Neuroscience 6e 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. 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 and other electrical signals. A large number of ion channel genes creates channels with a correspondingly wide range of functional characteristics, thus allowing different types of neurons to have a remarkable spectrum of electrical properties. Voltage-gated ion channels are responsible for the voltage-dependent conductances that underlie the action potential. These channels 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. 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 sensitivity to neurotoxins. Other types of channels have specialized structures that enable detection of chemical signals, such as neurotransmitters or second messengers, or other types of signals such as heat or mechanical distortion of the plasma membrane. Active transporter proteins are quite different from ion channels because they move ions against a concentration gradient. The energy required for ion translocation is provided either by the hydrolysis of ATP or by the electrochemical gradient of other ions, such as Na+. The Na+ pump produces and maintains the transmembrane gradients of Na+ and K+ by ATP-dependent phosphorylation of the pump, which causes structural changes that enable ion movement. Other transporters, both ATPase pumps and co-transporters, are responsible for the electrochemical gradients for other physiologically important ions, including Cl–, Ca2+, and H+. Together, transporters and channels provide a comprehensive and satisfying molecular explanation for the ability of neurons to generate electrical signals.