Flashcards in Fundamentals of Neuronal Function/Ion Channels Deck (7)
What is plasma membrane potential, Vm?`
The resting membrane potential (Vm), is due to the separation of electrical charges across the cell membrane (inside -70 mV, outside 0mV).
How is Vm maintained?
By the Na+/K+ ATPase 'sodium pump'; actively transporting 2 K+ into the cell for every 3 Na+ pumped out.
What is the Na+/K+-ATPase cycle? (6 steps)
1.) Cytoplasmic (intracellular) Na+ binds to ATPase stimulating phosphorylation by ATP
2.) Phosphorylation causes conformational change of the ATPase protein
3.) Conformational change expels 3 x Na+ outside, whilst extracellular K+ (2 x) bind
4.) K+ binding triggers release of a phosphate group into the cytoplasm
5.) Loss of phosphate group restores OG ATPase conformation
6.) K+ is released (into the cell) and Na+ sites are receptive again.
How is electrical neutrality achieved inside and outside the cell, and what does this mean?
Outside: Na+ balanced by Cl- (1:1)
Inside: K+ balanced by A- (negatively charged proteins, where other anions e.g. PO4- and Cl- are also present too )
- Equal positive and negative charges.
- 70mV potential difference across but sides are 1:1
Describe the passive movement of K+ across the membrane and what it results in.
- Some K+ goes out of the cell through leaky channels (selective for K+) down its concentration gradient
- But the negative charge inside the cell (-70 mV) attracts it back in; an equilibrium is established where electrical potential balances chemical potential (the electrochemical equilibrium).
What is the equilibrium potential?
- The voltage at which the electrical force experienced by an ion is equal and opposite to the chemical force produced by the concentration gradient; any chemical gradient can be balanced by an electrical potential.
- Also called the Nernst potential (Ek), described by the Nernst equation.