Ionotropic cell surface receptors are channels that open when a specific chemical binds to the extracellular face of the channel protein. The nicotinic acetylcholine receptor (Fig. 17.1), so named because the drug nicotine binds to it, is one example. In the absence of a transmitter called acetylcholine, the channel is closed. When acetylcholine binds, the channel opens and allows sodium and potassium ions to pass through. The electrochemical gradient pushing sodium ions into the cell is much greater than that pushing potassium ions out of the cell, so when the channel opens, sodium ions pour in carrying positive charge and depolarizing the plasma membrane. This mechanism is similar to that of two channels we met in Chapter 16, the inositol trisphosphate-gated calcium channel and the cAMP-gated channel. There is, however, a major difference: Those two channels were opened by cytosolic solutes, while ionotropic cell surface receptors are opened by extracellular solutes.
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