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.

Outlines

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🧠 Neurotransmitter Effects on Postsynaptic Potentials

This paragraph explains how neurotransmitter chemicals can cause either depolarizing or hyperpolarizing postsynaptic potentials, leading to excitatory (EPSP) or inhibitory (IPSP) responses. It discusses how these potentials move along the dendritic membrane and diminish over distance, affecting the likelihood of generating action potentials. The text also describes how simultaneous excitatory and inhibitory signals can interact, with the net effect being the difference between them. The concept of temporal summation, where closely timed postsynaptic potentials can sum up, and spatial summation, where potentials from different locations can also sum, are introduced. The paragraph concludes by explaining that an action potential is triggered only when the combined EPSPs and IPSPs depolarize the cell to threshold at the axon hillock.

Keywords

💡Neurotransmitter

Neurotransmitters are chemical messengers that transmit signals across a synapse, the junction between two nerve cells. They play a crucial role in the communication between neurons and are essential for the functioning of the nervous system. In the script, neurotransmitters are responsible for causing either depolarizing or hyperpolarizing effects on the postsynaptic neuron, which can lead to either excitation or inhibition.

💡Depolarizing

Depolarization refers to the process where the membrane potential of a neuron becomes less polarized or more positive. This is often due to the influx of positively charged ions. In the context of the video, depolarizing neurotransmitters cause an excitatory postsynaptic potential (EPSP), which can bring the neuron closer to the threshold needed to generate an action potential.

💡Excitatory Postsynaptic Potential (EPSP)

An EPSP is a temporary positive shift in the membrane potential of a postsynaptic neuron, which makes it more likely to fire an action potential. It is caused by depolarizing neurotransmitters. The script mentions that EPSPs can result from neurotransmitter chemicals and are part of the process by which neurons integrate signals to potentially trigger an action potential.

💡Hyperpolarizing

Hyperpolarization is the process where the membrane potential of a neuron becomes more negative. This is typically due to the efflux of positively charged ions or the influx of negatively charged ions. In the script, hyperpolarizing neurotransmitters lead to inhibitory postsynaptic potentials (IPSPs), which make it less likely for the neuron to generate an action potential.

💡Inhibitory Postsynaptic Potential (IPSP)

An IPSP is a temporary negative shift in the membrane potential of a postsynaptic neuron, which makes it less likely to fire an action potential. It is caused by hyperpolarizing neurotransmitters. The script explains that IPSPs help to stabilize the membrane potential below the threshold, preventing the neuron from reaching the threshold necessary for an action potential.

💡Passive Spread

Passive spread refers to the way in which electrical signals, such as postsynaptic potentials, naturally decrease in amplitude as they travel along the neuron's membrane. This is due to the resistance of the membrane and the leakiness of ions through ion channels. The script mentions that postsynaptic potentials move passively along the dendritic membrane, becoming smaller as they spread.

💡Axon Hillock

The axon hillock is a specialized region of a neuron where the cell's axon begins, and it plays a crucial role in the initiation of action potentials. If the membrane potential reaches a certain threshold at the axon hillock, an action potential is generated. The script discusses how the integration of EPSPs and IPSPs at the axon hillock can determine whether an action potential is triggered.

💡Threshold

The threshold is the level of membrane potential at which a neuron will generate an action potential. If the sum of excitatory and inhibitory inputs reaches this threshold, the neuron will fire. The script explains that most synapses produce sub-threshold postsynaptic potentials, which means they alone are not enough to reach the threshold for an action potential.

💡Spatial Summation

Spatial summation is the process by which multiple postsynaptic potentials from different locations on the neuron's dendrites are combined. If the sum of these potentials is sufficient to depolarize the cell to threshold at the axon hillock, an action potential can be triggered. The script uses the example of two excitatory endings being activated to illustrate how spatial summation can lead to an action potential.

💡Temporal Summation

Temporal summation occurs when postsynaptic potentials that are not absolutely simultaneous are summed because they last for a few milliseconds before fading away. The closer in time these potentials occur, the greater the overlap and the more complete the summation. The script mentions temporal summation as a mechanism by which the neuron can integrate signals over time to reach the threshold for an action potential.

💡Sub-threshold

Sub-threshold refers to the condition where the membrane potential of a neuron is not sufficient to trigger an action potential. The script discusses how individual synapses often produce postsynaptic potentials that are sub-threshold, meaning they do not by themselves cause the neuron to fire. However, through spatial and temporal summation, these sub-threshold potentials can combine to reach the threshold.

Highlights

Postsynaptic potentials can be either depolarizing (EPSP) or hyperpolarizing (IPSP) based on neurotransmitter chemicals.

EPSPs are often excitatory, while IPSPs are inhibitory.

Postsynaptic potentials move passively along the dendritic membrane, diminishing in size as they spread.

Potentials from distant synapses take longer to reach the axon hillock compared to those from closer synapses.

Most synapses produce sub-threshold postsynaptic potentials that alone do not generate action potentials.

Two excitatory endings, when activated, can combine to depolarize the membrane to threshold if taken individually they are insufficient.

Inhibitory synapses stabilize the membrane potential below threshold through hyperpolarizations or sub-threshold depolarizations.

The effects of excitatory and inhibitory postsynaptic potentials can partially cancel each other out at the axon hillock.

Neurons subtract IPSPs from EPSPs to determine the net effect on the membrane potential.

Postsynaptic potentials that are not simultaneous can be summed due to their duration of a few milliseconds.

The overlap and summation of potentials are more complete when they are closer in time, a process known as temporal summation.

The summation of potentials from different locations across the cell body is called spatial summation.

An action potential is triggered only if the overall sum of EPSPs and IPSPs depolarizes the cell to threshold at the axon hillock.

The integration of postsynaptic potentials is crucial for the transmission of information in neurons.

The axon hillock acts as an integration zone where the net effect of EPSPs and IPSPs determines if an action potential is generated.

The passive spread and gradual diminution of postsynaptic potentials are key to understanding their role in synaptic transmission.

The ability of neurons to sum EPSPs and IPSPs, both spatially and temporally, is essential for their computational function.

The threshold for action potential generation is a critical factor in the integration of synaptic inputs.

Transcripts

the postsynaptic potentials that are

caused by neurotransmitter chemicals can

be either depolarizing often but not

always resulting in an excitatory

postsynaptic potential or epsp or

hyperpolarizing resulting in an

inhibitory postsynaptic potential or

IPSP

postsynaptic potentials generally move

passively along the dendritic membrane

gradually becoming smaller as they

spread therefore the postsynaptic

potentials from more distant synapses

will take a more than PSPs from synapses

closer to the integration zone at the

axon hillock

the postsynaptic potentials produced at

most synapses are usually well below the

threshold for generating postsynaptic

action potentials how then can synapses

transmit information if their

postsynaptic potentials are

sub-threshold

suppose that two excitatory endings are

activated causing local depolarizations

of the cell body taken alone neither

would be sufficient to trigger an action

potential but when combined the two

depolarization some to depolarize the

membrane in the hillock region to

threshold when inhibitory synapses are

active the membrane potential tends to

be stabilized below threshold because

they induce hyper polarizations or sub

threshold depolarizations that cannot

reach threshold these postsynaptic

effects also spread passively

dissipating as they travel because some

potentials excite and others inhibit the

hillock these effects partially cancel

out each other thus the net effect is

the difference between the two the

neuron subtracts the ipsps from the

epsps postsynaptic effects that are not

absolutely simultaneous can also be

summed because the postsynaptic

potentials last a few milliseconds

before fading away the closer they are

in time the greater is the overlap and

the more complete is the summation which

in this case is called temporal

summation the summation of potentials

originating from different physical

locations across the cell body is called

spatial summation only if the overall

sum of all the potentials both the epsps

and ipsps is sufficient to depolarize

the cell to threshold at the axon

hillock is an action potential triggered