Assessment of Network States: Local Hemodynamics

Neural activity utilizes energy resources and requires replenishment of metabolites through vascular dilation. During wake, cortical neurons usually have depolarized membrane potentials and exhibit frequent spontaneous action potentials, requiring an increased metabolic delivery to activated tissue and causing blood vessels to dilate. Quiet sleep (QS) is characterized by alternating membrane potential between a depolarized and hyperpolarized state. The hyperpolarized state has a lower membrane potential and exhibits few action potentials, which may be less metabolically demanding. In order to investigate the relationship between evoked neural and metabolic responses across wake and sleep states, we combined electrical and optical imaging techniques. We implanted rats with screw electrodes to measure evoked response potentials (ERPs), and used a light emitting diode (LED) and photodiode to measure evoked changes in local hemodynamics based on hemoglobin absorption properties. During QS, hemodynamic changes were larger in amplitude compared to wake and rapid eye movement (REM) sleep. In this review, we explore the potential mechanisms for the larger hemodynamic changes. Wake periods may correspond to decreased vessel compliance as they expand to supply tissue with metabolites while sleep periods may decrease metabolic demand and allow vessels to relax and restore compliance.