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IB Biology > 6.5 Neurons & Synapses > Flashcards

6.5 Neurons & Synapses Flashcards

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1
Q

What are neurons?

A

Neurons are specialised cells that function to transmit electrical impulses within the nervous system

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2
Q

What is the role of the nervous system?

A

The nervous system converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli

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3
Q

Why may neurons differ?

A

neurons may differ according to role (sensory, relay or motor)

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4
Q

What are the 3 basic components of all neurones?

A

dendrites
axon
soma

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5
Q

What are dendrites?

A

Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals

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6
Q

What is an axon?

A

An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors

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7
Q

What is a soma?

A

A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

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8
Q

What may the axon be surrounded by?

A

In some neurons, the axon may be surrounded by an insulating layer known as a myelin sheath

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9
Q

What is the role of the myelin sheath?

A

The myelin sheath improves the conduction speed of electrical impulses along the axon, but require additional space and energy

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10
Q

How do neurons generate and conduct electrical signals?

A

Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane

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11
Q

What does the unequal distribution of ions cause?

A

The unequal distribution of ions on different sides of the membrane creates a charge difference called a membrane potential

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12
Q

What is a resting potential?

A

A resting potential is the difference in charge across the membrane when a neuron is not firing

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13
Q

In a typical resting potential, which part of the neurone is more negative?

A

In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV)

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14
Q

What type of process is the maintenance of a resting potential?

A

The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps

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15
Q

What is a sodium-potassium pump and what is its role?

A

The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)

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16
Q

What amount of ions are exchanged in a sodium-potassium pump?

A

It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)

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17
Q

What gradient does the sodium-potassium channel create?

A

This creates an electrochemical gradient

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18
Q

What does the electrochemical gradient create in terms of cell potential?

A

the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)

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19
Q

What does the exchange of sodium and potassium require?

A

The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)

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20
Q

What are action potentials?

A

Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing

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21
Q

What are the 3 stages of action potentials?

A

Action potentials occur in three main stages: depolarization, repolarization and a refractory period

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22
Q

What is depolarisation?

A

Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge

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23
Q
  1. What occurs in response to a signal initiated at a dendrite?
    (depolarisation)
A

In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon

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24
Q
  1. What does the opening of sodium channels?
A

As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium

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25
Q
  1. How does the influx of sodium ions affect membrane potential?
A

The influx of sodium causes the membrane potential to become more positive (depolarisation)

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26
Q

What is repolarisation?

A

Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge)

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27
Q

What channels after the influx of sodium ions?

A

Following an influx of sodium, potassium channels open within the membrane of the axon

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28
Q

By what mechanism do potassium ions in/out the neuron?

A

As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium

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29
Q

What does the efflux of potassium cause?

A

The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarisation)

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30
Q

What is the refractory period?

A

The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again

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31
Q

Where are sodium ions in a normal resting state?

A

In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential)

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32
Q

What is reversed following depolarisation?

A

Following depolarisation (sodium influx) and repolarisation (potassium efflux), this ionic distribution is largely reversed

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33
Q

What restores the resting potential during the refractory period?

A

Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump

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34
Q

What are nerve impulses?

A

Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation

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35
Q

When does depolarisation occur?

A

Depolarisation occurs when ion channels open and cause a change in membrane potential

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36
Q

What can the channels along the axon be categorised as?

A

The ion channels that occupy the length of the axon are voltage-gated (open in response to changes in membrane potential)

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37
Q

What does the ion channels being voltage-gated mean for depolarisation?

A

Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon

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38
Q

Therefore how does depolarisation spread?

A

This causes depolarisation to spread along the length of the axon as a unidirectional ‘wave’

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39
Q

According to what principle are action potentials generated?

A

Action potentials are generated within the axon according to the all-or-none principle

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40
Q

What is needed for an action potential to be propagated?

A

An action potential of the same magnitude will always occur provided a minimum electrical stimulus is generated

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41
Q

What is the threshold potential?

A

This minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels

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42
Q

What happens if the threshold potential is not reached?

A

If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire

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43
Q

When are threshold potentials triggered?

A

Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation

44
Q

What happens if the threshold potential is reached?

A

If the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section

45
Q

What are oscilloscopes?

A

Oscilloscopes are scientific instruments that are used to measure the membrane potential across a neuronal membrane

46
Q

How is data from an oscilloscope presented?

A

Data is displayed as a graph, with time (in milliseconds) on the X axis and membrane potential (in millivolts) on the Y axis

47
Q

For how long will a typical action potential last?

A

A typical action potential will last for roughly 3 – 5 milliseconds and contain 4 key stages

48
Q

What 4 stages are shown on the graph?

A

resting potential
depolarisation
repolarisation
refractory period

49
Q

What is the resting potential of an oscilloscope trace?

A

Before the action potential occurs, the neuron should be in a state of rest (approx. –70 mV)

50
Q

What is depolarisation on an oscilloscope trace?

A

A rising spike corresponds to the depolarisation of the membrane via sodium influx (up to roughly +30 mV)

51
Q

What is repolarisation on an oscilloscope trace?

A

A falling spike corresponds to repolarisation via potassium efflux (undershoots to approx. –80 mV)

52
Q

What is refractory period on an oscilloscope trace?

A

The oscilloscope trace returns to the level of the resting potential (due to the action of the Na+/K+ pump)

53
Q

When will an action potential occur?

A

An action potential will only occur if the initial depolarization exceeds a threshold potential of approximately –55 mV

54
Q

Are all neurones covered with myelin?

A

NO
In certain neurons, the axon may be covered by a fatty white substance called myelin which functions as an insulating layer

55
Q

What is myelin and what produces it?

A

Myelin is a mixture of protein and phospholipids that is produced by glial cells (Schwann cells in PNS; oligodendrocytes in CNS)

56
Q

What is the main purpose of myelin?

A

The main purpose of the myelin sheath is to increase the speed of electrical transmissions via saltatory conduction

57
Q

How is an action potential propagated along unmyelinated neurons?

A

Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation

58
Q

How is an action potential propagated along myelinated neurons?

A

In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier

59
Q

By what factor does myelination increase the speed of electrical conduction?

A

This results in an increase in the speed of electrical conduction by a factor of up to 100-fold

60
Q

What is the advantage of myelination?

A

The advantage of myelination is that it improves the speed of electrical transmission via saltatory conduction

61
Q

What is the disadvantage of myelination?

A

The disadvantage of myelination is that it takes up significant space within an enclosed environment

62
Q

What appears as white matter?

A

Regions of the nervous system composed of myelinated axon tracts appear as white matter, all other areas appear as grey matter

63
Q

What appears as grey matter?

A

Grey matter consists of neuronal cell bodies and dendrites, as well as support cells (glial cells) and synapses

64
Q

How do nerves transmit electrical impulses?

A

Nerves transmit electrical impulses by changing the ionic distribution across the neuronal membrane (membrane potential)

65
Q

When are electrical signals unable to be propagated?

A

Therefore, electrical signals are not able to be conducted when a semi-permeable membrane is absent

66
Q

What are synapses?

A

Synapses are the physical gaps that separate neurons from other cells (other neurons and receptor or effector cells)

67
Q

How do neurons transmit information across synapses?

A

Neurons transmit information across synapses by converting the electrical signal into a chemical signal

68
Q
  1. What happens when an action potential reaches the axon terminal?
    synaptic transmission
A

When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels

69
Q
  1. What is there an influx of?
    synaptic transmission
A

Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitter) with the cell membrane

70
Q
  1. What is released into the synapse and by what process?
    synaptic transmission
A

The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft

71
Q
  1. What do neurotransmitters bind to?
    synaptic transmission
A

Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels

72
Q
  1. What does the opening of ion channels cause?
    synaptic transmission
A

The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal

73
Q
  1. What happens to the neurotransmitters?
    synaptic transmission
A

The neurotransmitters released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)

74
Q

What are neurotransmitters?

A

Neurotransmitters are chemical messengers released from neurons and function to transmit signals across the synaptic cleft

75
Q

When are neurotransmitters released?

A

Neurotransmitters are released in response to the depolarisation of the axon terminal of a presynaptic neuron

76
Q

Where do neurotransmitters bind?

A

Neurotransmitters bind to receptors on post-synaptic cells and can either trigger (excitatory) or prevent (inhibitory) a response

77
Q

What 3 cells can neurotransmitters trigger?

A

neuron
glandular cell
muscle fibre

78
Q

What is the response of a neuron when triggered by neurotransmitters?

A

stimulation or inhibition of an electrical signal (nerve impulse)

79
Q

What is the response of a glandular cell when triggered by neurotransmitters?

A

stimulation or inhibition of secretion (exocrine or endocrine)

80
Q

What is the response of muscle fibre when triggered by neurotransmitters?

A

stimulation or inhibition of muscular contraction/relaxation

81
Q

What is an example of a neurotransmitter?

A

One example of a neurotransmitter used by both the central nervous system and peripheral nervous system is acetylcholine

82
Q

Where can acetylcholine be released?

A

neuromuscular junctions
autonomic nervous system

83
Q

What is the role of acetylcholine in neuromuscular junctions?

A

It is commonly released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction

84
Q

What is the role of acetylcholine in the autonomic nervous system?

A

It is also commonly released within the autonomic nervous system to promote parasympathetic responses (‘rest and digest’)

85
Q

Where is acetylcholine created and how?

A

Acetylcholine is created in the axon terminal by combining choline with an acetyl group (derived from mitochondrial Acetyl CoA)

86
Q

Where is acetylcholine stored and when is it released?

A

Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse

87
Q

What does acetylcholine activate?

A

Acetylcholine activates a post-synaptic cell by binding to one of two classes of specific receptor (nicotinic or muscarinic)

88
Q

What must be done to acetylcholine in the synapse, continuously?

A

Acetylcholine must be continually removed from the synapse, as overstimulation can lead to fatal convulsions and paralysis

89
Q

What breaks down acetylcholine?

A

Acetylcholine is broken down into its two component parts by the synaptic enzyme acetylcholinesterase (AChE)

90
Q

Where can acetylcholinesterase be found?

A

AChE is either released into the synapse from the presynaptic neuron or embedded on the membrane of the post-synaptic cell

91
Q

What is done with the degraded components of acetylcholine?

A

The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine

92
Q

What are neonicotinoid pesticides?

A

Neonicotinoid pesticides are able to irreversibly bind to nicotinic acetylcholine receptors and trigger a sustained response

93
Q

Why do neonicotinoid pesticides persist?

A

Neonicotinoid pesticides cannot be broken down by acetylcholinesterase, resulting in permanent overstimulation of target cells

94
Q

What does overstimulation of acetylcholine receptors produce?

A

While low activation of acetylcholine receptors promotes nerve signalling, overstimulation results in fatal convulsions and paralysis

95
Q

Why are neonicotinoids more persistent in insects?

A

Insects have a different composition of acetylcholine receptors which bind to neonicotinoids much more strongly

Hence, neonicotinoids are significantly more toxic to insects than mammals, making them a highly effective pesticide

96
Q

What are 3 disadvantages of neonicotinoids?

A

Neonicotinoid use has been linked to a reduction in honey bee populations (bees are important pollinators within ecosystems)

Neonicotinoid use has also been linked to a reduction in bird populations (due to the loss of insects as a food source)

Consequently, certain countries (including the European Union) have restricted the use of neonicotinoid pesticides

97
Q

What is the role of neurotransmitters?

A

Neurotransmitters bind to neuroreceptors on the post-synaptic membrane of target cells and open ligand-gated ion channels

98
Q

What is a graded potential?

A

The opening of these channels cause small changes in membrane potential known as graded potentials

99
Q

When is a nerve impulse initiated?

A

A nerve impulse is only initiated if a threshold potential is reached, so as to open the voltage-gated ion channels within the axon

100
Q

What do excitatory neurotransmitters cause?

A

Excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation by opening ligand-gated sodium or calcium channels

101
Q

What do inhibitory neurotransmitters cause?

A

Inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation by opening ligand-gated potassium or chlorine channels

102
Q

What determines whether a threshold potential is reached?

A

The combined action of all neurotransmitters acting on a target neuron determines whether a threshold potential is reached

103
Q

Considering depolarisation and hyperpolarisation, when will threshold potential be reached?

A

If overall there is more depolarisation than hyperpolarisation and a threshold potential is reached, the neuron will fire

104
Q

Considering depolarisation and hyperpolarisation, when will threshold potential NOT be reached?

A

If overall there is more hyperpolarisation than depolarisation and a threshold potential is not reached, the neuron will not fire

105
Q

What is the typical threshold potential?

A

For a typical neuron, the threshold potential (required to open voltage-gated ion channels) is approximately –55 mV

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