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Announcements
• In case you weren’t here last time, Rachel’s email address is not rachel_kaplan@brown.edu but instead rakaplan@brown.edu
• Exam 1 is in 2 weeks!
Introduction
• Membrane Hypothesis on the RMP– Arises from high resting selective
permeability to K+ and a concentration gradient for K+ ions across the membrane. • Changes in K+ should change RMP
Giant Squid Axons
• An experimental tool that helped Hodgkin and Huxley to figure out relationships between ion concentrations and the membrane potential - like testing the Nernst equation– found that predicted Vm did not match their
measured Vm at low concentrations of K+– this was because the axon is not just permeable
to K+ but also other ions!• therefore, the membrane hypothesis isn't wholly
right, but it's a pretty good guess for ye olde science
A Model Cell
• Requires us to make following assumptions– Must be electroneutral (equal - and + charges). – Osmotically balanced. – No net movement of ions.
• There is no ATPase
What determines the equilibrium potential?
• Determined by the two opposing forces:
• Electric gradient – (like charges repel, opposites attract)• Concentration gradient – (too much of one
ion doesn’t like to stay in one place if it has more room to go somewhere else)
So how do equilibrium potentials determine membrane potential?
• If your membrane is only permeable to one ion, you’re in the clear.– Vm = Eion
• But usually, it’s not.– So use the GHK Equation, which takes
into account relative permeabilites of ions and their equilibrium potentials to determine overall Vm.
NB: Make sure to flip Cl! - Z causes reverse ratio in ln(Cl-)
What’s everyone’s job, and what does GHK help us determine?
• Na+ : Brings membrane potential more positive than Eion of K+– But because membrane is not as
permeable to Na as it is to K+, it does not contribute as much as K does.
• K+ : Heavy weight determinant of membrane potential– Permeability of K+ is highest of all ions,
so Vm will be closest to Eion of K+
What’s everyone’s job?
• Cl- : Stabilizes the membrane potential– This is due to shunting effect – complicated,
don’t ask too much about it…
• Na+/K+ Pump: Establishes concentration gradients– Because it exchanges more positive ions out
than in (3Na+ out / 2K+ in), excess of negative charge builds on intracellular side of membrane, and subtracts 5 mV from the RMP. • electrogenic
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