# How many transmissions does it normally take for a neuron to reach threshold for action potential?

This video illustrates and explains how the dendrite connections being activated causes the neuron to be polarized until it reaches "threshold", and firing. My question is how many times might a neuron's dendrites need to receive "fires" from other neurons in order to reach threshold. I think I understand that it isn't a binary matter, so I realize the answer isn't going to be 5 or 5,000 but a general range.

As a related question, how long does it take for polarization to wear off if additional transmissions don't come in, and if the polarization does wear off, is this wear-off mechanic important to the neural system? Is wearing off a significant aspect of the neural process, in other words?

To give a rough idea, according to my calculations it takes 153 inputs (individual action potentials) from CA3 pyramidal neurons to produce a depolarisation above threshold in the CA1 pyramidal neuron. This is of course a very simplistic division that does not consider spatial and temporal summation or any other complicating factor. Also note that each CA3 pyramidal neuron forms a variable number of synapses onto each CA1 pyramidal neuron (~5? according to the hippocampus book), and for each action potential only some of these synapses release a vesicle. Thus 153 inputs = xxx number of activated synapse (activated synapse = synapse receiving action potential).

So how did I arrive at 153?

According to this paper [1], a single action potential from a CA3 neuron produces an average depolarisation of 131 uV, presumably at the cell body, which we can also take, for simplicity, to be the depolarisation produced at the axon hillock. So assuming the threshold is 20mV (I'm not sure exactly but it should be in the 20-30mV range), the number of CA3 neuron each producing a depolarisation of 131 uV required to reach threshold is 20/0.131=153.

Again this is a simplistic division and may not be a good estimate once you take into account all the complicating factors in summation, but hopefully it gives some idea. Also bear in mind that synapses in different parts of the brain can have very different properties, so 153 might not be applicable to them.

[1] The time course and amplitude of EPSPs evoked at synapses between pairs of CA3/CA1 neurons in the hippocampal slice. RJ Sayer, MJ Friedlander and SJ Redman. Journal of Neuroscience 1 March 1990, 10 (3) 826-836

How many action potential are needed to cause an action potential in the postsynaptic neuron depends on the strength of the synapse(s) involved. It's not possible to answer this question in general.

It is not just the number of action potentials received but also the firing pattern in which they arrive and the location of the synapse(s) on the dendrites/soma (proximal/distal, on a spine or not, on which kind of spine,...) of the postsynaptic neuron.

However, for some neurons it is possible to trigger an action potential in a postsynaptic neuron with a single action potential. But in this case it is very probable that multiple synapses were involved between the two neurons (example paper).

To your second question: the time until polarization wears off is called the time constant tau of a neuron. It is a property that varies from neuron to neuron and is one of the things that can be measured when recording from a neuron. It is one of the many factors that affects how incoming action potentials are translated into a neuron's output firing.

• What would be a minimum number? The smallest number of transfers it can take? 2 or 3? In other words how big do the synapses get in terms of polarizing the neuron? 30%? 80%? 100%? Also is it proper to use the word "synapse" the way you did, to refer to the transfer rather than simply the location of the transfer? That's a topic of confusion for me. – J.Todd Feb 20 '17 at 23:47
• The synapse is the structure that connects the presynaptic to the postsynaptic neuron. The effect on the postsynaptic neuron depends on the activity of the presynaptic neuron, but also on the strength of the synapse (see my answer to your other question: biology.stackexchange.com/questions/56402/…) – a tiger Feb 21 '17 at 14:49
• Your question about how far in % a synapse can get in terms of depolarizing a neuron is misleading. It depends on so many other factors such as the location of the synapse (look up dendritic integration), the proteins expressed on the postsynaptic neuron and so on. You can't answer this question with a single number. It's more complex than that. – a tiger Feb 21 '17 at 14:53
• Let me change the question for the OP so that we can get a quantitative answer: In a CNS neuron, what is the threshold for action potential initiation at the axon hillock, what is the typical depolarisation produced by an activated synapse on the dendrite at the dendrite and what is the resulting depolarisation at the axon hillock. I know it varies with different neurons and with different distance of synapse from soma, but something like ~20mV at the dendrite and ~1mV at the axon hillock will help us get some idea about how difficult it is to excite a neuron. – liyuan Apr 1 '17 at 16:04