Synaptic Transmission

Question 1

Neurotransmitter released by cholinergic synapse

Answer 1

acetylcholine

Question 2

Contains vesicles with neurotransmitter

Answer 2

Pre-synaptic neurone

Question 3

Contains receptors on cell-surface membrane for neurotransmitter

Answer 3

Post-synaptic neurone

Question 4

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.

Describe how (3 marks)

Answer 4

1. Depolarisation of membrane causes calcium ion channel proteins to open;
2. Calcium ions enter by (facilitated) diffusion;
3. Causes vesicles to fuse with presynaptic membrane

Question 5

Describe the sequence of events involved in transmission across a cholinergic synapse (5 marks).

Answer 5

1. Depolarisation of / action potential moves into presynaptic membrane;
2. Calcium ion channels open AND calcium ions enter pre-synaptic terminal;
3. (cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;
4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
5. Acetylcholine binds to receptors on the postsynaptic membrane;
6. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 6

TRUE or FALSE:

Acetylcholinesterase breaks down acetylcholine after it has binded to its receptor to prevet further depolarisation of the post synaptic neurone

Answer 6

TRUE

Question 7

Inhibitory synapses cause hyperpolarisation in postsynaptic neurones.

Explain how this inhibits synaptic transmission (2 marks).

Answer 7

1. Inside postsynaptic neurone is more negative

OR postsynaptic neurone potential is below resting potential;

2. More sodium ions required to enter for depolarisation / action potential

OR prevents sodium ions causing depolarisation;

Question 8

Synaptophysin is a protein involved in the production of synaptic vesicles.

Scientists can use the presence or absence of synaptophysin to identify presynaptic and postsynaptic membranes in synapses.

Explain why they are able to use synaptophysin for this purpose (1 mark).

Answer 8

(Synaptic) vesicles only found in presynaptic neurone

Question 9

Dopamine is a neurotransmitter. Production of too much dopamine is associated with schizophrenia. A drug used to treat schizophrenia binds to dopamine receptors in synapses. This binding does not lead to the formation of an action potential.

Suggest why the drug used to treat schizophrenia is able to bind to the same receptor as dopamine (1 mark).

Answer 9

Has similar tertiary structure / shape to dopamine

OR

Complementary (to binding site on receptor);

Question 10

It is important that a neurotransmitter such as serotonin is transported back out of synapses.

Explain why (2 marks).

Answer 10

1. If not removed keeps binding to receptors;
2. Keeps causing action potentials / depolarisation of post-synaptic membrane;

Question 11

Glutamate is a neurotransmitter involved in the transmission of nerve impulses from pain receptors to the brain. Ziconotide is a drug that can reduce severe, constant pain. Ziconotide blocks the calcium ion channels at some of the synapses which use glutamate.

The transmission of glutamate at synapses is similar to that of acetylcholine.

Explain how ziconotide reduces severe, constant pain (5 marks).

Answer 11

1. No/less calcium ions enter presynpatic neurone via calcium ion channels;
2. No/fewer synaptic vesicles move to/fuse with presynaptic membrane and no/less glutamate is released;
3. No/less glutamate diffuses across synaptic cleft;
4. No/less glutamate binds to receptors on the postsynaptic membrane;
5. No/fewer sodium ions enter (postsynaptic neurone) so no/ fewer impulses (sent to brain);

Question 12

Ziconotide is a polypeptide and acts on synapses in the spinal cord. Scientists investigated the effectiveness of ziconotide in reducing severe, constant pain.

Ziconotide was injected into each patient’s cerebrospinal fluid that bathes the brain and spinal cord.

Suggest two reasons why the patients had ziconotide injected into their cerebrospinal fluid rather than taking a pill containing the drug (2 marks).

Answer 12

1. Rapidly/quickly/directly reaches synapses / spinal cord;
2. Is broken down by enzymes / proteases

OR is broken down by stomach acid

OR is too large to be absorbed;

Question 13

Dopamine is a neurotransmitter released in some synapses in the brain. The transmission of dopamine is similar to that of acetylcholine.

Dopamine stimulates the production of nerve impulses in postsynaptic neurones.

Describe how (3 marks).

Answer 13

1. Dopamine diffuses across synapse;
2. Binds to receptors on postsynaptic membrane;
3. Stimulates entry of sodium ions and depolarisation/action potential;

Question 14

GABA is a neurotransmitter released in some inhibitory synapses in the brain. GABA causes negatively charged chloride ions to enter postsynaptic neurones.

Explain how this inhibits postsynaptic neurones (3 marks).

Answer 14

1. Inside of postsynaptic neurone becomes more negative/hyperpolarisation;
2. More sodium ions required (to reach threshold)

OR Not enough sodium ions enter (to reach threshold);

3. For depolarisation/action potential;

Question 15

Describe how synapses are unidirectional (2 marks)

Answer 15

1. Neurotransmitters are only made in and released from the presynaptic neurone
2. Receptors for the neurotransmitter are only found on the membrane postsynaptic neurone

Question 16

Explain spatial summation involving synapses (3 marks)

Answer 16

Several presynaptic neurones connect to a single postsynaptic neurone;

Low amounts of neurotransmitter are released from each presynaptic neurone;

Having several presynaptic neurones increases the likelihood of the threshold potential being reached in the postsynaptic neurone;

Question 17

Explain temporal summation involving synapses (2 marks)

Answer 17

A single presynaptic neurone releases neurotransmitter many times over a short period;

Over time this can accumulate and the threshold potential is reached in the postsynaptic neurone;

Question 18

A neuromuscular junction is a specific synapse of a _____________ neurone and a muscle cell.

Answer 18

motor

Question 19

Neurotransmitter involved at the neuromuscular junction

Answer 19

acetylcholine

Question 20

TRUE or FALSE:

Following events at the neuromuscular junction, the muscle is depolarised.

Answer 20

TRUE

Question 21

Compare the cholinergic synapse and neuromuscular junction

Answer 21

1. Action potential arrives at pre-synaptic bulb / knob;
2. Calcium ions enter pre-synaptic bulb and are needed for release of acetylcholine;
3. Acetylcholine receptors;
4. Neurotransmitter binds to receptor causing depolarisation;
5. Acetylcholinesterase needed
   to hydrolyse neurotransmitter

Question 22

Contrast the cholinergic synapse and neuromuscular junction

Answer 22

1. Cells of muscle fibre is depolarised rather than a post-synaptic neurone
2. Muscle cell surface membrane has folds that form clefts, so larger surface area
3. So more receptors and acetylcholinesterase
4. Motor neuron firing an action potential always triggers a response in a muscle, not always the case in the brain (i.e. requires spatial summation)

Question 23

Following the release of acetylcholine into a neuromuscular junction, a muscle contraction occurs.

Describe the sequence of events, following the release of acetylcholine, that leads to stimulation of this contraction.

Do not include in your answer the events following the release of calcium ions in the myofibril.

Answer 23

1. Acetylcholine diffuses across synaptic cleft;
2. Acetylcholine binds to receptors on the sarcolemma;
3. Sodium ions channels open AND sodium ions diffuse into muscle fibre leading to depolarisation/action potential;
4. Calcium ions released by sarcoplasmic reticulum;

Question 24

Myasthenia gravis (MG) is an autoimmune disease caused when antibodies bind to the sarcolemma postsynaptic membrane of neuromuscular junctions. This can weaken contraction of muscles.

Suggest and explain how MG can weaken contraction of muscles (2 marks).

Do not include details of myofibril or muscle contraction in your answer

Answer 24

1. Less/no acetylcholine/neurotransmitter binds to receptors;
2. Less/no depolarisation

OR Fewer/no action potential(s)

OR Fewer/no sodium ions enter to reach threshold;

Question 25

The black mamba’s toxin kills prey by preventing their breathing. It does this by inhibiting the enzyme acetylcholinesterase at neuromuscular junctions. Explain how this prevents breathing (2 marks).

Answer 25

1. Acetylcholine not broken down / stays bound to receptor; 2. Sodium ions continue to enter / continued depolarisation / Na+ channels (kept) open / action potentials / impulses fired (continuously); 3. Intercostal muscles / diaphragm stay contracted / cannot relax;