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biosci S1 Pt2 > Lec 27 and 28 - Action potentials > Flashcards

Lec 27 and 28 - Action potentials Flashcards

1
Q

What is the concept behind hyperpolarisation and depolarisation?

A

Potential inside neurons is not always constant- it changes when membrane permeability or ion concentrations change

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2
Q

What factors determine the potential inside neurons?

A

membrane permeability changes
ion concentration changes

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3
Q

What is hyperpolarisation?

A

Where the inside of the cell becomes more negative (e.g -70 to -75mV)

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4
Q

What usually causes hyperpolarisation?

A

The inside potential moving closer to the K equilibrium potential (EK)

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5
Q

What is depolarisation?

A

When the inside of the cell becomes more positive (e.g -75mV to -70mV)

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6
Q

What usually causes hyperpolarisation?

A

The inside potential moving closer to the Na equilibrium potential (ENa)

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7
Q

What is an action potential?

A

A brief fluctuation in membrane potential causes by a transient opening of voltage-gated ion channels which spreads, like a wave, along an axon.

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8
Q

Are voltage gated channels and leak channels the same thing?

A

No

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9
Q

What is the threshold potential

A

The potential a membrane has to reach before an action potential can occur

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10
Q

Why do we need action potentials?

A
  • The frequency encodes information (a language by which neurons communicate)
  • AP are a key element of signal transmission across (usually very long) axons
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11
Q

Fill the boxes

A

Starts with RMP
1: Stimulus - here it is depolarisation
2: Fast depolarisation - It reaches the threshold and very rapidly depolarises
3: Repolarisation - gradually becomes more negative
3.5: is at RMP
4: After-hyperpolarisation - becomes slightly more negative than RMP

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12
Q

What is the section across 1 and 2?
What is the section at 3?

A

1 -> 2 is the absolute refractory period (even if we have another stimulus, action potential will not change)
3 is the relative refractory period
(we can produce another action potential if the next threshold is larger than your original)

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13
Q

Why is the top of the main peak curved and not pointy? (overshoot point)

A

Because there is a reduction in the steepness of the electrical gradient

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14
Q

Explain parts 2, 3 and 4 of the line- describe what is happening

A

At 2, the voltage-gated Na+ channels open/activate. they move along electrochemical and concentration gradients.
At 3, the voltage gated Na+ channels begin to deactivate and the voltage gated K+ channels begin to open
At 4, the voltage gated Na+ channels are at rest, but the K+ channels are still open, hence the increasing negative value

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15
Q

What is the permeability ratios of K+:Na+ in each step of the graph?

A

1: 40:1 (before)
2: 1:20 (activation of Na+ channels)
3: 100:1 (hyperpolarisation)

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16
Q

What is an inactivation gate?

A

A gate for a voltage gated ion channel that closed the gate faster than waiting for the activation gate to close on its own

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17
Q

What is the average amplitude of an action potential?

A

100 mV

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18
Q

What is a subthreshold depolarisation?

A

Aka the stimulus on the action potential graph

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19
Q

Does the amplitude of an action potential depend on stimulus intensity?

A

No, as long as the stimulus is ‘suprathreshold’

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20
Q

What does the action potential ‘all or nothing’ mean?

A

It will either fire up to +30 mV or not fire at all. you can’t get a 1/2 action potential

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21
Q

What are the two ways we can start an action potential?

A

Internally (underphysiological situations, post synaptic potentials build up (neurotransmitters))
externally (under experimental conditions - such as stimulation of the giant squid AP via electrodes/battery)

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22
Q

What is the simple process of evoking an action potential?

A

Synaptic potentials which start from the axon hillock (initial axon segment). This leads to a change in membrane potential, and it this passes the threshold, then an action potential will happen

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23
Q

what are the two main paths current can take when trying to evoke an action potential externally?

A

Most of it actually flows along the outside of the axon of the neuron, but the current that enters through the cell membrane from the anode to the cathode is the one that changes to the RMP

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24
Q

Where will you have local depolarisation and hyperpolarisation?

A

Local depolarisation is under the cathode (inside to outside), local hyperpolarisation is under the anode (outside to inside)

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25
Q

How do action potentials move across axons?

A

local depolarization reaches threshold, Na+ gated channels open at this part of the membrane and stimulate action potentials across and other Na+ gated channels opening as you move down the axon

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26
Q

What potentials travel on the dendrites and cell body?

A

the synaptic potentials, which are passive and graded.

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27
Q

Where is the first point on a neuron where there are voltage gated ion channels?

A

The axon hillock, or initial axon segment.
There is a high density of VGIC

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28
Q

Where is an AP first generated and how is it different to the rest of the cell?

A

In the Axon hillock, or initial axon segment.
It has the lowest threshold, so all action potentials start here (so its a trigger zone)

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29
Q

How is the depolarization to threshold evoked internally?

A

excitatory postsynaptic potentials which spread mainly passively from the dendrites

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30
Q

Once transmitted, action potentials are transmitted _________ along the axon

A

Actively

31
Q

What causes the action potentials to only move in one direction?

A

The high density of voltage gated ion channels at the axon hillock. It can only go down the axon. There are no VGIC in the other direction

32
Q

Imagine an inhibitor bound to the Na+ channels, preventing them from closing. What would happen to the action potential cycle?

A

Inside of the cell will increase
It will depolarise and meet threshold.
Won’t repolarise, even if K+ channels are open, since the Na+ channels cant close.

33
Q

imagine an inhibitor bound to the Na+ channels, preventing them from closing. What would happen to the subsequent signals sent to the neuron?

A

Nothing will happen. It can’t send further action potentials.
Muscles will be contracted, because it has been depolarized and neurotransmitters have attached.
physiogically, you will have spastic paralysis.

34
Q

What is the inhibitor that prevents the voltage gated Na+ channels from closing?

A

Bactrachotoxin

35
Q

When stimulating an action potential with a battery, which direction will the aciton potentials go?

A

Both directions, not just one like physiologically.

36
Q

What happens during the stimulus step of an action potential?

A

Cation channels (NOT voltage gated - e.g ligand) cause the graded potential to meet threshold.
Permeability to cations will increase
membrane potential increases.

37
Q

Describe the fast depolarisation step of an action potential?

A

Voltage gated sodium channels opening
Permeability to sodium increases
membrane potential becomes more positive.

38
Q

Describe the repolarisation step of an action potential?

A

Voltage gated Na+ channels are inactivating - inactivation gates are shutting
Voltage gated K+ channels are opening.
Permeability to K+ increases
Membrane potential becomes more negative.

39
Q

Describe the hyperpolarisation step of an action potential?

A

Voltage gated sodium channels activation gates are closing, inactivation will eventually be opening
Voltage gated K+ channels are still open
Permeability higher to K+
Membrane potential becomes more negative.

40
Q

Name and describe the two types of axons

A

Unmyelinated axons
- fairly small
- Transmission of APs are slow but continuous
Myelinated axons
- Much larger 5-10x
- Much faster transmission of APs, but they are saltatory (in large steps)

41
Q

What are the two stages of action potential transmission and in which type of axons do they occur?

A

Passive spread
Generation of action potentials
The two stages happen in both types of axons

42
Q

What is the Axolemma and Axoplasm?

A

The axolemma is the cell membrane, the axoplasm is the cell cytoplasm

43
Q

How does the passive spread of current work in action potentials?

A
  1. there is a sub threshold (depolarization - a positive charge) in one region of the inside of the axon
  2. Positive charges (both inside and outside the cell) move to where there is negative charge
44
Q

Does passive spread of current in action potential transmission go far?

A

No, it can only travel short distances, the current dissipates along the axon usually at ~1mm.

45
Q

What is a microelectrode typically made of?

A

Glass

46
Q

Why can’t passive current keep moving for longer?

A

Charges are leaving through leak channels due to diffusion of ions

47
Q

What do action potentials help with that passive current cannot do?

A

carrying on the electric potential for longer

48
Q

What causes local depolarisation of the membrane/axolemma?

A

The small amount of current that enters the cell when externally stimulating an action potential

49
Q

Does depolarisation happen under the anode or the cathode?

A

Cathode (positive charge accumulates underneath a negatively charged cathode)

50
Q

Describe the process of action potential transmission in an unmyelinated axon

A

small amount of current passing through the cell membrane causes local depolarization.
If this reaches threshold, it will open voltage gated ion channels and allows + ions in
This charge passively (current) flows.
The section current is flowing to, if the charge reaches threshold again, a new action potential will occur.

51
Q

What causes the conduction of an unmyelinated axon to be slow?

A

Passive flow between two points is fast, but action potentials must be regenerated at every point on the membrane, and so conductance is slow

52
Q

What is the conduction velocities of myelinated and unmyelinated axons?

A

unmyelinated = 1m/s
myelinated = 20-100m/s

53
Q

Myelination is not continuous along the axon. What causes this discontinuation?

A

Myelination is interrupted at the nodes of Ranvier

54
Q

are oligodendrocytes or schwann cells a part of the CNS?

A

Oligodendrocytes

55
Q

Schwann and oligodendrocytes are types of ______ cells

A

Glial

56
Q

Why are myelinated axons better for conducting passive spread?

A

Because myelin is insulating, it prevents too much dissipation of charge

57
Q

Does passive conduction act both left and right?

A

Yes, both ways

58
Q

At which point in myelinated axons can current dissipate out of the cell?

A

At the nodes of Ranvier, where there is no insulating myelin

59
Q

How does myelination increase the speed of action potential conduction?

A

Increasing the efficiency of passive spread.
& because action potentials do not need to be regenerated at every point of the cell membrane, only at the nodes of ranvier (this is called saltatory conduction)

60
Q

Which is the only point on a myelinated axon that action potentials are generated?

A

At the nodes of ranvier (current flows passively between nodes)

61
Q

Do myelinated or unmyelinated spend more energy on the concentration gradient?

A

Unmyelinated uses more energy

62
Q

Why do we have unmylinated axons if myelinated is so much better?

A

Sometimes we want slow conduction.
MORE MYELINATED LEAVES LESS SPACE SO WE WOULD HAVE LESS NEURONS IN TOTAL IF WE ONLY HAD MYELINATED
costs more energy to make myelinated

63
Q

Why can’t action potentials travel backwards in physiological conditions?

A

Due to the absolute refractory period

By the time the ARP is over, the AP is already at node 4

Imagine there are 3 nodes. By the time the current reaches the third one, the first nodes’ voltage gated Na+ channels are still activated, and cannot do another action potential. Once it gets to node four, the first node channels are at rest, but the passive current can’t generate an action potential that far away that will meet the threshold

64
Q

In physiological conditions, the action potentials travel ____ while the passive current travels _______

A

One way, both ways

65
Q

What type of neurons does the PNS specifically contain?

A

sensory neurons

66
Q

What are the two types of neurons and their purposes?

A

Sensory neurons to detect stimuli from an external environment and motor neurons whose stimulus are neurotransmitters and the output is movement

67
Q

Where is the muscle spindle?

A

At your joints

68
Q

How does the muscle spindle work?

A

The more it stretches, the more mechanically gated ion channels it opens, allowing more ions through

69
Q

What does it mean by a graded potential?

A

The action can happen at any level but must happen at a certain level to activate something else
Graded potentials are dependent on the stimulus and are not voltage gated.

70
Q

When a stimulus acts on receptors in sensory neurons, does it immediately evoke an action potential?

A

No

71
Q

What is the receptor potential?

A

Muscle spindle evokes a graded depolarization known as the ‘receptor potential’

72
Q

What is the trigger zone in sensory neurons?

A

Where action potentials are generated from passively spread receptor potentials (located more distal from the muscle spindle) Then APs go toward CNS along axon.
Acts similar to axon hillock

73
Q

What happens if we have a very large stimulus? (such as major stretch of muscle spindle)

A

Larger amplitude of receptor potentials means higher frequency of action potentials.
Amplitude of action potentials is always the same

biosci S1 Pt2 (16 decks)

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