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The canonical definition of action potentials found in textbooks states that action potentials are all alike in shape. From techniques such as spike sorting which are used to attribute electrophysiological recordings to individual cells, we know, however, that shapes and amplitudes of action potentials vary.

Synaptic strength is defined as the average amplitude of postsynaptic potential evoked following a presynaptic action potential.

Given that the incoming action potentials might differ for different synapses (and for instance lead to more or less vesicles being released), how can we hence have a uniform definition of synaptic strength?

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  • $\begingroup$ What do you mean by uniform definition of synaptic strength? Synaptic strength has nothing to do with shape or amplitude of action potentials so why do you think that is relevant? $\endgroup$
    – Bryan Krause
    Nov 22 '20 at 23:45
  • $\begingroup$ Why not? Say the amplitude of an AP is larger, then could it not be that the number of vesicles being released at the axon terminal is larger as well, hence leading to a larger PSP by a stronger depolarization at the axon terminal? $\endgroup$
    – TestGuest
    Nov 23 '20 at 2:28
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    $\begingroup$ Ok, if that's your confusion I understand now, I'll move my comments into a short answer. Let me know if this covers it. $\endgroup$
    – Bryan Krause
    Nov 23 '20 at 18:05
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    $\begingroup$ Although action potentials do vary from neuron to neuron between different classes of neurons and neurons from different areas, the differences that appear in extracellular recordings are due to recording conditions, such as the distance of the electrode from the neuron and the part of the neuron that is being recorded. Hence, the different action potential shapes observed in extracellular recordings are not inherent to the neurons. Spike sorting takes advantage of the different conditions that each neuron is recorded in an attempt to separate them into different units, i.e. candidate neurons. $\endgroup$
    – vkehayas
    Nov 24 '20 at 8:35
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    $\begingroup$ @vkehayas Some of those AP characteristics can also be measured extracellularly, in particular the spike width differences between cortical pyramidal cells and fast spiking interneurons. $\endgroup$
    – Bryan Krause
    Nov 27 '20 at 0:17
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Synaptic strength is defined as the average amplitude of postsynaptic potential evoked following a presynaptic action potential.

This is a correct definition. Synaptic strength is a fairly abstracted measure. It can be influenced by both presynaptic (for example: number of vesicles/concentration of neurotransmitter per vesicle) and postsynaptic effects (for example: receptor number and composition). So yes, changes presynaptically such as the character of an action potential can influence synaptic strength, but that isn't a problem: we still represent this as part of the synaptic strength if it affects the average postsynaptic potential.

Biologically, action potential amplitude in the axon does not really matter much for synaptic release, but local conditions in the presynaptic terminal that influence calcium influx do. The section on presynaptic inhibition in the answer here may help a bit.

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