Membrane Potentials and Action Potentials

Question 1

what is voltage?

Answer 1

generated by ions to produce a charge gradient = potential difference

Question 2

what is current?

Answer 2

movement of ions due to potential difference

Question 3

what is resistance?

Answer 3

barrier that prevents the movement of ions

Question 4

which three things can cause ion channels to open/close?

Answer 4

• transmembrane voltage
• mechanical force
• binding of ligands

Question 5

what is an electrochemical equilibrium?

Answer 5

when the concentration gradient exactly balances the electrical gradient ie. electric force= concentration force

Question 6

what does the Nernst equation enable you to calculate?

Answer 6

electrochemical equilibrium potential

eg. potential at which the concentration of potassium moving out of the cell is exactly balanced by the electrical gradient that occurs as you move potassium out of the cell

Question 7

what is the concentration of sodium extracellularly and intracellularly, respectively?

Answer 7

150mM and 10mM

Question 8

what is the concentration of potassium extracellularly and intracellularly, respectively

Answer 8

5mM and 150mM

Question 9

why is the resting potential -70mV and not -90mV?

Answer 9

some sodium can still enter the cell

Question 10

what does the Goldman-Hodgkin_Katz (GHK) equation describe?

Answer 10

resting membrane potential

Question 11

depolarisation meaning

Answer 11

membrane potential moves towards 0mV

Question 12

repolarisation

Answer 12

membrane potential becomes more negative towards resting membrane potential

Question 13

overshoot

Answer 13

membrane potential becomes more positive from 0

Question 14

hyperpolarisation

Answer 14

when potential becomes more negative than resting potential

Question 15

what happens to the charge as a small depolarisation propagates down an axon? What is this called?

Answer 15

charge leakage resulting in the depolarisation fading away - graded potentials

Question 16

Where does a graded potential occur? What do they do in terms of action potentials? What are they in response to?

Answer 16

• synapses and sensory receptors
• initiate or prevent
• stimulus

Question 17

where do action potentials occur?

Answer 17

exciteable cells - muscle, cardiac tissue, neurons (nerve impulse) and some endrocrine tissue

Question 18

what does membrane potential depend on

Answer 18

movement of potassium through CHANNELS not pumps

Question 19

what are the 5 phases of an action potential?

Answer 19

1. resting potential
2. depolarising stimulus
3. upstroke
4. repolarisation
5. after - hyperpolarisation

Question 20

how is resting potential maintained?

Answer 20

K+ flow out of cell, small amount of Na+ into cell. Permeability of K+ is greater than Na+. Eq potential of K+ = 90mV

Question 21

what happens during a stimulus?

Answer 21

depolarisation, if it reaches a threshold value then an action potential is produced

Question 22

at the threshold potential, what happens to the permeability of the membrane to sodium?

Answer 22

increases permeability

Question 23

What happens during upstroke?

Answer 23

VGSC open quickly, sodium enters cell, VGPC open slowly, potassium leaves cell. Membrane potential moves towards Na+ equilibrium potential.

Question 24

what happens during repolarisation?

Answer 24

Sodium channels inactivated (plug stops flow), Na+ entry stops. More K+ channels open and remain open as K+ moves down its electrochemical gradient. Membrane potential moves towards K+ equilibrium potential

Question 25

what happens during the absolute refractory period?

Answer 25

nerve cannot be restimulated as sodium channels are inactivated, sodium cannot exit cell.

Question 26

what happens during hyperpolarisation?

Answer 26

K+ channels still open and K+ continues to leave cell down electrochemical gradient. As membrane potential reaches closer to K+ equilibrium, potential some K+ channels close. Membrane potential returns to resting.

Question 27

what is the name of the refractory period during hyperpolarisation?

Answer 27

relative refractory period

Question 28

what happens during the relative refractory period?

Answer 28

inactivation gate of Na+ channels opens, so a stronger than normal stimulus is needed to stimulate an action potential

Question 29

what happens to the decay in action potential depending on the diammeter / myelation of the neuron and why?

Answer 29

slower decay with a larger diammeter or small myelinated neuron because there is lower resistance

Question 30

at what speed does an action potential flow in a small diammeter, non myelinated axon?

Answer 30

1m/s

Question 31

at which speed does an action potential spread in a large diammeter, myelinated axon?

Answer 31

120m/s

Question 32

what can decrease conduction velocity?

Answer 32

* reduced axon diammeter eg. regrowth after injury * reduced myelination eg. multiple sclerosis and diptheria, cold, anoxia, compression and drugs (some anaesthetics)

Question 33

what are the three main factors that influence the movement of ions accross a membrane?

Answer 33

* concentration of ion on both sides of the membrane * charge on ion * voltage accross membrane

Question 34

Why is the Na+ equilibrium potential -ve eg. -70mV and the K+ equilibrium potential +ve eg. +40mV, when both are positive ions?

Answer 34

* More K+ inside, so tend to flow out * More Na+ outside, so tend to flow in * A potential of -70mV is needed to attract K+ and stop net outward flow * A potential of +40mV is needed to repel Na+ from entering the cell

Question 35

which ion is important for the upstroke and which is important for the falling portion of the action potential? In which directions do these ions move?

Answer 35

Na+ - in K+ - out respectively

Question 36

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

Answer 36

* myelination - linear relationship between conduction and myelin thickness * axon diammeter - lower resistance, ions move faster * Conduction velocity equivalent to square root of axon diammeter

Question 37

Which equation is used to calculate real membrane potential?

Answer 37

Goldman-Hodgkin-Katz equation