Object Type:
nernst
Description:
Calculates the Nernst potential for the given
ionic concentrations and temperature.
Author:
M. Wilson, Caltech (3/89)
----------------------------------------------------------------------------ELEMENT PARAMETERS
DataStructure:
nernst_type
Size:
96 bytes
Fields:
E
T
valency
scale
Cin
Cout
constant
[in src/device/dev_struct.h]
equilibrium (Nernst) potential
temperature in degrees Celsius
ionic valency z
voltage scale factor
internal ionic concentration
external ionic concentration
scale*(R/zF)*(T + 273.15)
This is set by the element,
and should not be set by the user.
----------------------------------------------------------------------------SIMULATION PARAMETERS
Function:
Nernst
Classes:
device
channel
Actions:
PROCESS
Messages:
[in src/device/nernst.c]
RESET
CHECK
sets the fields from incoming messages
and calculates constant and E.
similar to a single PROCESS step.
checks for valid messages and parameters.
CIN Cin
COUT Cout
TEMP T
sets the Cin field.
sets the Cout field.
sets the T field, and calculates constant.
----------------------------------------------------------------------------Notes:
A nernst element calculates E = constant * ln(Cout/Cin)
= scale*(R/zF)*(T + 273.15)*ln(Cout/Cin), where R is the
universal gas constant (1.9872 cal/mol deg) and F is
Faraday's
constant (23061 cal/volt mol). Note that the temperature in
degrees Celsius is internally converted to degrees Kelvin. A
scale factor of 1 gives E in volts, and a scale factor of
1e3 gives E in millivolts.
A nernst element is usually used with a channel and a
Ca_concen element to calculate the channel equilibrium
potential (Ek) from the internal ionic concentration in a
shell near the compartment surface. The equilibrium
potential
is sent from the nernst element to the channel with an EK
message, and the nernst element receives the concentration
from the Ca_concen element with a Cin message. The Ca_concen
element calculates this concentration from ionic currents
delivered with an I_k message.
It is also possible to perform a "one-time" calculation
of E
with fixed values of the ionic concentrations by setting
these
fields instead of setting up messages, and performing a
reset.
If this is done, the nernst element should be disabled to
avoid wasting time performing the PROCESS action.
Example:
// assume that a tabchannel `Ca' has been set up
create Ca_concen conc
setfield conc
\
B
{5.2e-6/(area*shell_thick)} \
Ca_base 4.0e-5
\
tau
1e-4
\
thick
{shell_thick}
create nernst nernst
setfield nernst \
Cout
4.0 \
//
T
21 \
//
valency 2
\
//
scale
1
//
addmsg Ca
addmsg conc
addmsg nernst
See also:
Ca_concen
conc
nernst
Ca
external Ca concentration of 4 mM.
near room temperature
Ca++
E in volts
I_Ca
Cin
EK
Ik
Ca
E