View source for Up and down states

===Ionic mechanism of the down state in striatal spiny neurons===

The stability of the Down state is apparent from examination of the current-voltage (I-V) curve for the spiny neuron.  At membrane potentials associated with the Down state, the I-V curve slope (the input conductance) increases with hyperpolarization. This is called inward rectification, or anomalous rectification (anomalous because it isn’t predicted by the Goldman-Hodgkin-Katz  equation).  In the Down state, the input resistance of the spiny neuron  is low (10-30 MOhms), making the cell relatively insensitive to small synaptic inputs.  The origin of inward rectification in the spiny neuron is a hyperpolarization-activated potassium channel, KIR2 (Nisenbaum and Wilson, 1995).  The KIR2 channel is regenerative in the sense that it is a hyperpolarizing channel that is activated by hyperpolarization.  In the Down state, when synaptic input is low, the conductance contributed by this channel is much larger than any other influence on the spiny neuron.  The spiny cell’s membrane potential moves close to the potassium reversal potential, and the cell enters a high-conductance state, in which it is relatively insensitive to synaptic input.  Because the membrane potential in the Down state is more hyperpolarized than the chloride reversal potential, GABA<sub>A</sub>-mediated inhibitory synapses from striatal interneurons and from recurrent collaterals of other spiny neurons are all depolarizing in nature (Plenz, 2003). 

[[Image:cjwUpDownStatesFig2.jpg|thumb|400px|right|F2|Measurement of input resistance and rectification with a small depolarizing or hyperpolarizing pulse in the Up and Down states]]