L2 Flashcards
(85 cards)
Why is the resting membrane potential -70 mV instead of -90 mV?
- The membrane is most permeable to K+ at rest
- But Na+ and Cl- ions are also diffusing somewhat, counteracting the K+ effect and making the potential less negative than K+’s equilibrium potential
How much more permeable is the membrane to K+ compared to Na+ at rest?
The membrane is 35-40 times more permeable to K+ than to Na+ at rest.
What counteracts the outward movement of K+ at the resting membrane potential?
- The influx of Na+ ions counteracts the outward movement of K+, making the membrane potential less negative than -90 mV
What does the Goldman equation calculate?
The actual membrane potential (Em) by accounting for the contributions of multiple ion species (K+, Na+, Cl-) and their relative permeabilities
Why does the resting membrane potential sit closer to the equilibrium potential of K+ than Na+?
Because at rest, potassium is the most permeable ion, so the membrane potential sits closer to K+’s equilibrium potential (-90 mV) than to Na+’s equilibrium potential
What happens to the membrane potential if Na+ permeability becomes dominant?
If Na+ permeability becomes dominant, the membrane potential can change drastically, moving toward the Na+ equilibrium potential
What is the direction of Na+ current when membrane becomes highly permeable to Na+?
When the membrane becomes highly permeable to Na+, there is a net Na+ current inward (into the cell)
Why would opening Na+ channels make the inside of the cell more positive?
Opening Na+ channels allows Na+ ions to flow into the cell along their concentration gradient (more outside than inside), bringing positive charges into the cell and making the inside more positive
What is the equilibrium potential for Na+?
+60 mV
- Meaning the membrane potential is positive inside with respect to the outside
Why does sodium diffuse into the cell?
- It diffuses into the cell along its concentration gradient because there is more sodium on the outside than inside of the cell
What happens to the inside of the cell when sodium channels open?
- When sodium channels open, the inside of the cell becomes more positive as Na+ ions flow inward, carrying positive charges
What limits the influx of Na+ into the cell?
The influx of Na+ only occurs until there is a build-up of positive charge on the inside that creates an electrical force that repels further Na+ entry, reaching electrochemical equilibrium
Why doesn’t the resting membrane potential reach +60 mV despite Na+ influx?
The resting membrane potential is closer to K+’s equilibrium potential (-90 mV) because the membrane is more permeable to K+ than to Na+ at rest
What would happen if “sodium had its way” in determining membrane potential?
If sodium had its way (if permeability was good and all channels were open), the membrane potential would be sitting at +60 mV instead of -70 mV
Why are Cl- ions more concentrated in the extracellular space?
Cl- ions are pushed out of the cell by the negatively charged proteins trapped inside the cell, as like charges repel each other
How do large intracellular proteins affect Cl- distribution?
Large intracellular proteins have negative charges that repel Cl- ions (which are also negatively charged anions), pushing them out of the cell
What creates the concentration gradient for Cl- ions?
The concentration gradient for Cl- ions is created by electrical repulsion from negatively charged intracellular proteins, not by active pumps
How do Cl- ions differ from Na+ and K+ in terms of how their concentration gradients are established?
Unlike Na+ and K+ gradients which are established by the Na+/K+ ATPase pump, the Cl- gradient is established purely by electrical forces (repulsion from negative proteins) without requiring active transport
What is the role of Na+ channels in neurons?
They generate signals by increasing membrane conductance to Na+ ions.
What type of channel is the Na+ channel?
A voltage-gated channel
What is the state of Na+ channels at resting membrane potential?
Closed/shut
What membrane potential value are Na+ channels closed at?
-70 mV
What process is needed to open Na+ channels?
Depolarization (less negative) of the membrane
What happens to the S4 segment when the membrane depolarizes?
It moves upward, allowing Na+ passage from outside to inside
