Amount of Neurotransmitters affects what?

[JReiss]

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Does the physical quantity of neurotransmitter in the cleft affect only the frequency of the action potential? In other words, if you have more n.t. will you fire more frequently and get more action potentials?

Or, if you have more n.t. will you have a larger depolarization --- meaning a larger positive voltage due to an increase in the amount of sodium entering the cell.

My gut tells me that it's the former --- more n.t. ---> greater frequency as action potentials are usually all or nothing. However, I recently heard that muscle cells do not function in an 'all or nothing' way. On this last point, has anyone else heard this?

 

[Pisiform]

Good question.

I don't think there will be any change in frequency if you increase the concentration of the NT. For e.g. if you increase the concentration of ACh, then our body will produce more AChE to degrade the NT from the synaptic cleft (assuming a healthy person).

Also, remember that if you add an inhibitor to the AChE, then ACh will not be degraded and will remain in the synaptic cleft for greater time resulting in longer depolarization everytime time action potential is fired.

However, if you want to increase the intensity of the signal by a presynaptic neuron, then the more frequently the action potential is fired, the greater would be the intensity because intensity would be determined by Temporal summation in which all EPSPs and IPSPs are piled up resulting in increased response.

And as far as I know ... I think all neurons follow all or none response.

 

[Ssina]

I'm confused as to what you mean by longer depol!? You can't become more positive than what you get with a regular AP, given the electrochemical relationship of inside vs. outside of the cell, at least it's my understanding.... And since we are all-or-nothing then it would just increase frequency of AP given the fact that you have more NT in the synaptic cleft... This is like having more AP that released more NT, but here you just have more of them left over (e.g. Inhibiting AChE)!?

 

[Pisiform]

By longer I mean the muscle cells remain depolarized/remain contract for a longer period of time causing your muscles to become paralyzed and we die.

Intensity is related to AP frequency but not NT conc. directly. As I said, the concentration of neurotransmitter near the dendrite of a neuron is proportional to the amplitude (Intensity) of a graded potential and the number of receptors on the target cell.

 

[Ssina]

O kk.. yeah that's what I was trying to say also, since neurons can't change size or speed of AP, they change the frequency, which dictates the intensity.

 

[MedPR]

Does the physical quantity of neurotransmitter in the cleft affect only the frequency of the action potential? In other words, if you have more n.t. will you fire more frequently and get more action potentials?

Or, if you have more n.t. will you have a larger depolarization --- meaning a larger positive voltage due to an increase in the amount of sodium entering the cell.

My gut tells me that it's the former --- more n.t. ---> greater frequency as action potentials are usually all or nothing. However, I recently heard that muscle cells do not function in an 'all or nothing' way. On this last point, has anyone else heard this?

All action potentials are "all or nothing," regardless of where or what they are signaling.

As far as muscles go, individual muscle fibers (e.g 1 muscle cell) do function in an "all or nothing" way, but the entire muscle (which is made up of hundreds, thousands, millions, whatever amount of muscle fibers) do not function in an "all or nothing" way. Higher frequency = stronger contraction.

I don't know if it's relavent to mcat, but there is spatial summation (more muscle cells contracting = stronger contraction by the whole muscle) and temporal summation (very rapid overlap of action potentials = muscle cell doesn't get to relax = cramping, tetanus, etc). Spatial summation is what's important here in regard to the "all or nothing" idea behind action potentials -- again, individual muscle cells are "all or nothing" (it contracts or it doesn't) since an action potential is all or nothing, but obviously we can control how much force we put into lifting, throwing, punching, etc hence whole muscles (a bicep, for example) do not act in "all or nothing" fashion. For example, curling a 5lb weight might require us to fire 5 muscle fibers in our bicep, while lifting a 100lb weight would require us to fire 100 muscle fibers. More fibers = more action potentials, not "stronger" or more depolarized action potentials.

In the case of a neuromuscular junction, I don't know if the amount of NT matters, but if the NT is not removed from the synaptic cleft, the Na+ channels will never close and that particular muscle cell will never stop contracting (Na+ channels continuously open = continuous depolarization = continuous action potential = continuous contraction).

 

[typicalindian]

afaik, the rate of action potentials should increase if there is more NT in the cleft, more NT = more ion channels open = more Na+ influx, this is essential for increasing the rate because if you remember the action potential curve, after the peak the Na+ channels first go into the absolute refractory period, which is when they can't be opened no matter what, but after the absolute refractory period comes the relative refractory period, this is when the Na+ channels CAN open if a strong enough stimulus is applied (the NT is the stimulus in this case).

Also, the strength of the original stimulus does not determine the rate of action potentials, once the threshold is reached, the AP will occur normally and after its over you will need a new stimulus to induce a new AP.

However, at the neuromuscular junction if you have too much ACh for an extended period of time, the post synaptic cell becomes desensitized to the neurotransmitter and will stop responding to it.
Hope this made sense as I typed it at 1am after a long day of studying :X

 

[TXKnight]

All action potentials are "all or nothing," regardless of where or what they are signaling.

As far as muscles go, individual muscle fibers (e.g 1 muscle cell) do function in an "all or nothing" way, but the entire muscle (which is made up of hundreds, thousands, millions, whatever amount of muscle fibers) do not function in an "all or nothing" way. Higher frequency = stronger contraction.

I don't know if it's relavent to mcat, but there is spatial summation (more muscle cells contracting = stronger contraction by the whole muscle) and temporal summation (very rapid overlap of action potentials = muscle cell doesn't get to relax = cramping, tetanus, etc). Spatial summation is what's important here in regard to the "all or nothing" idea behind action potentials -- again, individual muscle cells are "all or nothing" (it contracts or it doesn't) since an action potential is all or nothing, but obviously we can control how much force we put into lifting, throwing, punching, etc hence whole muscles (a bicep, for example) do not act in "all or nothing" fashion. For example, curling a 5lb weight might require us to fire 5 muscle fibers in our bicep, while lifting a 100lb weight would require us to fire 100 muscle fibers. More fibers = more action potentials, not "stronger" or more depolarized action potentials.

In the case of a neuromuscular junction, I don't know if the amount of NT matters, but if the NT is not removed from the synaptic cleft, the Na+ channels will never close and that particular muscle cell will never stop contracting (Na+ channels continuously open = continuous depolarization = continuous action potential = continuous contraction).

This.
Amount does not matter since AP's are all or nothing. However, where it does matter is in the frequency of AP's...continuous nt/receptor binding will lead to fatigue. I guess in some other cases it might induce downregulation of the receptor to effect tolerance. Correct me if i'm wrong.

 

[MedPR]

This.
Amount does not matter since AP's are all or nothing. However, where it does matter is in the frequency of AP's...continuous nt/receptor binding will lead to fatigue. I guess in some other cases it might induce downregulation of the receptor to effect tolerance. Correct me if i'm wrong.

Yea, I haven't learned any specific cases of downregulation, but I don't see why not.

 

[Erythropoietin]

Does the physical quantity of neurotransmitter in the cleft affect only the frequency of the action potential? In other words, if you have more n.t. will you fire more frequently and get more action potentials?

Or, if you have more n.t. will you have a larger depolarization --- meaning a larger positive voltage due to an increase in the amount of sodium entering the cell.

My gut tells me that it's the former --- more n.t. ---> greater frequency as action potentials are usually all or nothing. However, I recently heard that muscle cells do not function in an 'all or nothing' way. On this last point, has anyone else heard this?

A presynaptic neuron increases the intensity of its signal by only increasing the frequency of action potentials; so it can increase it by increasing the number of action potentials it is transmitting in a given time (hence, its frequency).