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Chapter 4 Summary

Ion channels and active transporters have complementary functions. The primary purpose of transporters is to generate transmembrane concentration gradients, which are then exploited by ion channels to generate electrical signals. Ion channels are responsible for the voltagedependent conductances of nerve cell membranes. The channels underlying the action potential are integral membrane proteins that open or close ion-selective pores in response to the membrane potential, allowing specific ions to diffuse across the membrane. The flow of ions through single open channels can be detected as tiny electrical currents, and the synchronous opening of many channels generates the macroscopic currents that produce action potentials. Molecular studies show that such voltage-gated channels have highly conserved structures that are responsible for features such as ion permeation and voltage sensing, as well as the features that specify ion selectivity and toxin sensitivity. Other types of channels are sensitive to chemical signals, such as neurotransmitters or second messengers, or to heat or membrane deformation. A large number of ion channel genes create channels with a correspondingly wide range of functional characteristics, thus allowing different types of neurons to have a remarkable spectrum of electrical properties. Active transporter proteins are quite different in both structure and function. The energy needed to move ions against a concentration gradient (e.g., in maintaining the resting potential) is provided either by the hydrolysis of ATP or by the electrochemical gradient of co-transported ions. The Na+/K+ pump produces and maintains the transmembrane gradients of Na+ and K+, while other transporters are responsible for the electrochemical gradients for other physiologically important ions, including Cl, Ca2+, and H+. Together, transporters and channels provide a reasonably comprehensive molecular explanation for the ability of neurons to generate electrical signals.