Summation of reversal potentials in neurons

[Simfish]

Homework Statement

Consider a neuron with resting potential of -65 mV and threshold of -55 mV. It receives two
synaptic inputs with similar synaptic conductances, one with reversal potential of -10 mV and the
other with reversal potential of -58 mV. Draw the predicted postsynaptic response (change in
membrane potential) to stimulation of each synapse alone, and then to simultaneous stimulation of
both synapses. Briefly explain what’s going on and why the results might at first be confusing.

Homework Equations

The Attempt at a Solution

So the neuron will reach an action potential at -55 mV. So if the neuron gains 10 mV, it will depolarize and reach an action potential. Now, how will the synaptic inputs affect the neuron? Input 1 might have a reversal potential of -10 mV, but that doesn't say how much current it sends to the neuron, nor does it say the proportion of total positive/negative ions in the input compared to that of the neuron. So that confuses me. How do we add potentials? Do we just take some average of the resting potential of the neuron with the reversal potential of the inputs? Which would be sort of like adding up concentrations or ratios. Or do we add-55 to -10 and -55 to -58? Ions sum up additively (and don't involve taking ratios) but I highly doubt the question wants me to say that since then there would just be hyperpolarization and no action potential for either input.

Thanks

 

[nate808]

for your question and for thinking critically about the problem! You are correct in thinking that the reversal potential alone does not tell us the full story of how the inputs will affect the neuron. We also need to consider the synaptic conductance, which is a measure of how easily ions can flow through the synapse. This conductance will determine the amount of current that is sent to the neuron from each input.

To answer your question about adding potentials, yes, we can add them just like we would add concentrations or ratios. In this case, we are adding the resting potential of the neuron (-65 mV) to the reversal potentials of the inputs (-10 mV and -58 mV). This will give us the total potential change that the neuron will experience from each input.

Now, let's look at the predicted postsynaptic responses. When input 1 is stimulated alone, the neuron will depolarize by 55 mV (-65 + -10 = -55 mV). This is above the threshold of -55 mV, so an action potential will be triggered. Similarly, when input 2 is stimulated alone, the neuron will hyperpolarize by 3 mV (-65 + -58 = -123 mV). This is below the threshold, so no action potential will be triggered.

When both inputs are stimulated simultaneously, we need to consider the combined effects of their potentials. The total potential change will be -118 mV (-65 + -10 + -58 = -133 mV). This is below the threshold, so no action potential will be triggered. However, the neuron will still be more depolarized than when input 2 was stimulated alone, so it will be in a state of reduced excitability.

The reason this might be confusing at first is because we are used to thinking of action potentials as being "all or nothing" events. However, in reality, the strength and timing of synaptic inputs can greatly influence whether or not an action potential is triggered. In this case, the combined effects of the two inputs result in a subthreshold potential change, meaning no action potential is triggered. This is a common occurrence in neural circuits and highlights the complexity of how neurons integrate and process information.

 

[Blueshift5]

for your question! This is a great example of the complexities involved in understanding the effects of synaptic inputs on a neuron's membrane potential. Let's break down the problem and see if we can make sense of it.

First, let's consider the predicted postsynaptic response to stimulation of each synapse alone. In this case, the neuron's membrane potential will change based on the difference between the reversal potential of the input and its resting potential. So, for input 1 with a reversal potential of -10 mV, the membrane potential will depolarize by 55 mV (since -10 mV - (-65 mV) = 55 mV). This will bring the membrane potential to -10 mV, which is still below the threshold of -55 mV, so no action potential will be generated.

For input 2 with a reversal potential of -58 mV, the membrane potential will hyperpolarize by 7 mV (since -58 mV - (-65 mV) = 7 mV). This will bring the membrane potential to -72 mV, which is further below the threshold and will not generate an action potential.

Now, let's consider the simultaneous stimulation of both synapses. In this case, the membrane potential will change based on the net effect of the two inputs. The reversal potential of input 1 is closer to the resting potential of the neuron, so it will have a larger effect in depolarizing the membrane potential. However, input 2 will also have an effect in hyperpolarizing the membrane potential. So, the net effect will depend on the relative strengths of the two inputs.

If input 1 is stronger, it will depolarize the membrane potential more than input 2 can hyperpolarize it, and an action potential will be generated. If input 2 is stronger, it will hyperpolarize the membrane potential more than input 1 can depolarize it, and no action potential will be generated. If the inputs are equal in strength, they will cancel each other out and the membrane potential will remain at -65 mV.

So, the results might be confusing at first because they depend on the relative strengths of the inputs and the differences between their reversal potentials and the neuron's resting potential. The key is to consider the net effect of the inputs and how they interact with each other to determine the overall response of the neuron.