synaptic transmission

[sashakal]

Hi,
Can someone tell me the duration of synaptic transmission ? (approximate order  - milliseconds, nano seconds ...).
I found a lot of information about the speed of the action potential propagation (It depends on the type of the axon but ranges from 10 to 100 meters\second). But if understand correctly, this is the speed of propagation only along the axon of one neuron. I assume that the time required for synaptic transmission is considerably longer, since it requires molecules crossing the gap junction. So the actual time required for an action potential to travel across several neurons should be calculated considering the total distance traveled plus the time for crossing the gap junction.
I would be glad to receive comments on the validity of what I wrote here.
Thanks.

[voynarazuma]

sashakal,
       so at chemical synapses, the delay of transmission can be 2-5 ms long depending on what kind of neuron is releasing the transmitter. delay generally seems to be longer at peptidergic synapses as the molecules are large and take more time to diffuses across the cleft. but size of neurotransmitter is not the only factor that contributes to duration of delay. kinetics of Ca2+ receptors on the presynaptic terminal and binding of Ca2+ to synaptotagmin, diffusion of transmitter across cleft, and kinetics of post-synaptic receptor all contribute to the delay. This delay is virtually absent in electrically coupled cells with gap junctions and most of the miniscule delay is due to the actual presynaptic propagation of action potential. hope this helps.

[sashakal]

Many thanks !
Are electrically coupled neurons common in the human cerebral cortex ? (I read somewhere that this type of synapse is not common in the mammalian brain)
So a delay of 2 - 5 milli seconds seems to be very long. If a human reacts to a visual stimuli after ~400 ms, it means that the longest path of a nervous signal from the eyes to the mussels can contain no more than 100 neurons ? (although there can be many such parallel pathways)
Thanks.

[voynarazuma]

gap junctions are not dominant but do occur within mammalian CNS.
the response time to visual stimuli is not dependent on the number of neurons in the pathway, but the number of synapses because many neurons can be transmitting in parallel, but it's the number of synapses that determines the fastest route. cheers.

[sashakal]

But each synapse connects only two neurons , so the the number is the same. By a single pathway I meant a path as it is defined in Graph : <v1,v2,...,vn> where each vertex is a neuron, and each edge is a synapse.
Your answers were very helpful, so I would to ask you another related question :
Do you know of methods that trace the pathway of electrical signals in neural tissues (in vitro or in vivo)  ?
I know of methods such as diffusion imaging that map anatomical connectivity but do not trace an action potential. On the other hand I know of many single cell recordings, but again they record the activity of only several cells. I have also heard of optogenetics but if I understand it correctly, though is has high temporal resolution, it traces an activity of a single cell, and it is not possible to target all the cells in the brain and thus trace a neural signal.
Thanks,
Sasha.

[Kevin Manz]

But each synapse connects only two neurons , so the the number is the same. By a single pathway I meant a path as it is defined in Graph : <v1,v2,...,vn> where each vertex is a neuron, and each edge is a synapse.
Your answers were very helpful, so I would to ask you another related question :
Do you know of methods that trace the pathway of electrical signals in neural tissues (in vitro or in vivo)  ?
I know of methods such as diffusion imaging that map anatomical connectivity but do not trace an action potential. On the other hand I know of many single cell recordings, but again they record the activity of only several cells. I have also heard of optogenetics but if I understand it correctly, though is has high temporal resolution, it traces an activity of a single cell, and it is not possible to target all the cells in the brain and thus trace a neural signal.
Thanks,
Sasha.

Yep, there are several methods scientists use to light up certain connectivity. Jumping into genetics here a bit (pathway tracing), it's possible to inject a reporter gene for a visualizable substance into the genome of certain cell types. Promoters can then turn that particular gene "on", but only in cells containing that specific promoter. As a result, the visualizable substance lights up like a Christmas tree and, voila, you have an outline of certain cells with their routes. Aside from lesion studies, cell density, and so forth, this seems to me like the most exact approach in outlining neuronal connectivity. But of course, I'm only 17, so I could be wrong.

Also to add to what someone said above, electrical synapses although very scarce in the mammalian brain, are conveniently located at places where fast transmission is a must. Hypothalamic, hormone secreting neurons there contains ES's, interneurons within the hippocampal region, concentrations in the cerebellum and areas in the brainstem all hold some amount of gap junctions. Why? Well quite simply, some physiological tasks need to be synchronized, otherwise we'd be a huge mess.

Hope this helps!

Kevin

[voynarazuma]

KM, expressing genetically engineered markers such as GFP in subsets of neurons may show the anatomical connectivity, but not functional connectivity. For example I use GFP mice under control of GAD67 promoter so that I can visualize interneurons in the cortex, but it tells me nothing about which ones are active at any given instance. Functional connectivity (especially in cortex) is a big area of research right now and is actually the focus of my infantile thesis project. To answer Sasha's question, there are techniques such as using voltage sensitive dyes or calcium imaging to look at functional connectivity, however, these techniques (at least this is my understanding at this point) provide poor spatial resolution, thus you're not able to see individual neurons light up as much as you do groups of neurons.

[sashakal]

Thanks,
Calcium imaging seems a promising method if it is achieves high resolution. I googled the term and I found only references for it's usage in vitro  but not in vivo.
I am surprised that even today no methods exist to monitor the activity of even small number (hundreds) of neurons in high spatial and temporal resolution. I am thinking now of a possible area of research in neuroscience (by which I mean : understand how the brain works). Since I come from the computational field, a reverse engineering seems the most simple way to attack this question. But with the current methods, it seems impossible because it is like uncovering only a small aspect of the brain at each research without being able to see the whole picture.
Can you recommend me areas of research which try this direction ?
Sasha.

[StriatumPDM]

I am momentarily in a lab were exactly this problem - mapping neuronal activity at a single cell level - is tackeled by using the thallium autometallography method (http://www.ncbi.nlm.nih.gov/pubmed/19682585?dopt=Citation). It is similar to 2-DG methods in that the uptake of the tracer is activity-dependent, however, the spatial resolution is higher and it gets on without radioactivity.

[voynarazuma]

Sasha, here's a paper you may find useful.

http://www.ncbi.nlm.nih.gov/pubmed/18478091

Hope I've helped. Peace

[Kevin Manz]

KM, expressing genetically engineered markers such as GFP in subsets of neurons may show the anatomical connectivity, but not functional connectivity. For example I use GFP mice under control of GAD67 promoter so that I can visualize interneurons in the cortex, but it tells me nothing about which ones are active at any given instance. Functional connectivity (especially in cortex) is a big area of research right now and is actually the focus of my infantile thesis project. To answer Sasha's question, there are techniques such as using voltage sensitive dyes or calcium imaging to look at functional connectivity, however, these techniques (at least this is my understanding at this point) provide poor spatial resolution, thus you're not able to see individual neurons light up as much as you do groups of neurons.

Thanks man, I didn't know that.