Simulable objects

In NRV, all simulations can be launched by objects that have a physiological meaning:

1. axons or individual fibers. These fibers can be myelinated or unmyelinated and few models are already implemented.

2. fascicles: containing one or several axons.

3. nerves: containing one or several fascicles.

All these object have in common a method called simulate that automatically performs the simulation, handles computation of extracellular fields if needed, processes the stimuli, records internal variables. For fascicles and nerves, if the script is launched on several CPU cores, the parallelization is automatically handled transparently for the user.

Note

Simulable objects are callable. Calling any simulable object will call the simulate-method. In other words:

my_simulable.simulate(*myargs)

is equivalent to:

my_simulable(*myargs)

Axons

The axon class is abstract, meaning that the end user cannot directly instantiate an axon. Instead, two daughter classes are accessible for the end-user for myelinated and unmyelinated fibers.

Generic parameters for all axons are defined by default and can be changed, as both unmyelinated and myelinated axons inherit from them.

Some useful methods are inherited by both unmyelinated and myelinated fibers:

• The simulate method that solves the neural model.

  Arguments of simulate method

  Parameter	Type	Comment
  t_sim	float	total simulation time (ms), by default 20 ms

  The simulate method is also controlled by axon attributes as described in the next table:

  Attributes of axon involved in simulate

  Attribute	Type	default	Comment
  record_V_mem	bool	True	if true, the membrane voltage is recorded
  record_I_mem	bool	False	f true, the membrane current is recorded
  record_I_mem	bool	False	if true, the ionic currents are recorded
  record_particules	bool	False	if true, the particle states are recorded
  loaded_footprints	dictionary	None	Dictionary composed of extracellular footprint array, the keys are int value of the corresponding electrode ID, if None, footprints calculated during the simulation,

  The simulate method for axons generate results are stored and returned in a axon_results object.

• A method to link an axon to an extracellular stimulation. This object provides the description of the extracellular context. It is required for computing the external fields and footprint used to evaluate the axon response. A dedicated chapter of this user’s guide s dedicated to extracellular stimulation. This method, called attach_extracellular_stimulation has a single parameter, the stimulation object instance.

• A method to change the stimulation waveform from a specific electrode already linked to the axon with the attach_extracellular_stimulation method. This method is called change_stimulus_from_electrode and has for arguments:

  Arguments of the change_stimulus_from_electrode method

  Parameter	Type	Comment
  ID_elec	int	ID of the electrode which should be changed
  stimulus	stimulus	New stimulus to set

Unmyelinated axons

Unmyelinated axons object implements automatically Hodgkin-Huxley-like cable formalism in NEURON. Axons are oriented along the x-axis as depicted bellow. The user can specify the y and z coordinates, directly in microns.

The main property of the axon is the diameter, that the user specifies in microns, and the length of the fiber, also specified in microns. Different models are pre-implemented in NRV:

Available models for unmyelinated axons

Name	Instantiation string	Comment
Hodgkin-Huxley	"HH"	model from Hodgkin and Huxley 1952
Rattay-Aberham	"Rattay_Aberham"	Comment
Sundt	"Sundt"	Comment
Tigerholm	"Tigerholm"	Comment
Schild 1994	"Schild_94"	Comment
Schild 1997	"Schild_97"	Comment

A scientific review of these models is available in the following paper: Pelot, N. A., Catherall, D. C., Thio, B. J., Titus, N. D., Liang, E. D., Henriquez, C. S., & Grill, W. M. (2021). Excitation properties of computational models of unmyelinated peripheral axons. Journal of neurophysiology, 125(1), 86-104.

The complete list of tunable parameters for unmyelinated axons is:

Public attributes of unmyelinated axons

Property	type	default	Comment
y	float	0	y coordinate for the axon, in µm
z	float	0	z coordinate for the axon, in µm
d	float	1	axon diameter, in um
L	float	1000	axon length along the x axons, in µm
model	str	“Rattay_Aberham”	choice of conductance based model (table above)
dt	float	0.001	computation step for simulations, in ms. By default equal to 1 us
Nrec	int	0	Number of points along the axon to record for simulation results. Between 0 and the number of segment, if set to 0, all segments are recorded
Nsec	int	1	Number of sections in the axon, by default 1. Useful to create umnyelinated axons with a variable segment density
Nseg_per_sec	int	0	Number of segment per section in the axon. If set to 0, the number of segment is automatically computed using d-lambda rule and following parameters. If set by user, please use odd numbers
freq	float	100	Frequency used for the d-lambda rule, corresponding to the maximum membrane current frequency, by default set to 100 Hz
freq_min	float	0	Minimal frequency for the d-lambda rule when using irregular number of segments along the axon, if set to 0, all sections have the same frequency determined by the previous parameter
mesh_shape	str	“plateau_sigmoid”	Shape of the frequencies’ distribution for the d-lmabda rule along the axon, pick between:
alpha_max	float	0.3	Proportion of the axon set to the maximum frequency for plateau shapes, by default set to 0.3
d_lambda	float	0.1	value of d-lambda for the d-lambda rule,
v_init	float	None	Initial value of the membrane voltage in mV, set None to get an automatically model attributed value
T	float	None	temperature in C, set None to get an automatically model attributed value
ID	int	0	axon ID, by default set to 0,
threshold	float	-40	voltage threshold in mV for further spike detection in post-processing, by default set to -40mV, see post-processing files for further help

For the end-user, two specific methods for intracellular stimulation of unmyelinated axons are available:

• insert_I_Clamp to perform current clamp stimulation. For the moment only single pulse waveform are available.

  Arguments of current clamp method

  Parameter	Type	Comment
  position	float	relative position over the axon
  t_start	float	starting time, in ms
  duration	float	duration of the pulse, in ms
  amplitude	float	amplitude of the pulse (nA)

• insert_V_Clamp to perform voltage clamp stimulation.

  Arguments of voltage clamp method

  Parameter	Type	Comment
  position	float	relative position over the axon
  stimulus	Stimulus object	stimulus for the clamp, see corresponding page for more information

Myelinated axons

Similarly, myelinated axons implements the double cable fiber description in NEURON. Axons are oriented along the x-axis as depicted bellow. The user can specify the y and z coordinates, directly in microns.

The main property of the axon is the diameter, that the user specifies in microns, and the length of the fiber, also specified in microns. The succession of myelinated regions and Nodes-of-Ranvier is automatically computed, and the axon can be shifted along its axes so that for similar diameter fibers, nodes-of-Ranvier are not aligned. Different models are pre-implemented in NRV:

Available models for myelinated axons

Name	Instantiation string	Comment
MacIntyre-Grill-Richardson	"MRG"	First model of double cable axon described in [1], ionic channels on NoR and passive myelin
Gaines motor fibers	"Gaines_motor"	Double cable described in [2], ionic channels on NoR and adjacent myelinated regions for motor fibers
Gaines sensory fibers	"Gaines_sensory"	Double cable described in [2], ionic channels on NoR and adjacent myelinated regions for sensory fibers

Details of model can be found in the following scientific contributions:

[1] McIntyre CC, Richardson AG, and Grill WM. Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle. Journal of Neurophysiology 87:995-1006, 2002.

[2] Gaines, J. L., Finn, K. E., Slopsema, J. P., Heyboer, L. A., Polasek, K. H. (2018). A model of motor and sensory axon activation in the median nerve using surface electrical stimulation. Journal of computational neuroscience, 45(1), 29-43.

The complete list of tunable parameters for unmyelinated axons is:

Public attributes of unmyelinated axons

Property	type	default	Comment
y	float	0	y coordinate for the axon, in um
z	float	0	z coordinate for the axon, in um
d	float	1	axon diameter, in um
L	float	1000	axon length along the x axis, in um
model	str	“MRG”	choice of conductance based model (table above)
dt	float	0.001	computation step for simulations, in ms. By default equal to 1 µs
node_shift	float	0	shift of the first node of Ranvier to zeros, as a fraction of internode length (0<= node_shift < 1)
Nseg_per_sec	int	0	Number of segment per section in the axon. If set to 0, the number of segment is automatically computed using d-lambda rule and following parameters. If set by user, please use odd numbers
freq	float	100	Frequency used for the d-lambda rule, corresponding to the maximum membrane current frequency, by default set to 100 Hz
freq_min	float	0	Minimal frequency for the d-lambda rule when using irregular number of segment along the axon, if set to 0, all sections have the same frequency determined by the previous parameter
mesh_shape	str	“plateau_sigmoid”	Shape of the frequencies distribution for the d-lmabda rule along the axon, pick between:
alpha_max	float	0.3	Proportion of the axon set to the maximum frequency for plateau shapes, by default set to 0.3
d_lambda	float	0.1	value of d-lambda for the d-lambda rule,
rec	str	"nodes"	recording zones for the membrane potential, eiter: “nodes”: record only at the nodes of Ranvier “all”: all computation points in nodes of Ranvier and over myelin
v_init	float	None	Initial value of the membrane voltage in mV, set None to get an automatically model attributed value
T	float	None	temperature in C, set None to get an automatically model attributed value
ID	int	0	axon ID, by default set to 0,
threshold	float	-40	voltage threshold in mV for further spike detection in post-processing, by default set to -40mV, see post-processing files for further help

Again, for the end-user, four specific methods for intracellular stimulation myelinated axons are available:

• insert_I_Clamp_node, for which the current clamp is directly applied at a node-of-Ranvier, given its number

  Arguments of current clamp at a node method

  Parameter	Type	Comment
  index	int	node number of the node to stimulate
  t_start	float	starting time, in ms
  duration	float	duration of the pulse, in ms
  amplitude	float	amplitude of the pulse (nA)

• insert_I_Clamp, for which the current clamp is applied in the fiber with a normalized position

  Arguments of current clamp method

  Parameter	Type	Comment
  position	float	relative position over the axon
  t_start	float	starting time, in ms
  duration	float	duration of the pulse, in ms
  amplitude	float	amplitude of the pulse (nA)

• insert_V_Clamp_node, for which the voltage clamp is directly applied at a node-of-Ranvier, given its number

  Arguments of voltage clamp method

  Parameter	Type	Comment
  index	int	node number of the node to stimulate
  stimulus	Stimulus object	stimulus for the clamp, see corresponding page for more information

• insert_V_Clamp, for which the voltage clamp is applied in the fiber with a normalized position

  Arguments of voltage clamp method

  Parameter	Type	Comment
  position	float	relative position over the axon
  stimulus	Stimulus object	stimulus for the clamp, see corresponding page for more information

Fascicles

In NRV, fascicles consist in an aggregation of axon objects, on which we can perform logical/arithmetical operations, specify properties, and simulate. Fascicle constructor takes two initialization parameters:

Fascicle initialization parameters list

Property	type	default	Comment
d	float	None	fascicle diameter, in um
ID	int	0	fascicle unique identifier

Once created, the fascicle can be filled with a population of axon. The axon population can be either generated with NRV (see xxx), or by any external means. Axons are added to the fascicle with the fill_with_population method.

fill_with_population parameters

Property	type	default	Comment
axons_diameter	np.array[float]		Array of axon diameter, in um
axons_type	np.array[bool]		Array of axon type. True means myelinated, False means unmyelinated
delta	float	1um	axon-to-axon and axon to fascicle border minimal distance
y_axons	np.array[float]	None	Optional, y-position of each axon
z_axons	np.array[float]	None	Optional, z-position of each axon
fit_to_size	bool	False	If true, axons position will be dilated to fit the entire fascicle area
n_iter	int	20 000	Optional, number of iteration of the packing algorithm

Note

If the y_axons and z_axons parameters are not specified, the fill_with_population-method will automatically called the NRV’s packing algorithm to place them.

Axon simulation parameters can be specified in a dictionary and pass to the fascicle object with the set_axons_parameters method:

set_axons_parameters parameters

Property	type	default	Comment
unmyelinated_only	bool	False	Parameters are for unmyelinated axons only
myelinated_only	bool	False	Parameters are for myelinated axons only
kwargs	kwargs		kwargs for axon parameters

To simulate the fascicle, one option is to use the insert_I_Clamp method.

insert_I_Clamp parameters

Property	type	default	Comment
position	float		Clamp position along the axon’s x-axis
t_start	float		Pulse start time, in ms
duration	float		Pulse duration, in ms
amplitude	float		Pulse amplitude, in nA
ax_list	np.array[int]	None	Array to filter axon for I clamp. If None, I clamp is applied to all axons.

Extracellular context is attached to a fascicle with the attach_extracellular_stimulation method, as for the axon (see above). Stimulus can also be changed with the change_stimulus_from_electrode method. NRV also provides several other methods to manipulate fascicle objects, such as remove_myelinated_axons, remove_axons_size_threshold, rotate_fascicle, translate_fascicle, plot, etc. Full documentation can be found in the API documentation section.

Before running simulation, some flags can be set: save_results, return_parameters_only and save_path:

• If save_results is True, then fascicle simulation results are saved on the hard-drive.

• save_path specifies where to save the results

• return_parameters_only removes results from the returned fascicle_results object.

Setting correctly those flags are particularly useful for large simulations. It relaxes RAM memory usage and facilities heavy post-processing. Additionally, post-processing scripts can be run during fascicle simulation. Post-processing scripts are set in the fascicle simulate method with the PostProc_Filtering parameter. Available post-processing script and how to make a custom one is described in the post-processing section of the documentation.

Warning

We do not recommend attaching extracellular context and running simulation of fascicle directly. Instead, we recommend using nerve object (see below), even for monofascicular nerve.

Nerves

A nerve object in NRV serves two purposes:

• Gathering one or multiple fascicles object

• Ensuring consistency between the FEM model and the neural model

The nerve object is initialized with the following list of parameters:

nerve object initialization parameters list

Property	type	default	Comment
length	float	10_000	nerve length, in µm
diameter	float	100	nerve diameter, in µm
Outer_D	float	5	outer saline box diameter, in mm
ID	int	0	nerve unique identifier

fascicle objects are incorporated to the nerve with the add_fascicle method:

add_fascicle parameters list

Property	type	default	Comment
fascicle	fascicle		fascicle object to add to the nerve
ID	int	None	optional, forces new fascicle unique identifier
y	float	None	optional, forces fascicle new y coordinate
z	float	None	optional, forces fascicle new z coordinate

Warning

Aggregation of fascicle objects in the nerve object uses the Python’s deep-copy mechanism. Any modification of one of the fascicle after adding them to the nerve will also affect the copy added in the latter.

The nerve class includes most of the method available in the fascicle class: set_axons_parameters, plot, insert_I_Clamp, attach_extracellular_stimulation, etc. The simulate method runs the simulation and returns a nerve_results object (see post-processing section).