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What is a membrane potential?
What is in high concentration in the Extracellular fluid, what is in high concentration in the intracellular fluid?

A difference in charge (potential difference) between inside and outside of a cell
ECF: Na, Cl, Ca2+
ICF: K+ and protein high


Measuring the membrane potential
What instrument is used and how?

Two wire electrodes measure the charge at two different places
-Oscilloscope displays the difference in charge between the two electrodes in voltage
-when one electrode is inside the cell and the other is outside, it will display the membrane potential (difference in charge across the cell membrane)


Cell membrane currents:
Ions moving across the cell membrane

The lipid layer is impermeable to charged particles like ions so they must flow through protein channels.
-channels can be selective for a particular ion
-channels can be open all the Time (leak channels) or gated (only opening in response to mechanical, electrical or chemical stimuli)


Electrochemical forces and equilibrium:
Movement of ions (and therefore current flow) in solutions is determined by two forces:

Chemical gradient: ions move from a high concentration to a low concentration
Electrical gradient: opposite charges attract, like charges repel


What is the electrochemical gradient?

It's the overall force on an ion due to combination of chemical and electrical driving forces.
To determine the net movement of a particular ion at a certain membrane potential, we need to know it equilibrium potential.

Equilibrium potential= value of Vm at which electrical gradient is equal in magnitude and opposite in direction to the chemical gradient -> no net movement of the ion


What is the equilibrium potential?

The equilibrium potential = value of Vm at which electrical gradient is equal in magnitude and opposite in direction to the chemical gradient =no net movement of ions


Calculating equilibrium potential (Ex): the Nernst equation
The equilibrium potential is calculated from the Nernst equation which uses an ions

A) intracellular concentration (Ci)
B) extracellular concentration (Co)
C) valence (Z)

Ex= 61log/z x Co/Ci


Resting membrane potential: what is it?
What determines the RMP?

Overall voltage across the cell membrane when the cell is not transmitting an electrical signal
RMP is usually negative inside relative to outside
RMP depends on:
-concentration gradient of all ions across membrane
-differing permeability of cell membrane to those ions (number of open channels)


Dominance of RMP by K+ ions

Concentration gradient for K+ is slightly bigger than for Na+
-at rest the number of open channels for K+ is much higher (25x) than the number of open channels for Na+


Electrochemical gradient acting at resting membrane potential

-large effflux (down chem gradient) leaves net neg charge on the inside of cell because your loosing more positive ions than your gaining.
-this creates an electrical gradient for both Na+ and K+ influx because they are both positively charged ions. So it will generate an inwards electrical gradient for both.
-K+ continues to leave the cell down its chemical gradient, creating an increasingly strong electrical gradient for Na and K to move into the cell
-at a particular value of Vm, K+ efflux (chemical gradient) will be opposite and equal to Na+ influx (chemical and electrical gradient) and K influx (electrical gradient)
-the membrane will be mostly stable at this value of Vm called the RMP!!


No equilibrium: net Na+ and K+ movement at RMP

There is net movement of both of those ions in one direction or the other at the RMP.