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Introduction to neuroscience - L1 > Lecture 8 - synapses & neurotrasmitter > Flashcards

Lecture 8 - synapses & neurotrasmitter Flashcards

1
Q

What is a synapse?

A

a junction between 2 neurons allowing signals to pass from one to the other

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2
Q

What is evidence for neurons?

A
  • Golgi stain (Ramon & Cajal, late 1800s)
  • Physiological evidence from study of reflexes (Sherrington around 1900)
  • Final evidence from electron microscopy (1950s)
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3
Q

Explain how synapses enable flexible processing

A

A very simple animal: sensory input –> behaviour

More specialisation, flexibility: sensory input –> muscle –> behaviour

Even more flexibility integration: sensory input (multiple) –> motor neuron –> muscle –> behaviour

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4
Q

How are electrical synapses are formed of gap junctions and what does this allow?

A
  • allow current to pass directly between neurons

The membranes of the 2 neurons must be close together as the gap is very small. Gap junction about 1 nanometer big - allows charge & ions to pass through

When 2 neurons are connected by gap junctions, electrical changes in one neuron pass directly to the other

  • stimulation of the first neuron (action potential) will lead to a smaller action potential and a smaller hyper-potential in the second neuron. This is because they are electrically connected

Whatever is going on in neurone 1 will be faintly reflected in neuron 2, as they are electrically connected by gap junctions

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5
Q

How do you tell if neurons are connected by gap junctions?

A
  1. small molecules like dyes diffuse from one neurone to the other, as they travel through the pore.
  2. both hyperpolarisation & depolarisation stimuli are passed from one neuron to the other. This is blocked by deleting a connexin gene (shakB2 mutation) - as connexin genes create a gap junction
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6
Q

What are electrical synapses good for?

A
  • fast communication
  • synchronising neurons
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7
Q

What is the first evidence of chemical synapses 100 years ago?

A

Otto Loewi - demonstrated using 2 isolated frog hearts that nerves release a chemical which slows the heartbeat

  1. Stimulate vagus
  2. Heart rate slows
  3. Remove fluid sample
  4. Add fluid to recipient heart
  5. Heart rate slows
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8
Q

What is a prototypical chemical synapse?

A

The post-synaptic cell could be another neuron, or a non-neuronal cell:
- Motor neuron –> skeletal muscle
- Autonomic neuron –> hormonal gland smooth muscle, or heart

Synaptic vesicles found at active zone (very electron-dense) - ready to exocytose

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9
Q

What are the steps in chemical synapse transmission?

A
  1. Package neurotransmiiter in vesicles
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10
Q

What are the 2 major categories of vesicle - defined by what they store?

A

1 type - clear synaptic vesicles - small (40-50nm) - these store small molecule transmitters (e.g. GABA, glutamate & acetylcholine). Filled by transporter proteins at the presynaptic terminal. Recycled by endocytosis.

2 type - dense synaptic vesicle - large (100nm) - peptide neurotransmitter - created & filled by the ER (endoplasmic reticulum)/Golgi secretory apparatus. ‘One & Done’.

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11
Q

What happens after there is an influx of calcium?

A

vesicle fuses to membrane

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12
Q

What has a larger synapse - chemical or electrical synapses?

A

Chemical synapse

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13
Q

What do vesicles fuse to?

A

SNAREs

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14
Q

What does Ca2+ bind to, and what happens after?

A

when Ca2+ binds to synaptotagmin, a conformational change makes the SNAREs ‘zipper’ together, forcing the vesicle to fuse to the plasma membrane

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15
Q

What are SNAREs the target for?

A

toxins (Botulinum toxin, tetanus toxin)
- SNAREs also used in other cellular processes

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16
Q

What happens when neurotransmitters bind to postsynaptic?

A

neurotransmitters affect the postsynaptic neurone by binding to receptors

  1. Ligand-gated ion channels (ionotropic receptors) –> directly depolarise then hyperpolarise the postsynaptic cell.
  2. G-protein-coupled receptors (metabotropic receptors) –> more complex effects
17
Q

Does the neurotransmitter itself enter the postsynaptic cell?

A

NO

18
Q

What are different ways of removing neurotransmitters?

A
  1. They diffuse away
  2. They are actively taken up by transporters for recycling (into the presynaptic neuron or glia)
  3. They are destroyed in the synaptic cleft by enzymes
19
Q

Describe features of electrical synapses

A
  • electrical signals pass in both directions
  • signals are passed directly, can only be attenuated
  • fast (<0.3ms)
20
Q

Describe the features of chemical synapses

A
  • signals pass in one direction
  • signals can be radically transformed - presynaptic may inhibit postsynaptic (inverted, amplified, modulated)
  • slower (0.3-5ms)
21
Q

Are both electrical & chemical synapses plastic?

A

YES

22
Q

Which type of synapse is more plastic?

A

chemical synapses

23
Q

Do both types of synapses allow for summing?

A

YES - both allow summing up inputs by the post-synaptic neuron

24
Q

What is the most common type of synapse?

A

most synapses are chemical synapses

25
Q

Describe the features of a neuromuscular junction

A
  • fast & reliable neurotransmission
  • motor neuron action potentials always cause muscle cell action potentials
  • uses the neurotransmitter acetylcholine
26
Q

What are the parts of the Neuromuscular junction?

A
  • synaptic vesicles
  • active zone
  • synaptic cleft
  • receptors
  • junctional fold
  • postsynaptic muscle fibre
27
Q

How does the NMJ achieve such efficient transmission?

A

one of the largest synapses in the body

Presynaptic:
- large number of active zones

Postsynaptic (motor end-plate)
- contains junctional folds, densely filled with neurotransmitter receptors

Active zones and junctional folds are precisely aligned

28
Q

How did we figure out than neurotransmitters are released from vesicles?

A

Bernard Katz - stimulated motor nerve and recorded muscle. He noticed how the evoked responses are all integer multiple ps of a spontaneous potential

29
Q

What is each peak?

A

each peak is each quantum (vesicle), which is full of neurotransmitter

30
Q

Describe varieties of CNS synapses

A
  • the shape of the synapse is adapted to the function - e.g. NMJ have folds that make it sensitive to acetylcholine
  • dendrosomatic
  • dendrodendritic

Introduction to neuroscience - L1 (10 decks)

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