4 Resting and Action Potentials

Question 1

Q: What does an AP allow?

Answer 1

A: nerves to electrically communicate from one part of body to another

Question 2

Q: What is diffusion? Energy?

Answer 2

A: movement of molecules down concentration gradient from high to low concentration until it reaches a dynamic equilibrium (concentration is equal throughout vessel)

spontaneous, no energy input

Question 3

Q: What does flux mean? Unit? What is it when a dynamic equilibrium has been reached?

Answer 3

A: numerical term

number of molecules that cross a unit area per unit of time (number of particles) ie. molecules.m^-2.s^-1

no net flux

Question 4

Q: What is Ohm’s law?

Answer 4

A: V= I x R

Question 5

Q: What is voltage? Units?

Answer 5

A: potential difference generated by ions to produce a charge gradient

volts

Question 6

Q: What is a current? Units?

Answer 6

A: movement of ions due to a potential difference

amps

Question 7

Q: What is resistance? Units?

Answer 7

A: barrier that prevents the movement of ions (current)

ohms

Question 8

Q: How is a membrane potential measured? Which cells have a membrane potential? Average?

Answer 8

A: reference electrode is placed outside the cell. This is the zero volt level.

Another electrode is placed inside the cell. It measures a voltage difference that is negative compared with the outside (i.e. reference).

All cells have a membrane potential

-70mV

Question 9

Q: Describe general cell membrane permeability. How can it allow ion movement? (2) Due to? (3)

Answer 9

A: Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments.

• can contain ion channels to allow ion diffusion
• membrane can selectively allow ions to cross the barrier by changing its permeability

-Permeable pores in the membrane (ion channels) open and close depending on trans-membrane voltage, presence of activating ligands or mechanical forces (stretch sensitive)

Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+) and movement across the membrane will occur when the concentration of the permeant species is different on one side of the membrane

Question 10

Q: Scenario: 2 compartments- CASE 1

1: 0.15M NaCl
2: 0.15M KCl

Membrane between is impermeable

Why is the membrane impermeable?
What does this result in? (2)
What is the membrane potential?

Answer 10

A: No channels in the membrane

• So… no diffusion across the membrane despite concentration gradients (osmotic gradient is the same but not the ion types)
• No separation of charge

Membrane potential = 0 mV

Question 11

Q: Scenario: 2 compartments- CASE 2

1: 0.15M NaCl
2: 0.15M KCl

Membrane between is permeable to K+ ions

What happens? (6)

Answer 11

A: 1. K+ crosses the membrane from compartment 2 to 1 and the direction of flux is driven by its concentration gradient

2. get charge separation between the compartments:
   - compartment 1 gains +ve charge
   - compartment 2 gains -ve charge
3. no Na+ movement as membrane is impermeable
4. like charges start repelling: movement of K+ into +ve compartment slows/reduces as it comes up against +ve potential that has built up
5. reach point where concentration gradient pushing K+ from 2 to 1 is balanced by electrical gradient forming across the membrane -> some ions are even pulled back : called= EQUILIBRIUM POTENTIAL
6. stable trans-membrane potential is achieved

Question 12

Q: Scenario: 2 compartments- CASE 3

1: 0.15M NaCl
2: 0.15M KCl

Membrane between is permeable to Na+ ions

What happens? (4)

Answer 12

A: 1. Na+ crosses the membrane and the direction of flux is driven by its concentration gradient

2. Charge separation occurs
   Compartment 2 gains +ve charge
   Compartment 1 becomes more –ve
3. Enough +ve charge accumulates in compartment 2 to prevent further net movement of Na+
4. Electrochemical equilibrium is achieved when electrical force prevents further diffusion across the membrane

Question 13

Q: Compare case 2 and 3:

1: 0.15M NaCl
2: 0.15M KCl

2: Membrane between is permeable to K+ ions
3: Membrane between is permeable to Na+ ions

What do they both have? but? why?
What can be reached in both cases? when?

Answer 13

A: In both cases a membrane potential exists, but its sign is opposite

The difference in sign arises because of the selectivity of the membrane
Case 2 permeable to K+
Case 3 permeable to Na+

In both cases the electrochemical equilibrium has been reached at which the concentration gradient exactly balances the electrical gradient = point of the equilibrium potential

Question 14

Q: What is the equilibrium potential? What does it cause?

Answer 14

A: The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient

Question 15

Q: How can you calculate the equilibrium potential? How can this be simplified and made useful? (4) How is value given?

Answer 15

A: nernst equation

………RT………[X2]
E = —— ln ——–
………zF……….[X1]

R = gas constant
T = Temp.  Kelvin
Z = charge on ion -> -1 for Cl-, +2 for Ca2+
F = Faraday’s number -> charge per mol of ion
ln = log to base e
X2= ion on one side of membrane 
X1= same ion on other side of membrane

• Assume T = 37C
• Convert natural log to common log
• State E in mV
• Make compartment 2 the inside of the cell and compartment 1 the outside

……..-61…………[X]inside
E = —— log —————-
……….z………….[X]outside

in mV

Question 16

Q: What are the 2 most important ions for the resting membrane potential of neurons? What are their concentrations extracellularly and intracellularly? At rest, how does their membrane permeability vary?

Answer 16

A: Na+ and K+

Na+: extra= 150mM, intra= 10mM

K+: extra= 5mM, intra= 150mM

K+ permeability&raquo_space; Na+

Question 17

Q: Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV). Typical Em is -70 mV. Why?

Answer 17

A: Membranes have mixed K+ and Na+ permeability

biological membranes are not uniquely permeable to only one ion

Question 18

Q: What equation is used to calculate real membrane potential? Explain. What is it based on? (2)

Answer 18

A: Goldman-Hodgkin-Katz (GHK) equation

…………………P(k)[K]i . P(Na)[Na]i . P(Cl)[Cl]o
E= -61.log.——————————————
………………..P(k)[K]o . P(Na)[Na]o . P(Cl)[Cl]i

P=permeability/ion channel open probability
0=100% closed
0.5=50% open
1=100% open

K Na and Cl all contribute to real MP

size of each ions contribution is proportional to how permeable the membrane is to the ion and the concentration of ions on either side of the membrane

Question 19

Q: What does sodium entering a cell only permeable to potassium ions do?

Answer 19

A: decreases negative potential

Question 20

Q: Define on a graph:

depolarisation
overshoot
repolarisation
hyperpolarisation

Answer 20

A: REFER

Question 21

Q: What are small changes in voltage that can take place? (3) To what stimuli can these changes occur? (4) What do these changes result in?

Answer 21

A: 1. small depolarisation or hyperpolarisation

2. weak stimulus=small depolarisation or strong stimulus= larger
3. either measured at stimulus site or 1mm away (smaller depolarisation)-> because these events decay over the length of the axon

touch, smell, taste, sight

graded potential (graded small changes in potential)

Question 22

Q: Describe the decremental spread of graded potentials? Where does this occur? (2) Purpose? (2)

Answer 22

A: charge is dissipated as you go across which means voltage is lost

At synapses
Sensory receptors

Contribute to initiating (depolar…) or preventing (hyper…) action potentials (all or nothing event)

Question 23

Q: Where do action potentials occur? (3) What are they also known as? allow?

Answer 23

A: excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)

In neurons they are also known as “nerve impulses” and allow the transmission of information reliably and quickly over long distances

Question 24

Q: What do AP play a central role in? Examples (2).

Answer 24

A: They play a central role in cell-to-cell communication and can be used to activate intracellular processes

Eg – muscle cells, an action potential is the first of a series of events leading to contraction.
Eg – beta cells of the pancreas, they provoke release of insulin

Question 25

Q: What does membrane permeability generally depend on? Changes depend on? What happens when membrane permeability of an ion increases? What does this do?

Answer 25

A: conformational state of ion channels ****Changes in membrane potential during the action potential are NOT due to ion pumps**** ion increases it crosses the membrane in a direction dictated by its electrochemical gradient This movement changes the membrane potential toward the equilibrium potential for that ion

Question 26

Q: What generally causing ion channels to open? inactivated? closed? Which is related to the refractory period?

Answer 26

A: Opened by membrane depolarisation Inactivated by sustained depolarisation (really fast) *** Closed by membrane hyperpolarisation/repolarisation

Question 27

Q: How long does an action potential last in an axon? What are the 5 phases? Draw a graph.

Answer 27

A: 1-1.5 ms 1. resting membrane potential 2. depolarising stimulus 3. upstroke (and overshoot) 4. repolarisation 5. hyperpolarisation REFER inc equilibrium K and Na potential lines

Question 28

Q: Phase 1 of an action potential. Membrane permeability? Membrane potential position?

Answer 28

A: resting membrane potential - membrane is much more permeable to K than Na (PK >> PNa) - Membrane potential nearer equilibrium potential for K+ than that for Na+

Question 29

Q: Phase 2 of an action potential. What can cause it? (2) What happens? (3) Membrane potential position?

Answer 29

A: depolarising stimulus -stimulus could be voltage, stretch (muscle) - small number of sodium channels open - Na moves down concentration gradient into cell - membrane depolarises (becomes more positive) Moves it in the +ve direction towards threshold

Question 30

Q: Phase 3 of an action potential. Where does it begin? what happens are this point? Result? (2) What is happening at the same time (little effect)? (2) Compared to first event? Membrane potential position?

Answer 30

A: upstroke (and overshoot) -Starts at threshold potential= system reaches point where voltage across membrane is around 55mV -> causes response from VG sodium channels (open quickly) = get large permeability change - Na+ ions enter the cell down their electrochemical gradient - membrane potential increases to 0 and get overshoot - PK increases as the voltage-gated K+ channels start to open slowly - K+ ions leave the cell down their electrochemical gradient Less than Na+ entering Membrane potential moves toward the Na+ equilibrium potential= net effect

Question 31

Q: Phase 4 of an action potential. What happens? (4) Membrane potential position? What also occurs during this phase? result? (2)

Answer 31

A: repolarisation - PNa decreases because the voltage-gated Na+ channels inactivate (rapid) - Na+ entry stops - PK increases 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= net effect absolute refractory period - stops nerve going into constant impulse conduction - only get discrete AP

Question 32

Q: How do sodium channels become inactivated? When are they reactivated? What does this cause during phase 4 of an action potential?

Answer 32

A: part of protein sticks in the permeability pore and blocks sodium movement when membrane repolarises and protein blocking it has moved away 4. REPOLARISATION gives rise to the absolute refractory period -

Question 33

Q: Phase 5 of an action potential. What happens at the start? (2) Membrane potential position? What happens in the second half? Membrane potential position? What else occurs during this phase? What happens during this? (2) What can it allow?

Answer 33

A: hyperpolarisation - 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 relative refractory period 2 stage process - inactivation gate is opened by the removal of the protein blocking it (due to the change in membrane potential) - but the repolarisation means that the Na channel shuts too (no longer stimulated since it's VG) -between these steps, a stronger than normal stimulus can trigger an action potential

Question 34

Q: Draw a timecourse graph of the changes in membrane permeability to various ions. (3) Include equilibrium potential lines. (2)

Answer 34

A: REFER

Question 35

Q: What is the nature of AP? what does this mean? Draw a diagram to show this. When does it end? What happens for a while after?

Answer 35

A: “All-or-Nothing” nature once triggered, a full sized action potential occurs REFER - small circle: resting potential-> depolarisation-> if less than threshold, graded (small changes in) potential -> REP - depolarisation -> if threshold -> depolarisation (part of big circle) - depo-> opening of VG Na+ C -> increase in Na+ permeability-> increase in Na+ entry -> depo ^ positive feedback loop - Cycle continues until the voltage-gated Na+ channels inactivate (closed and voltage-insensitive) - Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation

Question 36

Q: What does it mean by threshold when it comes to AP?

Answer 36

A: once this potential is reached an action potential is triggered- voltage at which huge number of Na channels open -> point where process can't be stopped

Question 37

Q: Why is the refractory period needed? Main mechanism?

Answer 37

A: WAY OF RESTORING electrochemical equilibrium -need a way of getting Na and K back to where they need to be (otherwise they will be depleted- signals depends on it) Na K ATPase which puts Na out of cell and brings K in

Question 38

Q: Describe passive propogation. Compare myelination. What does it show? (2) In reference to?

Answer 38

A: from site of depolarisation, there is an outward movement either side of membrane potential change as you move further from the site, the potential decays/degrades small, unmyelinated neuron decays faster than large-diameter myelinated neuron Internal (or axial) & Membrane Resistance alters propagation distance and velocity graded potential not action potential

Question 39

Q: Describe the process active propogation of the action potential. (3) Direction?

Answer 39

A: 1. active area at peak of action potential= starting site 2. local current flow depolarises adjacent region toward threshold (where adjacent area was at resting potential) 3. adjacent area becomes new active area at peak of action potential 4. repeat can only occur in one direction- the old active area is now in refractory stage

Question 40

Q: What is saltatory conduction? Speed?

Answer 40

A: action potential jumping from node of ranvier to next (between pieces of myelin) start with passive propogation from initial site-> if reaches node AND THRESHOLD HAS BEEN REACHED-> creates AP in next node much faster than non

Question 41

Q: Compare the velocity of AP travel in 2 types of mammalian axon. What influences conduction velocity?

Answer 41

A: Large diameter, myelinated axons 120 m/s Small diameter, non-myelinated axons 1 m/s Both axon diameter and myelination

Question 42

Q: How does conduction velocity change with: axon diameter? myelination? 4 others?

Answer 42

A: Increases with axon diameter- less resistance to current flow inside large diameter axons Higher in myelinated than non-myelinated axons of the same diameter- action potentials only occur at nodes of Ranvier Conduction velocity is slowed by reduced axon diameter reduced myelination cold, anoxia, compression and drugs (some anaesthetics)

Question 43

Q: What can cause reduced axon diameter? What can cause reduced myelination? (2)

Answer 43

A: re-growth after injury multiple sclerosis and diphtheria