W2 L1 (Membrane potential) Flashcards
Membrane potential
The potential difference (voltage) is the distribution of positive and negative charges (ions) across the membrane with the inside being slightly negative. Also described as the separation of charges across a membrane.
Millivolt (mV)
The units for measuring membrane potential. One millivolt = 1/1000 of a volt
Ohm’s Law
V=IR
V=Voltage across the membrane
I=The flow of ions across the membrane
R=Resistance by the membrane
Capacitor
A device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator.
Explain how the membrane functions as a capacitor
Outside and inside the cell are conductors (the salt water) and in the middle is an insulator (the membrane). This separation of conductors by an insulator stores electric charge.
Resting membrane potential
The state of an excitable membrane when there is not displaying an electric signal
What leads to membrane potential?
The uneven distribution of ions is primarily responsible for the generation of the resting membrane potential. These ions are sodium, potassium, and anions; large negatively charged intercellular proteins
Where are the concentrations of K and Na higher?
What maintains this gradient?
K is higher inside the cell, while Na is higher outside the cell. This gradient is maintained by the sodium-potassium pump.
What is the range of normal resting potentials of most cells?
-5mV to -90mV
What is the range of normal resting potentials for neurons?
-50mV to -75mV (-60mV is what we assume)
Hyperpolarization
A decrease in Vm (ex. -60mV to -75mV)
Depolarization
An increase in Vm (ex. -60mV to -50mV)
What does Vm stand for?
Membrane voltage
How do we record voltage
- Place a cell with a prong and another prong in a liquid-filled beaker
- Run the prongs through an amplifier and computer
- The amplifier will measure the difference between outside the cell and inside the cell
Explain the formula for calculating Vm
Vm=(Q/C)
Vm=Membrane voltage
Q=Ionic charge
C=Membrane capitance
What do selective channels do?
Separate specific ions
Typical inside and outside concentrations (in mM) and permeability of K+
In: 125 Out: 5 Permeability: High
Typical inside and outside concentrations (in mM) and permeability of Na+
In: 12 Out: 120 Permeability: Low
Typical inside and outside concentrations (in mM) and permeability of Cl-
In: 10 Out: 125 Permeability: Very Low
Typical inside and outside concentrations (in mM) and permeability of Ca 2+
In: 0.0002 Out: 2 Permeability: Ultra Low
Typical inside and outside concentrations (in mM) and permeability of Large Anions (A-)
In: 130 Out: 0ish Permeability: Non existent (0)
What determines resting permeability?
The number of open passive channels
What are A-?
Large anions, amino acids, proteins, RNA, SO4, and PO4.
How many times more permeable is K than Na?
50-75X
Equilibrium potential
The Vm where movement of an ion down its concentration gradient is equal to the force of an opposing electric potential force
Explain the basis for membrane resting potential
When the ions (charge) are separated across the membrane (capacitor) it creates the membrane potential
(voltage). This refers to the charge separation that occurs because K can flow out at rest, while the large
anions cannot. The natural loss of K due to diffusion to the ECF pulls some of the intracellular anions to the inside of the membrane, which
then attracts the K that has left (and other positive charges in the ECF) to the outside of the membrane.
The charge is now separated AT THE MEMBRANE and a membrane potential or voltage is created that
reflects the equilibrium potential for K (-90).
The small amount of Na entry does the same thing but in
reverse, and this pulls the membrane potential towards the equilibrium potential for Na (+60).
The net result is a membrane potential of somewhere between -50 and -75 (closer to Vk (-90) than Vna (+60)
because K is more permeable at rest than Na).
What is an electrochemical cell?
A device that converts chemical energy into electrical energy or vice versa when a chemical reaction is
occurring in the cell. It typically consists of two metal electrodes immersed into an aqueous solution
(electrolyte) with electrode reactions occurring at the electrode-solution surfaces. As an electrical
current passes, it must change from electrical current to ionic current and back to electrical current.
These changes of conduction mode are always accompanied by oxidation/reduction reactions and the
electrodes (loss or gain of electrons)