Action Potentials

Question 1

Depolarisation definition?

Answer 1

Membrane potential becomes LESS negative (even positive)

Question 2

Hyperpolarisation definition?

Answer 2

Membrane potential becomes MORE negative

Question 3

What does increasing membrane potential do?

Answer 3

Make it more negative

Question 4

Factors causing change in V_m?

Answer 4

Depolarisation/hyperpolarisation depends on:

Direction of ion movement - influx (into cell) or efflux (out of cell)

Charge carried by ions (+ve or -ve)

Question 5

Methods of ion movement across membranes?

Answer 5

Via ion channels/transporters

Passive movement of an ion through an ion channel is driven by its ELECTROCHEMICAL gradient

Question 6

How does Na⁺ move through the membrane?

Answer 6

Response to opening of cell membrane sodium-selective channels Na⁺ flows INWARDLY as conc. gradient and electrical gradient are inward

Question 7

Equilibrium potential of Na⁺?

Answer 7

E_Na = +60 mV

Question 8

What is the membrane potential and how does it compare to E_Na?

Answer 8

V_m = -80 mV

This is negative compared to E_Na (+60 mV)

Question 9

Driving force of a sodium ion?

Answer 9

Driving force for Na⁺ influx: V_m - E_Na

When negative, inward movement of Na⁺ occurs

Question 10

How is the current carried by an ion calculated?

Answer 10

I_Na = g_Na (driving force or V_m - E_Na) or I_K = g_K (driving force or V_m - E_K)

Question 11

How does K+ move through the membrane?

Answer 11

Response to opening of cell membrane potassium-selective channels

K+ flows OUTWARDLY as conc. gradient is outward and electrical gradient is inward; conc. gradient energy exceeds energy of electrical gradient

Question 12

Equilibrium potential of potassium compared to membrane potential?

Answer 12

E_K = -90 mV; more negative than the membrane potential (-80 mV)

Question 13

Driving force of a potassium ion?

Answer 13

Driving force for K⁺ efflux: V_m - E_K, when positive, outward movement of K⁺ occurs

Question 14

Effect of Na⁺/ K⁺ channel opening on V_m?

Answer 14

Na⁺ channel opening - V_m driven towards E_Na

K⁺ channel opening - V_m driven towards E_K

Question 15

Describe ion channels and structure

Answer 15

Protein complexes spanning lipid bilayer to form a central pathway, allowing rapid flow of SELECTED ions

Have 4 modules

Question 16

Regulation of ions channels?

Answer 16

Most are regulated and exist in:

Open (o)

Closed (c)

…and additional conformations cycling between these

Question 17

Opening of ion channels?

Answer 17

Opened (“gated”) by:

Membrane voltage - voltage-gated ion channels

Chemical substances - ligand-gated ion channels

Physical stimuli - like mechanical/thermal

Question 18

Ion channels responsible for depolarisation and hyperpolarisation?

Answer 18

Voltage-activated Na+ channels - depolarising

Voltage-activated K+ channels - hyperpolarising

Question 19

Steps of a neurone action potential?

Answer 19

Resting potential (for neurones, 60-70 mV)

Threshold - depolarisation to threshold which opens channels

Upstroke - rapid depolarisation moves V_m close to E_Na, due to opening of voltage-activated Na⁺ channels

Downstroke - rapid hyperpolarisation moves V_m close to E_K, due to opening of voltage-activated K⁺ channels and inactivation of voltage-activated Na⁺ channels

Undershoot - after action potential and is more negative than resting membrane potential, due to delayed closure of voltage-activated K⁺ channels

Question 20

Action potential properties?

Answer 20

Brief electrical signals in which polarity of nerve cell membrane is MOMENTARILY ( approx 2ms) REVERSED

“All or none” - action potentials generated when threshold is reached

Question 21

Describe “All of None” nature of action potentials

Answer 21

Cannot have half an action potential

Even if size of depolarising stimulus is large, action potential has a fixed amplitude

Question 22

What occurs at rest?

Answer 22

K⁺ channels are leaky

Question 23

Describe similarities between voltage-activated Na⁺ and K⁺ channels

Answer 23

Highly selective for respective cations

Activated by membrane depolarisation - Na⁺ channels rapidly and K⁺ channels with a slight DELAY

Question 24

Difference between voltage-activated Na⁺ and K⁺ channels?

Answer 24

Activation of Na⁺ channels is self-reinforcing - opening of a few channels causes more channels to open, causing more depolarisation, etc (POSITIVE FEEDBACK)

Activation of K⁺ channels is self-limiting - outward movement of K⁺ causes repolarisation, turning off the stimulus for opening (NEGATIVE FEEDBACK)

Question 25

Na⁺ channels during maintained depolarisation? Reversal?

Answer 25

Enter a non-conducting, inactivated state, even though they initially open due to depolarisation Repolarisation causes the channel to enter a closed state in readiness for opening, to generate another action potential

Question 26

Why is maintained depolarisation of Na⁺ channels important?

Answer 26

Na⁺ channels will be inactivated, which contributes to the repolarising phase of action potential and is responsible for REFRACTORY PERIOD

Question 27

Repolarisation definition?

Answer 27

Returning the membrane potential to normal resting value

Question 28

Absolute refractory period definition?

Answer 28

No stimulus, however strong, can elicit a second action potential immediately after generation of an action potential, as all Na+ channels are inactivated

Question 29

Relative refractory period definition?

Answer 29

A stronger than normal stimulus may elicit a second action potential (mixed population of inactivated and closed channels)

Question 30

When do absolute and relative refractory periods occur?

Answer 30

During downstroke and undershoot respectively

Question 31

Briefly, how is impulse propagation achieved with unmyelinated and myelinated axons?

Answer 31

Unmyelinated axons - passive spread of current Myelinated axons - saltatory conduction

Question 32

Why do passive signals not spread far from origin?

Answer 32

Nerve cell membrane is "leaky", not a perfect insulator, so passive signals do not spread far (current is lost from the membrane along the axon)

Question 33

How can passive electrical signals be conducted over large distances without decaying?

Answer 33

Via action potentials

Question 34

Passive current spread increase causes?

Answer 34

Decrease ri - axial resistance (increase axon diamater) Increase rm - membrane resistance (by adding insulating material like myelin)

Question 35

Describe myelin and creation

Answer 35

Insulating material created by SCHWANN cell in PNS and by OLIGODENDROCYTES in CNS Conduction in myelinated axons is much faster than in non-myelinated axons of similar diameter

Question 36

Node of Ranvier definition?

Answer 36

Points of neurones at which myelin sheath disappears

Question 37

What is saltatory conduction?

Answer 37

Action potential jumps from one node of ranvier to next