chloride ions continue to cross the membrane in both directions equally, but the overall effect remains inhibitory. ‘IPSP at Equilibrium’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

If the resting membrane potential of the cell was more negative than chloride’s equilibrium potential, for example, at -70 mV, then chloride would leave the cell, in order to move the membrane potential toward -65 mV. This would result in a depolarization of the membrane potential. However, the overall effect is still inhibitory because once the cell reaches -65 mV, the driving forces acting on chloride would try to keep the cell at that membrane potential, making it more difficult for the cell to depolarize further and fire an action potential.

A good rule of thumb is to remember that opening of sodium channels is excitatory whereas opening of chloride channels is inhibitory.

 

Animation 5.5. If the cell’s resting membrane potential is more negative than chloride’s equilibrium potential of -65 mV, opening chloride channels causes chloride to leave the cell. The loss of negative charge produces a depolarization, but the effect is still inhibitory because chloride movement drives the membrane potential toward -65 mV, opposing further depolarization. ‘Inhibitory Depolarization’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Summation of Inputs

Postsynaptic potentials combine when more than one stimulus is present 

If an excitatory stimulus is followed by additional excitatory stimuli, the sodium channels will either remain open or additional sodium channels will open. The increased sodium conductance will cause the EPSPs to summate, depolarizing the cell further than one EPSP alone. Each neuron has a threshold membrane potential at which the cell will fire an action potential. The summation of EPSPs causes the neuron to reach that threshold.

 

Animation 5.6. Excitatory stimuli that occur quickly in succession lead to summation of EPSPs. This leads to increased depolarization of the membrane potential compared to a single EPSP. ‘Summated EPSP Ion Flow’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Summation can occur in two ways. Temporal summation occurs when one presynaptic input stimulates a postsynaptic neuron multiple times in a row. Spatial summation occurs when multiple presynaptic inputs each stimulate the postsynaptic neuron at the same time. Both types of summation result in a depolarization of a higher magnitude that when only on excitatory input occurs.

Diagram showing temporal summation (repeated inputs from one neuron) and spatial summation (simultaneous inputs from multiple neurons) creating larger EPSPs. Link to detailed alternative text in caption.
Figure 5.1 EPSP summation mechanisms. Temporal summation occurs when a single presynaptic input stimulates the postsynaptic neuron multiple times in rapid succession. Spatial summation occurs when multiple presynaptic inputs stimulate the postsynaptic neuron simultaneously. Both result in larger depolarizations than a single EPSP alone. ‘Synaptic Summation” by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

In addition to the summation of excitatory inputs, EPSPs can also summate with inhibitory inputs. The addition of an inhibitory stimulus will result in either a weaker depolarization compared to a single excitatory stimulus or possibly no depolarization at all, depending on the strength of the inhibitory input.

Diagram showing how simultaneous excitatory and inhibitory inputs can reduce or prevent depolarization in the postsynaptic neuron. Link to detailed alternative text in caption.
Figure 5.2. Interaction between excitatory and inhibitory inputs. When an inhibitory input occurs simultaneously with an excitatory input, the resulting depolarization is reduced or potentially eliminated, depending on the strength of the inhibitory input relative to the excitatory input. ‘EPSP and IPSP Summation’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

In the case of combined inhibitory and excitatory stimuli, both chloride and sodium channels will open. As sodium enters the cell trying to move the membrane potential to +60 mV, the equilibrium potential of sodium, chloride will also enter, trying to keep the cell near -65 mV, the equilibrium potential of chloride.

 

Animation 5.7. When excitatory and inhibitory inputs arrive simultaneously, sodium and chloride channels open. Sodium influx drives the membrane potential toward depolarization, while chloride influx opposes that change by driving the potential toward chloride’s equilibrium potential. The net result depends on the relative strength of each input. ‘EPSP and IPSP Ion Flow’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Conclusion

The interaction between excitatory and inhibitory postsynaptic potentials determines whether a neuron reaches the threshold to fire an action potential. This balance underlies neural communication and integration of stimuli.

Key Takeaways

  • Postsynaptic potentials occur in the dendrites or cell body
  • Excitatory postsynaptic potentials (EPSPs) result from sodium influx, depolarizing the membrane and increasing the likelihood of action potential firing.
  • Inhibitory postsynaptic potentials (IPSPs) occur due to chloride influx, reducing the likelihood of firing.
  • Temporal summation arises from repeated inputs from one presynaptic neuron, while spatial summation involves inputs from multiple neurons.
  • Combined EPSPs and IPSPs can modulate the net depolarization, influencing whether the neuron reaches the threshold.
  • Ion flow through stimulus-gated channels is brief, and the membrane potential returns to rest unless additional stimuli occur.

Important Terminology

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