From connelly.bill from gmail.com Fri Aug 7 04:15:31 2009 From: connelly.bill from gmail.com (Bill) Date: Fri Aug 7 08:44:28 2009 Subject: [Neuroscience] Differential Attenutation of EPSPs in Voltage Clamp vs Current Clamp Message-ID: <1217c413-c684-46a1-bf89-159901332eac@u16g2000pru.googlegroups.com> I was reading a paper by Stephen Williams that showed that you get more distance dependent attenuation of EPSPs when you record in voltage clamp than in current clamp. I modelled it using NEURON and I got the exact same result. Why is this? I would have thought the (albiet partial) voltage control along the cable would mean that voltage clamp would reduce capacitive currents (i.e. less C*dv/dt), and therefore would retain more longitudial current and you would get less attenuation. From r_s_norman from comcast.net Fri Aug 7 08:33:45 2009 From: r_s_norman from comcast.net (r norman) Date: Fri Aug 7 12:35:18 2009 Subject: [Neuroscience] Re: Differential Attenutation of EPSPs in Voltage Clamp vs Current Clamp References: <1217c413-c684-46a1-bf89-159901332eac@u16g2000pru.googlegroups.com> Message-ID: On Fri, 7 Aug 2009 02:15:31 -0700 (PDT), Bill wrote: >I was reading a paper by Stephen Williams that showed that you get >more distance dependent attenuation of EPSPs when you record in >voltage clamp than in current clamp. I modelled it using NEURON and I >got the exact same result. Why is this? > >I would have thought the (albiet partial) voltage control along the >cable would mean that voltage clamp would reduce capacitive currents >(i.e. less C*dv/dt), and therefore would retain more longitudial >current and you would get less attenuation. It is easier to see this doing the rather complex math, but here is an attempt at an explanation. With current clamp, there is essentially no "loading" or alteration of the conditions for voltage and current spread down a cable and the voltage decays with distance in a particular fashion. With voltage clamp, there is essentially a "short circuit" across the cable at the point of the clamp. This causes a rather drastic alteration in the pattern of current flow pattern when the clamp is close to the synapse. However when the clamp is farther down the cable, the current flow near the synapse is virtually unchanged. As a result, when the clamp is near the synapse it "steals" pretty much all the current, far more than would pass that way unclamped. When the clamp is far from the synapse, it only gets what current would normally end up passing that far anyway. Hence an apparent "faster decay with distance" in voltage clamp than unclamped. From connelly.bill from gmail.com Fri Aug 7 18:22:30 2009 From: connelly.bill from gmail.com (Bill.Connelly) Date: Fri Aug 7 21:48:06 2009 Subject: [Neuroscience] Re: Differential Attenutation of EPSPs in Voltage Clamp vs Current Clamp References: <1217c413-c684-46a1-bf89-159901332eac@u16g2000pru.googlegroups.com> Message-ID: <46b30746-1311-48b1-9cee-93384ce52bf8@x6g2000prc.googlegroups.com> Oh, so when the current is generated far away, the voltage clamp kind of acts like a lot of open channels, i.e that end of the cable appears to have low resistance? On Aug 8, 1:33?am, r norman wrote: > With voltage clamp, there is essentially a "short circuit" across the > cable at the point of the clamp. ?This causes a rather drastic > alteration in the pattern of current flow pattern when the clamp is > close to the synapse. ?However when the clamp is farther down the > cable, the current flow near the synapse is virtually unchanged. ?As a > result, when the clamp is near the synapse it "steals" pretty much all > the current, far more than would pass that way unclamped. ?When the > clamp is far from the synapse, it only gets what current would > normally end up passing that far anyway. ?Hence an apparent "faster > decay with distance" in voltage clamp than unclamped. From r_s_norman from comcast.net Fri Aug 7 22:28:31 2009 From: r_s_norman from comcast.net (r norman) Date: Sat Aug 8 10:30:23 2009 Subject: [Neuroscience] Re: Differential Attenutation of EPSPs in Voltage Clamp vs Current Clamp References: <1217c413-c684-46a1-bf89-159901332eac@u16g2000pru.googlegroups.com> <46b30746-1311-48b1-9cee-93384ce52bf8@x6g2000prc.googlegroups.com> Message-ID: A voltage clamp always acts like a low resistance at that point on the cell. It is just that when the synapse is far away, it doesn't "see" the clamp very much so the longitudinal flow of currents down the dendrite are not much affected. However when the synapse is very close, then a major portion of synaptic current is shunted through the clamp and the pattern of longitudinal current is seriously altered. On Fri, 7 Aug 2009 16:22:30 -0700 (PDT), "Bill.Connelly" wrote: >Oh, so when the current is generated far away, the voltage clamp kind >of acts like a lot of open channels, i.e that end of the cable appears >to have low resistance? > >On Aug 8, 1:33?am, r norman wrote: > >> With voltage clamp, there is essentially a "short circuit" across the >> cable at the point of the clamp. ?This causes a rather drastic >> alteration in the pattern of current flow pattern when the clamp is >> close to the synapse. ?However when the clamp is farther down the >> cable, the current flow near the synapse is virtually unchanged. ?As a >> result, when the clamp is near the synapse it "steals" pretty much all >> the current, far more than would pass that way unclamped. ?When the >> clamp is far from the synapse, it only gets what current would >> normally end up passing that far anyway. ?Hence an apparent "faster >> decay with distance" in voltage clamp than unclamped. From nin from neurohost.org Sun Aug 16 07:31:12 2009 From: nin from neurohost.org (Jose Guzman) Date: Sun Aug 16 12:56:42 2009 Subject: [Neuroscience] Re: Warm slicing In-Reply-To: References: Message-ID: Bill wrote: > Hi Guys, > > I thought I would post the result of a little experiment I tried > today. After reading: > http://www.precisionary.com/products_faq.html#q9 > I tried making some cortical slices (250um thick) in a cutting chamber > heated with a 35 degree water bath (in sucrose aCSF) from 20 day old > tissue. While the brain was so much more fluid than at 4 degrees, but > the cells were wonderfully visible and healthy looking. > Just a suggestion that I thought someone might be interested in. Hi Bill, I though already about cutting with a warm bath... but I did not want to risk... on the other hand P20 are quite easy to get, what about older animals? (ie. 1/2 months?) From connelly.bill from gmail.com Sun Aug 16 21:08:14 2009 From: connelly.bill from gmail.com (Bill.Connelly) Date: Mon Aug 17 07:34:35 2009 Subject: [Neuroscience] Re: Warm slicing References: Message-ID: <6ff4667a-05a3-477b-b1d4-266035b6b39e@18g2000yqa.googlegroups.com> For me it was death city when I tried it on 30 day old animals. On Aug 17, 12:31?am, Jose Guzman wrote: > Bill wrote: > > Hi Guys, > > > I thought I would post the result of a little experiment I tried > > today. After reading: > >http://www.precisionary.com/products_faq.html#q9 > > I tried making some cortical slices (250um thick) in a cutting chamber > > heated with a 35 degree water bath (in sucrose aCSF) from 20 day old > > tissue. While the brain was so much more fluid than at 4 degrees, but > > the cells were wonderfully visible and healthy looking. > > Just a suggestion that I thought someone might be interested in. > > Hi Bill, > > I though already about cutting with a warm bath... but I did not want to > risk... on the other hand P20 are quite easy to get, what about older > animals? (ie. 1/2 months?) From connelly.bill from gmail.com Tue Aug 18 21:22:53 2009 From: connelly.bill from gmail.com (Bill.Connelly) Date: Wed Aug 19 10:00:49 2009 Subject: [Neuroscience] Re: If brain part is removed and kept alive, would it still work if stimualted electrically, I.e. postcentral gyrus (main sensory cortex) References: Message-ID: If a piece of the brain was removed from your head; and then you stimulated that piece of tissue, you would not feel anything. The piece of tissue is no connected to your body in any way, and hence it can not interact with your body in any way. This is no more possible that if you cut of someones arm, and then gave tactile stimulation to that arm, that person could not feel it. Hopefully this answers your question. On Aug 17, 8:25?pm, chief12 wrote: > Thanks, so if for example the post central gyrus was removed and > stimulated electrically would you still feel the same sensations? > Thanks soo much. From connelly.bill from gmail.com Tue Aug 18 21:38:05 2009 From: connelly.bill from gmail.com (Bill) Date: Wed Aug 19 10:00:54 2009 Subject: [Neuroscience] How do you deal with equations when the denominator tends to 0? Message-ID: <418a025b-8b78-4ed8-a14a-ca4bcc45922b@a39g2000pre.googlegroups.com> This is more of a maths question than a neuroscience question, but I've come across it twice when dealing with neuroscience problems 1) I was trying to solve the Goldman Hodgkin Katz field equation, which I shan't type out here in full, but it has a denonimator term of 1-e^(-z.V.F/RT) so when V=0, the denominator is 0. Obviously, I could calculate the value fractionally above 0, and fractionally below 0, and average the result to get the value for 0; but I was wondering if there was a smarter way 2) Now here is the real problem. I've got some voltage ramp data, I wanted to convert the current trace to a conductance trace using G = I/ (Vm-Ve). However as Vm approaches Ve the trace goes crazy (obviously again, at Vm=Ve I couldn't calculate G, but even as Vm-Ve gets very small, presumabley the noise of the trace is amplified, so you have a rectangular hyperbola overlaid on a bolztman style curve). Is there anything I can do about this? (and filtering doesn't work).