Units specified by NEURON

[gabrielleg]

If I've put calcium current in my ASSIGNED block with units of nA/cm2, does NEURON override my choice of units and calculate ica in terms of mA/cm2 anyway?

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ASSIGNED
  {
  ica (nA/cm2)
  eca (mV)
  F (mM/nA)
  C0 (mM)
  tauca (ms)
  L (um)
  diam (um)
  }

-g

[ted]

"Go not there; that way lies error and confusion."

Currents that are declared in the WRITE clause of USEION statements must be in units
of mA/cm2. If you really truly need other units, then in addition to ica (which you declare
with a USEION ca . . . WRITE ica statement), declare a surrogate variable i in the
NEURON block, e.g.

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RANGE i

give it the desired units in the ASSIGNED block, e.g.

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ica (mA/cm2)
i (nA/cm2)

and assign it the proper value in the BREAKPOINT block, with whatever scale factor is
needed to make units consistent., e.g.

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ica = expression that returns results in (mA/cm2)
i = ica*(1e6)

or alternatively

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i = expression that returns results in (nA/cm2)
ica = i*(1e-6)

And always check with modlunit to make sure that no units inconsistencies slip by.

[gabrielleg]

Hey, thanks for clearing that up. I didn't want to have to go back and write in scale factors everywhere until I was sure I'd have to. Also, modlunit was helpful. I'm starting to accumulate many scale factors in my mod files. Out of curiosity, why does NEURON use mA/cm2 instead of nA/um2? Biological currents are typically measured in nA and the dimensions diam and L are in terms of um. Is there an advantage to the system of units that NEURON uses that I haven't yet realized?

-g

[ted]

NEURON has its roots in membrane biophysics, and its units are those that have long been
standard in that field.