Synaptic Transduction and Integration

Question 1

What intercellular connections are involved in electrical synapses?

Answer 1

Gap junctions. Each is made of six connexon subunits.

Question 2

In which cells are electrical synapses likely to be found?

Answer 2

In cells that require synchrony, for example, some neurons in motor pathways (not in the brain), cardiac muscle and some glands. Allow synchrony as there is not delay in signal travel the synapses are biderectional.

Question 3

What are the smallest neurotransmitters? List examples and their likely effect on the post-synaptic neuron.

Answer 3

Amino acids.
Glutamate- excitatory through increasing membrane potential by opening Navs and calcium channels.
GABA- inhibitory through decreasing membrane potential.
Glycine.

Question 4

Which neurotransmitters are intermediate in size?

Answer 4

Amines:

Dopamine, acetocholine (motor neurons to muscles), histomines.

Question 5

Which neurotransmitters are the largest in size?

Answer 5

Peptides- short amino acid chains stored in and released from secretory granules (100nm vesicles):
Dynorphin, Enkephalis.

Question 6

Chemical synapses can be classified based on what?

Answer 6

Location of connections; axo-dendritic, axo-somatic and axo-axonic.
Microscopic structure; Gray’s type I and Gray’s type.
Size
Neurotransmitter; amino acides, amine and peptides.
Location; neuron to neuron, neuron to muscle/gland.

Question 7

In which part of the neuron are amino acid and amine neurotransmitters packaged?
In which part of the cell are peptides packaged into secretory granules?

Answer 7

In the terminals.

In the nucleus.

Question 8

What is the difference in the effect on post-synaptic membrane potential of excitatory and inhibitory neurotransmitter?

Answer 8

Excitatory neurotransmitter causes sodium influx and causes membrane depolarisation.
Inhibitory neurotransmitters cause influx on calcium ions, which lowers the membrane potential to approximately -65mV following depolarisation or keeps the membrane potential at -65mV

Question 9

What does the size of the post-synaptic membrane potential depend on?

Answer 9

As all vesicles are approximately 50nm in diameter, they contain approximately the same size of neurotransmitter. Hence, the size of post-synaptic potential depends on the number of vesicles released.

Question 10

Which is true?
Excitatory neurotransmitter causes opening of sodium-specific channels in the post-synaptic cells or
Excitatory neurotransmitter causes opening of non-specific cation channels?

Answer 10

Non-specific cation channels. Only sodium has an effect on depolarisation, however, as potassium ions are always open anyway.

Question 11

How can neurotransmitter be removed from the synapse?

Answer 11

Three ways:

• Neurotransmitter diffuses away
• Re-uptake of neurotransmitter by pre-synaptic neuron
• Digestion of neurotransmitter, e.g. digestion of acetylcholine by acetylcholinesterase.

Question 12

Signals propagate in one direction/bidirectionally in neurons and decay/grow over distance.

Answer 12

Bidirectionally, decay.

Question 13

Most dendrites use passive conduction, though some utilise active conduction. What is the difference between the two?

Answer 13

Passive conduction depends exponentially with distance due to leak across membranes through K2P channels. Active conduction depends on voltage-gated channels which boost membrane depolarisations. This is not as reliable as action potentials and does not cause as much depolarisation.

Question 14

How can the EPSP be altered to increase its chance of producing an action potential as the EPSP source is moved further from the soma?

Answer 14

The initial EPSP (depolarisation) can be increased. As the depolarisation decreases exponentially, by increasing initial depolarisation, the decay is reduced and membrane depolarisation remains the same.

Question 15

The trigger zone for AP is at the:
- axon hillock/cell body/dendrites
and have the lowest/highest threshold for initiation of AP.

Answer 15

Axon hillock

Lowest.

Question 16

What makes the trigger zone have the lowest threshold for AP initiation?

Answer 16

The high density of Navs.

Question 17

What determines the size of EPSPs?

Answer 17

Size of initial depolarisation and decay and the threshold at the trigger zone.

Question 18

How does shunting inhibition stop EPSP from propagating?

Answer 18

It causes Cl ion influx, which counteracts the influx of Na ions and so there is no relative change in charge and hence, no depolarisation.

Question 19

What does the length constant, lander, depends on?

Answer 19

Thickness and length of axon (internal resistance) and membrane resistance.

Question 20

Is back propagation possible in dendrites? Why is it important?

Answer 20

Basis of learning- provides a signal to dendritic synapses that an anterograde spike has occurred, which strengthens synapses.

Question 21

Are all vesicles released at once when AP reaches the axon terminal?

Answer 21

No

Question 22

In what part of the nervous system are pre-synaptic inhibition and post-synaptic inhibition normally found?

Answer 22

In the sensory system.

Question 23

How far apart do two stimuli need to be (in time) to produce two separate peaks and not summate?

Answer 23

100ms

Question 24

What is the difference in effect of paired-pulsation in synapses where the probability of vesicle release is much lower than 1 and in synapses where the probability of vesicle release is close to 1?

Answer 24

1: Most vesicles have been used up from first stimulus, leaving very few for the second, and so inhibition occurs, whereby the second stimulus produces a low EPSP.

Question 25

What is the result of noredrenaline binding to its receptor in metabotrobic modulation?

Answer 25

Close potassium channels, reducing leakage of current, increasing the distance constant.