[15.5-6] synapses: structure, function, and transmission

Question 1

what is a synapse?

Answer 1

point where one neurone communicates with another or with an effect

Question 2

what are all the features of the structure of a synapse? (7)

Answer 2

• mitochondria
• Ca²⁺ ion protein channel
• synaptic vescicle containing neurotransmitter acetylcholine
• Na⁺ ion protein channels
• synaptic cleft
• receptor site
• acetylcholinesterase enzymes

Question 3

what are the names given to the two neurons involved in synaptic transmission?

Answer 3

• pre-synaptic neuron
• post-synaptic neuron

Question 4

in what way are synapses unidirectional?

Answer 4

synapses can only pass information in one direction, from the pre-synaptic neuron to the post-synaptic neuron

Question 5

why is summation needed?

Answer 5

• low-frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the post SN
• summation allows a rapid build-up of neurotransmitter in the synapse

Question 6

what are the two versions of summation?

Answer 6

• spatial summation
• temporal summation

Question 7

what is spatial summation?

Answer 7

• many different pre SNs release enough neurotransmitter together at the same time to exceed the TV of the post SN
• this therefore triggers a new AP

Question 8

what is temporal summation?

Answer 8

• a single pre SN releases neurotransmitter many times over a short period
• if the concentration of NT exceeds the TV of the post SN, a new AP is triggered

Question 9

what do inhibitory synapses do?

Answer 9

they make it less likely that a new AP will be created on the post SN

Question 9

how do inhibitory synapses work?

Answer 9

• pre SN releases NT which binds to chloride ion protein channels on post SN
• NT causes chloride IPC to open
• chloride ions move into post SN by FD
• binding of NT causes nearby potassium PCs to open
• potassium ions move out of post SN into synapse
• combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out makes inside of post-synaptic membrne more negative and outside more positive
• membrane potential increases to as much as -80 mV compared with usual -65 mV at resting potential
• this is called hyperpolarisation and makes it less likely that a new AP will be created as a larger influx of sodium ions is needed to produce one

Question 10

what are excitatory synapses?

Answer 10

synapses that produce new action potentials when neurotransmitter binds with the receptor proteins

Question 11

what is the importance of synpases acting as junctions?

Answer 11

• allows a single impulse along one neuron to initiate new impulses in a number of different neurons at the synapse. this allows a single stimulus to create a number of simultaneous responses
• many impulses can be combined at a synapse, which allows nerve impulses from receptors reacting to different stimuli contribute to a single response

Question 12

describe transmission across a synapse

Answer 12

1. AP arrives at pre SM
2. calcium ion channels open in pre SM
3. calcium ions enter membrane via FD
4. vesicles move to and use with pre SM due to membrane fluidity
5. neurotransmitter molecules (acetylcholine) are released into synaptic cleft
6. NT diffuses across SC and bind to complementary receptor proteins on post SM
7. sodium ion channels open in post SM
8. sodium ions enter membrane via FD
9. if enough sodium ions enter, TV will be reached, establishing an AP in the post SM
10. to prevent permanent APs, acetylcholine is broken down by acetylcholinesterase
11. products of this breakdown are re-absorbed, re-combined and re-packaged into vesicles
12. this requires ATP, which is released from the mitochondria
13. recombined vesicles go back to pre SM via FD
14. sodium IPC close in absence of acetylcholine in receptor sites