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Is there any data on how long it takes for signals to propagate from one neuron to its neighbors in complex networks, such as the brain (particularly the neo-cortex)?

If not, is there any reasonable way to estimate at least the order of magnitude (for example by considering synaptic delay, which is in the order of 0.5 ms and the firing delay)?

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  • $\begingroup$ do you mean what the latency is in a two-neuron network? In that case, the synaptic delay is definitely the most time consuming step because it's dependent on passive diffusion of neurotransmitter. $\endgroup$
    – AliceD
    Aug 6, 2015 at 14:47
  • $\begingroup$ @AliceD What I am interested in is to get some idea about the (average/lowest/highest) latency between any two directly connected neurons in a network as complex as the brain. $\endgroup$
    – back2dos
    Aug 6, 2015 at 15:43
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    $\begingroup$ Are you mainly interested in the synaptic delay or the propagation delay? The later depends a lot on whether the axon is myelinated and the length of the connection. Are you mainly interested in delays within a cortical area or between cortical areas? Note that the the brain is NOT a dense network. $\endgroup$
    – Jonathan
    Aug 10, 2015 at 11:56
  • $\begingroup$ @Jonathan Ok, good to know. I'm a little shaky on terminology, but propagation delay sounds fitting, if it is the correct term for the time it takes for a "signal" to propagate from one neuron to the "next", i.e. the delay between the first neuron firing and the second neuron's potential being affected. $\endgroup$
    – back2dos
    Aug 10, 2015 at 12:50

2 Answers 2

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Short answer
A rough indication of the delay in monosynaptically connected cortical neurons is 6 - 14 ms.

Background
As @Jonathan correctly states, there are many variables involved in neuronal delays. However, because you ask for an estimate of the order of magnitude of the neuronal delay between two coupled neurons in the neocortex, I will suffice by providing the latencies reported form one study that seems to meet your demands very well. In this study the latency data on horizontally connected neurons in monkey prefrontal cortex in layer III were reported González-Burgos et al (2000).

The authors inserted stimulating electrodes to evoke action potentials and used distant recording electrodes to measure the responses in cells located distally from the stimulation site (Fig. 1).

González-Burgos et al (2000)
Recording setup used in González-Burgos et al (2000).

They report that delays between monosynaptic connections range between 6 - 14 ms. Fig. 1 shows that these latency estimates depend on the position of the stimulating and recording electrodes, i.e., which part of the presynaptic cell is stimulated and which part of the postsynaptic cell is recorded. Most notably in the recording setup used, longer axon collaterals will have resulted in longer latencies. The authors conclude that the conduction velocity of the horizontal axon collaterals was 0.14 ± 0.04 m/s and the synaptic delay was estimated at 2.29 ± 0.49 ms.

Reference
- González-Burgos et al., Cereb Cortex (2000); 10(1): 82-92

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The conduction velocity can vary by orders of magnitude based on many factors, so more information would be needed for a useful answer. Unmyelinated axon conduction velocities range from about 0.5 to 10 m/s, whereas myelinated axon conduction velocities can reach up to 150 m/s1.

I'm also not sure what you mean by "dense network". The brain is not a dense network and it sounds like you are including networks that are not brains.

[1]: Neuroscience (Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Sunderland (MA): Sinauer Associates; 2001. 2nd edition).

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