An electrophysiology quesiton

Matt Jones jonesmat at physiology.wisc.edu
Fri Feb 13 17:08:19 EST 2004


Howdy,

Several responses rolled into one post here.



Xiaoshen Li <xli6 at gmu.edu> wrote in message news:<c0gl1r$beo at portal.gmu.edu>...

> I am curious if I hope to develop a hobby of playing the 
> electronic circuit at the elementary level in my apartment, are there 
> places  for shopping and getting instructions? I confess that I know 
> very little about electronic things.


I suggest getting:

1)  "The Art of Electronics" by Horowitz and Hill
2) The lab manual that accompanies the book above.
3) A breadboard for quickly building and testing circuits, an
assortment of jumper wires for the breadboard, and an assortment of
simple components (i.e., resistors, capacitors, inductors and bipolar
junction transistors (i.e., they'll say "NPN" or "PNP" on the package)
to start with.

You can probably get a "starter kit" online. Mouser is a good place to
get all sorts of stuff (http://www.mouser.com). You can also get
"hobby kits", which are limited in their scope but actually very easy
to use and experiment with (e.g.,
http://www.hobbytron.net/labssub.html).

If you can possibly avoid it, stay away from Radio Shack. They suck.
("You've got questions, we've got blank vacuous stares...")

The books above are a serious introduction to understanding how
circuitry works, with excellent lab excercises and clear explanations.
A hobby kit probably won't be as good a learning experience, but it
might result in building things that work faster. I had one of these
when I was ten, and it was great fun.

Also, I think building audio-related circuits is good background for
understaqnding how electrophysiological stuff works, since mostly
we're using the same sorts of circuits (e.g., amplifiers, filters,
bipolar power supplies, op-amps, etc).





BilZ0r <BilZ0r at TAKETHISOUThotmail.com> wrote in message news:<Xns948E851496825BilZ0rhotmailcom at 202.20.93.13>...
> Isn't that why you use ultra high impedance/resistance clamps? So that you 
> minimise current flow, and keep the neuron in as normal a condition as 
> possible?

Yes, exactly. But, the "ultra high" impedance of a typical patch clamp
amplifier might be a few gigaohms, even with all the fancy
electronics. That's not that much higher than the input impedance of a
granule cell from hippocampus, for example. So it's still potentially
a problem.




fburton at nyx10.nyx.net (Francis Burton) wrote in message news:<1076672636.204715 at irys.nyx.net>...
> In article <b86268d4.0402120950.6931264d at posting.google.com>,
> Matt Jones <jonesmat at physiology.wisc.edu> wrote:
> >First, Francis Burton said he heard that dynamic clamp was injecting
> >voltage clamp waveforms that look like action potentials.  This
> >actually ISN'T what  dynamic clamp means, but it is a very common
> >confusion.
> 
> I am grateful for the correction. This isn't my field, and I was
> only reporting what I had heard - I should have refrained from
> muddying the water!
> 
> Thanks for posting such a clear description of what it actually is.
> 
> Francis


Sorry Francis, I wasn't trying to slam you. It really is a very common
confusion, because the method you referred to has been used a lot, and
may indeed have been called "dynamic clamp" by somebody, including
some electrophysiologists.



Xiaoshen Li <xli6 at gmu.edu> wrote in message news:<c0gdbu$9ng at portal.gmu.edu>...
> Matt Jones wrote:
> > 
> > HOWEVER, V IS CHANGING DURING THE EPSP (by definition)!
> > Therefore, Isyn will also be changing during an EPSP in real life. In
> > fact, if the cell fires a spike at the top of the EPSP, then V will
> > actually change sign during the spike, and Isyn will also change sign.
> > This means that the flow of ions actually reverses direction through
> > the synaptic channels during a spike (cool, huh?).
> 
> Matt,
> 
> I think this part needs some more discussion:
> 
> Soma fires action potential, so the membrane potential at soma will 
> shoot up to a positive value, say +30mV. Only if the synapse is located 
> at the soma and it is opened by presynaptic firing, then the flow of 
> ions will reverse direction. Most synapses are at dendritic 
> tree(proximal or apical location). The local membrane potential will 
> rise to be closer to 0mV but may not exceed it to become positive value. 
> So the direction of the ion flow on those synapses will not flip. (The 
> value of the current flowing through the synapses will change due to 
> smaller driving force).

I agree with all that.

> If the dendrite also fires a spike locally, then the direction of ion 
> flow will reverse. However, "dendrite fires spikes" is very new research 
> information. 

Yes. Relatively new, but has now been demonstrated over and over again
by many different labs.

>(My impression is those spikes are calcium spikes, not 
> classical Hodgkin-Huxley Na-K spike). Are those called "spikelet"?

Dendritic spikes can have a large calcium conductance, but there are
also lots of sodium channels in the dendrites, and you can get almost
full blown sodium spikes in the dendrites. They tend to be *initiated*
near the soma, but can propagate into the dendrites pretty
efficiently.

> 
> I am also curious if this direction reverse has any physiology meaning.

Yes. The reversal is the mechanism by which the conductance is
attempting to *clamp* the voltage at the equilibrium potential. If V <
E, then an inward current flows, which will tend to depolarize the
cell (i.e., attempting to force V = E). If V > E, then an outward
current flows, which will tend to hyperpolarize the cell (i.e., again,
attempting to force V = E).



Cheers,

Matt



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