Presence of a Chaotic Region Between Subthreshold Oscillations and Rhythmic Bursting in a Simulation Of Thalamocortical Relay and Reticular Neurons

Abstract

The complex interplay of ionic conductances within the thalamo-reticular network give rise to multiple modes of activity. They include the spindle oscillations in the 7–14 Hz range observed during drowsiness and the slow 0.5–4 Hz oscillations that is observed during deep sleep. This activity is a function of both the circuitry as well as the intrinsic properties of the neurons. The functional connectivity of the thalamic cells (TC) and reticular neurons (RE) are of particular importance in the synchrony and other network properties. The thalamus and the cortex have reciprocal excitatory connections and also send excitatory collaterals to the RE. The RE have inhibitory projections to TC1’2. The TC cells show a post-inhibitory rebound bursting property that is mediated by a low-threshold Ca²⁺ current (I_T) ³. The voltage range of activation and inactivation of the low – threshold Ca²¹ current (I_T) is between -80 and -50 mV. A hyperpolarizing current results in the deinactivation of the low-threshold Ca²⁺ conductance and a rebound burst is produced at the end of the hyperpolarizing pulse due to the activation of the lowthreshold Ca²⁺ conductance that depolarizes the membrane potential to the threshold for the fast Na⁺ spikes. The interplay of I_T and a mixed Na⁺/K⁺ current I_H which is activated by hyperpolarization in a subthreshold range has been shown to be essential for the intrinsic oscillations of TC neurons². A similar low-threshold Ca²⁺ current (I_TS) is also observed in RE cells with slower kinetics. The RE cells also show intrinsic rhythmic firing properties which are mediated by a set of three currents, I_TS, a Ca²⁺ activated K⁺ current I_K[Ca2+] and a non-specific cation current I_CAN ²