Synaptic transmission

Question 1

where are synapses found

Answer 1

between 2 neurons or between a neurone and effector cell

Question 2

why does the synaptic knob many mitochondria and large amounts of SER

Answer 2

mitochondria
- to produce ATP for synthesis reactions

SER
- make lipids and carbohydrates

Question 3

approximately how wide is the synaptic cleft

Answer 3

20-30 nm

Question 4

what is the role of the synaptic vesicles

Answer 4

to carry neurotransmitters to synaptic cleft

Question 5

summarise how an action potential reaching the PREsynaptic neurone causes a new action potential in the POSTsynaptic neurone

Answer 5

• when action potential reaches synaptic knob, synaptic vesicles fuse with the presynaptic membrane
• neurotransmitter diffuses across cleft and binds to specific receptor proteins on post synaptic membrane, causing depolarisation
• this can lead to a new action potential

Question 6

define synapse

Answer 6

gap between 2 neurons where chemical diffusion occurs

Question 7

define cholinergic synapse

Answer 7

releases neurotransmitter acetyl choline (Ach)

Question 8

explain the process of transmission across a cholinergic synapse

Answer 8

• action potential arrives at the presynaptic neurone
• causes voltage gated Ca2+ protein channels to open and Ca2+ enters by facilitated diffusion
• Ca2+ ions cause synaptic vesicle to fuse with presynaptic membrane
• Acetylcholine (Ach) is released into synaptic cleft by exocytosis
• Ach diffuses across cleft
• At postsynaptic membrane Ach binds to receptor sites on Na+ ion protein channels causing them to open
• Na+ ions diffuse down concentration gradient into post synaptic neurone causing membrane to be depolarised
• an action potential is generated (if threshold is reached)

Question 9

define spatial summation

Answer 9

a number of postsynaptic neurones together release neurotransmitter to exceed the threshold value of the post synaptic neurone and together they trigger a new action potential

Question 10

explain the process of spatial summation

Answer 10

• neurone A releases neurotransmitter but concentration below threshold to trigger action potential in the post synaptic neurone
• neurone B releases neurotransmitter but concentration below threshold to trigger action potential in the post synaptic neurone
• neurone A + B release neurotransmitter so concentration is above threshold so an action potential is triggered in post synaptic neurone

Question 11

define temporal summation

Answer 11

1 presynaptic neuron releases neurotransmitter many times over a short period. If the concentration of neurotransmitter exceeds the threshold value of post synaptic neurone so an action potential is triggered

Question 12

explain the process of temporal summation

Answer 12

• low frequency action potential leads to neurotransmitter being broken down quickly. Concentration is below threshold to trigger action potential in post synaptic neuron
• high frequency action potential leads to release of a neurotransmitter in a short time. Concentration is above threshold to trigger an action potential in post synaptic neurone.

Question 13

explain why impulses at a synapse are unidirectional (3 marks)

Answer 13

• neurotransmitter is only released from the presynaptic membrane
• neurotransmitters bind to specific receptors found only in post synaptic membrane
• so neurotransmitter can’t stimulate an action potential on pre synaptic membrane

Question 14

define inhibitory synapse

Answer 14

action potentials are not generate in post synaptic membrane due to hyperpolarisation

Question 15

what is the mechanism for an inhibitory synapse

Answer 15

action potential triggers release of inhibitory neurotransmitter from the postsynaptic membrane

neurotransmitter diffuses across cleft to post synaptic membrane

neurotransmitter binds to Cl- ion channel proteins causing….

Cl- ions to move into postsynaptic membrane by facilitated diffusion…

Cl- ions bind to K+ ion channels causing…

K+ ions to move out of post synaptic neurone….

this causes hyperpolarisation of membrane
(inside becomes more negative than resting)

this prevents an action potential occurring (as a very large concentration of Na+ ions would be required to depolarise membrane)

Question 16

why does a neurotransmitter bind to a specific receptor protein

Answer 16

complementary shapes

Question 17

what are the 2 types of neurotransmitters and define both

Answer 17

excitatory - leads to a new action potential in the post synaptic membrane

inhibitory - make action potentials less likely in post synaptic membrane

Question 18

what 2 ways can drugs act on a synapse

Answer 18

stimulants

inhibitors

Question 19

what effect do drugs have on neurotransmitter action (stimulant)

Answer 19

action

• mimics action of neurotransmitter so binds to receptors on postsynaptic membrane (agonist) as have a similar shape
• stimulate the release of neurotransmitters
• decrease/ inhibit the action of enzyme which breaks down the neurotransmitter

Question 20

what effect do drugs have on neurotransmitter action (inhibitor)

Answer 20

action

• inhibits release of neurotransmitter
• blocks receptors on postsynaptic membrane (antagonists)

Question 21

· Explain how acetylcholine is recycled

Answer 21

Acetylcholine is hydrolysed by acetylcholinesterase into choline and ethanoic acid (acetyl) this prevents a continuous action potential in the postsynaptic neurone
Choline and ethanoic acid (acetyl) diffuse back into the presynaptic neurone to be recombined into acetylcholine using ATP