Ion channel "quantization"

Richard Vickery Richard.Vickery at
Wed Aug 14 20:20:47 EST 1996

   radams2000 at (RAdams2000) wrote:

>I have read that at the dendrites of a neuron, the current flow into the
>neuron across the synaptic junction consists of some large number
>on ionic channels that are either completely open or completely closed.
>So it would seem that, according to how many channels are involved,
>the current flow into the soma is highly "quantized", meaning that only a 
>finite number of possible currents can exist. Does this contribute to 
>a fundemental noise mechanism in the process of neural firing? Can this
>noise mechanism be measured accurately? Is it modelled in any of the
>popular neural simulation packages??

We need to be very careful about terminology here.  Although the contribution 
of channels will be to quantize current flow, the concept of a quantum (lets 
call it a packet) in neurophysiology is inextricably linked with a transmitter 
vesicle and its postsynaptic effect.  This level of quantization (release of x 
vesicles from y active zones (synaptic release regions)) has been extensively 
studied, and is still the subject of a lot of debate.  Quantization by channel 
current flow (lets call it micro-quanta) has also been examined, and figures 
in the argument about whether the fairly constant size of a packet is due to 
constant amounts of transmitter in vesicles or due to saturation of the 
postsynaptic site so that all channels are opened.

At the level of a neuron, the micro-quantization is unlikely to be 
significant, as the varying distance of synapses from the soma will produce a 
smearing of micro-quantal size, which makes it unclear whether even packets 
can be reliably observed under normal conditions.

This reference might get you started:

  Faber DS.  Young WS.  Legendre P.  Korn H.
  Intrinsic quantal variability due to stochastic properties of
  receptor-transmitter interactions.

  Science.  258(5087):1494-8, 1992 Nov 27.

Richard Vickery                \ /     Our quest is for Meaning,
Physiology & Pharmacology     (oVo)    but the meaning is The Quest.
UNSW, Australia               )   (
ph. 61 2 3851676               ^ ^     Life's a Hoot!

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