The interference of the infra slow oscillations controls the size of the pupil of the urethane anaesthetised rat
Infra slow oscillations (ISO) are low frequency (<0.1Hz) fluctuations detected at the various levels of the brain organisation. In the urethane model of sleep at least a few ISOs can be detected in the rat brain: (1) urethane sleep structure – cyclic alternation of brain state between activated and deactivated EEG patterns; (2) rhythmic, neuronal bursting in the the subcortical visual system – e.g., olivary pretectal nuclei (OPN). Our preliminary observation has revealed that, under constant illumination, the pupil size of the anaesthetised rat oscillates with the period in the range of ISO. The present study was aimed to: (1) determine the relationship between the changes in the pupil size and the ISOs observed in the brain; (2) elucidate the neuronal mechanism of observed pupil size changes. The following signals were simultaneously recorded from urethane anaesthetised rats: multisite ECoG, neuronal firing in left and right OPN, video of the left and right eye. Results revealed that changes of the size of the pupils are synchronised with each other and characterized by two dominant ISO frequencies (~0.01Hz and <0.001Hz). A simple mathematical model of iris smooth muscle constriction and relaxation was proposed to verify the hypothesis that observed changes of the size of the pupils are result of the interference of the three ISOs. The model is governed by the linear first order Ordinary Differential Equation that expresses the muscle relaxation and integration of three ISOs. The parameters of the model were fit to the measured area changes using nonlinear least squares algorithm. It is shown that the area changes predicted from the model correlate well with the observed values. Moreover the analysis revealed that taking all three ISOs into account produces significantly better fit of the model to observed data than any one or two signals, which means that observed pupillary oscillations are result of the interference of all three ISOs recorded in the brain.
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