Synapses

Question 1

Synapse def.

Answer 1

• The gap between end of axon of one neurone and dendrite of another neurone.

Question 2

True or False

Action potentials of neurone can move across synapses.

Answer 2

• False.
• Action potentials are transmitted as neurotransmitters that diffuse across synapse.

Question 3

What is end of neurone before synapse called?

Answer 3

• Synaptic knob.

Question 4

STATE the different parts of cholinergic synapse within AXON terminal of one neurone. Another word to describe this terminal?

Answer 4

Another word: synaptic knob.
1.) Synaptic vesicles.
2.) Neurotransmitters.
3.) Voltage-gated Ca2+ channels (within presynaptic membrane.)
4.) Presynaptic membrane.

Question 5

Name of membrane separating the axon terminal of 1 neurone and dendrite of another.

Answer 5

• Presynaptic membrane.

Question 6

Overall, what are the 3 MAIN parts of cholinergic synapse?

Answer 6

1.) Axon terminal of presynaptic neurone has presynaptic membrane.
2.) Synaptic cleft (gap between axon terminal and dendrite spine of 2 neurones.)
3.) Dendrite spine of post synaptic neurone has post synaptic membrane.

() - extra info

Question 7

What are the main structures found on dendritic spine of cholinergic synapse?

Answer 7

1.) Postsynaptic membrane (surrounding the dendrite.)
2.) Neuro-transmitter receptors (on postsynaptic membrane.)

Question 8

What separates axon terminal of one neuron/ dendrite spine of another neurone?

Answer 8

• Small gap called: **synaptic cleft. **

Question 9

STAGE 1

What happens once an action potential reaches a synaptic knob?

up till release to synaptic cleft

Answer 9

1.) Depolarisation of synaptic knob leads to opening Ca²⁺ channels/ Ca²⁺ diffuses into synaptic knob.
2.) Triggers vesicles containing neurotransmitter move to/ fuse with pre-synaptic membrane.
3.) Neurotransmitter released to synaptic cleft.

Question 10

STAGE 2

What happens once the neurotransmitter is released from synaptic knob to synaptic cleft?

Answer 10

1.) Neurotransmitter diffuses down conc gradient across synaptic cleft to post-synaptic membrane.
2.) Neurotramsitter binds to receptors comp in shape on post-synaptic surface membrane.

Question 11

What do we call the neuron with receptors comp on surface to neurotransmitter? Hb. the one releasing neurotransmitter?

Answer 11

• With receptors: post-synaptic neuron.
• Release neurotransmitter: pre-synaptic neuron.

Question 12

STAGE 3

What happens once neurotransmitters bind to receptors comp in shape on post-synaptic neuron membrane? Where are these receptors?

Answer 12

1.) Receptors are attached to Na+ channel proteins on post-synaptic membrane.
2.) If enough neurotransmitter binding, Na+ channel proteins widen.
3.) Causes Na+ in synaptic cleft to diffuse into post-synaptic neurone.
4.) If enough Na+ diffuse in to meet threshold (-55mV), post-synaptic membrane = depolarised.
5.) Action potential generated.

Question 13

STAGE 4

What happens after an action potential is generated in post-synaptic neurone?

Answer 13

1.) Neurotransmitter released from receptor on Na+ channel protein into synaptic cleft.
2.) Neurotransmitter is broken down by enzymes
3.) Na+ channel protein = closed/ re-establish resting potential.
4.) Break down products of neurotransmitter –> transported back into pre-synaptic neurone.
5.) Recycled/ resynthesised into new neurotransmitter (in pre-synpatic neurone.)

() - extra info.

Question 14

Why is it important that neurotransmitter is released from receptors on post-synaptic membrane?

Answer 14

• Ensures action potential isn’t always generated!

Question 15

Where are neurotransmitters broken down?

Answer 15

• In synaptic cleft (ie. in the gap.)

Question 16

What is neurotransmitter/ enzyme used to break this neurotransmitter down in cholinergic synapse?

Answer 16

• Neurotransmitter: acetylcholine.
• Enzyme: acetylcholineterase.

Question 17

True or False

The neurotransmitter: acetylcholine will enter pre-synaptic neuron after being broken down by enzymes in synaptic cleft.

Answer 17

• False.
• Breakdown products of acetylcholine will enter pre-synaptic neuron.

Question 18

2 types of synapses.

Answer 18

1.) Cholinergic synapse.
2.) Neuromuscular junction.

Question 19

Def: summation.

Answer 19

• Summation is rapid build-up of neurotransmitters in synapse to help generate an action potential.

Question 20

2 forms of summation.

Answer 20

1.) Spatial.
2.) Temporal.

Question 21

What is spatial summation?

Answer 21

1.) Many different pre-synaptic neurones collectively trigger new action potential in 1 post-synaptic neurone.
2.) Combine all the neurotransmitters they release.
3.) More Na+ channel proteins open.
4.) Exceed threshold value.

Question 22

What is temporal summation?

Answer 22

1.) 1 pre-synaptic neurone releases neurotransmitter repeatedly over short peiod of time.
2.) Adds up enough for more Na+ channel proteins (on post SM) to open.
3.) Exceeds threshold value.

Question 23

Why is summation needed?

Answer 23

• Some action potentials do not result in sufficient concentrations of neurotransmitter being released to generate a new action potential.

Question 24

What do inhibitory synapses do?

Answer 24

1.) Cause Cl⁻ ions to move into post-synaptic neurone.
2.) Cause K⁺ ions to move out post-synaptic neurone.
3.) Combined effect of negative ions moving in/ positive ions moving out leads to hyperpolrisation.
4.) Action potential = very unlikely.

Question 25

What is a neuromuscular junction?

Answer 25

- Synapse that occurs between motor neurone and muscle.

Question 26

Give 1 similarity between neuromuscular junction and cholinergic synapse.

Answer 26

- Unidirectional as neurotransmitter receptors only on post-synaptic membrane. (unidirectional --> means impulse only flows in 1 direction.) ## Footnote () - extra info

Question 27

4 differences between neuromuscular junction (NJ)/ cholinergic synpase (CS)

Answer 27

1.) NJ: only excitatory. CS: excitatory/ inhibatory. 2.) NJ: connects motor neurone to muscles. CS: connects two of any neurones (ie. sensory, motor, relay.) 3.) NJ: end point for action potential. CS: new action potential generated in next neurone. 4.) NJ: acetylcholine binds to receptors on sarcolemma. CS: acetylcholine binds to receptors on post-synaptic membrane of neurone,

Question 28

What is meant by: "Neuromuscular junctions are only excitatory?"

Answer 28

- Neuromuscular junctions will only TRIGGER action potential, never inhibit it.

Question 30

# Exam Q 2022-2024 Inhibitory synapses cause hyperpolarisation in postsynaptic neurones. Explain how this inhibits synaptic transmission.

Answer 30

- Inside postsynaptic neuron = more negative. - More Na+ required to enter for depolarisation.

Question 31

# Past paper Following acetylcholine release into neuromuscular junction, muscle contraction occurs. Describe sequence of events following release of acetylcholine that leads to stimulation of this contraction. DO NOT include the events following release of Ca2+ in the myofibril. (4 marks.)

Answer 31

- Acetylcholine DIFFUSES across synaptic cleft. - Acetylcholine attaches to receptors on sarcolemma. - Na+ ions enter leading to depolarisation. - Ca2+ ions released by sarcoplasmic reticulum.

Question 32

Sarcolemma def.

Answer 32

- Specialised cell-surface membrane that surrounds muscle cell.

Question 33

What is: 1.) Endoplasmic reticulum in muscle cell called. 2.) Cell membrane of muscle cell called.

Answer 33

1.) Sarcoplasmic reticulum. 2.) Sarcolemma.