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> Neural circuit divergence and convergence
ettaD
post Dec 09, 2009, 09:38 AM
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Hi!
I have several linked questions about propagation of signals through a biological neural network. Namely, how one signal from a pre-synaptic neuron diverges into many post-synaptic neurons. Conversely, what happens to many signals which funnel into one cell where neurons converge. For divergence, say there is one pre-synaptic neuron connected to 10 post-synaptic neurons. Does the pre-synaptic neuron fire just once to trigger all ten downstream neurons or does it need to fire ten times? If the former is the case, then the number of neurotransmitter molecules produced by the triggering neuron must be independent of the signal but dependent upon the number of synapses. How is this controlled energetically? This also means that for every single presynaptic neuron with many linked post-synaptic neurons, signal propagation would multiply exponentially around a circuit. Surely, the neuron circuit would have a major problem in regulating itself? Does convergence achieve this regulation and what happens to all the excess signals?

If the alternative is the case (ie ten firings to trigger the ten post-synaptic neurons), the choice of which downstream neuron is triggered per firing has to be probabilistic and maintained at a high level to ensure equal downstream propagation? Sorry, there are quite a few questions here! I would also be very grateful for any "further reading" suggestions that tackle these points since none of the many articles I have read seem to cover them. Perhaps they are so basic it is assumed everyone knows the answer!

Many thanks,

EttaD
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lucid_dream
post Dec 09, 2009, 10:01 AM
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neurons connect to other neurons through their axons, which branch and arborize, so that a single neuron can connect with more than 10,000 other neurons. A single neuronal input onto a neuron is typically not sufficient to induce that neuron to fire an action potential. Generally, several presynaptic action potentials have to arrive within a narrow time window (about 10 ms), in order for that neuron to fire an action potential. Thus, signal propagation does not multiply exponentially. Also, signal propagation through axons is not believed to be 100%, with some action potential propagation failures occurring at axon branch points. Hope this helps.
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Hey Hey
post Dec 09, 2009, 10:08 AM
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In terms of anatomy, how are synapses enclosed? In other words are they encapsulated in some way? Diagrams never show that. Why don't neurotransmitters flow sideways out of synapses? How are the enzymes that break down NT's after usage regulated?
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lucid_dream
post Dec 10, 2009, 04:28 PM
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synapses are enclosed in astrocytes, a type of glial cell. The synaptic cleft is only several nanometers wide, so there is little room for transmitter to diffuse away, though there are suggestions and some reports of volume transmission. I'm not sure about regulation of enzymes that breakdown NTs. No doubt negative feedback is involved
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Paul King
post Aug 20, 2010, 08:10 PM
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"yes" to what lucid_dream said about neuron signal propagation and the fact that one input is not enough to trigger the next neuron.

More importantly, neurons are not relay conductors of signal. Each neuron receives a constant barage of 100,000 or so impulse inputs per second on its 10,000 or so dendritic inputs. Internally the neuron integrates those signals and occasionally (maybe 10 times per second) produces a 1 millisecond output that is a single pulse (a spike) that is sent simultaneously to the 10,000 or so neurons it contacts.

Also importantly, 20% of neurons are inhibitory, meaning that they reduce activation to balance the others that increase activation.

QUOTE(Hey Hey @ Dec 09, 2009, 11:08 AM) *
In terms of anatomy, how are synapses enclosed? In other words are they encapsulated in some way? Diagrams never show that. Why don't neurotransmitters flow sideways out of synapses?

This is a great question! As lucid_dream says, the synapse is a contact point between two surfaces that is maybe 200 nm in diameter with a 20 nm gap between the surfaces -- the size of a few protein molecules. The neurotransmitter released on one side goes pretty directly to the other side.

Most neurotransmitter is captured and recycled by reuptake proteins of the releasing neuron or by catalytic degradation processes. Some is cleaned up by neighboring glial cells (astrocytes). This varies by neurotransmitter.

However some neurotransmitter does flow sideways out of the synapse. This extrasynaptic leakage can be a regulating signal for measuring local neural activity levels. The leakage is measured by extrasynaptic receptors and captured by reuptake and degradation proteins outside the synapse.

A lot of neurotransmitter is not released at the synapse at all. "Extrasynaptic release" or "volume transmission" is when neurotransmitter is released into the intercellular matrix and diffuses, activating extrasynaptic receptors until it is eventually captured and recycled. This is generally how neuromodulators work (serotonin, dopamine, noradrenalin, acetylcholine).

Certain retrograde signaling molecules, such as the cannabinoids (related to cannabis), transmit in the opposite direction, from the post-synaptic neuron to the presynaptic one.

QUOTE(Hey Hey @ Dec 09, 2009, 11:08 AM) *
How are the enzymes that break down NT's after usage regulated?

All cellular enzymes, including those that break down neurotransmitter, are typically regulated. There are numerous mechanisms of regulation that vary by enzyme type. In general, there is usually a molecule-scale feedback mechanism of some sort that speeds up or slows down the reuptake process to maintain a certain transmitter level over time.

The mechanism sometimes involves the temporary binding of a molecule to the reuptake protein that changes its shape and disables it. Sometimes it involves a phosphorylation/dephosphorylation reaction that attaches or removes a phosphate group (PO4), which acts like an on-off switch on the enzyme. Sometimes regulation is by synthesizing more enzymes or degrading excess ones, or by adding or removing reuptake protein molecules from the cell membrane.

SSRIs (serotonin selective reuptake inhibitors) initially increase extrasynaptic serotonin by slowing down the recycling process. This causes serotonin neuron firing to actually reduce to compensate for the excess serotonin. Also the number of serotonin receptors is eventually reduced in compensation as well by removing them from the cell membrane. It is this last side-effect step, which takes a couple weeks, that is believed to produce the antidepressent effects.
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