Mechanistic aspects of the generation of subthreshold oscillations, the onset of spikes and related phenomena in a medial entorhinal cortex stellate cell model
The study of rhythmic activity in various areas of the brain has been the object of many experimental and theoretical investigations. This talk concerns the generation of rhythmic mixed-mode oscillations at theta frequencies (4-12 Hz) in a biophysical model of medial entorhinal cortex stellate cells (SCs). Mixed-mode oscillatory temporal patterns consist of a combination of subthreshold oscillations and spikes. The SC model consist of a multiscale, high-dimensional system of nonlinear ordinary differential equations describing the evolution of voltage and other biophysical variables.
During the interspike interval, where subthreshold oscillations are generated, the SC model can be reduced to a three-dimensional fast-slow system. Voltage is the fast variable and the two slow variables are the two components (fast and slow) of the hyperpolarization-activated (h-) current. Using dynamical systems arguments we provide a mechanism for the generation of subthreshold oscillations and the onset of spikes. This mechanism is based on the three-dimensional canard phenomenon. We show that, in this biophysical SC model, the subthreshold oscillatory phenomenon is intrinsically nonlinear and three dimensional, involving the participation of both components of the h-current. We discuss some consequences of this mechanism for the interaction between SCs and other cell and external inputs. If time permits, we will show some new results related to epileptic activity of stellate cells.