Summation of Postsynaptic Potentials

Summary

This script delves into the intricacies of neurotransmission, focusing on the dynamics of postsynaptic potentials. It explains how neurotransmitters can induce either depolarizing (EPSP) or hyperpolarizing (IPSP) responses. The script highlights the passive spread of these potentials along the dendritic membrane, diminishing with distance, and how their sub-threshold nature can still influence neuronal firing through temporal and spatial summation. The concept of summation, where multiple EPSPs and IPSPs combine to reach the threshold for an action potential, is crucial for understanding how neurons integrate and transmit information.

Takeaways

• 🧠 Neurotransmitters cause postsynaptic potentials that can be either depolarizing (EPSP) or hyperpolarizing (IPSP).
• 🔋 Depolarizing potentials are excitatory, while hyperpolarizing potentials are inhibitory.
• 🌊 Postsynaptic potentials diminish as they move along the dendritic membrane due to passive spread.
• 📉 Distant synapses produce smaller PSPs compared to those closer to the axon hillock.
• 🚫 Most synapses generate sub-threshold potentials, which alone cannot trigger an action potential.
• 🤝 Combined excitatory inputs can summate to depolarize the membrane at the axon hillock to threshold, initiating an action potential.
• 🛡️ Inhibitory synapses stabilize the membrane potential below threshold, preventing action potential generation.
• ⏱️ Temporal summation occurs when postsynaptic potentials that are not simultaneous overlap in time, enhancing their combined effect.
• 🌐 Spatial summation is the summation of potentials from different locations across the cell body.
• 🔑 An action potential is triggered only if the net sum of EPSPs and IPSPs depolarizes the cell to threshold at the axon hillock.

Q & A

What are the two types of postsynaptic potentials caused by neurotransmitter chemicals?

The two types of postsynaptic potentials are depolarizing, which often results in an excitatory postsynaptic potential (EPSP), and hyperpolarizing, which results in an inhibitory postsynaptic potential (IPSP).

How do postsynaptic potentials move along the dendritic membrane?

Postsynaptic potentials move passively along the dendritic membrane, gradually becoming smaller as they spread.

Why do postsynaptic potentials from distant synapses take longer to reach the axon hillock?

Postsynaptic potentials from distant synapses take longer to reach the axon hillock because they gradually diminish as they spread, making them weaker by the time they reach the integration zone.

What is the threshold for generating postsynaptic action potentials?

The threshold for generating postsynaptic action potentials is the level of depolarization at which the neuron's membrane potential is sufficient to trigger an action potential.

How can synapses transmit information if their postsynaptic potentials are sub-threshold?

Synapses can transmit information through the summation of multiple sub-threshold EPSPs, which when combined, can depolarize the membrane to threshold and trigger an action potential.

What happens when two excitatory endings are activated simultaneously?

When two excitatory endings are activated, they cause local depolarizations that, when combined, can depolarize the membrane in the hillock region to threshold, potentially triggering an action potential.

How do inhibitory synapses affect the membrane potential?

Inhibitory synapses stabilize the membrane potential below threshold by inducing hyperpolarizations or sub-threshold depolarizations that cannot reach the threshold for an action potential.

What is the difference between temporal and spatial summation of postsynaptic potentials?

Temporal summation refers to the summation of potentials that are not absolutely simultaneous and are closer in time, leading to greater overlap and more complete summation. Spatial summation is the summation of potentials originating from different physical locations across the cell body.

Why do some postsynaptic effects partially cancel each other out?

Some postsynaptic effects partially cancel each other out because some potentials excite (EPSPs) while others inhibit (IPSPs), and their opposing actions at the axon hillock result in a net effect that is the difference between the two.

What is necessary for an action potential to be triggered at the axon hillock?

An action potential is triggered at the axon hillock when the overall sum of all the potentials, both EPSPs and IPSPs, is sufficient to depolarize the cell to threshold.