lecture 18 - action potentials Flashcards

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1
Q

What is the ‘essential foundation’ for electrical signalling?

A

The establishment of a resting membrane potential

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2
Q

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

A

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

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3
Q

What is the electrical gradient of a cell membrane?

A

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

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4
Q

What is the value of the resting membrane potential?

A

-70mV

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5
Q

How are changes in membrane permeability controlled?

A

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

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6
Q

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

A

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.

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7
Q

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

A

Potential difference decreases - the cell is depolarised.

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8
Q

What is the process of chemical ion channel gating?

A

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.

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9
Q

What is voltage ion channel gating?

A

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.

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10
Q

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

A

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

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11
Q

What is mechanical ion channel gating?

A

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

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12
Q

What zone are chemical gated channels found in?

A

Input zone

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13
Q

What zone are voltage gated ion channels found in?

A

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

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14
Q

What is a local potential?

A

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)

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15
Q

What are the two types of local potential?

A

Inhibitory and excitatory

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16
Q

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

A

Depolarisation

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17
Q

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

A

Hyperpolarization

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18
Q

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

A

They will sum to give no net change in potential.

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19
Q

Where is the action potential generated?

A

The initial segment/axon hillock

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20
Q

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

A

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)

21
Q

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

A

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

22
Q

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

A

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

23
Q

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

A

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

24
Q

What is the process of repolarisation in an action potential?

A

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.

25
Q

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

A

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

26
Q

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

A

Near the threshold potential when potassium channels close.

27
Q

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

A

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.

28
Q

What are the 2 refractory periods of an action potential?

A

Absolute and Relative refractory periods.

29
Q

What is the absolute refractory period in an action potential?

A

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

30
Q

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

A

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

31
Q

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

A

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

32
Q

What is the relative refractory period of an action potential?

A

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

33
Q

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

A

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.

34
Q

What is propagation in terms of action potentials?

A

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.

35
Q

How are action potentials propagated in an unmyelinated axon?

A

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.

36
Q

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

A

1-5m/s

37
Q

What is the speed of action potentials in myelinated axons?

A

20-100m/s

38
Q

What myelinates axons?

A

Oligodendrocytes (CNS), Schwann Cells (PNS)

39
Q

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

A

At the nodes of ranvier (between the myelinated regions)

40
Q

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

A

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.

41
Q

Why does myelin speed up action potentials?

A

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

42
Q

What ion has the greatest influence on resting membrane potential?

A

K+

43
Q

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

A

They have the largest membrane permeability at rest

44
Q

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

A

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

45
Q

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?

A

At the equilibrium potential for the K+ ion

46
Q

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

A

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.

47
Q

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

A

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.

48
Q

What do refractory periods limit?

A

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

49
Q

What is a graded potential?

A

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.