Anomalous Spike in A-conductance
Posted: Mon May 23, 2011 5:08 am
Hi NEURON Forum,
I am using a .mod file called borgka.mod, which is a model of the A-current membrane mechanism. This transient outward potassium current activates during the rising phase of an afterhyperpolarisation, and consequently prolonges the AHP.. delaying the return to resting potential/increasing the interspike interval.
borgka.mod:
http://senselab.med.yale.edu/ModelDb/sh ... borgka.mod
The trouble with this .mod file is that the nature of the timeconstants means that the A-current also activates on the falling phase of the AHP, and indeed the action potential; during this period, the membrane potential is both strongly activated and deinactivating. This, combined, with the relative response speeds of the gates (as specified by the time-constants), results in an anomalous spike in A-conductance. This results in counterintuitive results; the spike in A-conductance happens way before it biologically occurs, and consequently the AHP tends to shorten as, for example, the maximal conductance density of the channel is increased.
I was wondering whether there was a way of specifying in this .mod file that A-conductance could only flow on the upstroke of an AHP? I am not familiar with how to modify NMODL files, so it would be difficult for me to implement this. I thought that perhaps constraining A-current to flow when the derivative of the membrane potential is positive would mean that it would only flow during the upstroke of the afterhyperpolarisation (assuming that the parameter values didn't mean that it could potential activate during the upstroke of an action potential, too)... how would I implement this? Is there a more robust way to insure A-current only flows from an inactivated state?
Thanks,
lb
I am using a .mod file called borgka.mod, which is a model of the A-current membrane mechanism. This transient outward potassium current activates during the rising phase of an afterhyperpolarisation, and consequently prolonges the AHP.. delaying the return to resting potential/increasing the interspike interval.
borgka.mod:
http://senselab.med.yale.edu/ModelDb/sh ... borgka.mod
The trouble with this .mod file is that the nature of the timeconstants means that the A-current also activates on the falling phase of the AHP, and indeed the action potential; during this period, the membrane potential is both strongly activated and deinactivating. This, combined, with the relative response speeds of the gates (as specified by the time-constants), results in an anomalous spike in A-conductance. This results in counterintuitive results; the spike in A-conductance happens way before it biologically occurs, and consequently the AHP tends to shorten as, for example, the maximal conductance density of the channel is increased.
I was wondering whether there was a way of specifying in this .mod file that A-conductance could only flow on the upstroke of an AHP? I am not familiar with how to modify NMODL files, so it would be difficult for me to implement this. I thought that perhaps constraining A-current to flow when the derivative of the membrane potential is positive would mean that it would only flow during the upstroke of the afterhyperpolarisation (assuming that the parameter values didn't mean that it could potential activate during the upstroke of an action potential, too)... how would I implement this? Is there a more robust way to insure A-current only flows from an inactivated state?
Thanks,
lb