Structure and Function of Synapses

Question 1

What is a synapse?

Answer 1

junction between two neurones

Question 2

Describe synaptic transmission at a cholinergic synapse.

Answer 2

1. arrival of action potentials at the pre-synaptic knob causes depolarisation of presynaptic membrane
2. this stimulates influx of calcium ions into synaptic knob
3. this causes synaptic vesicles to fuse with the presynaptic membrane
4. neurotransmitter is released and diffuses across the synaptic cleft
5. acetylcholine attaches to specific protein receptors on the postsynaptic membrane
6. this stimulates Na+ channels in the postsynaptic membrane to open and Na+ diffuse into the post-synaptic neurone, causing depolarisation
7. if enough acetylcholine attach to the receptor proteins and enough Na+ diffuses in, the threshold is passed and the neurone is depolarised to trigger an action potential

Question 3

How is synaptic transmission stopped?

Answer 3

• acetylcholine s broken down by acetylcholinesterase to form acetyl and choline
• acetyl and choline are taken up by the presynaptic neurone by active transport and acetyl choline is resynthesised
• this requires energy from ATP hydrolysis

Question 4

Why are synapses described as unidirectional?

Answer 4

• vesicles containing neurotransmitter are only found in the presynaptic knobs
• receptors for neurotransmitters are only found on the postsynaptic membrane

Question 5

What are excitatory synapses?

Answer 5

cause synaptic transmission to occur by making the membrane potential in the postsynaptic neurone more positive

Question 6

What are inhibitory synapses?

Answer 6

prevent synaptic transmission by making membrane potential more negative (hyperpolarisation), so it is harder for the threshold to be reached

Question 7

In inhibitory synapses, what happens when neurotransmitter binds?

Answer 7

• K+ channels in the postsynaptic membrane open and K+ diffuses out of the neurone
• Cl- channels open and Cl- diffuses into the neurone

Question 8

What is spatial summation?

Answer 8

• several presynaptic neurones synapsing with one post synaptic neurone
• so enough neurotransmitter is released to pass the threshold, triggering an action potential

Question 9

What is temporal summation?

Answer 9

• a single presynaptic neurone that conducts a high frequency of action potentials, releases enough neurotransmitter to pass the threshold, triggering an action potential

Question 10

What are the effects of drugs on synapses?

Answer 10

1. Stimulates the nervous system by creating more action potentials in postsynaptic neurones
   - similar shape to neurotransmitter
   - stimulate release of more NT
   - inhibiting NT enzyme
   - inhibiting re-uptake of NT
2. Inhibit the nervous system by creating fewer action potentials in postsynaptic neurone
   - inhibit NT release
   - blocking Na+ channels from opening