[ExI] ANN question
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.
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