Hi there,

i was just wondering, with this vast connectivity that exists within the brain and between layers of neurons, do we have any idea how many synapses connect two neurons together? Is there any research that suggests that a neuron only connects to aother neuron through exactly one synapse?
I would guess, that there is no reason why two neurons aren't connected together via several synapses.
Does anyone know more?

Thanks!

neuronics

I know there are multiple synapses between two neurons, as simple tracer experiments reveal this information. I don't know offhand who has quantitated this, though.

Well, it's definitely not just one; you can rationalize this out by thinking of axon collaterals (telodendria). Also, you have dendrodendritic, axoaxonic, axosomatic and of course axodendritic synapses. I'd imagine there is quite a bit of connectivity between two neurons. You can obtain this information through any neuroscience book basically. 'Neuroscience: Exploring the Brain' by Bear, Connors & Paradiso (2007) or 'Biological Psychology' by Kalat (2007) and many more academic books. It may be difficult to search for such a topic in journal articles, because of the vast array of tracing studies that go on.

There is one Axon and several thousand dendrities for every neuron. An Axon sends a signal out, a dendrite recieves it.

I dont think there is an exact set number of dendrities, I think it varies depending on the brain development and the area of the brain the neuron is in.

but each single axon gives rise to hundreds to thousands of collateral branches and makes thousands to tens of thousands of synapses.

Good point

Exactly.

Also, if you get in the realm of LTP, the influx of calcium of the NMDA receptors encourage more axon collaterals to branch off (through retrograde signaling of NO) -- not to mention splicing of the post-synaptic dendrites.

Yes. Theoretically, it could be one. You only need one synapse to get the neurotransmitter to send along the action potential, but of course, in reality, this doesn't occur. They just keep branching. And branching. And branching.
-OddDuckNash4348-

I think one of reasons why need so much synapses between neurons is that for correcting errors if neurons connected with more than one synapse two same neurons...

This varies by type of neuron and by the physical proximity of the respective neuron axonal and dendritic arbors.

The number I have heard for randomly chosen proximal pairs of excitatory neurons in the neocortex is 1 - 6 synapses per connected pair.

Inhibitory basket cells supposedly make 4-5 synapses onto each pyramidal cell they contact.

Purkinje cells are geometrically arranged so that they might receive only 1 "en passant" synapse from a given input cell via the parallel fibers.

When synapses are forming, a neuron's axonal branch doesn't necessarily know that another of its axonal branches has already synapsed onto that same neuron already, so duplicated synapses are statistically likely to occur if the neurons are physically near each other.

I wonder what keeps a neuron from over-synapsing onto the same neuron though? If a synapse between two neurons is desirable, one might imagine that the system could go haywire and synapse over and over to the detriment balanced information processing.

That provokes the question "what would happen if a neuron sent signals to itself?" Does this occur? Why not? Etc.

It's an interesting question.

Most of the time, it seems that neurons synapse onto neurons of a slightly different type or in a different tissue layer or brain area. So it seems that there is some natural physical and type segregation that would make self-synapsing unlikely.

However there are cases of neurons that synapse onto the same class via "horizontal" or "lateral" connections. So what keeps them from synapsing onto themselves? Or do they?

Spike Timing Dependent Plasticity (STDP) is model of synaptic change that is being heavily studied these days. In STDP, connections are strengthened when the presynaptic spike is followed by a post-synaptic spike. Connections are weakened when the opposite occurs.

If an excitatory neuron synapsed onto itself, the spike order would always be the reverse of what STDP wants (the synaptic target would always spike before the axon). The result would be a synapse that would weaken and presumeably atrophy. Or perhaps the synapse would never form in the first place because of the undesirable direction of spike timing.

That makes sense as a neuron can receive no new information from itself, so self-synapsing ought not to be evolutionarily advantageous, and would therefore be selected out.

Although autoreceptors are a common occurrence, which is a little bit like self-synapsing.

Autoreceptors are common on the axonal terminals of neuromodulatory neurons (serotonin and dopamine, for example). When the neuron fires, the axon's synaptic terminal releases neurotransmitter intended for a target cell. However the presynaptic terminal contains receptors for the same neurotransmitter it is releasing, called autoreceptors. These provide an inhibitory feedback signal to the sending neuron so that as neurotransmitter accumulates in the extracellular space, the axon becomes less and less likely to release more neurotransmitter.

Eventually the extracellular neurotransmitter is cleaned up via either reuptake (for reuse) or degradation. This deactivates the presynaptic autoreceptors.

In general, inhibitory self-synapsing would be okay for self-regulation, and excitatory self-synapsing would be a big problem due to self-reinforcing feedback loops (like microphone-to-speaker feedback).

When a neuron spikes, which is what causes neurotransmitter release at the synapse, news of that spike travels backwards to all dendrites (called backpropagating action potentials). So as Rick points out, there would seem to be nothing a neuron could learn from a self-synapse that it wouldn't already know via the intracellular backpropagating action potential mechanism.