1.2 Insert Newer Ionic Channels and Mechanisms
Since the 1952 Hodgkin and Huxley description of the sodium and
potassium channels in squid axons, many other voltage-sensitive
channels and transport mechanisms have been described. Mike Hines
has made a very useful tool, called "Inserter", for insertion and
removal of the multitude of new channels and mechanisms which can
be described by his Model Description
Language. This should be very useful for comparing how new or
modified channels or mechansims affect the membrance currents
and action potential shapes seen with HH channels.
For our purposes here, we are providing a prepackaged simulation of a single compartment
with a limited number of
previously described and representative channels as choices for the
experimenter. The channels and mechanisms are inserted inot
or removed from the membrane by "clicking" on the button beside
the named mechanism. A red checkmark shows beside the name when that mechanism is in the membrane.
The channels available so far are
- "hh": the reference standard or normal Hodgkin & Huxley set of sodium, potassium and leakage
These are also available as individual channels for comparison with alternative descriptions and/or other channel types. Test this by "clicking" all three to insert them, "clicking" on hh to remove the normal ensemble of channels;
"keep lines" in the action potential graph and see that an identical shape is produced by "init & run" as that for the full hh.
- "HHk"
- "HHna"
- "pas" passive or voltage insensitive channel whose conductance and reversal
potential have been set equal to those parameters for the HH leakage channel
The Moore-Cox kinetic model for the sodium channel was manually fitted (by many, many iterations) to the HH sodium currents at several levels in a voltage clamp. The published version of the MC model turn
s out to be spontaneously regenerative. A better fit has been made recently with the new fitter widget described in Chapter 9.. If one chooses "keep li
nes" in the voltage
trace panel, one can compare action potentials generated by the HH na channel and the MC na
channel by toggle between them in the insert panel.
Several calcium ion flux mechanisms can be inserted also:
calcium channel,
intracellular calcium binding, diffusion and pumping, and
a sodium/calcium exchanger
The rate constants of these mechanisms have not yet been
matched to experimental data. Experimentation with them is very useful in appreciating
their roles and magnitudes.
An area of 100 um2 was chosen for the "soma" so that the
value of the current density mA/cm2 is identical to the total current in nA,
the range of units you would expect to encounter in a simple spherical cell.
Thus this single compartment has quite small dimensions:
{diam=10 L=10/PI}.
This example needs more polishing but is
RUN as it is.
Please come back later for additional channels and text.