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Electrical Signalling Flashcards

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1
Q

What is the synapse (electrically) ?

A

The point where electrical conduction is converted to chemical conduction.

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2
Q

What are the two kinds of synapse?

A

electrical (gap junctions)and chemical (neurotransmitters)

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3
Q

What is is called when changes in the charge of the dendrite/cell body region are variable in amount?

A

Graded potential

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4
Q

Incremental changes are always _ and can initiate action potential

A

Identical

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5
Q

What are intrinsic electrical responses?

A

Where neurons are always active, generate action potentials in a specific way

o Silent – they don’t
o Beating – regular pulse
o Bursting – many at a time

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6
Q

What are electrical responses to external stimuli?

A

o Sustained response – normal response
o Accommodation – modified
o Delay - sometimes it just doesn’t respond

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7
Q

What causes charge separation?

A

The membrane’s ability to selectively prevent movement of ions

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8
Q

What is impalement (electrodes)?

A

Stick electrode into the cell – measures charge movement

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9
Q

What is patch clamping?

A

larger electrode onto surface of cell – measures channel activity

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10
Q

Equation relating potential difference, resistance and current

A

V=IR

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11
Q

What can patch clamping be used to measure?

A

Either voltage or amplitude. Either V or A is set so that it is held constant – clamped.
Clamping voltage allows current to be measured.

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12
Q

Features of an electrical synapse

A
  • 2nm
  • continuous
  • Gap junctions
  • No delay
  • Ionic transport agents
  • Bidirectional
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13
Q

Features of a chemical synapse

A

20-40nm
Not continuous
Vesicles, receptors
Chemical Transmitters
short delay
Unidirectional

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14
Q

What is the process of fast chemical transmission?

A

an action potential arrives at the synapse, causing the opening of Ca2+ channels and the depolarising the presynaptic membrane.
This releases transmitter into the synaptic cleft, and it diffuses across before binding to receptors located on the presynaptic membrane.
This opens ligand-gated Na+ channels on the post synaptic membrane.
The transmitter is removed from the cleft and the postsynaptic ions channels close.

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15
Q

What is the motor end-plate model?

A

Neurons are surrounded by Schwann cells that directly contact muscles.
Therefore active zones in the neuron are in close contact with the junctional folds of the muscle, so current moves quickly from one to the other.

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16
Q

What happens in slow chemical transmission?

A

large vesicles release transmitters, but not necessarily towards the postsynaptic cell.
Ligand binding functions through G-proteins which then force ion channels to open.

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17
Q

What is the primary difference between fast and slow chemical transmission?

A

Slow chemical transmission uses second messenger molecules rather than ligand-gated ion channels

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18
Q

What is the threshold?

A

the membrane potential that results in the neurone generating the action potential.
Potential = current (ion movement) x resistance to that movement (closed channels)

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19
Q

What causes a post-synaptic potential?

A

generated when a signal crosses the synapse and results in the opening of post-synaptic ion channels. Resulting in charge movement and a change in potential.

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20
Q

What are ion channels?

A

Protein tubes that span the membrane, some are non-gated and stay open all the time, others are gated and open when there is an action potential, causing a change in the permeability of the membrane.
Ion channels let specific ions through via facilitated diffusion (no ATP needed).
Most gated channels open intermittently and briefly.

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21
Q

What causes electrically/voltage gated ion channels to open?

A

membrane potential changes .
They are associated with action potential, Na+ channels cause depolarisation. K+ channels cause repolarisation. Ca2+ channels cause neurotransmitter release.

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22
Q

What is Vr?

A

The Nernst potential.
Also called the zero current potential or the equilibrium potential.

When there is only one ion in the system it is equal to the reversal potential

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23
Q

What is the equilibrium potential?

A

The voltage required to oppose the flow of ion x across a membrane.

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24
Q

What is the reversal potential?

A

The membrane potential at the point at which there is no movement of a specific ion across the membrane (current)
The value of the reversal potential gives a clue as to the ion that stimulates action potential for the cell

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25
Which equation do we use to determine which ion carried the action potential?
Nernst if there was just one, Goldman if multiple
26
What is resting potential?
-40 to -90mV.
27
Excitatory postsynaptic potentials (epsp)
increase the likelihood that an action potential will be initiated in the post synaptic cell. Result from Na influx
28
Inhibitory postsynaptic potentials (ipsp)
Decrease the likelihood that an action potential will be initiated in the post synaptic cell. Result of either Cl influx or K efflux
29
K+ leak channels
At resting potential K+ leak channels are open therefore K+ is at equilibrium. When axon is impaled with the electrode, negative potential is detected. When K+ is added to the outside, potential decreases.
30
What is summation?
when many different EPSPs and IPSPs at different synapses are integrated and if it is sufficiently excitatory to raise the membrane potential to the threshold.
31
What is temporal summation?
An additive process involving multiple instances of stimulation at one synapse over a short period of time.
32
What is spatial summation?
It involves simultaneous stimulation from two or more nearby synapses where each is excitatory or inhibitory. This can have reinforcing or opposing effects – equal and opposite graded potentials cancel each other out.
33
What causes the action potential?
Sequential opening of voltage-sensitive channels which cause an “all or none” spike
34
What causes the brief hyperpolarisation period?
Open potassium channels
35
What are the 4 phases of the action potential?
Depolarisation Repolarisation Refractory period Return to resting potential
36
What happens to Na channels during the refractory and return periods?
Na channels are inactivated during the refractory period and become closed during the return period.
37
What does the refractory period govern?
How the action potential propagates throughout the axon
38
What is the absolute refractory period?
The downwards slope after peak depolarisation.
39
What is the relative refractory period?
The hyperpolarised period after the spike.
40
How does AP move down the axon?
Through passive spread of current via the opening and closing of Na channels
41
What does the Na/K antiporter pump do?
It exchanges 2 K ions from outside with 3 Na from inside the cell Uses ATP Makes inside 1 more negative and outside 1 more positive. Causes repolarization Present at the node of Ranvier
42
Why does AP not travel backwards?
Because the sodium channels go through a brief inactive phase before closing
43
Why is action potential self-propagating?
Because electrically gated ion channels open in response to depolarization
44
True or false: The action potential passes undiminished as a wave down the axon; a local, transient event.
True Will be the same at every point along the axon and occurs at every node of Ranvier
45
What happens during repolarisation?
Voltage-gated potassium channels open, causing an outflow of potassium, down its electrochemical gradient. The voltage-gated K+ channels then close, and so much Potassium accumulates outside the cell that repolarization occurs.
46
How is the hyperpolarisation remedied?
The Sodium – Potassium Pump
47
Action potential in myelinated vs unmyelinated axons
Myelinated neurons conduct signals at the same speed as 83-times larger unmyelinated neurons. Ion channels only need to act at the nodes of Ranvier. Action potential “jumps” the gaps between nodes 120 times faster than in unmyelinated.
48
How does calcium enter the cell?
Through VSCC (Voltage Sensitive Ca Channels).
49
What is the action of calcium inside the cell?
Calcium binds to calmodulin, protein kinase. Calmodulin phosphorylates synapsin I which cannot bind to neurotransmitter vesicles in a phosphorylated form. Vesicles can now be released into synapse and release neurotransmitter
50
Where are L-type calcium channels found and what are they blocked by?
Skeletal muscle and the cortex 1,4-dihydropyridine
51
Where are N-type calcium channels located and what blocks them?
CNS/PNS ω-conotoxin
52
Where are P-type calcium channels found and what blocks them?
Cerebellum ω-agatoxin
53
What are ionotropic receptors with examples?
Ligand-gated ion channels For example AMPA, NMDA, GABAA and glycine Nicotinic (on muscle) acetyl-choline receptors
54
What are Metabotropic receptors?
Linked to ion channels through G-proteins second messenger (also G-protein mediated) coupled to ion channels open Ca2+ channels
55
Examples of metabotropic receptors
Glutamate (mGluR) , beta-adrenergic, histamine, GABAB muscarinic (bind muscarin) acetyl-choline receptors
56
Inputs from excitatory and inhibitory synapses are received in __ and response is produced in the ___
Integrating segment Spike initiation zone
57
What are the types of short term synaptic modulation?
Facilitation Depression Temporary potentiation
58
What are the long term forms of synaptic modulation?
Long term potentiation Long term depression
59
What is fascilitation?
Two or more action potentials reach the presynaptic terminals in a short period of time. Causes more neurotransmitter to be released per action potential and so a stronger response in the post synaptic neurone. Frequently used pathways become more effective pathways. The closer together the signals the greater the amplitude.
60
What is depression?
A tetanic train can result in the depletion of neurotransmitters and therefore depression as release is decreased. Depression causes a change in Ca-dependent processes, afterwards the number of available vesicles per action potential increases – called recovery.
61
What is a tetanic train?
A rapid succession of action potentials
62
What is post-tetanic potentiation?
Short term enhancement of the synapse’s activity after recovery. A high rate of stimulation of the presynaptic neuron resulting in a gradual increase in the amplitude of the postsynaptic potential. The enhancement in the strength of the synapse represents storage of information about previous activity.
63
True or false: Short Term Plasticity is identical for all synapses.
False Short Term Plasticity is specific for individual synapses and may be different for two branches of the same axon.
64
What is the Hebbian synapse?
The synapses associated with the same kind of input will be strengthened while those that are not acting in the same way would be weakened. Following this, stimulation by a single input of the more common type would result in a greater post-synaptic response. Memory.
65
What is long-term potentiation?
Stimulation results in enhanced synaptic activity. LTP formation - NMDA receptors, Ca/calmodulin-dependent protein kinases and changes in protein synthesis. Retrograde acting factors are also involved.
66
What is long term depression?
Similar to LTP LTD requires lower levels of calcium to be induced. LTP and LTD are the basis of memory.
67
What kind of memory does the Hippocampus hold?
Spatial memories Its synapses are modifiable and strengthen through an LTP-like mechanism.
68
What can we learn from hippocampus synapses?
Circuits in hippocampus allow us to study long term potentiation. Schaffer collaterals and CA1 pyramidal cells.
69
What kind of memory does the Amygdala hold?
Fear memory Synapses modified with learning
70
What kind of memory does the cortex hold?
Motor skill Synapses strengthen through an LTP-like mechanism.
71
What is the Perforant pathway?
Entorhinal cortex to DG. Granule cells
72
What is the Mossy fibre pathway?
DG to CA3. Pyramidal cells
73
True or false: Transmission between individual neurons is highly variable
True Fluctuations in synaptic responses recorded intracellular from a pyramidal cell by one or few afferents. Responses occur in what appear to be discrete steps.
74
True or false: LTP increases the probability of neurotransmitter release presynaptically
True
75
What does LTP do to AMPA receptors?
Increases the number of AMPA receptors and alters their function. Phosphorylation increases their conductance and new receptors are delivered that initially express NMDA not AMPA. GFP-GluR1 is inserted after the activation of NMDAR.
76
What does LTP do to protein kinases?
Alters them For example CaMKII is normally calcium/calmodulin-dependent but there is an autophosphorylated form that is active without calcium. Molecular switch.
77
Does a non-potentiated synapse have active NMDA?
No NMDA receptor is blocked by Mg2+ ion at resting potential.
78
What does the gate opening of NMDA-receptor complex require?
a second amino acid, glycine, to be present and to bind to an allosteric site on the receptor complex As well as occupation of the agonist site by NMDA or glutamate
79
What do endogenous polyamines do?
They modulate the NMDA-receptor complex, with low µM and high µM potentiating and inhibiting its function, respectively. e.g. spermine and spermidine
80
Activation of intracellular protein kinase C (PKC) _ NMDA-receptor mediated responses.
Enhances
81
How can NMDA-receptor mediated responses be modulated by redox changes?
Exposure to reducing agents (e.g. dithiothreitol) potentiates, whereas oxidation reduces NMDA-receptor activation.
82
What is the structure of NMDA receptors?
They are large hetero-oligomeric complexes (tetrameric) including at least two copies of an NR1 subunit and two copies of an NR2 subunit.
83
Examples of retrograde messengers
* Nitric oxide * Carbon monoxide * Arachidonic acid
84
What do retrograde messengers do?
They return messages to the presynaptic side of the synapse. Modifing the quantal release that occurs there

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