<p>Neurons connected by chemical synapses do not &quot;contribute voltage&quot; to other neurons.</p>
<p>They release <a href="https://en.wikipedia.org/wiki/Neurotransmitter" rel="nofollow noreferrer">neurotransmitters</a>, neurotransmitter diffuses within the synaptic cleft and binds to receptors on the post-synaptic cell. Those receptors are often themselves channels (for example, <a href="https://en.wikipedia.org/wiki/AMPA_receptor" rel="nofollow noreferrer">AMPA receptors</a> bind the neurotransmitter glutamate), and opening those channels allows ions to flow which changes the postsynaptic potential.</p>
<p>The size of the change in voltage will depend on many things: the amount of neurotransmitter released, the number and type of receptor, the voltage of the post-synaptic cell, the capacitance of the post-synaptic cell, the resistance of the post-synaptic cell (that is, what other conductances are open), the presence of active (i.e., voltage-gated) channels, the distance between the synapse and the soma, etc.</p>
<blockquote>
<p>neurons B
, C
, and D
has only one type of ion channel (e.g. sodium channels)</p>
</blockquote>
<p>This doesn't make any sense, because no neuron will have only &quot;one type of ion channel&quot;. If it did, it would never change its potential and would be useless as a neuron.</p>
<blockquote>
<p>as well as the same electrical properties (e.g. same resistance, capacitance, etc). In addition, let's assume all synapse sizes/strengths are the same</p>
</blockquote>
<p>If you make everything the same, then everything will be the same.</p>
<blockquote>
<p>if neuron A
is connected to neuron B
through 100 synapses redundantly, but connected to C
through only 1 synapse, would neuron A
create a potential onto B
100 times stronger than that onto C
?</p>
</blockquote>
<p>Additive differences in receptors tend to not lead to additive changes in voltage because the relationship between resistance and membrane voltage is nonlinear. But generally yes, you'd expect more synapses on one cell to have a bigger effect. For modeling, you might just combine them all conceptually and think of it as if there is one larger synapse rather than many separate ones; the distinction will only matter if you take into account more complicated aspects of the scenario (presence and position of other synapses, dendritic morphology, etc).</p>
<blockquote>
<p>I was trying to read up on the Hodgin-Huxley model (but don't understand it really well as it is very dense and I am self-taught), and noticed the chart near equation 2.5. In the chart, it shows that each type of ion channel (e.g. sodium channels) have the same voltage constant, so I would naturally guess that A
would impact the same potential onto all of its connected neurons.</p>
</blockquote>
<p>I do not think you are understanding what is being discussed here. I'd recommend first studying physics and understanding concepts like current and voltage. And then next I would recommend an entire comprehensive textbook about neuroscience used by undergraduates, like Purves or Kandel.</p>

CC BY-SA 4.0
2024-08-12T19:19:19.860Z
Thanks! This post is very helpful, and I'll follow up on your suggestion on studying physics + grabbing an undergrad neuroscience book for further reading. I just have a quick curiosity-related follow-up question: "But generally yes, you'd expect more synapses on one cell to have a bigger effect." --> do you mean there are some cases where more synapses make the effect smaller? Is it reasonable to assume that synapses between two specific neurons, A and B, could be a mixture of both excitatory and inhibitory, so combining them together could cancel some of the effect out?
user3667125
CC BY-SA 4.0
2024-08-12T19:46:31.620Z
@user3667125 In most cases at least in vertebrate nervous systems I would expect synapses between A and B to be all excitatory or all inhibitory. I can't think of a case where adding a synapse would make a response smaller, but not all synapses have equal efficacy; one "big" synapse might have a larger evoked response than two smaller synapses. Also, in a real nervous system, signaling is probabilistic, related to vesicle release. Most synapses might not release neurotransmitter at all during a given presynaptic AP, so having more synapses between two cells is also about *reliability*.
Bryan Krause
CC BY-SA 4.0
2024-08-12T14:31:07.797Z
2024-08-12T14:31:07.797Z
Bryan Krause
<p>My understanding is that a neuron's resting potential is somewhere between <span class="math-container">$-65mV$</span> to <span class="math-container">$-70mV$</span>, and each neuron has some variable difference in their threshold for firing an action potential: for example, some neurons fire when they reach <span class="math-container">$-55mV$</span>, some fire when they reach <span class="math-container">$-50mV$</span>.</p>
<p>Let's say we have a neuron <span class="math-container">$A$</span> connected to neurons <span class="math-container">$B$</span>, <span class="math-container">$C$</span>, and <span class="math-container">$D$</span>, and for simplicity, let's say that neuron <span class="math-container">$A$</span> can only emit one type of neurotransmitter, and neurons <span class="math-container">$B$</span>, <span class="math-container">$C$</span>, and <span class="math-container">$D$</span> has only one type of ion channel (e.g. sodium channels) as well as the same electrical properties (e.g. same resistance, capacitance, etc). In addition, let's assume all synapse sizes/strengths are the same.</p>
<p>The main question is: when <span class="math-container">$A$</span> fires an action potential, is any meaningful difference between the change in voltage potential impacted onto <span class="math-container">$B$</span>, <span class="math-container">$C$</span>, and <span class="math-container">$D$</span>, or are the differences negligible and so <span class="math-container">$A$</span> contributes roughly the same amount of potential (say <span class="math-container">$+1 mV$</span>) to each of the connected neurons? If there is any difference, what are some of the key variables to consider?</p>
<p>I was trying to read up on the <a href="https://neuronaldynamics.epfl.ch/online/Ch2.S2.html" rel="nofollow noreferrer">Hodgin-Huxley model</a> (but don't understand it really well as it is very dense and I am self-taught), and noticed the chart near equation 2.5. In the chart, it shows that each type of ion channel (e.g. sodium channels) have the same voltage constant, so I would naturally guess that <span class="math-container">$A$</span> would impact the same potential onto all of its connected neurons.</p>
<p>However, I came across some papers like <a href="https://www.pnas.org/doi/full/10.1073/pnas.1803274115#:%7E:text=Recent%20experimental%20studies%20suggest%20that,are%20realized%20by%20multiple%20synapses." rel="nofollow noreferrer">this one</a> where it mentions that two neurons could be connected through multiple redundant synapses. Does this affect the voltage potential? For example, if neuron <span class="math-container">$A$</span> is connected to neuron <span class="math-container">$B$</span> through 100 synapses redundantly, but connected to <span class="math-container">$C$</span> through only 1 synapse, would neuron <span class="math-container">$A$</span> create a potential onto <span class="math-container">$B$</span> 100 times stronger than that onto <span class="math-container">$C$</span>?</p>
<p>I tried Googling an answer for this question, but it was difficult to find an answer because the search results only covers general introductions to synapses or ion channels.</p>

<neuroscience><neuro-transmitters>
When a neuron fires an action potential, does it contribute the same amount of voltage to each connected neuron?
user3667125
# When a neuron fires an action potential, does it contribute the same amount of voltage to each connected neuron?

(Asked by: username_0 on 2024-08-10)

My understanding is that a neuron's resting potential is somewhere between $-65mV$ to $-70mV$, and each neuron has some variable difference in their threshold for firing an action potential: for example, some neurons fire when they reach $-55mV$, some fire when they reach $-50mV$.

Let's say we have a neuron $A$ connected to neurons $B$, $C$, and $D$, and for simplicity, let's say that neuron $A$ can only emit one type of neurotransmitter, and neurons $B$, $C$, and $D$ has only one type of ion channel (e.g. sodium channels) as well as the same electrical properties (e.g. same resistance, capacitance, etc). In addition, let's assume all synapse sizes/strengths are the same.

The main question is: when $A$ fires an action potential, is any meaningful difference between the change in voltage potential impacted onto $B$, $C$, and $D$, or are the differences negligible and so $A$ contributes roughly the same amount of potential (say $+1 mV$) to each of the connected neurons? If there is any difference, what are some of the key variables to consider?

I was trying to read up on the [Hodgin-Huxley model](https://neuronaldynamics.epfl.ch/online/Ch2.S2.html) (but don't understand it really well as it is very dense and I am self-taught), and noticed the chart near equation 2.5. In the chart, it shows that each type of ion channel (e.g. sodium channels) have the same voltage constant, so I would naturally guess that $A$ would impact the same potential onto all of its connected neurons.

However, I came across some papers like [this one](https://www.pnas.org/doi/full/10.1073/pnas.1803274115#:%7E:text=Recent%20experimental%20studies%20suggest%20that,are%20realized%20by%20multiple%20synapses.) where it mentions that two neurons could be connected through multiple redundant synapses. Does this affect the voltage potential? For example, if neuron $A$ is connected to neuron $B$ through 100 synapses redundantly, but connected to $C$ through only 1 synapse, would neuron $A$ create a potential onto $B$ 100 times stronger than that onto $C$?

I tried Googling an answer for this question, but it was difficult to find an answer because the search results only covers general introductions to synapses or ion channels.

**Tags:** neuroscience, neuro-transmitters


---

## 1 answers

### Answer by username_1 on 2024-08-12. Score: 1 | Accepted answer

Neurons connected by chemical synapses do not "contribute voltage" to other neurons.

They release [neurotransmitters](https://en.wikipedia.org/wiki/Neurotransmitter), neurotransmitter diffuses within the synaptic cleft and binds to receptors on the post-synaptic cell. Those receptors are often themselves channels (for example, [AMPA receptors](https://en.wikipedia.org/wiki/AMPA_receptor) bind the neurotransmitter glutamate), and opening those channels allows ions to flow which changes the postsynaptic potential.

The size of the change in voltage will depend on many things: the amount of neurotransmitter released, the number and type of receptor, the voltage of the post-synaptic cell, the capacitance of the post-synaptic cell, the resistance of the post-synaptic cell (that is, what other conductances are open), the presence of active (i.e., voltage-gated) channels, the distance between the synapse and the soma, etc.

> neurons B
> , C
> , and D
> has only one type of ion channel (e.g. sodium channels)

This doesn't make any sense, because no neuron will have only "one type of ion channel". If it did, it would never change its potential and would be useless as a neuron.

> as well as the same electrical properties (e.g. same resistance, capacitance, etc). In addition, let's assume all synapse sizes/strengths are the same

If you make everything the same, then everything will be the same.

> if neuron A
> is connected to neuron B
> through 100 synapses redundantly, but connected to C
> through only 1 synapse, would neuron A
> create a potential onto B
> 100 times stronger than that onto C
> ?

Additive differences in receptors tend to not lead to additive changes in voltage because the relationship between resistance and membrane voltage is nonlinear. But generally yes, you'd expect more synapses on one cell to have a bigger effect. For modeling, you might just combine them all conceptually and think of it as if there is one larger synapse rather than many separate ones; the distinction will only matter if you take into account more complicated aspects of the scenario (presence and position of other synapses, dendritic morphology, etc).

> I was trying to read up on the Hodgin-Huxley model (but don't understand it really well as it is very dense and I am self-taught), and noticed the chart near equation 2.5. In the chart, it shows that each type of ion channel (e.g. sodium channels) have the same voltage constant, so I would naturally guess that A
> would impact the same potential onto all of its connected neurons.

I do not think you are understanding what is being discussed here. I'd recommend first studying physics and understanding concepts like current and voltage. And then next I would recommend an entire comprehensive textbook about neuroscience used by undergraduates, like Purves or Kandel.

> **Comments:**
> **username_0**: Thanks! This post is very helpful, and I'll follow up on your suggestion on studying physics + grabbing an undergrad neuroscience book for further reading. I just have a quick curiosity-related follow-up question: "But generally yes, you'd expect more synapses on one cell to have a bigger effect." --> do you mean there are some cases where more synapses make the effect smaller? Is it reasonable to assume that synapses between two specific neurons, A and B, could be a mixture of both excitatory and inhibitory, so combining them together could cancel some of the effect out?  
> **username_1**: @username_0 In most cases at least in vertebrate nervous systems I would expect synapses between A and B to be all excitatory or all inhibitory. I can't think of a case where adding a synapse would make a response smaller, but not all synapses have equal efficacy; one "big" synapse might have a larger evoked response than two smaller synapses. Also, in a real nervous system, signaling is probabilistic, related to vesicle release. Most synapses might not release neurotransmitter at all during a given presynaptic AP, so having more synapses between two cells is also about *reliability*.  

---

