Synapses 1

Question 1

Define the cholinergic synapse

Answer 1

a synapse that uses acetylcholine as its neurotransmitter.

Question 2

Define neurotransmitter

Answer 2

a chemical used as a signalling molecule between two neurones in a synapse.

Question 3

Define synapse

Answer 3

a gap between two neurones where one neurone can communicate with or signal to another neurone.

Question 4

What is between two neurones?

Answer 4

synaptic cleft approximately 20nm wide.

Question 5

action potential across a synapse.

Answer 5

The action potential cannot bridge the gap between two neurones so the action potential in the pre synaptic neurone causes the release of the neurotransmitter that diffuses across the synaptic cleft and generates a new action potential in the posy synaptic neurone.

Question 6

What is the presynaptic bulb?

Answer 6

the presynaptic neurone ends in a swelling called the presynaptic bulb.

Question 7

What are the presynaptic bulbs specialised features?

Answer 7

• many mitochondria- meaning an active process that needs ATP involved.
• large amount of smooth endoplasmic reticulum which packages the neurotransmitter into vesicles.
• large numbers of vesicles containing molecules of a chemical called acetylcholine- the transmitter that will diffuse across the synaptic cleft.
• a number of voltage gated calcium ion channels on the cell surface membrane.

Question 8

What does the post synaptic membrane contain?

Answer 8

specialised sodium ion channels that can respond to the neurotransmitter.
these channels consist of 5 polypeptide molecules.

Question 9

What do 2 out of the 5 polypeptide molecules contain?

Answer 9

they have a special receptor site specific to acetylcholine.

Question 10

How are the receptor sites specialised

Answer 10

they have a shape that is complementary to the shape of the acetylcholine molecule.

Question 11

What happens when acetylcholine is present in the synaptic cleft?

Answer 11

it binds to the two receptor sites and causes the sodium ion channels to open.

Question 12

The first sequence of events in the transmission of a signal across the synaptic cleft?

Answer 12

• an action potential arrives at the synaptic bulb
• The voltage-gated calcium ion channels open.
• calcium ions diffuse into the synaptic bulb
  -the calcium ions cause the synaptic vesicles to move to and fuse with the pre synaptic membrane.
• acetylcholine is released in exocytosis
• acetylcholine molecules diffuse across the cleft

Question 13

The second sequence of events in transmission of a signal across the synaptic cleft?

Answer 13

• acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post synaptic membrane.
• the sodium channels are open
  -sodium ions diffuse across the post-synaptic neurone.
• a generator potential or excitatory post synaptic potential is created.
• if sufficient generator potentials combine then the potential across the post synaptic membrane reaches th threshold potential.
• a new action potential is created in the post synaptic neurone.

Question 14

What happens if acetylcholine is left in the synaptic cleft?

Answer 14

it will continue to open the sodium ion channels in the post synaptic membrane and will continue to cause action potentials .

Question 15

What is acetylcholinesterase

Answer 15

an enzyme found in the synaptic cleft.

Question 16

What does acetylcholinesterase do?

Answer 16

it hydrolyses the acetylcholine to ethnic acid and choline.
This stops the transmission of signals, so that the synapse does not continue to produce action potentials in the post synaptic neurone.

Question 17

What happens to the ethic acid and the choline

Answer 17

it is recycled. they re enter the synaptic bulb by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria. The recycled acetylcholine is stored in the synaptic vesicles for future use.