Lectures 4 and 5

Rachel A. Kaplan and Elbert Heng

2.11.14

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!

LECTURE 4: RESTING MEMBRANE POTENTIAL

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)

Nernst Equation

• This is exactly what the Nernst equation is considering.

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

Membranes as Circuits

Membranes as Circuits

• Membranes can be modeled as an electrical circuits– Resistors: ion channels– Battery: membrane potential– Capacitors: membrane