Action Potential Question

[September24]

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So I know that many excitatory post synaptic potential cause action potentials. I also know that the ACTION POTENTIAL is GENERATED at the axon hillock. How does this exactly work?

At the presynaptic neuron, an excitatory neurotransmitter will bind to "post synaptic dendrites" right? These will open sodium channels which dont generate an AP, but small post synaptic potentials. Do these EPSP's COMBINE or sum up at the axon hillock and from there, an official AP is generated?

I'm just trying to figure out if a neurotransmitter actually causes an action potential directly at the dendrite or just small EPSP's which sum up at another place.

 

[dyspareunia]

So I know that many excitatory post synaptic potential cause action potentials. I also know that the ACTION POTENTIAL is GENERATED at the axon hillock. How does this exactly work?

At the presynaptic neuron, an excitatory neurotransmitter will bind to "post synaptic dendrites" right? These will open sodium channels which dont generate an AP, but small post synaptic potentials. Do these EPSP's COMBINE or sum up at the axon hillock and from there, an official AP is generated?

I'm just trying to figure out if a neurotransmitter actually causes an action potential directly at the dendrite or just small EPSP's which sum up at another place.

EPSPs converge at the axon hillock -> threshold is reached (in the case of an AP) in the trigger zone (lots of Na channels) -> AP.

 

[NextStepTutor_1]

I agree fully with dyspareunia that the general concept is that neurotransmitters are summed at the axon hillock which, if the trigger voltage is reached will cause an action potential to fire. However, if you go into more depth, you have to remember that there are also Inhibitory signals that decrease the voltage at the axon hillock. Together with the exitatory AND inhbitiory signals, the frequency is summed to get the final voltage based on whether an action potential fires.

Furthermore, you need to note that certain types of nerves or even neurotransmitters might serve as excitatory signals in one case, but inhibitory signals in another. This all depends on the post-synaptic cell. An example of this is the sympathetic vs parasympathetic nervous systems. A parasymphatic nerve might actually excite the cells for the digestive system, while a sympathetic nerve would inhibit it. If we look at another organ, such as the heart, we would see the opposite relation is true. Parasympathetic nerves would inhibit while sympathetic nerve would excite the post-synaptic nerve.

 

[September24]

Thanks guys! That's actually what I thought. I was just confused as how neurotransmitter act at the dendrite but an AP is caused at a hillock. Now I know that EPSPs are created at dendrite and many EPSPS converge at hillock to cause and AP.

And NextStepTutor, I think I understand what you mean. Isn't that actually caused by the type of receptors instead of the neurotransmitters themselves. For example, in the autonomic nervous system, specifically the parasympathetic system, the post synaptic "ganglionic cells" contain nicotinic choline receptors which are excitatory while the the post ganglionic cell innervate an effector organ which contains muscuranic choline receptor which are inhibitory yet still bind acetylcholine.