NEURON Extension to NMODL

This section describes the special NEURON block that has been added to the standard model description language in order to allow translation of a model into a form suitable for linking with NEURON.

The keyword NEURON introduces a special block which contains statements that tell NMODL how to organize the variables for access at the NEURON user level. It declares:

  • Which names are to be treated as range variables.
  • Which names are to be treated as global variables.
  • The names of all the ions used in the model and how the corresponding concentrations, current, and reversal potential are to be treated.
  • The suffix to be used for all variables in the model so that they do not conflict with variables in other models.
  • Whether the model is for a point process such as a synapse or a distributed process with density along an entire section such as a channel density.
  • Which names will be connected to external variables. (See “Importing variables from other mechanisms”.)

The syntax is (each statement can occur none or more times) :



   SUFFIX ...
   RANGE ...
   GLOBAL ...
   USEION ... READ ... WRITE ... VALENCE real
   POINTER ...


The suffix, “_name” is appended to all variables, functions, and procedures that are accessible from the user level of NEURON. If the SUFFIX statement is absent, the file name is used as the suffix (with the addition of an underscore character). If there is a Point Processes and Artificial Cells statement, that name is used as the suffix. Suffixes prevent overloading of names at the user level of NEURON. At some point in the future I may add something similar to the access statement which will allow the omission of the suffix for a specified mechanism. Note that suffixes are not used within the model description file itself. If the SUFFIX name is the word, “nothing”, then no suffix is used for variables, functions, and procedures explicitly declared in the .mod file. However, the mechanism name will be the base file name. This is useful if you know that no conflict of names will exist or if the .mod file is primarily used to create functions callable from NEURON by the user and you want to specify those function names exactly.


These names will be become range variables. Do not add suffixes here. The names should also be declared in the normal PARAMETER or ASSIGNED statement outside of the NEURON block. Parameters that do not appear in a NEURON RANGE statement will become global variables. Assigned variables that do not appear in this statement or in the NEURON GLOBAL statement will be hidden from the user. When a mechanism is inserted in a section, the values of these range variables are set to the values specified in the normal PARAMETER statement outside the NEURON block.


These names, which should be declared elsewhere as ASSIGNED or PARAMETER variables, become global variables instead of range variables. Notice here that the default for a PARAMETER variable is to become a global variable whereas the default for an ASSIGNED variable is to become hidden at the user level.


This signifies that we are calculating local currents which get added to the total membrane current but will not contribute to any particular ionic concentration. This current should be assigned a value after any SOLVE statement but before the end of the BREAKPOINT block. This name will be hidden at the user level unless it appears in a NEURON RANGE statement.


The ELECTRODE_CURRENT statement has two important consequences: positive values of the current will depolarize the cell (in contrast to the hyperpolarizing effect of positive transmembrane currents), and when the extracellular mechanism is present there will be a change in the extracellular potential vext. TODO: Add existing example mod file (iclamp1.mod)



This statement declares that a specific ionic species will be used within this model. The built-in HH channel uses the ions na and k. Different models which deal with the same ionic species should use the same names so that total concentrations and currents can be computed consistently. The ion, Na, is different from na. The example models using calcium call it, ca. If an ion is declared, suppose it is called, ion, then a separate mechanism is internally created within NEURON, denoted by ion, and automatically inserted whenever the “using” mechanism is inserted. The variables of the mechanism called ion are outward total current carried by this ion, iion; internal and external concentrations of this ion, ioni and iono; and reversal potential of this ion, eion. These ion range variables do NOT have suffixes. Prior to 9/94 the reversal potential was not automatically calculated from the Nernst equation but, if it was used it had to be set by the user or by an assignment in some mechanism (normally the Nernst equation). The usage of ionic concentrations and reversal potential has been changed to more naturally reflect their physiological meaning while remaining reasonably efficient computationally.

The new method governs the behaviour of the reversal potential and concentrations with respect to their treatment by the GUI (whether they appear in PARAMETER, ASSIGNED, or STATE panels; indeed, whether they appear at all in these panels) and when the reversal potential is automatically computed from the concentrations using the Nernst equation. The decision about what style to use happens on a per section basis and is determined by the set of mechanisms inserted within the section. The rules are defined in the reference to the function ion_style(). Three cases are noteworthy.


Assume only one model is inserted in a section.

        USEION ca READ eca

Then eca will be treated as a PARAMETER and cai/cao will not appear in the parameter panels created by the gui.

Now insert another model at the same section that has

        USEION ca READ cai, cao

Then 1) eca will be “promoted” to an ASSIGNED variable, 2) cai/cao will be treated as constant PARAMETER’s, and 3) eca will be computed from the Nernst equation when finitialize() is called.


Lastly, insert a final model at the same location in addition to the first two.

        USEION ca WRITE cai, cao

Then eca will still be treated as an ASSIGNED variable but will be computed not only by finitialize but on every call to fadvance(). Also cai/cao will be initialized to the global variables cai0_ca_ion and cao0_ca_ion respectively and treated as STATE’s by the graphical interface.

The idea is for the system to automatically choose a style which is sensible in terms of dependence of reversal potential on concentration and remains efficient.

Since the nernst equation is now automatically used as needed it is necessary to supply the valence (charge carried by the ion) except for the privileged ions: na, k, ca which have the VALENCE 1, 1, 2 respectively.

Only the ion names na, k, and ca are initialized to a physiologically meaningful value — and those may not be right for your purposes. Concentrations and reversal potentials should be considered parameters unless explicitly calculated by some mechanism.


The READ list of a USEION specifies those ionic variables which will be used to calculate other values but is not calculated itself. The WRITE list of a USEION specifies those ionic variables which will be calculated within this mechanism. Normally, a channel will read the concentration or reversal potential variables and write a current. A mechanism that calculates concentrations will normally read a current and write the intracellular and/or extracellular; it is no longer necessary to ever write the reversal potential as that will be automatically computed via the nernst equation. It usually does not make sense to both read and write the same ionic concentrations. It is possible to READ and WRITE currents. One can imagine, a large calcium model which would WRITE all the ion variables (including current) and READ the ion current. And one can imagine models which READ some ion variables and do not WRITE any. It would be an error if more than one mechanism at the same location tried to WRITE the same concentration.

A bit of implementation specific discussion may be in order here. All the statements after the SOLVE statement in the BREAKPOINT block are collected to form a function which is called during the construction of the charge conservation matrix equation. This function is called several times in order to compute the current and conductance to be added into the matrix equation. This function is never called if you are not writing any current. The SOLVE statement is executed after the new voltages have been computed in order to integrate the states over the time step, dt. Local static variables get appropriate copies of the proper ion variables for use in the mechanism. Ion variables get updated on exit from these functions such that WRITE currents are added to ion currents.


Optionally provide CURIE (Compact URI) to annotate what the species represents e.g. CHEBI:29101 for sodium(1+).

TODO: Add existing example mod file (src/nrnoc/hh.mod)





Point models are used for synapses, electrode stimuli, etc. They are distinguished from standard mechanisms in that instead of inserting the mechanism into a section and accessing parameters via range variables, point mechanisms are created as interpreter objects, eg.

    objref stim
    stim = new IClamp(x)

Values are accessed via the standard object syntax, eg. stim.amp = 2. Since standard mechanisms are considered in terms of density, the appropriate current units for standard mechanisms are mA/cm2 and conductance units are mho/cm2. However, point process current units are nA and conductance units are umho. These conventions ensure that the simulation is independent of the number of segments in a section (assuming the number of segments is large enough so spatial discretization error is small).

At the NEURON user level, all variables and functions associated with a POINT_PROCESS are accessed via the normal object syntax. A point process, call it pnt is inserted into (or moved to) the currently specified section at location, 0 < x < 1, with the function, pnt.loc(x). See pnt.get_loc()

If a point process is created with no argument then it is not located anywhere. If an argument is present and there is a currently accessed section then the point process is placed there. At this time, point processes are placed at the center of the nearest segment.

pnt.has_loc() returns 1 if the point process is located in a section and returns 0 if not located. If a point process has no location then attempts to access its variables or get its location will produce an error message. See pnt.has_loc()

One finds the location of a point process via the function, x = pnt.get_loc(). See pnt.get_loc()

The function returns the x location at the center of the segment where the process was placed and pushes the section name onto the stack so that it becomes the currently accessed section. The stack must be popped with pop_section() at a subsequent time. BE SURE TO POP THE SECTION STACK! This can be a dangerous function in the sense that if the stack is not popped, then section access is completely screwed up.

The POINT_PROCESS mechanism can be used to implement classes written in c/c++ for use by the interpreter. To aid in this the special block CONSTRUCTOR is called when a point process is created with the new command in the interpreter. Just before the memory associated with a point process instance is freed the users DESTRUCTOR block (if any) is called.


These names are pointer references to variables outside the model. They should be declared in the body of the description as normal variables with units and are used exactly like normal variables. The user is responsible for setting these pointer variables to actual variables at the hoc interpreter level. Actual variables are normal variables in other mechanisms, membrane potential, or any hoc variable. See below for how this connection is made. If a POINTER variable is ever used without being set to the address of an actual variable, NEURON may crash with a memory reference error, or worse, produce wrong results. Unfortunately the errors that arise can be quite subtle. For example, if you set a POINTER correctly to a mechanism variable in section a. And then change the number of segments in section a, the POINTER will be invalid because the memory used by section a is freed and might be used for a totally different purpose. It is up to the user to reconnect the POINTER to a valid actual variable.



See: Memory Management for POINTER Variables

TODO: Add description (?) and existing example mod file (provided by link)



These names, which should be declared elsewhere as ASSIGNED or PARAMETER variables allow global variables in other models or NEURON c files to be used in this model. That is, the definition of this variable must appear in some other file. Note that if the definition appeared in another mod file this name should explicitly contain the proper suffix of that model. You may also call functions from other models (but do not ignore the warning; make sure you declare them as

extern double fname_othermodelsuffix();

in a VERBATIM block and use them with the proper suffix.



See: Multithreaded paralellization and Thread Safe MOD Files

TODO: Add description and existing example mod file


TODO: Add description and existing example mod file


TODO: Add description and existing example mod file



FOR_NETCONS (args) means to loop over all NetCon connecting to this target instance and args are the names of the items of each NetCon’s weight vector (same as the enclosing NET_RECEIVE but possible different local names).

TODO: Add existing example mod file (test/coreneuron/mod/fornetcon.mod)


    GLOBAL var

    PROTECT var = var + 1

Mod files that update values to GLOBAL variables are not considered thread safe. In case of multi-threaded/SIMD/GPU execution, such updates can result in a race condition. To avoid this, one needs to use PROTECT keyword. Note that PROTECT internally uses atomic operations on CPU or GPU execution and hence the statement needs to be of a simple form such as:

var1 = var1 binary_operator expression
var1 = expression binary_operator var1

If the mod file is using the GLOBAL essentially as a file scope LOCAL along with the possibility of passing values back to hoc in response to calling a PROCEDURE, make sure to use the THREADSAFE keyword in the neuron block to automatically treat those GLOBAL variables as thread specific variables. NEURON assigns and evaluates only the thread 0 version and if FUNCTION and :PROCEDURE are called from Python, the thread 0 version of these globals are used.


For the performance reason, we recommend to reduce or remove the use of PROTECT construct.


LOCAL factors_done

    if (factors_done == 0) {
          factors_done = 1

PROCEDURE factors() {
    : ...

Similar to PROTECT, MUTEXLOCK and MUTEXUNLOCK are two constructs to handle thread-safety in case update of updates to GLOBAL variables in multi-threaded execution. Internally it uses mutex mechanism to avoid race condition.


This construct is not supported in the case of GPU execution via CoreNEURON. For the performance reason and compatibility with GPU execution, either avoid the usage of this construct or check alternatives using PROTECT construct.

Connecting Mechanisms Together

Occasionally mechanisms need information from other mechanisms which may be located elsewhere in the neuron. Connecting pre and post synaptic point mechanisms is an obvious example. In the same vein, it may be useful to call a function from hoc which modifies some mechanism variables at a specific location. (Normally, mechanism functions callable from HOC should not modify range variables since the function does not know where the mechanism data for a segment is located. Normally, the pointers are set when NEURON calls the BREAKPOINT block and the associated SOLVE blocks.)

One kind of connection between mechanisms at the same point is through ionic mechanisms invoked with the USEION statement. In fact this is entirely adequate for local communication although treating an arbitrary variable as an ionic concentration may be conceptually strained. However, it does not solve the problem of communication between mechanisms at different points.



Basically what is needed is a way to implement the hoc statement

section1.var1_mech1(x1) =  section2.var2_mech2(x2)

efficiently from within a mechanism without having to explicitly connect them through assignment at the HOC level everytime the var2 might change.

First of all, the variables which point to the values in some other mechanism are declared within the NEURON block via

   POINTER var1, var2, ...

These variables are used exactly like normal variables in the sense that they can be used on the left or right hand side of assignment statements and used as arguments in function calls. They can also be accessed from HOC just like normal variables. It is essential that the user set up the pointers to point to the correct variables. This is done by first making sure that the proper mechanisms are inserted into the sections and the proper point processes are actually “located” in a section. Then, at the hoc level each POINTER variable that exists should be set up via the command:

        setpointer pointer, variable

where pointer and variable have enough implicit/explicit information to determine their exact segment and mechanism location. For a continuous mechanism, this means the section and location information. For a point process it means the object. The variable may also be any hoc variable or voltage, v.

For example, consider a synapse which requires a presynaptic potential in order to calculate the amount of transmitter release. Assume the declaration in the presynaptic model



objref syn
somedendrite {syn = new Syn(.8)}
setpointer syn.vpre, axon.v(1)

will allow the syn object to know the voltage at the distal end of the axon section. As a variation on that example, if one supposed that the synapse needed the presynaptic transmitter concentration (call it tpre) calculated from a point process model called “release” (with object reference rel, say) then the statement would be

setpointer syn.tpre, rel.AcH_release

The caveat is that tight coupling between states in different models may cause numerical instability. When this happens, merging models into one larger model may eliminate the instability.



Sections of code surrounded by VERBATIM and ENDVERBATIM blocks are interpreted as literal C/C++ code. This feature is typically used to interface with external C/C++ libraries, or to use NEURON features (such as random number generation) that are not explicitly supported in the NMODL language.

PROCEDURE set_foo() {
/* literal C/C++ */
  foo = 42

This is, by its nature, more fragile than exclusively using NMODL language constructs, but it can be necessary. NEURON versions newer than 8.1 (#1762) provide some C/C++ preprocessor macros that make it easier to follow incompatible changes in external libraries or the internal workings of NEURON.

#if NRN_VERSION_EQ(9, 0, 0)
/* NEURON version is exactly 9.0.0 */
#if NRN_VERSION_NE(8, 2, 3)
/* NEURON version is not 8.2.3 */
#if NRN_VERSION_GT(9, 1, 0)
/* NEURON version is >9.1.0 */
#if NRN_VERSION_LT(10, 2, 0)
/* NEURON version is <10.2.0 */
#if NRN_VERSION_GTEQ(8, 2, 1)
/* NEURON version is >=8.2.1 */
#if NRN_VERSION_LTEQ(8, 2, 2)
/* NEURON version is <=8.2.2 */
#ifndef NRN_VERSION_GTEQ_8_2_0
/* NEURON version is <8.2.0 */
/* NEURON version is >=8.2.0 so NRN_VERSION_{EQ,NE,GT,LT,GTEQ,LTEQ}(...)
 * are defined. */

VERBATIM should be used with caution and restraint, as it is very easy to introduce dependencies on the implementation details of NEURON and the NMODL language compilers and end up with MOD files that are only compatible with a limited range of NEURON versions.