Contributed by Lauren Jones: ljones@wesleyan.edu
This is a model of slow oscillatory bursting in thalamic relay cells in
GENESIS, based on one by McCormick et. al. (1991). This is a single
compartment lumped-soma model which includes four active membrane
conductances: a fast Na+ current, a delayed rectifier K+ current, a
low-threshold Ca+2 current (T-current), and a hyperpolarization-activated
K+ current (H-current). The interaction of these last 3 conductances
produces oscillatory bursting at 2-5 Hz when the resting membrane potential
of the cell is more negative than -65 mV. The resting membrane potential is
regulated by a passive K+ leak conductance. I obtained the kinetics for the
T-current from Lytton & Sejnowski (1991) rather than using those of
McCormick, because I feel that their inactivation time constant is more
accurate. The GENESIS model is called THALMODES and exists in a directory of
the same name. It illustrates the two distinct firing modes of
thalamocortical relay cells: the single-spike relay mode at depolarized
resting potentials, and the oscillatory bursting mode at hyperpolarized
potentials. The switching between these two modes is dependent on the
magnitude of the K+ leak conductance. This K+ leak conductance is in turn
regulated by various neurotransmitters, but for the purposes of this model,
the leak conductance is a parameter that can be set by hand.
If the simulation is run at the default values, the cell will remain at the
baseline resting membrane potential of -50 mV. But if the resting membrane
potential is depolarized by reducing the leak conductance, then repetitive
spiking will result. This same effect can be achieved by depolarizing
current injections (e.g. 0.5 nA). However, if the resting membrane potential
is made more negative by increasing the leak conductance to 31 nS, then the
neuron will burst at an oscillatory frequency of about 3 Hz. This frequency
can be adjusted by varying the size of the H-current: larger values will
give higher frequencies, while smaller values will generate lower
frequencies.