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RUNNING THE NEURON TUTORIAL SIMULATION
(Don't forget to move the cursor into this window and use the mouse
buttons to scroll through this text.)
DESCRIPTION OF THE MODEL
------------------------
The help menu selection "Neuron Inputs" shows the configuration of the model
neuron, which consists of three compartments. The "Soma" represents the
axon hillock of the soma where action potentials are generated. This
compartment contains two ionic channels for sodium (Na) and potassium (K)
with voltage dependent conductances obeying Hodgkin-Huxley kinetics. The
two dendritic compartments, "Dendrite #1" (closest to the soma), and
"Dendrite #2" each have a chemically activated excitatory channel and
inhibitory channel. The parameters which determine the equilibrium
potentials of the channels and the time dependence of the channel
conductances may be modified by a sub-menu of the Cell Parameters menu,
which is described in the "Cell Parameters" selection of the help menu.
The dendrite channel parameters are currently set so that the excitatory
channel is a non-selective sodium/potassium channel and the inhibitory
channel is a potassium channel.
In addition, it is possible to add any number of passive "cable"
compartments between the two dendrite sections in order to explore the
effects of stimulating the neuron at spatially separated locations.
The "Neuron Inputs" diagram indicates that a step pulse of current may
be injected to the soma and/or the two dendrite compartments. It also shows
the two different sources of spike trains which may be applied to the
synaptic inputs of the dendrite compartments. (It may be helpful to call up
this diagram from the help menu now. You can make it disappear by clicking
on the "DONE" box at the top of the diagram.)
THE CONTROL BUTTONS
-------------------
The control panel at the lower left of the screen contains buttons which
control the starting and stopping of the simulation. If you are eager to
get going, click the left mouse button on the box labeled "STEP" to start
the simulation while you are reading the help messages.
The various buttons are:
HELP - brings up the help menu.
RESET - resets the simulation to time step 0 and clears the graphs (unless
in overlay mode).
STOP - temporarily stops the simulation. You may then change parameters
and click on "STEP" to continue, or "RESET" and "STEP" to start
over.
QUIT - exits the simulation.
Inputs - calls up a menu which allows you to inspect or change the initial
delay, duration (width) and spike interval for the two sources of
spike trains which are applied to the dendrite inputs. It also
governs the delay and width of the injection current pulse.
Defaults - This restarts the simulation with the default values of the
various parameters. Use this if you have changed a lot of things
and don't remember what the initial values were.
Overlay OFF/ON - toggles overlay mode between "OFF" and "ON". When overlay
is ON, RESET followed by STEP causes the next plot to be plotted
over the previous plot.
Plot Soma/Plot Dend2 - acts as a toggle to determine whether the set of
graphs on the right will be for the soma or for dendrite
compartment #2.
STEP - in addition to initiating the simulation, this also functions as a
"dialog box" for setting the number of simulation steps which are
performed each time. To change the numeric field of a dialog box,
use the mouse to move the cursor into the dialog box. Then, use
the keyboard right and left arrow keys to move the "^" symbol to
the right of the place where you wish to make the changes. Then
use the "Delete" key to back up over anything you wish to change,
and type in the changes. To cause the changes to be entered, you
must then hit "Return" while the cursor is in the numeric field.
Cable Compts. - This is a dialog box which allows you to specify the
number of passive cable compartments between the two dendrite
sections. The cable sections have the same size, resistances
and capacitance as the default values for the dendrites.
dt - is a dialog box used for setting the simulation time step. The default
value of 0.01 milliseconds was chosen for accuracy in reproducing
the action potentials. You can speed up the operation of the simulation
by chosing a larger value if you just want to get a rough idea of the
effect of changing various neuron parameters. When you are ready for
serious work, change it back to the original value.
INJECTION CURRENT
-----------------
This set of three dialog boxes may be used to set the magnitude of the
injection current pulses. The default value of the soma injection current
(0.0002 microamperes, or 0.2 nA) is a good starting point to trigger action
potentials in the soma. You may wish to experiment in order to find which
value produces just enough depolarization to cause firing. The "INPUTS"
control button may be used to bring up the menu which sets the duration and
delay of the current pulse. Don't forget to hit "Return" after modifying
the contents of a dialog box.
DENDRITE INPUTS
---------------
These four lines allow you to set the source and the synaptic weights of the
spikes applied to the excitatory and inhibitory inputs of each of the two
dendrite compartments. The boxes at the right are toggles which switch the
inputs between spike train sources A and B. (The "INPUTS" control button
may be used to change the timing parameters for the two sources.) The
dialog box fields toward the center are used to set the weights of the
connections. The default values of zero indicate that the inputs are
initially disconnected. The parameters which govern the channel conductances
were chosen to model the behavior for a single synaptic input. To produce a
post-synaptic potential which is sufficiently depolarizing to produce action
potentials in the soma, a patch of dendrite may require on the order of one
hundred or more excitatory synaptic inputs. This may be approximated by
setting the connection weight to a number on the order of 100.
CELL PARAMETERS
---------------
This group of boxes at the right calls up menus which allow you to vary
compartment parameters such as length, diameter, membrane capacitance,
membrane resistance and axial resistance. The resistances and
capacitances may be specified either in absolute terms (Kohms and
microfarads) or in terms of the specific resistances and capacitances.
Setting the axial resistance of a dendrite compartment to a very large
value allows it to be effectively decoupled from the soma, allowing PSPs
to be examined without interence from action potentials generated in the
soma. Submenus for the two types of channels allow you to set equilibrium
potentials and other channel parameters. For example, you may wish to
change the inhibitory channel from a potassium channel (the default) to a
chloride channel. These parameters are described in more detail under the
"Cell Parameters" help menu selection.
GRAPHS
------
The graph at the upper left shows the time course of the injection current
pulse and the two spike trains from sources A and B. Each plot is shown
with unit amplitude and is offset vertically from the previous one by 2.0.
If you have a color display, you can tell one plot from the other by
relating its color to the color of the label at the right. Otherwise, be
sure to note that the first label (0 Inject) is for the lowest plot, and
that the next plot (1 SourceA) is plotted ABOVE the previous one. Another
thing to note is that, as these input plots are scaled to unit amplitude, a
negative (hyperpolarizing) injection current will still be plotted as a
positive pulse. These inputs are plotted whether or not they are actually
connected to the cell.
The graph at the middle left plots the conductances for the excitatory and
inhibitory channels of dendrite section #1. The units are in milli-Siemens,
where a Siemen is an inverse Ohm. All time scales are in milliseconds.
The bottom left graph plots the membrane potential in millivolts for
dendrite section #1.
The corresponding quantities are plotted at the right for either dendrite
section #2 or the soma, depending on the state of the Plot Dend2/Plot Soma
toggle button.
In addition, the Hodgkin-Huxley activation parameters m, h and n are
plotted for the soma in the upper right graph. As with the graph for the
cell inputs, each successive plot is offset upwards by 2.0. To interpret
the activation parameter plots, you should recall that the Hodgkin-Huxley
model for the voltage activated Na channel gives a conductance which is
proportional to m cubed and to h to the first power. Also note that the
"inactivation parameter", h, is the probability that the channel is NOT
closed. Thus the channel has maximum conductance when both m and h are
near unity. The K channel has no inactivation parameter, and its
conductance is proportional to n to the fourth power.
The scales of the graphs can be changed by clicking on the 'scale' button
associated with the graph, changing the values in the appropriate dialogs
and pressing the 'DONE' button. For example, you may wish to increase the
time scale (xmax) if you click on "STEP" more than once without resetting,
or you may wish to view a more limited range of data in order to increase
the resolution. Note that the existing data will be replotted whenever the
scale is changed.
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