[Neuroscience] Re: number of ion channels in a neuron, type and probability of open/closed

r norman via neur-sci%40net.bio.net (by r_s_norman from comcast.net)
Sat Oct 9 12:57:40 EST 2010


On Sat, 9 Oct 2010 10:11:29 -0700 (PDT), "pennsylvaniajake from gmail.com"
<pennsylvaniajake from gmail.com> wrote:

>On Oct 7, 5:07 pm, Bill <connelly.b... from gmail.com> wrote:
>> Wow, that is a lot more of a complicated question than I suspect you
>> know.
>>
>> The best answer I can give you is to look up the Hodgkin and Huxley
>> equations. Theres are arbitrary functions that phenomenologically
>> explain the excitable behaviour of neurons. They do not work on a
>> single ion channel level, but model the total conductance of ions that
>> pass across a neurons membrane.
>>
>> We could do a pretty rough back of an envelope calculation though.
>> Lets say a "typical" neuron has a resting input conductance of 10
>> nS... a twin pore K channel that mediates a lot the resiting membrane
>> conductance has a single channel conductance of about 30 pS and I
>> think, quite a high open probability at rest, of about 0.5... while
>> Ih, another important leak channel is about 500 fS and an open
>> probability of about 0.2 at resting membrane potentials... lets just
>> say the conductance is half and half of these two... so 50,000 Ih
>> channels and 350 Twin Pore channels... this is just at the perisomatic
>> region...
>>
>> That's my attempt anyway. I've probably made some fundamental flaw in
>> my logic, which I'm sure someone will correct.
>>
>> I think Greg Stuart has a paper where he estimates the density of Ih
>> expression in channels per um^2 from the soma out to the dendrite
>>
>> On Oct 5, 7:15 am, "pennsylvaniaj... from gmail.com"
>>
>> <pennsylvaniaj... from gmail.com> wrote:
>> > How many ion channels are in a typical neuron?  And of these, how many
>> > are K, Cl, Na and so on?
>> > Also, is there an equation that give the probability of how many are
>> > open and closed based on time?
>
>I appreciate you trying to answer my question.  I know it is a hard
>one.  I have seen so many equations including the H & H and K
>equations.  But I have never run across in 25 years any one taking
>about the summation of all of the channels and types in a single
>neuron.
>
>Due to my education, engineer and mathematician, with the engineer
>coming out in me, too wonder if any one has ever looked at "the total
>picture" vs just one ion channel or 3 at the most (K,Na,Cl).
>Thanks once again.

In general, single channel dynamics is the key to understanding the
membrane mechanisms underlying nerve activity.  This is the
"microscopic" level.  The "macroscopic" level of summed channel
activity is really what determines cell-level activity.  Patch clamp
studies clearly show that both synaptic currents and voltage clamp
currents from whole cells are most definitely the result of summed ion
channel events.  But it really isn't useful to consider single channel
activity in most levels of neurobiology just as it really isn't useful
to consider single molecules reacting in most levels of biochemistry.
Reaction rates result from the summation of large numbers of
elementary actions:  nerve potentials are the same.

Here is one web site (out of many) that discusses the relation between
single channel events and summed membrane currents
  http://nerve.bsd.uchicago.edu/med98b.htm





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