Backpropagation in biological neurons

[Fredo]

I've been reading a bit about neurophysiology and neurobiology. A number of
texts refer to backpropagation in real neurons, where the signal
backpropagates, though significantly attenuated, up the dendrites.

I can't seem to find out much detail on the matter, beyond the fact that it
exists. In particular, what is the significance? The backpropagation might
affect gap junction-connected neurons, but it can't back-propagate through
synapses, can it? If it can, what's the mechanism? Surely not
neurotransmitter release in the dendrites...

Thanks

[r norman]

Yes, most surely neurotransmitter release in the dendrites. There are
an enormous number of dendro-dendritic synapses. There are an
enormous number of "local" neurons with no axon or with very short
axons where the local propagation of potentials does all the work.
There are an enormous number of "microcircuits", closely interacting
groups of synapses working as a unit within the dendritic field. There
are reciprocal synapses : A synapses on B and B synapses back on A
right next door. There are serial synapses: A on B and B on C, all
in the same microciruit area.

G. Shepherd, "The Synaptic Organization of the Brain" is a good
resource.

[Fredo]

"r norman" <r_s_norman@_comcast.net> wrote in message
news:1icm84hv01a1ed1nf...@4ax.com...

Okay, that makes sense, I guess. I wasn't thinking in terms of
dendrite-to-dendrite communication so much as dendrite-to-axon communication
which didn't make as much sense to me. I also wasn't aware that dendrites
could release neurotransmitters though after reading your response and doing
some quick searching, came across the text, "Dendritic Neurotransmitter
Release" edited by M. Ludwig.

I'll try to get a copy of "The Synaptic Organization of the Brain".

Thanks a ton.

[Christian Wilms]

While dendritic neurotransmitter release is one function,
backpropagating APs are also considered important in other phenomena.

One important thing to consider is that backpropagation can have two
separate mechanisms: A merely passive electrotonic spread of
actionpotentials from the soma/initiation site into the dendrites. This
is often weak and not found in all neurons. Alternatively, voltage
sensitive ionchannels in the dendrites can cause an active
backpropagation.

One role of backprops is the unblocking of NMDA-channels at synapses,
allowing them to open when they are activated by glutamate released from
the presynapse. This coincidence of pre- and post-synaptic activity can
lead to changes in the strength of the involved synapses.

You might want to have a look at:

M. London and M. Häusser. Dendritic computation. Annual Reviews
Neuroscience, 28:503-532, 2005.

Best, Christian

[r norman]

Yes, that one I forgot about completely. My bad, it is a very
important phenomenon.

[David Olmsted]

Fredo <fr...@hotmail.com> wrote: I've been reading a bit about neurophysiology and neurobiology. A number of

Thanks

Back-propagation is the natural result of injecting any ionic current into a neuron. These ions will spread out in both directions. In neurons 3 ionic currents do this: Sodium ions, Potassium ions, and Calcium ions. Sodium ions are the "normal" neural charge, Potassium ions reverse the Sodium effects and thus are considered an inhibitory response, Calcium ions are involved in neural modulation and adaptability.

As others have mentioned the Sodium back-propagation can be actively amplified (the NMDA receptors) and modulated further via dendritic micro-circuits. Yet you asked about the significance of all this. The answer is that the interactions of these back-propagating currents determine the response characteristics of the neuron which in turn is governed by the purpose of the neuron its local circuit which is in turn governed by the behavioral needs of the animal. By response characteristics I mean control over latency, time horizon, burstiness, frequency, connective type (more Sum-like or OR-like) etc. for a given set of input types,

If you are really interested in how these back-propagating currents interact and want to play with their various control parameters I have a demo brain circuit simulation program for windows computers at my site (softstatemagic.com) where you can create your own neuron or use a neuron in an example brain circuit that you can download.

And in an answer to Stephen Wolstenholme I hope you can see that the backpropagation technique used in artificial neuron networks is nothing like the neural backpropagation in question here.

Dave

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Brain Circuit Simulation Resources: http://www.softstatemagic.com