[ExI] ANN question

Ben bbenzai at yahoo.com
Thu Mar 16 22:39:57 UTC 2017

William Flynn Wallace <foozler83 at gmail.com> wrote:

 >addition - come to think of it, whether the neuron is firing or not at 
a specific point in time, is a function of where on its body you measure 
the chemical exchange;  we know that the spike travels the length of the 
cell body, a traveling wave, such that at the receiving end - the 
dendrite - you can measure the influx of ions into the body, whereas at 
its opposite end nothing is happening yet.  Maybe this makes no 
difference.  What do I know?

If I may, I'd like to weigh in on this, knowing a little about neurobiology.

Whether a neuron is firing or not is not a function of where you measure 
it. It's well-defined as the launch of an action potential down the axon 
of the neuron. Whether or not that action potential is launched is 
dependent on the sum of the depolarisations across the cell body 
(synaptic potentials), integrated at the axon hillock, which is the 
'root' of the axon.

Apart from the axon and axon hillock, the depolarisations fade away over 
time and distance, and only if they exceed a certain critical value at 
the axon hillock, does a self-perpetuating action potential arise, which 
then travels down the axon with no attenuation.

The synaptic potentials and the action potential are distinct things. 
One fades away with time and distance, the other doesn't.

So the 'spike' does not traverse the cell body, only the axon. The cell 
body acts as an integrator in both space and time (spatial and temporal 
summation) to decide whether or not a spike is generated at the axon 
hillock. There are different types of ion channels in the cell body and 
the axon, which ensure this.

The simple version is that the cell body (including the dendrites) 
decides whether or not the neuron will fire, based on a summation of 
incoming excitatory and inhibitory signals from other neurons, as well 
as the intrinsic depolarisation of the neuron, and these signals 
converge on the axon hillock. If the total depolarisation at the axon 
hillock exceeds the threshold voltage, then an action potential is 
produced which shoots off down the axon.

Neuron cell body = leaky integrator. Axon = lossless transmission of signal.

Ben Zaiboc

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