Week 3: Synapses/Neurotransmitters

Question 1

Describe how chemical transmission at a synapse results in the action potential in a presynaptic neuron producing an EPSP or IPSP in the post-synaptic neuron?

Answer 1

Depends on the ions eg sodium going in = EPSP vs potassium exiting = IPSP

Question 2

What is an ionotrophic receptor?

Answer 2

For ionotropic receptors, the neurotransmitter binds to the receptor.

This causes a change in the channel allowing ions to flow in/out of the neuron (or disallows ions to flow in/out of the neuron). As the receptor is located on the channel itself it means that the neurotransmitter acts quickly on the channel. This means that the channels are responsive to the binding and the release of the neurotransmitter. Quickly opening and closing as a result of these changes

Question 3

What is a metabotropic receptor?

Answer 3

For metabotropic receptors, the neurotransmitter binds to the G-protein linked receptor which is actually located away from the channel as we can see in the diagram.

Once the neurotransmitter connects to one of these receptors a molecule (also called a G protein) within the postsynaptic cell is released and either directly or indirectly through a series of reaction open and closes the ion channels

As a result of the increased processes that result in the opening and closing of the ion channels these receptors are a lot slower working on the channels than the ionotropic receptors because the mechanism of opening and closing the channel is less direct

Question 4

What is the role of a synapse?

Answer 4

It allows communication between neurons.

The action potential in one neuron will travel to the synapse, communication occurs via the neurotransmitters and this causes a change in the membrane potential of neuron 2.

Question 5

What is contained within the synaptic vesicles

Answer 5

Neurotransmitters

Question 6

Where in the neuron are the synaptic vesicles

Answer 6

Presynaptic terminal button

Question 7

What process is required to allow the neurotransmitter to enter the synaptic cleft?

Answer 7

Exocytosis - the synaptic vesicles fuse with the cell membrane which allows them to release their neurotransmitters into the presynaptic cleft.

Question 8

What happens to the neurotransmitter once it has bound to a receptor?

Answer 8

1. Reabsorbed by the sending neuron in a process called reuptake
2. Broken down by enzymes in the synaptic cleft
3. Neurotransmitter can be mopped up by astrocytes (glial cells)
4. Sent away from the synapse via diffusion

Question 9

What happens once the neurotransmitter enters the synaptic cleft?

Answer 9

The ligand-gated channels have receptors for neurotransmitters. The neurotransmitter binds to the receptor which causes the ion channel to open which allows ions to flow into or out of the post-synaptic neuron causing an EPSP or an IPSP.

Question 10

In an electrical synapse the ions flow directly from one neuron to another via….

Answer 10

Gap junctions

Question 11

Explain the process of a chemical synapse?

Answer 11

• When an action potential arrives at the axon terminal it activates voltage gated calcium channels which causes it to flow rapidly into the pre-synaptic neuron
• The synaptic vesicles which contain neurotransmitters have snare proteins which are responsible for joining the synaptic vesicle to the presynaptic membrane
• The calcium ions which have just flown into the presynaptic space bind to these snare proteins and change the configuration of the snare protein allowing exocytosis of the neurotransmitter into the synaptic cleft

Question 12

What is the role of a snare protein?

Answer 12

Drive the fusion of the synaptic vesicle (which contains the neurotransmitters) and the cell membrane to allow for the release of the neurotransmitter into the synaptic cleft.

The calcium ions bind to the snare proteins which changes their configuration and allows exocytosis of the neurotransmitter into the synaptic cleft.

Question 13

What is the mechanism of ligand gated channels

Answer 13

Lock & Key mechanism

Question 14

What is the lock & key mechanism?

Answer 14

This means only selected neurotransmitters can bind to a certain receptor protein, meaning that neurotransmitters are selective to the receptors they can bind to (or the receptors are selective to the neurotransmitters that bind to them)

Question 15

How do we turn off the signal so that the post synaptic neuron returns to its normal resting potential?

Answer 15

Neurotransmitter must be cleared otherwise the receptors will continually excite the post-synaptic neuron

Question 16

What type of glial cell can mop up the neurotransmitter?

Answer 16

Astrocytes

Question 17

What locations can you find electrical synapses? RLH

Answer 17

Retina, lateral vestibular nucleus & hippocampus

Question 18

The coupling potential is a lot smaller than the action potential in an electrical synapse? True or False

Answer 18

True

Question 19

Size of a synaptic cleft in an electrical vs chemical synapse?

Answer 19

These synapses are 0.2 nanometres in an electrical synapse & 10-20 nanometres in a chemical synapse

Question 20

Unlike chemical synapses, electrical synapses cannot turn an excitatory signal into an inhibitory signal in the target neuron – the sign of the signal in the presynaptic neuron is always the same as the sign in the post-synaptic neuron eg depolarising in pre-synaptic = depolarising in post-synaptic . True or False

Answer 20

True

Question 21

How do electrical synapses allow for the synchronised activity of groups of cells?

Answer 21

Electrical synapses can carry current in both directions so that the depolarisation of a post-synaptic neuron will lead to a depolarisation of the presynaptic neuron

Question 22

What is an electrical synapse

Answer 22

• Physical connection between presynaptic and postsynaptic neuron
• Ions directly flow from one neuron to another
• Faster than chemical synapse (direct link with pre/post-synaptic neuron due to gap junction)
• Very reliable although more signal attenuation than chemical synapses

Question 23

What are the three main groups of neurotransmitters?

Answer 23

Amino acids, monoamines & peptides

Question 24

What is the main excitatory transmitter in the whole nervous system?

Answer 24

Glutamate (AMPA, NMDA & Kainate)

Question 25

What is the main inhibitory neurotransmitter in the brain?

Answer 25

GABA (A & B)

Question 26

What is the main inhibitory transmitter in the spinal cord?

Answer 26

Glycine

Question 27

What happens when the glycine neurotransmitter binds to the receptor?

Answer 27

Opens chloride channels - negatively charged chloride will flow into the channels and into the neuron (causes IPSP).

Question 28

What happens when GABA A or B receptors bind?

Answer 28

GABA neurotransmitters will bind to GABA A receptor and this binding to the receptor opens channels specific to chloride. Based on the electrochemical gradient chloride will flow into the channels and reduce membrane potential.

GABA B receptors, GABA neurotransmitters bind to GABA B receptors which closes calcium channels resulting in less calcium positive ions flowing into the neuron
If there’s less positive ions going into the neuron it will reduce the membrane potential. It also opens potassium channels which will result in more potassium ions flowing out of the neuron . As potassium ions are positively charged it will result in a reduction in the membrane potential (IPSP/hyperpolarisation)

Question 29

What happens when glutamate receptors bind?

Answer 29

For AMPA (one of the glutamate receptors) both sodium and potassium channels open
The electrochemical gradient is for sodium to flow into the neuron and if sodium channels open and sodium enters the neuron there will be an increase in the membrane potential. But if potassium channels open the electrochemical gradient is for potassium to flow out of the neuron. If the channels open then there will be a decrease in the membrane potential and therefore opening them at the same time will mean you have positive ions flowing in and positive ions flowing out which means they are conflicting and redundant The electrochemical for sodium going into the neuron is actually greater than the electrochemical gradient for potassium to flow out of the neuron. Therefore the net effect is that the inside of the neuron becomes more positive because sodium ions are flowing into the neuron faster than potassium ions are flowing out of the neuron. The potassium efflux through these receptors make synaptic transmission energetically costlier as it requires more sodium ions to enter the cell or the neuron to generate an EPSP

For NMDA (one of the glutamate receptors) the calcium, sodium and potassium channels open. NMDA receptors are similar to AMPA receptors but they also cause calcium to flow with its strong electrochemical gradient. As well as sodium ions flowing into the neuron with its strong electrochemical gradient and potassium ions flowing out of the neuron with its weak electrochemical gradient

Question 30

Excitatory neurotransmitters

Answer 30

Epinephrine, norepinephrine, acetylcholine

Question 31

Inhibitory neurotransmitters

Answer 31

Dopamine, histamine, serotonin

Question 32

Key neurotransmitters & their effect on post synaptic pumps

Answer 32

Glycine: opens Cl channel
GABA (A): opens cl
GABA (B): Open K, closes Ca
Glutamate AMPA: opens NA & K
Glutamate NMDA: opens Ca, Na & K