Lecture 20

Question 1

What is a membrane potential?

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

Answer 1

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

Question 2

Measuring the membrane potential

What instrument is used and how?

Answer 2

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)

Question 3

Cell membrane currents:

Ions moving across the cell membrane

Answer 3

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)

Question 4

Electrochemical forces and equilibrium:

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

Answer 4

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

Question 5

What is the electrochemical gradient?

Answer 5

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

Question 6

What is the equilibrium potential?

Answer 6

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

Question 7

Calculating equilibrium potential (Ex): the Nernst equation

The equilibrium potential is calculated from the Nernst equation which uses an ions

Answer 7

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

Ex= 61log/z x Co/Ci

Question 8

Resting membrane potential: what is it?

What determines the RMP?

Answer 8

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)

Question 9

Dominance of RMP by K+ ions

Answer 9

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+

Question 10

Electrochemical gradient acting at resting membrane potential

Answer 10

• 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!!

Question 11

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

Answer 11

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