membrane and action potentials

Question 1

what is electrochemical equilibrium?

Answer 1

when electrochemical forces (positive charge repelling against more positive ions entering) balance diffusional forces

Question 2

what is electrochemical potential?

Answer 2

potential where electrochemical equilibrium has been reached

potential that prevents diffusion of the ion down its concentration gradient

Question 3

what equation is used to calculate electrochemical potential?

Answer 3

Nernst equation

Question 4

• What do the components in the Nernst Equation stand for?

Answer 4

R = Gas constant

T = Temperature in Kelvin

z - charge on ion

F = Faraday's number - charge per mol of ion

ln = natural logarithm

X2 = Intracellular ion concentration

X1 - Extracellular ion concentration

Question 5

what is the Nernst equation?

Answer 5

RT/ zF x ln (X2/X1

Question 6

what is the extra and intra cellular conc (mM) of

• Na
• K
• Ca

what is the osmolarity extra and intra cellularly?

Answer 6

150, 10

5, 150

2, 10^-2

285 mosmol/l

Question 7

what does the Nernst equation calculate

what does the GHK equation calculate

the movement of which ion determines the resting potential

Answer 7

Electrochemical potential can be calculated for single ion in ideal (control) system by Nernst equation

Membrane potentials can be estimated in more complex systems (many ions and variable permeability) by the GHK equation

Resting membrane potential due almost entirely to the movement of K+ ions out of the cell

Question 8

what is a graded potential

Answer 8

Change in membrane potential in response to external stimulation or neurotransmitters

Change in membrane potential 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 – they initiate or prevent action potentials

will decay if it does not reach threshold

Question 9

when do action potentials occur

Answer 9

Action potentials (AP) occur when a graded potential reaches a threshold for the activation (opening) of many Na+ channels resulting in an “all-or-nothing” event

Question 10

in what direction do ions move when permeability to that ion increases

what does the change in membrane potential not depend on

Answer 10

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

Changes in membrane potential during the action potential are not due to ion pumps

Question 11

what is phase one of an action potential

Answer 11

Resting membrane potential

permeability for PK > PNa

Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)

Question 12

what is phase one of an action potential

Answer 12

Resting membrane potential

permeability for PK > PNa

Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)

Question 13

What is phase two of an action potential

Answer 13

The stimulus depolarises the membrane potential

Moves it in the positive direction towards threshold

Question 14

what happens during depolarisation

Answer 14

Starts at threshold potential

inc Perm Na because voltage-gated Na+ channels open quickly [Na+ enters the cell down electrochemical gradient]
inc Perm K as the voltage-gated K+ channels start to open slowly [K+ leaves the cell down electrochemical gradient]
but less than Na+ entering

Membrane potential moves toward the Na+ equilibrium potential

Question 15

what happens during repolarisation

Answer 15

decreased perm Na because the voltage-gated Na+ channels close - Na+ entry stops
increased Perm K as more voltage-gated K+ channels open & remain open

K+ leaves the cell down its electrochemical gradient

Membrane potential moves toward the K+ equilibrium potential

Question 16

when does the Absolute refractory period occur and what is it?

what happens to the absolute refractory period later on in repolarisation

Answer 16

At the start of repolarisation

Na+ channel activation gate is open, inactivation gate is closed
New action potential cannot be triggered even with very strong stimulus
membrane is unresponsive to threshold depolarisation until voltage gated Na channels recover from inactivation

Absolute refractory period continues
Activation AND Inactivation gates closed

Question 17

What happens during after- hyperpolarisation?

Answer 17

At rest voltage-gated K+ channels are still open

K+ continues to leave the cell down the electrochemical gradient

Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close

Membrane potential returns to the resting potential

Question 18

When does the Relative refractory period occur and what is it

Answer 18

During after hyperpolarisation

activation gate is open, inactivation gate is closed

stronger than normal stimulus to trigger an action potential

Question 19

How does positive feedback work during all or nothing action potentials

how are Na channels inactivated

what two types of membrane potentials are there

Answer 19

depolarisation, opens Na+ channels, inc Na perm, Na Influx- causes more depolarisation

By sustained depolarisation

graded and action

Question 20

what happens during passive propagation

Answer 20

Only resting K+ channels open, when graded potentials decay

myelination of axon and diameter of axon alters propagation distance and velocity
(large diameter myelinated axon- small resistance)
(small diameter unmyelinated- large resistance)

Question 21

what factors affect conduction velocity

how fast does an AP travel in Small diameter, non-myelinated axons

how fast does an AP travel in Large diameter, myelinated axons

Answer 21

Both axon diameter and myelination influence conduction velocity

Small diameter, non-myelinated axons
1 m/s

Large diameter, myelinated axons
120 m/s

Question 22

what factors reduce conduction velocity in an axon

Answer 22

Decreases with reduced axon diameter (i.e. re-growth after injury)
reduced myelination (e.g. multiple sclerosis and diphtheria)
cold
anoxia
compression
drugs (some anaesthetics)

Question 23

What are the three main factors that influence the movement of ions across the membrane?

Answer 23

Concentration of the ion on both sides of the membrane, the charge on the ion and the voltage across the membrane.

Question 24

Why is the K+ equilibrium potential negative (e.g. -70mV) and the Na+ equilibrium potential positive(e.g. +40mV) when both are positive ions?

Answer 24

More K+ inside the cell than outside so tend to flow out of the cell, while more Na+ outside the cell than in, therefore tend to flow into the cell.

A potential of -70mV is needed to attract K+ and stop net outward flow, while a positive charge of +40mV is needed to repel Na+ from entering the cell.

Question 25

What factors influence the speed of propagation of an action potential along an axon?

Answer 25

Larger diameter axons have lower resistance, so ions move faster – conduction velocity is proportional to the square root of the axon diameter.

There is a linear relationship between conduction velocity and myelin thickness