Up and down states

Abstract

Operationally, up and down states refer to the observation that neurons have two preferred subthreshold membrane potentials, both subthreshold for action potential generation. The most common method of detecting these states is the so-called all-points histogram (Cowan and Wilson, 1994; Wilson and Kawaguchi, 1996; Pare et al., 1998). This method, borrowed from the study of single channel currents, plots the frequency of occurrence of various values of membrane potential for every point in a digitized record. Figure 1 shows a typical example for a striatal spiny neuron, and for a cortical pyramidal cell in layer V, recorded simultaneously. The histogram to the left of each one shows the amount of time the cell spends at each value of membrane potential. Both cells toggle between two preferred membrane potentials, one very hyperpolarized (Down state), and one more depolarized (Up state). In both cells, the Up state is only a few millivolts from the action potential threshold. Usually, membrane potential fluctuations around the Up state are of higher amplitude, whereas the Down state is relatively free of noise. Figure 1 Up and down states. Up and Down states have been most often studied in animals anesthetized with urethane or other anesthetics that induce slow coherent oscillations in the cortex similar to those seen during sleep or anesthesia (e.g. Steriade et al., 1993; Steriade et al., 2001; Mahon et al., 2006; Destexhe et al. 2007). Because of this, Up and Down states are sometimes used as a synonym for slow oscillations. That usage is avoided here, and, Up and Down states will refer only to the set of cellular and network properties that causes neurons to respond to synaptic input in a two-state manner. These cellular properties arise from ionic conductances that are always present in the cell, and can continue to influence cellular activity in other circumstances not associated with slow oscillations. For example, much of the work on Up and Down states has been obtained in tissue slices, which are neither anethetized nor asleep, and often not showing any slow oscillations. Neurons may exhibit two-state behavior because of their intrinsic properties, or because they are in a network that imposes it on them, or both, and may be expressed as a part of a variety of activity states.