lecture 18 - action potentials

Question 1

What is the ‘essential foundation’ for electrical signalling?

Answer 1

The establishment of a resting membrane potential

Question 2

What are the initial relative concentrations of K+ and Na+ across a cell membrane?

Answer 2

high [K+] in cytosol, high [Na+] in extracellular fluid

Question 3

What is the electrical gradient of a cell membrane?

Answer 3

A potential difference caused by a greater negative charge within the cytosol than in the extracellular fluid.

Question 4

What is the value of the resting membrane potential?

Answer 4

-70mV

Question 5

How are changes in membrane permeability controlled?

Answer 5

Gating of ion channels (the opening and closing of channels)

Question 6

What is chemical ion channel gating in terms of a neuron?

Answer 6

When a neurotransmitter (e.g. acetylcholine) is received by a neuron in its input zone, and binds to chemically gated channels allowing them to open and allow sodium ions to flow into the cytosol. The cell depolarises.

Question 7

When chemical ion channels allow Na+ to flow into the cytosol of a cell, what occurs to the membrane potential?

Answer 7

Potential difference decreases - the cell is depolarised.

Question 8

What is the process of chemical ion channel gating?

Answer 8

Neurotransmitters in extracellular fluid bind to chemically gated channels. Channel opens. Na+ flows down its concentration gradient into the cytosol. Neurotransmitter lost and channel closes.

Question 9

What is voltage ion channel gating?

Answer 9

Voltage gated channels detect a change in membrane potential (caused by chemically gated channels releasing Na+ into cell). They then open to cause more cations to flow into the cell. The channel is inactivated when the membrane potential becomes positive.

Question 10

Where is there a high density of voltage gated ion channels, and why?

Answer 10

In the axon hillock/initial segment, because it is where lots of chemical inputs are summed to create an action potential.

Question 11

What is mechanical ion channel gating?

Answer 11

In some membranes, channels will open to release Na+ only when a mechanical force is applied causing the physical distortion of the channel.

Question 12

What zone are chemical gated channels found in?

Answer 12

Input zone

Question 13

What zone are voltage gated ion channels found in?

Answer 13

Summation/conduction zone (Na+) and output zone (Ca2+)

Question 14

What is a local potential?

Answer 14

When ion channels open in receipt of a chemical signal, Na+ ions flow in causing a flow of local current away from the channel (K+ ions move towards more negative areas). This causes the area immediately surrounding the channel to become depolarised. (graded depolarisation, because it diminishes as the distance fro the source increases and there is no propagation)

Question 15

What are the two types of local potential?

Answer 15

Inhibitory and excitatory

Question 16

How does the membrane potential change when an excitatory local potential is initiated?

Answer 16

Depolarisation

Question 17

How does the membrane potential change when an inhibitory local potential is initiated?

Answer 17

Hyperpolarization

Question 18

What happens when an inhibitory and excitatory local potential occur at the same time?

Answer 18

They will sum to give no net change in potential.

Question 19

Where is the action potential generated?

Answer 19

The initial segment/axon hillock

Question 20

What must occur in order for an action potential to be initiated?

Answer 20

The potential difference at the initial segment, caused by the summation of local potentials, must meet the threshold (minimum depolarisation of approximately 10mV, giving potential of -60mV)

Question 21

What is minimal depolarisation needed for an action potential to be activated?

Answer 21

approximately +10mV above resting membrane potential (-60mV)

Question 22

What occurs when the depolarising local potentials reach threshold, and the action potential begins?

Answer 22

Voltage gated Na+ channels open, driving further depolarisation and a sudden large influx of cations.

Question 23

What occurs when the membrane potential reaches 30mV in an action potential?

Answer 23

Na+ channels close as maximum depolarisation is reached. Voltage gated potassium channels open and potassium ions move out of the cell to begin repolarisation

Question 24

What is the process of repolarisation in an action potential?

Answer 24

When the membrane potential reaches its maximum (approx. +30mV), the membrane potential will decrease as voltage gated potassium channels open to allow K+ to flow out of the cell to decrease membrane potential.

Question 25

At what stage do Potassium voltage gated channels close in an action potential?

Answer 25

Begin closing near the threshold potential (-60mV), but some may not close until after that.

Question 26

When are voltage-gated sodium channels reactivated in an action potential?

Answer 26

Near the threshold potential when potassium channels close.

Question 27

Why are Na+ channels activated at the end of action potential?

Answer 27

K+ channels may excessively repolarise the cell, i.e. decrease the membrane potential below -70mV, so some Na+ may need to enter the cell to increase it slightly to the resting potential.

Question 28

What are the 2 refractory periods of an action potential?

Answer 28

Absolute and Relative refractory periods.

Question 29

What is the absolute refractory period in an action potential?

Answer 29

A period in which no stimulus, no matter its size, will be able to generate another action potential.

Question 30

At what membrane potentials will the cell be in an absolute refractory period of an action potential?

Answer 30

At any point above a specific threshold (-60mV).

Question 31

Why can’t a second action potential be produced during the absolute refractory period?

Answer 31

Na+ channels are briefly inactivated, meaning they cannot release more Na+ to drive a second potential.

Question 32

What is the relative refractory period of an action potential?

Answer 32

A period when another action potential CAN be generate, but only in reponse to a very large stimulus, because sodium channels have been reactivated.

Question 33

Why do action potentials only travel down the axon in one direction?

Answer 33

When the action potentials propagate down the axon, they leave sections behind the potential where they membrane is still in a refractory period (i.e. inactivated sodium channels), so no actions potential can be activated in this area.

Question 34

What is propagation in terms of action potentials?

Answer 34

The process in which a potential travels down the axon after activation, by causing lots of depolarisation and repolarisation events all along the length of the axon to transmit the information to the end of the neuron.

Question 35

How are action potentials propagated in an unmyelinated axon?

Answer 35

An action potential occurs in the second segment (initiated by voltage-gated channels releasing sodium in the initial segment ). As it is repolarised, Na+ channels are reactivated, allowing the next section to reach threshold and then cause an influx of Na+ which is an action potential.

Question 36

What is the speed of action a potentials in an unmyelinated axon?

Answer 36

1-5m/s

Question 37

What is the speed of action potentials in myelinated axons?

Answer 37

20-100m/s

Question 38

What myelinates axons?

Answer 38

Oligodendrocytes (CNS), Schwann Cells (PNS)

Question 39

Where are large numbers of voltage gated Na+ channels found in a myelinated axon?

Answer 39

At the nodes of ranvier (between the myelinated regions)

Question 40

What is the process of action potential propagation in a myelinated axon?

Answer 40

Action potential develops at initial segment. Local current flows via myelinated section to node 1 where it exceeds the threshold and activates voltage gated channels to release Na+ and cause another action potential. Meanwhile, the initial segment repolarises and is in a refractory period. Process repeats as local current produces a grade depolarisation at node 2.

Question 41

Why does myelin speed up action potentials?

Answer 41

It preserves/insulates local current, allowing it to flow further and more rapidly along the axon before another action potential is initiated. Prevents leakage

Question 42

What ion has the greatest influence on resting membrane potential?

Answer 42

K+

Question 43

Why do K+ ions have the greatest influence on the resting membrane permeability?

Answer 43

They have the largest membrane permeability at rest

Question 44

What is the most important process in the development of a resting membrane potential?

Answer 44

Potassium ions diffuse out of the cell, down their concentration gradient, making the interior of the cell increasingly negative (until it reaches -70mV)

Question 45

In the development of the resting membrane potential, at what stage will the net flux of K+ into and out of the cell be zero?

Answer 45

At the equilibrium potential for the K+ ion

Question 46

How is the resting membrane potential maintained while K+ ions are constantly leaving the cell to repolarise it?

Answer 46

Cells maintain a high K+ and low Na+ concentration within the intracellular fluid via active transport using the Na+/K+-ATPase pump that exchanges 3Na+ and 2K+ out of and into the cell.

Question 47

What is ‘overshoot’ in terms of the resting membrane potential?

Answer 47

When the membrane potential reverses - i.e. becomes positive compared to the normally negative resting membrane potential. This switches the force acting on the cations in the cell.

Question 48

What do refractory periods limit?

Answer 48

Action potential frequency - the rate at which they can be produced in sequence

Question 49

What is a graded potential?

Answer 49

A potential that is not propagated along the axon, like an action potential, but instead declines with distance from the source. Are responsible for the the triggering of voltage gated ion channels that drive action potentials.