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FNP neuro > Synapses and neurotransmitters > Flashcards

Synapses and neurotransmitters Flashcards

1
Q

What is a synapse?

A

A point of contact between a neurone and another cell, or neurone.

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2
Q

What are the neuronal specialisations for communication?

A

Axon - impulse conduction
Axon terminal - releases neurotransmitter
Axon hillock - action potential generation
Dendrites - controls the excitability - whether it will fire an action potential or not.

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3
Q

What are the classifications of neurons?

A

Number of major processes
Dendrites - shape of tree, and presence of spines
Connections - motor, interneurons
Axon length
Neurotransmitter

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4
Q

What are the classifications of the number of major processes?

A

Unipolar - single e.g. invertebrate neurone
Bipolar - two e.g. retinal
Multipolar - multiple e.g. spinal motor (connects to muscle), purkinje cell
Psuedo-unipolar - single, major process splits, e.g. dorsal root ganglia cell

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5
Q

What is the electrical synapse?

A

Or gap junction.
Fastest and most primitive.
Bi-directional transfer of information.
Between physically attached cells.

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6
Q

What is the structure of an electrical synapse?

A

Proteins in cell membrane called connexons, lines up in one cell with one in another cell to form a pore which allows transfer of ions and small molecules between cells.
Bidirectional communication between cells.

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7
Q

What is a connexon?

A

1 connexon is made from 6 connexin sub-units.

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8
Q

What are characteristics of electrical synapses?

A

Allows synchronous activity between neurons.
Rare between neurons in CNS, but important in development.
Between glia-neuron and glia-glia cells.

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9
Q

How are gap junctions useful in the heart?

A

Gap junctions between cardiac myocytes allows depolarisation to spread across the whole tissue in the heart.
This allows coordinated contraction.

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10
Q

What direction does the chemical synapse travel?

A

Uni-directional transfer of information from the pre-synaptic membrane to the post-synaptic membrane.

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11
Q

What is the basic process of movement across the chemical synapse?

A

Vesicle contains neurotransmitter substance.
Exocytosis - Vesicle fuse with membranes and exposes content to extracellular space, it is not a physical connection to the postsynaptic membrane.
Neurotransmitter diffuses from vesicle to interact with receptors expressed on post synaptic membrane.

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12
Q

What is the first step of neurotransmission?

A

The action potential triggers the release of neurotransmitter.
The action potential is caused by depolarisation due to an influx of Na+ through VGNaC.

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13
Q

What is the influx of Ca2+ in neurotransmission?

A

Depolarisation triggers VGCaC opening, causing Ca2+ influx, because there is a low concentration inside the cell, and because it is negative inside the cell.
This triggers exocytosis.

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14
Q

What is the exocytosis step of neurotransmission?

A

Vesicle fuses with membrane and exposes itself to extracellular space.
Neurotransmitter diffuses out to the extracellular space.

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15
Q

What is the diffusion step of neurotransmission?

A

The neurotransmitter diffuses across the synapse and binds to the receptor (membrane-spanning protein) on the postsynaptic membrane.
This causes a conformational change on the receptor, which acts as a signal.

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16
Q

What is the postsynaptic effect?

A

The signal in the postsynaptic cell needs to go away, by taking the neurotransmitter away.
This is either through re-uptake of the neurotransmitter, or through enzymatic breakdown.
The method is dependent upon the neurone and which neurotransmitter is released.

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17
Q

What is termination by re-uptake of neurotransmitter?

A

The transporter protein pulls the neurotransmitter out of the synapse, and back to the pre-synaptic terminal.
The concentration is reduced to 0, which terminates the signal.
The neurotransmitter is then broken down or repackaged into vesicles.

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18
Q

What is termination by enzymatic breakdown?

A

Enzymes break down the neurotransmitter in the synapse.
This reduces the concentration to 0 and rapidly terminates the signal.

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19
Q

What is important about neurones?

A

Not all neurones are excitory - it sends a signal but can be inhibitory.
This is dependent on whether the neurotransmitter is excitatory or inhibitory.

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20
Q

What are the main amino acid neurotransmitters?

A

Glutamate - major excitatory
GABA - major inhibitory
Glycine - inhibitory

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21
Q

What are monoamine neurotransmitters?

A

Noradrenaline - excitatory, dopamine
5-hydroxytryptamine (serotonin) - inhibitory.

22
Q

What is Acetylcholine?

A

Major excitatory neurotransmitter in Peripheral Nervous System

23
Q

What are neuroactive peptides?

A

Neurotransmitters e.g. opioid peptides - endorphin
Tachykinins - Substance P

24
Q

What is the receptor in neurotransmission?

A

Membrane-spanning protein on postsynaptic cell which initiates the intracellular signal.
Specific to the neurotransmitter.
Whereas neurotransmitter can have several receptor subtypes.

25
What is glutamate?
Glutamate has multiple receptor sub-types. Glutamatergic receptors are receptive to glutamate.
26
What is receptor nomenclature?
The nomenclature of the receptor is based on the most potent, selective agonist. An agonist is a substance which switches a receptor on.
27
What are some types of receptor?
AMPA receptor NMDA receptor Kainate receptor These are each most affected by the agonist in the name. These are glutamatergic.
28
What is ionotropic signalling mechanism?
Or ligand gated channels. Ionotropic is the movement of ions.
29
What happens in ionotropic mechanism?
Neurotransmitter binds - causes a conformational change in the receptor, which opens up a pore/channel, which ions can move through. This is fast transmission.
30
What are excitatory ionotropic receptors?
Glutamergic - AMPA, NMDA, Kainate. ACh nicotinic receptor subtype.
31
What is the signalling by excitatory ionotropic receptor?
2 molecules bound to receptor opens up pore - excites - depolarises the membrane, which brings it closer to firing an action potential. The voltage is closer to where voltage gated channels can open, where Na+ influxes and depolarises the membrane.
32
What is signalling by inhibitory ionotropic receptor?
Substance binds to receptor, conformational change, the channel opens and Cl- enters the cell, so the membrane potential is more negative - hyperpolarisation. The Vm is now further from threshold, so is inhibitory.
33
What are inhibitory ionotropic receptors?
GABA a receptor Glycine receptor
34
What are metabotropic receptor signalling mechanisms?
Changes the metabolism of the cell. Transmitter binds - causes conformational change. Activates G-protein, activates effector system. Indirect effects on excitability, and slower transmission, but longer lasting effects.
35
What can Activated G-protein do?
Open or close ion channels. Stimulate or inhibit enzymes.
36
What are example of metabotropic receptors?
Ach muscarinic GABA B Monoamine receptors Can have fast and slow transmission by the same neurotransmitter.
37
What are the different types of synaptic arrangement in the CNS?
Axodendritic synapse - input goes to dendritic tree. Axosomatic - input goes directly to cell body. Axoaxonic - more than 2 synapses and inputs go to the cell body.
38
How does the point of synaptic contact influence neurones?
The closer the synapse to the axon hillock, the greater the influence on action potential generation. Inhibitory synapses are found on the soma and near the axon hillock, best positioned for controlling excitability.
39
What happens when a neurone is too excited?
This can lead to cytotoxic cell death, and epilepsy. Generalised seizures are caused by many neurones firing at the same time, rather than unsynchronised.
40
What is the electrical effect of ionotropic receptor activation?
Small excitatory post synaptic potential (EPSP) Produces a small depolarisation that fades away. Small inhibitory post synaptic potential (IPSP). Small hyperpolarisation.
41
What is an individual synaptic input?
A single input could be release of a single vesicle (quanta) containing a few thousand neurotransmitters. These neurotransmitter bind to even fewer receptors, which is not enough to cause an action potential. Slight depolarisation that does not reach threshold.
42
What are multiple synaptic inputs?
Used to make a post-synaptic neuron fire an action potential. 2 synaptic events happen at the same time. If both excitatory, there is double membrane polarisation, not yet at threshold. Increased EPSP through spatial summation.
43
What is spatial summation?
Summing of post synaptic potentials generated at separate synapses. Most effective way to get the post synaptic cell excited.
44
What happens when there are 3 synaptic inputs?
The threshold is reached because of the sum of the EPSP are enough to open VGNaC. Depolarisation - more VGNaC open, more Na+ influx, more depolarisation - positive feedback.
45
What is the effect of different synapses coinciding?
If an excitatory synapse and inhibitory synapse occur at the same time, nothing happens.
46
What is temporal summation?
Summing of post synaptic potentials, generated at the same synapse, occurring in rapid succession. Individual synaptic input at a high frequency. see picture
47
How do neurones communicate output signal?
The neuron receives messages from connected neurones. It integrates all the inputs. Sends a frequency encoded message.
48
How does a neurone respond to a stronger input?
If there is a stronger signal, the neurone increases the speed of the action potential firing.
49
How does a neurone communicate a bigger input?
The neurone fires the action potentials at a higher frequency.
50
What is the frequency coding of signalling?
Action potentials are all or nothing events - the amplitude is not increased at stimulations above threshold. The frequency of the action potential is directly related to the intensity of the stimulus - frequency-modulated, not amplitude-modulated.
51
What is sustained threshold stimulus?
The action potential frequency is limited by the sum of the Absolute Refractory Period and the Relative Refractory Period.
52
What is supra-threshold stimulus?
There is larger depolarisation, which is capable of moving the membrane potential from below resting to above the threshold. Sustained supra-threshold enables action potentials during the relative refractory period. The frequency of firing is only limited by the duration of the absolute refractory period.

FNP neuro (10 decks)

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