Gated Ion Channels:- Ion Channels Underlie Electrical Signaling in Excitable Cells

The excitability of sensory cells, neurons, and myocytes depends on ion channels, signal transducers that provide a regulated path for the movement of inorganic ions such as Na⁺, K⁺, Ca²⁺, and Cl^- across the plasma mem brane in response to various stimuli. these ion channels are “gated”; they may be open or closed, depending on whether the associated receptor has been activated by the binding of its specific ligand (a neurotransmitter, for example) or by a change in the transmembrane electrical potential, V_m. The Na⁺ K⁺ ATPase creates a charge imbalance across the plasma membrane by carrying 3 Na⁺ out of the cell for every 2 K⁺ carried in (Fig. 12–3a), making the inside negative relative to the outside. The membrane is said to be polarized. By convention, V_m is negative when the inside of the cell is negative relative to the outside. For a typical animal cell, V_m=-60 to -70 mV.

Because ion channels generally allow passage of either anions or cations but not both, ion flux through a channel causes a redistribution of charge on the two sides of the membrane, changing Vm. Influx of a positively charged ion such as Na⁺, or efflux of a negatively charged ion such as Cl^-, depolarizes the membrane and brings V_m closer to zero. Conversely, efflux of K hyperpolarizes the membrane and V_m becomes more negative. These ion fluxes through channels are passive, in contrast to active transport by the Na⁺ K⁺ ATPase. The direction of spontaneous ion flow across a polarized membrane is dictated by the electrochemical

FIGURE 12–3 Transmembrane electrical potential. (a)The electro genic Na⁺ K⁺ ATPase produces a transmembrane electrical potential of -60 mV (inside negative). (b)Blue arrows show the direction in which ions tend to move spontaneously across the plasma membrane in an animal cell, driven by the combination of chemical and electrical gradients. The chemical gradient drives Na⁺ and Ca²⁺ inward (producing depolarization) and K+ outward (producing hyperpolarization). The electrical gradient drives Cl^- outward, against its con centration gradient (producing depolarization).

potential of that ion across the membrane. The force (ΔG) that causes a cation (say, Na⁺) to pass spontaneously inward through an ion channel is a function of the ratio of its concentrations on the two sides of the membrane (Cin/Cout) and of the difference in electrical potential (Δ

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