Postsynaptic Potentials and Neural Summation

Types of Postsynaptic Potentials

• Neurotransmitters produce postsynaptic potentials ($PSP$) that are categorized based on their effect on the membrane potential.
• Excitatory Postsynaptic Potentials ($EPSP$): These result from depolarizing effects that move the membrane closer to an action potential.
• Inhibitory Postsynaptic Potentials ($IPSP$): These result from hyperpolarizing effects or subthreshold depolarizations that stabilize the membrane potential below the threshold.

Passive Conduction and Decay

• $PSPs$ move passively along the dendritic membrane.
• The magnitude of these potentials decreases with distance; therefore, $PSPs$ from distant synapses decay more significantly than those closer to the integration zone of the axon hillock.
• Individual $PSPs$ are typically subthreshold and insufficient on their own to trigger an action potential.

Mechanisms of Summation

• Spatial Summation: The process by which the neuron combines potentials originating from different physical locations across the cell body.
• Temporal Summation: The summation of potentials that occur at nearly the same time. Since postsynaptic effects last for several milliseconds, potentials that are closer in time exhibit greater overlap.
• Neural Integration: The neuron effectively subtracts $IPSPs$ from $EPSPs$ to determine the net effect.
• An action potential is only triggered if the net sum of all $EPSPs$ and $IPSPs$ is sufficient to reach the depolarization threshold at the axon hillock.