Membrane Potentials + Action Potentials

Question 1

Voltage
Current
Resistance
Definitions + Equation

Answer 1

Voltage - generated by ions to produce charge gradient
Current- movement of ions due to PD
Resistance -barrier that prevents movement of ions

Voltage = Current x Resistance

Question 2

Properties of Ion Channels

Answer 2

Lipid cell membrane acts as a barrier to ion movement and separates ionic environments
Cell membrane can selectively change permeability to specific ions
Permeable pores in membrane (ion channels) open and close depending of transmembrane voltage, presence of activating ligand or mechanical forces
Movement across membrane will occur when there is a concentration gradient between either side of membrane

Question 3

Electrochemical Equilibrium

Answer 3

Electrical forces (+/-) balance diffusional forces so stable transmembrane potential is achieved

Question 4

Equilibrium Potential

Answer 4

Potential at which electrochemical equilibrium has been reached which also prevents the diffusion of the ion down its concentration gradient

Question 5

How to calculate Equilibrium Potential (E)

Answer 5

Nernst Equation 
E= RT/zF lnX2/X1
R - gas constant  
T - temp in kelvin (310K)
Z = ion charge 
F = Faraday’s no° (charge per mol of ion)
ln = natural log
X2 = intracellular ion concentration
X1 = extracellular ion concentration

Question 6

Why is the Nernst equation limited?

Answer 6

As equilibrium potentials of K+ and Na+ are based on theoretic values but biological membranes aren’t uniquely selective for any one ion at a time and membranes have variable permeability to all ions.
Each ion’s contribution to MP is proportional to how permeable the membrane is to the ion at any time.
GHK is proposed instead

Question 7

Explain the Goldman-Hodgkin-Katz equation

Answer 7

Describes the Em more accurately by including P
This stands for He permeability or channel open probability ( 0=100% closed 0.5=50% time open 1=100% open)
Subscript on P indications the ion (K/Na/Cl) and subscript on conc of ion indicates inside or outside the cell (i/o)

Question 8

Define:
Depolarisation
Repolarisation
Overshoot 
Hyperpolarisation

Answer 8

D - membrane potential becomes more positive towards 0mV
R - membrane potential decreases towards resting potential
O - membrane potential becomes positive
H - membrane potential decreases beyond resting potential

Question 9

What are graded potentials?

Answer 9

External stimulations or neurotransmitters causes change in membrane potential
This change is graded in response to the type or strength of stimulation
Graded potentials produce the initial change in membrane potential that determines what happens next (initiate/prevent APs)

Question 10

Why do graded potentials decrease from one end to the other?

Answer 10

Moving down the axon from the site of initial depolarisation, charge ‘leaks’ from the axon into the extracellular fluid and so the size of the potential change decreases along the axon

Question 11

When do APs occur?

Answer 11

APs occur when a graded potential reaches a threshold for the activation of Na+ channels which occur in excitable cells aka nerve impulses allowing transmission of info reliably and quickly over long distances
Also used for intracellular processes

Question 12

What causes APs to form?

Answer 12

Permeability depends on conformational state of ion channels (open/closed/inactivated)
When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
Movement changes the membrane potential towards the equilibrium potential for that ion
These changes cause APs

Question 13

Name 5 phases of AP

Answer 13

Phase 1: Resting Membrane Potential 
Phase 2: Depolarising Stimulus
Phase 3: Upstroke
Phase 4: Repolarisation
Phase 5: After-hyperpolarisation

Question 14

Describe phase 1 RMP

Answer 14

Permeability for K+ is more than that for Na+

Membrane potential is also nearer equilibrium potential (-90mV vs +72mv respectively)

Question 15

Describe Phase 2 Depolarising Stimulus

Answer 15

The (graded potential) stimulus depolarises the membrane potential
Moved it in the positive direction towards threshold

Question 16

Describe phase 3 upstroke

Answer 16

Starts at threshold potential
Na+ potential increases as VGNC open quickly so Na+ enters the cell down the electrochemical gradient
K+ potential increases as VGPC start to open slowly so K+ leaves the cell down the electrochemical gradient (less K+ leaves than Na+ that enters)
Membrane potential moves towards the Na+ equilibrium potential

Question 17

Describe phase 4 Repolarisation

Answer 17

Na+ potential decreases as VGSC close so Na+ entry stops
K+ potential increases as more VGPC open and remain open so K+ leaves the cell down its electrochemical gradient
Membrane potential moves towards the K+ equilibrium potential

Question 18

Describe the absolute refractory period

Answer 18

Absolute refractory period is where Na+ channel activation gate is open but the inactivation gate is closed
New AP cannot be triggered as Na+ channels are closed
Absolute refractory period continues and so activation AND inactivation gates are closed

Question 19

Describe phase 5 after-hyperpolarisation

Answer 19

At rest VGPC are still open
K+ continues to leave the cell down the electrochemical gradient
Membrane potential moves closer to K+ equilibrium so then some VGPC close
Membrane potential returns to the resting potential

Question 20

Describe the relative refractory period

Answer 20

Some Na+ channels have recovered from inactivation and so the gate is open
An AP can be triggered at this stage but a stronger than normal stimulus is required

Question 21

How do APs move along an axon?

Answer 21

Internal and membrane resistance, diameter of axon alters propagation distance and velocity
Local current flow depolarises adjacent region towards threshold causing that area to activate from resting potential while the old active region returns to resting potential and this continues along the axon
Voltage gates channels are mostly located at nodes allowing Saltatory conduction down nerve fibres

Question 22

Describe factors that affect conduction velocity

Answer 22

Both axon diameter and myeloma toon influence conduction velocity
Small diameter + non myelinated axons - 1m/s
Large diameter + myelinated axons - 120m/s
Velocity decreases with:
reduced axon diameter and reduced myelination
Cold
Anoxia (absence of O2)
Compression
Drugs