Synapses and synaptic transmission

Question 1

What are pre and post synaptic neurons separated by?

Answer 1

gap (synaptic cleft)

Question 2

what does synaptic transmission across a cleft involve?

Answer 2

chemical messengers

Question 3

what are gap junction?

Answer 3

holes between cells allowing communication and transfer of substance

Question 4

Where are the synaptic vesicles contained?

Answer 4

In the presynaptic nerve.

Question 5

What would you find on the postsynaptic membrane?

Answer 5

Receptors which act in a lock and key mechanism with the chemical structure of the neurotransmitters released from the presynaptic terminal.

Question 6

What causes the vesicles to bind to presynaptic membrane?

Answer 6

SNAP snare complex. It is a process which docks these vesicle membranes, cleaves them and docks them to the presynaptic membrane allowing the release of neurotransmitter.

Question 7

how does botox work

Answer 7

Botox cleaves the snap snare complex resulting in paralysis of the neuromuscular junction because no neurotransmitter can be released.

Question 8

What are the events in synaptic transmission?

Answer 8

• Presynaptic action potential
• Depolarisation of synaptic terminal
• Opening of voltage gated Ca channels in the presynaptic membrane
• Allows for the entry of Ca and this results in calcium dependent exocytosis (activates snap snare complex).
• This binds vesicle to membrane and docks it to membrane of neuron and allows release of neurotransmitter across cleft.
• This diffuses across and acts upon a specific postsynaptic receptor of choice depending on the neurotransmitter that neuron releases.

Question 9

What receptor is usually for excitatory neurotransmitters?

Answer 9

Glutamate (allows Na- in)

Question 10

What receptor is usually for inhibitory neurotransmitters?

Answer 10

GABA (allows cl- in)

Question 11

Reduced Vesicle Release can occur due to what 4 things

Answer 11

Low calcium

High magnesium

Curare (poison)

Botox

Question 12

What are end plates?

Answer 12

postsynaptic region of single axon in muscle fibres, always excitatory

Question 13

What is an end plate potential?

Answer 13

Graded potential

causes depolarisation of skeletal muscle fibres due to neurotransmitters binding to post synaptic membrane in neuromuscular junction

If this above the threshold potential, then it will cause an Action potential

Question 14

What does the end plate potential depend on?

Answer 14

its size depends on number of vesicles that are released onto post synaptic site

Question 15

what happened if there is some threshold signal on an end plate?

Answer 15

If there is some threshold signal (not enough for AP) then it will simply dissipate the further the signal travels away from that synapse (along the muscle)

Question 16

What are EPSPs

Answer 16

EPSP’s add to generate depolarisation (more likely to fire an action potential).

Question 17

what are IPSP’s

Answer 17

IPSP’s add to generate hyperpolarisation (less likely to fire an action potential).

Question 18

what are EPSPs and IPSPs

Answer 18

EPSP’s and IPSP’s are sub threshold events which determine whether a cell will reach threshold to fire an action potential – or not.

EPSP’s and IPSP’s act to cancel each other out.

Question 19

Temporal Summation

Answer 19

slow accumulation of neurotransmitter over time to exceed threshold

Question 20

what is spatial summation

Answer 20

multiple pre-synaptic neurones synapse to one post-synaptic neurone

The closer the synapse is to the cell body, the more likely it will fire a A

Question 21

Outline the knee jerk reflex.

Answer 21

Hitting the patella tendon causes a stretch in the quadriceps muscle, that is sensed by the sensing organ, if it is strong enough it will send an action potential back into the spinal cord.

• That action potential will activate 2 types of muscles.
  
  • Motor neuron to quadriceps muscle causing contraction.
  • The other one via an inhibitory neuron, will inhibit the efferent pathway causing the relaxation of the hamstring muscle.
  • This results in the knee jerk reflex which is an antagonistic action.

Question 22

Explain the pathways that occur in the knee jerk reflex.

Answer 22

• When you trigger the reflex you generate an action potential in the sensory neuron that action potential propagates down the purple branch.
• It will cause neurotransmitter release in the ventral horn of the spinal cord which acts on the extensor motor neuron.
• Which travels to the extensor muscle which contracts and causes the flick.
• This is an excitatory pathway so we get an excitatory postsynaptic potential (EPSP) and it is a graded potential.
• In the other neuron the exact same input will trigger an action potential on an inhibitory interneuron which signals with chloride and releases GABA (- neg charged).
• This will activate GABA channels on the flexor motor neuron, they flux chloride ions which are negatively charged therefore hyper polarising this flexor motor neuron.
• Making it an inhibitory postsynaptic potential (IPSP).
• Summary: It is the summation of miniature end plate potentials to form end plate potentials to reach the threshold which then allows for an action potential to be fired in the postsynaptic muscle cell.

Question 23

What are the 3 termination mechanisms?

Answer 23

Main principle mechanism is re-uptake.

The second method (less common and less effective) is breakdown.

The last method is if the neurotransmitter simply diffuses away from the synapse where there is no receptor

Question 24

What can occur if the neurotransmitter release is too strong?

Answer 24

If the neurotransmitter release is too strong, and the nerve becomes too excitable and becomes too stable - that can be toxic to the nerve because it can drive it too fast and cause damage.

Question 25

What if the postsynaptic response is sub threshold?

Answer 25

• End-plate potential is a graded potential which will depend in size on the numbers of vesicles released on the postsynaptic site.
• The potential gets bigger as more transmitter is released.
• If the threshold is not reached then an action potential does not occur.
• Electrotonic decay
• These are called end-plate potentials because the structure of the nerve terminals splaying out onto the muscle is known as an end plate.

Question 26

Explain graded potentials.

Answer 26

• If you have sub-threshold potential - it will dissipate.
• The site of the end plate potential is highest at the synaptic site and dissipates with distance - this allows for summation.
• There is a wave of depolarisation throughout the cytoplasm.
• The potential will diminish quickly.
• It is a localised response.

Question 27

How can we record the propagation of the signal?

Answer 27

• Electrodes are inserted to record the voltage and the current across the cell.
• The response it strongest at the source and then it decays as it moves away from that voltage response.
• It will reactivate itself at the next node (in neurones) therefore allowing it to propagate along the neuron.

Question 28

What if the postsynaptic response in supra-threshold?

Answer 28

• If the stimulus arriving at the postsynaptic sites is supra-threshold - it gets to the threshold or above the threshold - then you get an action potential which is fired along the muscle cell.
• In the postsynaptic muscle cell, the action potential is much broader than it is for a normal axon potential.
• This is because in the postsynaptic site, calcium channels are also opened which allows for influx of calcium ions, giving rise to the plateau seen on the action potential.