In NEURON, artificial cells are point processes that serve as both a target
and source object for a network connection object. That they are targets mean
that they have a NET_RECEIVE block which handles discrete event input streams
through one or more NetCon objects.
That they are also sources means that the NET_RECEIVE block also generates
discrete output events which are delivered through one or more NetCon objects.
Generally, such cells are computationally very efficient (hundreds of times
faster than cells we have simulated up to now whose voltage response is a
consequence of membrane conductance) because their computation time does
not depend on the integration step, dt, but only on the number of events.
That is, handling 100000 spikes in one hour for 100 cells takes the same time as
handling 100000 spikes in 1 second for 1 cell. The total computation time
is proportional to the total number of spikes delivered
during a run and is independent
of the number of cells or number of connections or interval between spikes.
NEURON has four built-in point process classes which can be used to
construct artificial cell types:
-
NetStim produces a user-specified train of one or more
output events, and can also be triggered by input events
- IntFire1, which acts like a leaky integrator driven by delta function
inputs.
That is, the state variable m decays exponentially toward 0.
Arrival of an event with weight w causes an
abrupt change in m.
If m exceeds 1, an output event is generated and the cell
enters a refractory period during which it ignores further
inputs.
At the end of the refractory period, m is reset to 0 and the
cell becomes responsive to new inputs.
- IntFire2, a leaky integrator with time constant taum
driven by a total current that
is the sum of
      { a user-settable constant "bias" current }
plus
      { a net synaptic current }.
Net synaptic current decays toward 0 with time constant taus,
where taus > taum (synaptic current decays slowly compared
to the rate at which "membrane potential" m equilibrates).
When an input event with weight w arrives,
the net synaptic current changes abruptly by the amount w.
- IntFire4, with fast excitation current (rises abruptly,
decays exponentially)
and
slower alpha function like inhibition current
that is integrated by even slower membrane.
NEURON requires that all point processes be located in a section. To
meet this (in this context, conceptually irrelevant) requirement,
the Network Builder tool locates each point process of
its instantiated artificial cells in the dummy section called
acell_home_
NEURON hands-on course
Copyright © 2000-2010 by N.T. Carnevale and M.L. Hines,
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