SESSION 6LT 1 THE RESTING MEMBRANE POTENTIAL

Question 1

WHAT IS RESTING MEMBRANE POTENTIAL?

Answer 1

RESTING MEMBRANE POTENTIAL =
MAGNITUDE OF AN ELECTRICAL CHARGE WHICH EXISTS ACROSS A PLASMA MEMBRANE
- Always expressed as the potential INSIDE the cell relative to the extracellular solution.

Question 2

WHAT ARE MEAMBRANE POTENTIALS MEASURED IN?

Answer 2

• Milli volts ALWAYS

Question 3

What type of membrane potential do all animals cells have at rest?

Answer 3

ALL ANIMAL CELLS have a NEGATIVE resting membrane potential at REST

• Cardiac and SKM cells have largest resting potentials -80 to -95 mV
• Neurones- resting potential range -50-75mv
• Erythrocytes smallest resting potentials -9mv

Question 4

What is the purpose of membrane potentials ?

Answer 4

Membrane potentials provide the basis for signalling in all cell types.

• All cells have an electrical potential ( voltage) difference across their plasma membranes
• endocrine cells variable membrane potentials at ret some close to erythrocytes some close to neurones

Question 5

HOW ARE RESTING MEMBRANE POTENTIALS MEASURED?

Answer 5

• Using a ( glass)micropipette which is an electrode connected to a needle .
• Diameter of the needle less than 1 um
  ( still quite big compared to pm which is 10-20nm want to make sure membrane not damaged) Have a deflection to the negative side in the potentiometer because you have a negative membrane potential.
• When the electrode does no enter the membrane you have a reading of 0 mV which is the baseline then penetrate the membrane you have negative reading therefore the deflection.

Question 6

WHAT exactly establishes a resting MEMBRANE POTENTIAL?

Answer 6

2 factors- which is the minimum features that have to be present for membrane potential to be established.

• 1. Asymmetric distribution of ions across the plasma membrane (ion gradients)
• 2. Selective Ion Channels in the plasma membrane- Na+, K+, Cl- most important- have others such as H+ and Ca2+ channels

Question 7

HOW are resting membrane potentials set up?

Answer 7

• Because the membrane is selective permeable to different ions.
• This permeability of the membrane ions occurs by way of channel proteins = membrane spanning transport proteins.
• ALLOWS IONS TO permeate!
  Ion selectivity of channels and the type of channel makes the membrane selective permeable to ions

Question 8

What in general terms is a membrane permeable to ?

Answer 8

• Membrane is not permeable to small charged molecules - BASICALLY ions therefore need these transmembrane transport proteins= channels
• Membrane is permeable to small uncharged molecules eg O2, CO2, H20, benzene.

Question 9

WHAT ARE ION CHANNELS?

Answer 9

• Proteins which enable ions to cross the cell membrane
• Have an aqueous pore which ions flow by diffusion
• IONS FLOW IN BOTH DIRECTIONS!

Question 10

WHAT IS REFLECTIVE OF THE AMOUNT OF K+ in the BLOOD?

Answer 10

Extracellular Plasma K+ Concentration

- 4.5 mM

Question 11

WHAT ARE THE THREE CHARACTERISTICS OF ION CHANNELS?

Answer 11

1. SELECTIVITY- for one or few ons eg , Na+, K+ , Ca2+ , Chloride and anion channels
2. GATING- the pore can open or close by conformation change in the protein.
3. RAPID FLOW OF ion - always DOWN the electrochemical gradient.

Question 12

For one of the characteristics of ion channels the 3rd one - what is the difference we may see

Answer 12

• Where flow is not rapid - Na+ channels (voltage gated sodium channels)
• some sodium channels will open slowly where you have a conformational change allowing a small amount of sodium through then sodium will bind causing channel to completely open big flux of sodium through the channels and works in reverse .

Question 13

If the cell did not have chloride ion channel will chloride ions afect the membrane potential

Answer 13

No -the same if there are no sodium channels then Na+ ions will not have an effect

Question 14

How do we know for setting up resting membrane potential what type of membrane we need and the importance of these channels?

Think in context of KCL

Answer 14

Non permeable membrane - There is no charge on the membrane or no movement of ions and because there are = +ves and -ves cancel each other out there is no possibility of a voltage or potential occuring.
Totally permeable - No charge on the membrane because equal distribution of +ves and -ves - have movement of ions
SELECTIVELY PERMEABLE - Have charge separation which leads to electrical gradient which is the basis of membrane potential

Question 15

What is special about selectively permeable membranes in terms of forming a resting membrane potential?

Answer 15

• Have charge separation
• Which leads to the formation of an electrical gradient
• Basis of the resting potential.

Question 16

WHAT ENTAILS SETTING UP A RESTING POTENTIAL IN A REAL CELL?

Answer 16

• Chemical gradient and electrical gradient for K+ equilibrium therefore there is no net movement of ions but there is a negative charge across the membrane .
• This is resting membrane potential.

Question 17

WHAT IS THE EQUILIBRIUM POTENTIAL FOR K+?

Answer 17

Equilibrium potential is when the chemical gradient and electrical gradient balance.

• NO net movement of K+ ions but there is a negative charge across the membrane - RMP.
• USE the Nernst equation to calculate it !

Question 18

WHAT IS THE NERNST EQUATION?

Answer 18

Ek= 61/(Z)mV log 10 (ion out)/(ion out)
Use this because human temperature is 37 degrees
- Can write it for any ion
- The amount of K+ needed to set up a voltage across a semi permeable membrane is small.(1% change)
- IF membrane is just selective perm to k+ EK=RMP

Question 19

WHAT IS THE IMPORTANCE OF PERMEABILITY AND RMP?

Answer 19

• If cell is permeable to just one ion then the eq potential of that ion= RMP eg. If just perm to K+ EK=RMP
• But in a real cell other ion channels contribute to resting membrane potential
• That’s why its not EK=RMP but close to it because the membrane more permeable to K+ ions than Na+

Question 20

What channel dominates Resting membrane potential?

Answer 20

• Most cells OPEN K+ channels dominate the resting membrane potential.
• eg Cardiac and neurone cells RMP close to EK - becuase not just selective perm to K+
• LOWER RMP = lower selectivity for K+- SM (-50mv) and RBcs (-9mv)
• SKM- K+ and CL- so RMP close to EK and ECL

Question 21

What the difference with skeletal muscle in regards to RMP?

Answer 21

• Neurones RMP close to EK
• SKM membranes are equally permeable to K+ and CL- ions therefore RMP close to EK and ECL therefore RP moves closer to neutrality

Question 22

Describe how EK will have an effect on RP:

Answer 22

• Dependence of RP on K+ permeability means changing EK changes RP
• Increasing K+ concentration outside will increase EK =more positive membrane potential is more positive.
• Hyperkalemia - (K+) outside increases eating many food types eg kale, bananas can increase K+ ion concentration in the gut lumen.
  EK IS WHEN THE CONCENTRATION GRADIENT AND ELECTRICAL GRADIENT BALANCE THERE IS NO NET MOVEMENT OF IONS BUT THERE IS A NEGATIVE CHARGE ACROSS THE MEMBRANE.