Voltage-gated channels
Summary
Voltage-gated channels are ion-selective membrane proteins that open or close in response to changes in membrane potential, letting ions move down their electrochemical gradients. In a neuron, Na⁺ channels open at threshold and drive the upstroke; K⁺ channels open later and drive repolarization.
Detail
Meaning: Charged voltage-sensing regions in these channels shift when membrane potential changes, opening or closing the pore. Each type is selective for a particular ion, and flow is passive, down the electrochemical gradient. In an axon, depolarization to threshold opens Na⁺ channels; Na⁺ entry depolarizes further, opening more (positive feedback). Na⁺ channels then inactivate, and slower K⁺ channels open, so K⁺ leaves and the membrane repolarizes. Ca²⁺ channels at the presynaptic terminal trigger vesicle fusion.
Equation: I = g(Vm − Eion), where I is ion current (A), g is channel conductance (S), Vm is membrane potential (V), and Eion is the ion's Nernst potential (V).
On the MCAT: A passage shows a membrane-potential vs. time graph, a toxin or a mutation. Blocking Na⁺ channels (tetrodotoxin) prevents the action potential; blocking K⁺ channels slows repolarization.
Common confusion: Students credit the Na⁺/K⁺ ATPase for the upstroke, but that is passive Na⁺ flow through voltage-gated channels; the pump restores gradients slowly. An inactivated Na⁺ channel is also not the same as a closed one: a closed channel can open on depolarization, but an inactivated channel cannot open until the membrane repolarizes and resets it. This underlies the absolute refractory period.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.