Action potential

Question 1

What are the two types of changes in membrane potential

Answer 1

Action potentials and graded potentials.

Question 2

What is meant by depolarisation

Answer 2

Depolarisation = change in a positive direction.

Question 3

What is meant by repolarisation

Answer 3

Repolarisation = change in a negative direction towards the resting potential.

Question 4

What is meant by overshoot

Answer 4

Overshoot = change from 0 in a positive direction.

Question 5

What is meant by hyperpolarisation

Answer 5

Hyperpolarisation = voltage drops below the resting potential.

Question 6

Describe the characteristics of graded potentials

Answer 6

Is bi-directional – positive or negative depending on stimulus.
▪ Can have a weak stimulus = small potential, strong = large.
▪ Decreases in amplitude over time and distance from origin. o Due to leakage of charge along the axon 9decremental spread)
▪ Only occurs at SYNAPSES and SENSORY RECEPTORS.
They are distinct from action potentials
Shows a change in amplitude

Question 7

What is the function of graded potentials

Answer 7

Generate or prevent an action potential forming.

Question 8

What happens to graded potentials as you move further away from its site of origin.

Answer 8

The size of depolarisation decreases

Question 9

Why are graded potentials likely to be found at synapses and sensory receptors

Answer 9

Synapses- large number of inputs (can lead to summation or cancellation)
Sensory receptors- different magnitude of response depending on the strength of the stimuli.

Question 10

What is the key difference between graded and action potentials

Answer 10

Graded Potential = change in amplitude. ▪ Action Potential = Uniform amplitude (all-or-nothing event).

Question 11

Describe the roles of action potentials in cell-cell communication and activation of intracellular processes

Answer 11

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 12

Where do action potentials occur

Answer 12

Action potentials occur in 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 13

Define action potential

Answer 13

The change in electrical potential associated with the passage of an impulse along the membrane of a muscle or nerve cell.

Question 14

What does the permeability of the membrane to a particular ion depend on

Answer 14

Permeability depends on conformational state of ion channels
Opened by membrane depolarization
Inactivated by sustained depolarization
Closed by membrane hyperpolarization/repolarization

Question 15

What happens when the permeability of the membrane to that ion increases

Answer 15

When membrane permeability of an 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 16

What is the key thing to remember about changes in action potentials

Answer 16

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

Question 17

Where are axon potentials generated

Answer 17

At the axon hillock.

Question 18

Describe the state at resting membrane potential

Answer 18

PK&raquo_space; PNa therefore membrane potential nearer equilibrium potential for K+ than for Na+.

Voltage-gated Na+ channels (VGSC) and Voltage-gated K+ channels (VGKC): ▪ Sodium channel activation gate CLOSED. ▪ Sodium channel inactivation gate OPEN. ▪ Potassium channel CLOSED.

At rest, the voltage-gated ion channels (Na+ and K+) are closed (VGSC and VGKC).

Question 19

What is the key difference between voltage gated and sodium ion channels.

Answer 19

Only the sodium channel has an activation and inactivation gate.

Question 20

Describe the events that take place during the depolarising stimulus event

Answer 20

The stimulus (exaggerated here) depolarizes the membrane potential
 Moves it in the +ve direction towards threshold
if this depolarisation is above the threshold potential, it will cause the opening of VGSCs, which allow more Na+ to move into the cell, causing more depolarisation and thus generating an action potential.

Question 21

Describe the events that take place during the upstroke/depolarising phase

Answer 21

Starts at threshold potential
PNa  because the voltage-gated Na+ channels open quickly
Na+ ions enter the cell down their electrochemical gradient
PK  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
The net effect is the membrane potential moving towards the Na+ equilibrium potential.

Question 22

Describe the state of the voltage gated ion channels during the upstroke/depolarising phase

Answer 22

Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC): ▪ Sodium channel activation gate OPEN. ▪ Sodium channel inactivation gate OPEN. ▪ Potassium channel CLOSED.

Question 23

Describe the events that take place during the repolarisation phase

Answer 23

PNa  because the voltage-gated Na+ channels inactivate
Na+ entry stops
PK  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
The net effect is that membrane potential moves towards the equilibrium potential for K+.

Question 24

Describe the state of the voltage gated channels at the start of the repolarisation phase

Answer 24

Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC): ▪ Sodium channel activation gate OPEN. ▪ Sodium channel inactivation gate CLOSED. ▪ Potassium channel OPEN.
Hence no diffusion of Na+ into cell

Question 25

What is meant by the absolute refractory period

Answer 25

Inactivation gate is closed

New action potential cannot be triggered even with very strong stimulus

Question 26

Describe the state of the voltage gated channels at late repolarisation

Answer 26

Voltage-gated Na+ channels (VGSC) and Voltage gated K+ channels (VGKC): ▪ Sodium channel activation gate CLOSED. ▪ Sodium channel inactivation gate CLOSED. ▪ Potassium channel OPEN. The absolute refractory period continues.

Question 27

Describe the role of the inactivation peptide

Answer 27

Blocks the channel pore, hence reducing the permeability of the membrane to Na+. Happens as the membrane becomes more positive.

Question 28

What is the importance of the absolute refractory period

Answer 28

Prevents barrage of impulses flowing down into the cell- it acts as a safety net. Otherwise the cell would get stuck with most of its Na+ ion channels open.

Question 29

Describe the events that take place after-hyperpolarisation

Answer 29

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 30

What is the undershoot caused by

Answer 30

The undershoot takes place as the VGKCs remain open for a few milliseconds after repolarisation. More VGKCs open than at resting- they take a while to close- why we see the overshoot.

Question 31

Describe the state of the voltage gated channels during hyperpolarisation

Answer 31

Voltage-gated Na+ channels (VGSC) and Voltagegated K+ channels (VGKC): ▪ Sodium channel activation gate CLOSED. ▪ Sodium channel inactivation gate OPEN. ▪ Potassium channel OPEN.

Question 32

Describe the relative refractory period

Answer 32

The membrane enters a relative refractory period – A stronger stimulus is required to open the VGSCs as the membrane potential is already more negative than normal (hyperpolarisation).

Question 33

How long does the whole event take

Answer 33

About 2 ms

Question 34

What happens to the membrane potential during repolarisation

Answer 34

There is exponential decay of the membrane potential.

Question 35

What is meant by threshold potential

Answer 35

once this potential is reached an action potential is triggered

Question 36

What is meant by the all or nothing nature of the action potential

Answer 36

once triggered, a full sized action potential occurs

Question 37

What is meant by the refractory state

Answer 37

unresponsive to threshold depolarization

Question 38

Describe the Regenerative relationship between PNa and membrane potential

Answer 38

Initially, depolarisation is caused by an event outside the cell and if the potential is less than the threshold, graded potential returns to resting potential. 1. Once the threshold is reached, the cycle can continue – Positive feedback behaviour. 2. The cycle stops when the VGSCs are inactivated – closed and voltage insensitive (inactivation gate). 3. The membrane remains in an unresponsive state until the VGSCs recover from inactivation.

Question 39

When does the VGSCs remain in the inactivated state

Answer 39

Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation

Question 40

Describe ion movements during the action potential

Answer 40

▪ There are very small changes in concentration during an AP (<0.1%). ▪ Ion pumps are not directly involved in ion movements during the AP – This is a spontaneous event. ▪ Electrochemical equilibrium is restored following the AP by the ions moving through NON voltage-gated ion channels. ▪ Some ions move through pumps but this is a relatively slow process.

Question 41

Describe the propagation of an action potential

Answer 41

▪ Diameter of the neuron and myelination affects speed of AP. ▪ Myelination prevents loss of charge by acting as an insulator – allows the charge to travel further than with cable transport. ▪ The ABSOLUTE REFRACTORY PERIOD – Blocking the VGSCs by the inactivation gate means that the section of membrane which is hyperpolarised cannot be depolarised again and the AP cannot travel in the wrong direction. ▪ Speed of an AP can be up to 120m/s (and as low as 1m/s in small, un-myelinated axons). ▪ Action Potentials only occur at the Nodes of Ranvier. Na+ diffuses down conc gradient to depolarise membranes in the resting state.

Question 42

Describe passive propagation

Answer 42

Exponential decay of membrane potential as you move down axon away from the site of depolarisation. Lower rate of decay for larger diameter small myelinated neurones.
Internal (or axial) & Membrane Resistance alters propagation distance and velocity
Only resting K+ channels open

Question 43

Describe active propagation of the action potential

Answer 43

Local current depolarises adjacent region (at resting potential) to threshold

Active area is at the peak of its action potential

New area reaches action potential
Old area hyperpolarised- cannot be activated- eventually returns to resting potential.

Question 44

List two structural features that affect the conduction velocity along normal axons and briefly explain why they affect velocity as they do.

Answer 44

Diameter of neuron – Larger diameter = Lower resistance = Faster conduction speed due to more electronically charged ions. ▪ Degree of myelination – Myelination = Faster propagation, Non-myelination = Slower propagation (of same diameter axon). As action potentials only occur at nodes of ranvier.

Question 45

What can conduction velocity be reduced by

Answer 45

Reduced axon diameter – i.e. re-growth after injury. o Reduced myelination – i.e. Multiple Sclerosis. o Cold, anoxia, compression and drugs – i.e. some anaesthetics