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Nerve & Muscle > Neural Signalling > Flashcards

Neural Signalling Flashcards

1
Q

What is the Hodgkin & Huxley experiment?

A

Placing electrodes inside a neurone allows measurement of the potential difference between the inside and the outside of the cell

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2
Q

What is the resting potential of a neurone?

A

-65 mV

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3
Q

How do intracellular proteins contribute to the resting membrane potential?

A

There are large proteins within the cytoplasm that are too large to pass through channels in the membrane

They have a predominance of negatively charged groups on their surface

The lack of membrane permeability means the proteins are trapped within the cell, making it increasingly negatively charged

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4
Q

How does the sodium/potassium ion pump contribute to the resting membrane potential?

A

It moves 3 Na+ ions out of the cell for every 2 K+ ions it allows in

The inside of the cell becomes increasingly negative

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5
Q

What is the potassium ion gradient?

A

Potassium ions tend to diffuse out of the cell down a concentration gradient through K+ selective channels

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6
Q

How does the potassium ion gradient contribute to the resting membrane potential?

A

The large negatively charged protein molecules trapped in the cell cause an electrical gradient that tends to pull K+ ions back into the cell

The fluxes become balanced so that K+ distribution is in equilibrium

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7
Q

What is the potassium equilibrium potential?

Why is it not the same as the resting membrane potential?

A

-70 mV

The small leakage of sodium ions means the values are slightly different

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8
Q

How do you determine the equilibrium potential for any ion?

A

Use the Nernst Equation

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9
Q

How do sodium ion gradients influence the resting membrane potential?

A

Both the concentration and electrical gradients operate in the same direction to cause an inward flow of ions

This brings the resting membrane potential back up to -65 mV

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10
Q

Why may some neurones have resting potentials outside of the normal range?

A

Variation is due to differing levels of expression of ion channels in the membrane

Relative permeability of ions depends on the number of channels in the membrane through which ions can pass

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11
Q

What is an action potential?

A

It is an explosion of electrical activity created by a depolarising current

It is the means by which a neurone sends information down its axon, away from the cell body

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12
Q

What happens during the resting state of an action potential?

A

All the voltage-gated Na+ and K+ channels are closed

They remain closed until the threshold potential is reached

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13
Q

What happens once Na+ channels open?

A

Sodium ions enter the cell

The cell becomes depolarised as the membrane potential rises towards zero

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14
Q

What is the threshold potential and what happens when it is reached?

A

-40 mV

If it is reached, voltage-gated Na+ channel activation gates start to open

Na+ ions enter the cell, causing it to be more positive and open more activation gates

Large influx of Na+ ions leads to an action potential spike

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15
Q

What happens after the action potential spike?

A

Na+ channels close when Na+ equilibrium potential is reached which is +65mV

Inactivation gates begin to close as the interior of the cell becomes less negative

Voltage-gated K+ channel activation gates start to open and K+ ions flow out of the cell

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16
Q

What happens during repolarisation?

A

The membrane potential is reversed as K+ ions leave the cell

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17
Q

What happens during hyperpolarisation?

A

K+ ions continue to flow out of the cell whilst the Na+ channel inactivation gates are fully closed

This leads to the refractory period where no action potentials can be generated

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18
Q

How is resting potential re-established?

A

When the flow of ions stops, the ions are redistributed across the membrane by Na+/K+ pump

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19
Q

What is the example of positive feedback in the action potential?

A

The nerve cell only depolarises if it reaches the threshold potential

Voltage-gated Na+ channels open and more Na+ enter the cell

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20
Q

What is meant by the “all-or-nothing” response?

A

For each type of nerve cell, the amplitude of the action potential, the resting potential and the threshold potential are constant

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21
Q

What happens to the action potential if the stimulus intensity is increased?

A

The amplitude of the action potential will NOT change

There is a higher action potential frequency

There is a shorter latency period between the stimulus and the action potential

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22
Q

Why is the refractory period important?

A

It means that an action potential can only travel in one direction and cannot travel back towards its point of origin

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23
Q

What is the absolute refractory period?

A

No further action potentials can be elicited no matter how large the stimulus is

This ensures action potential propagation is unidirectional

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24
Q

What is the relative refractory period?

A

During this period, a larger stimulus can result in an action potential

Initiation of a second action potential is inhibited, but not impossible

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25
Q

How is an action potential propagated in a non-myelinated neurone?

A

Na+ influx will depolarise an area in the neurone and trigger voltage-gated Na+ channels to open further along

This generates an action potential in the next membrane segment

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26
Q

How is an action potential propagated in a myelinated neurone?

A

Local currents can only flow in and out of the axon at the nodes of Ranvier

This is the only area where the membrane can depolarise

The action potential “jumps” between the nodes of Ranvier by saltatory conduction

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27
Q

What is a sensory modality?

A

It is what is perceived after the stimulus

“sense”

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28
Q

What is sensory transduction and why is it needed?

A

It is the conversion of environmental or internal signals into electrochemical energy

All stimuli must be converted to electrochemical energy in order to be transported along the axon

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29
Q

What is a receptor potential?

A

The transmembrane potential difference produced by activation of a sensory receptor

It is a graded potential that causes an action potential if the threshold is reached

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30
Q

How are specific signals decoded in the CNS?

A

Through the rate and pattern of action potential firing

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31
Q

What are muscle spindles?

A

A bunch of modified skeletal muscle fibres (intrafusal fibres) enclosed in a connective tissue capsule

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32
Q

How do muscle spindles prevent the muscle from being over-stretched?

A

Intrafusal fibres detect stretch and initiate a reflex that causes the muscle to contract

33
Q

How does action potential frequency change when the muscle spindle is activated?

A

When a muscle is stretched passively the spindle is activated and initiates a reflex

Action potential frequency increases during stretch

When the muscle contracts and shortens, the reflex is switched off

Action potential frequency declines during contraction

34
Q

How is the knee-jerk reflex initiated?

A

Striking of the patellar ligament with a reflex hammer just below the patella

This stretches the intrafusal fibres of the muscle spindle in the quadriceps muscle

35
Q

How is the knee-jerk reflex conducted after the muscle spindle is stretched?

A

Stretching the muscle will stretch the spindle and increase discharge of the sensory neurones

Increased firing of the motor neurone

The quadriceps muscle contracts

36
Q

What is significant about the knee-jerk reflex?

A

There is no spinal interneurone

The bipolar sensory neurone synapses directly on a motor neurone in the spinal cord

37
Q

What is the role of the inhibitory interneurone in the knee-jerk reflex?

A

It is involved in relaxation of the antagonistic hamstring muscle

This helps to dampen the stretch of the muscle

38
Q

What is the Golgi Tendon organ and where is it found?

A

It is located in the tendon

It responds to tension

It is stimulated when the associated muscle contracts or is stretched and protects the muscle against excess load

39
Q

What is the structure of the Golgi Tendon organ?

A

Small bundles of tendon fibres (collagen) enclosed in a layered capsule

The terminal branches of a large diameter afferent fibre are intertwined with collagen bundles

40
Q

What is the reflex inhibition set up by the GTO?

A

It sets up a reflex that causes muscle to relax

This removes stimulation

41
Q

When is GTO active?

A

During both passive stretch and active contraction

42
Q

What type of receptors are muscle spindles and the GTO?

A

They are both proprioceptors and mechanoreceptors

43
Q

What are the two different types of synapses and how do they work?

A

Electrical synapses direct a passage of current via ions flowing through gap junctions

Chemical synapses release vesicles containing neurotransmitter which has an effect on receptors on the target cell

44
Q

How do gap junctions result in fast electrical transmission between neurones?

A

Gap junctions are present at points of contact between neurones with no synaptic cleft

There is only a very narrow gap between their membranes

Ions can pass directly from one cell to the next

45
Q

What are gap junctions formed by?

A

They are formed by channels called connexons

46
Q

What is the role of connexons?

A

They form pores that allow the cytoplasm of the two cells to be in continuity

The pores have some selectivity over ions and small molecules that can pass through

47
Q

What are connexons made of?

A

Protein molecules called connexins

48
Q

How are neurotransmitters released from the presynaptic neurone?

A

Action potential reaches the terminal bouton of the presynaptic cell

Voltage gated Ca2+ channels open and calcium enters the cell

Calcium influx causes the vesicle and synaptic membranes to fuse

Neurotransmitters are released into the synaptic cleft

49
Q

Why are vesicles docked and primed before the action potential arrives?

A

They are kept close to the plasma membrane

This allows release to be as rapid as possible upon increase in calcium concentration

50
Q

How can the probability of neurotransmitter release be increased and decreased?

A

Increased by increasing calcium concentration

Decreased by blocking depolarisation of the membrane and preventing calcium influx

51
Q

What happens once the neurotransmitter is released?

A

It diffuses across the synaptic cleft to act on specific receptors on the postsynaptic membrane of a cell body or dendrite

52
Q

How do postsynaptic receptors alter the properties of the postsynaptic cell?

A

They open gated ion channels

This allows action potential signals to be communicated from one neurone to the next

53
Q

What is a neurotransmitter?

A

A substance shown to be released by a neurone and have a physiological action on specific receptors on a target cell

54
Q

What is a neuromodulator?

A

A substance that is released and modifies the action of a neurotransmitter

55
Q

What is a neuroactive substance?

A

A neutral term for a substance that is known to have an effect on the CNS

Its precise action is not known

56
Q

What are the amine neurotransmitters?

A 
dopamine
noradrenaline
adrenaline
histamine
serotonin
57
Q

what are the amino acid neurotransmitters?

A

gamma-aminobutyric acid (GABA)
glutamate
glycine

58
Q

what are the peptide neurotransmitters?

A 
dynorphin
enkephalins
neuropeptide Y
calcitonin gene-related peptide (CGRP)
somatostatin
galanin
substance P
thyrotropin-releasing hormone
vasoactive intestinal polypeptide
59
Q

What are active zones? What is found there?

A

specialised areas on the presynaptic membrane

they guide the vesicles towards the membrane in a calcium-dependent fashion

synaptic boutons contain voltage-gated calcium channels near the active zone

60
Q

What is the process of docking?

A

Vesicles release their contents by exocytosis when the membrane of the synaptic vesicle fuses to the presynaptic membrane at the active zone

61
Q

How does priming allow for a rapid release of neurotransmitter?

A

Ca2+ enters the axon terminal directly at the active zone

This is precisely where the vesicles are primed for exocytosis

62
Q

What is significant about the local microdomain around the active zone?

A

It allows calcium to reach high concentrations

63
Q

How is the fused membrane vesicle taken back into the cell?

A

Endocytosis

64
Q

What does an ionotropic receptor comprise?

A

Ion channel that comprises 4 or 5 similar protein subunits arranged around a central pore

65
Q

What happens when a neurotransmitter binds to an ionotropic receptor?

A

It causes a conformational change that will briefly open the central pore

Ions can pass through and cause a rapid change in the resting potential of the cytoplasm

66
Q

How fast is the response from an ionotropic receptor?

A

They provide rapid responses

The movement of ions will depolarise or hyperpolarise the postsynaptic cell

67
Q

What is a metabotropic receptor comprised of?

A

Single, long protein molecule with 7 transmembrane domains

It has no ion pore

68
Q

What happens when a ligand binds to a metabotropic receptor?

A

There is a conformational change in the molecule, causing the intracellular part to interact with a G-protein

There is a chain of intracellular events that may lead to the opening of ion channels

69
Q

How fast is the response of a metabotropic receptor?

A

Much slower than ionotropic receptors

70
Q

What is an excitatory transmitter receptor?

A

It allows an influx of Na+ when it is activated

This causes a net inwards current and results in an EPSP

71
Q

What is an EPSP?

What does it do?

A

Excitatory postsynaptic potential

It brings the postsynaptic cell closer to the threshold for firing action potentials by depolarising it

72
Q

Give two examples of excitatory transmitters

A

Glutamate and acetylcholine

73
Q

What are inhibitory transmitter receptors?

A

They allow the efflux of K+ or influx of Cl- once activated

This causes a net outward current and leads to an IPSP

74
Q

What is an IPSP?

What does it do?

A

Inhibitory postsynaptic potential

It brings the postsynaptic cell further away from the threshold for firing action potentials by hyperpolarising it

75
Q

What happens to the EPSP/IPSP if a greater amount of transmitter is released?

A

More transmitter leads to more ion channels opening and a greater current flow

This leads to a greater IPSP/EPSP due to summation of individual presynaptic potentials

76
Q

How is an action potential related to an IPSP/EPSP?

A

One action potential will only cause a single EPSP/IPSP

Many action potentials will reach the threshold value and initiate an action potential in the postsynaptic neurone

77
Q

How is determined whether an action potential will fire in the postsynaptic neurone?

A

The same neurone can receive many excitatory and inhibitory inputs

It is the balance of the EPSPs and IPSPs that determine whether it will fire an action potential

78
Q

How do glial cells play a role in terminating the action of a neurotransmitter?

A

The transmitter can be taken up into glial cell processes lining the peri-synaptic zone by glial cell transporters

It is then shuttled back into neurones

Or it is broken down or converted by enzymes in the glial cells

The resulting metabolites are shuttled back into neurones

79
Q

How can a neurotransmitter’s actions be terminated in the synaptic cleft (not involving glial cells)?

A

It can be taken back up directly into neurones by transporters on the presynaptic membrane

It can be broken down by cell surface enzymes into constituent parts

Nerve & Muscle (5 decks)

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