Synaptic Transmission

Question 1

what is a synapse?

Answer 1

a junction between two neurones or between a neurone and an effector

Question 2

how can impulses cross from one neurone to the next?

Answer 2

using a chemical neurotransmitter

eg acetylcholine(AcH) which binds to cholinergic receptors

Question 3

why are synapses important?

Answer 3

give the nervous system control over the transmission of impulses, impulses can be:
- stopped
- passed on to more than one neurone
- directed along certain neuronal pathways

Question 4

what is a synaptic cleft?

Answer 4

a small gap between the neurones at a synapse which is 20-30nm

an impulse from one neurone is carried across a gap by a neurotransmitter which initiates (or inhibits) action potentials in the adjacent neurones

Question 5

label the diagram of a synaptic knob

Answer 5

Question 6

describe the transmission across a synapse

Answer 6

1. action potential arrives at presynaptic membrane
2. Ca2+ channels open and Ca2+ ions diffuse through the presynaptic membrane into the synaptic knob. They are pumped out afterwards by active transport
3. synaptic vesicles containing acetylcholine move towards presynaptic membrane (ATP required)
4. Vesicle fuses with presynaptic membrane and acetlcholine (ACh) is released from synaptic vesicles into synaptic cleft by exocytosis
5. acetylcholine diffuses across synaptic cleft to post synaptic membrane
6. acetylcholine binds with specific receptor protein on postsynaptic membrane
7. sodium ion channels in post synaptic membrane open and sodium ions enter and depolarise the membrane starting a new action potential
8. the enzyme acetylcholinesterase breaks down the acetylcholine and it diffuses back into the synaptic knob where it is resynthesises into neurotransmitter. This requires ATP.

It hydrolyses the acetylcholine to ethanoic acid and choline. This stops the transmission of impulses so that the synapse does not continue to produce action potentials in the postsynaptic neurone.

Question 7

what is an excitatory synapse?

Answer 7

at excitatory synapses the binding of neurotransmitter opens sodium channels in the post synaptic membrane initiating an action potential

(usually causes depolarisation in the postsynaptic neurone)

Question 8

what is an inhibitory synapse?

Answer 8

• binding of the neurotransmitter makes the resting potential of the post synaptic membrane more negative (hyperpolarise the post synaptic membrane)
• less likely that the threshold potential will be reached
• less likely action potentials will be initiated

Question 9

what is the role of synapses?

Answer 9

1. allows the nerve impulses to be transmitted between neurones and ensures the transmission of action potentials is in on direction only
2. summation = to produce depolarisation, several action potentials are needed to produce enough transmitter to overcome the threshold of the post synaptic membrane

Question 10

what are the two types of summation?

Answer 10

1. spatial summation
2. temporal summation

Question 11

what is spatial summation?

Answer 11

Each presynaptic neurone releases a small quantity of neurotransmitter which together results in an action potential in the post synaptic neurones

Question 12

what is temporal summation?

Answer 12

• several action potentials arrive very closely one after another and each releases a set volume of neurotransmitter which combines to initiate an action potential nin the post synaptic neurone
• prevents overstimulation and fatigue