Organisms Respond to Changes in their Internal and External Environments: Nervous Coordination - Synaptic Transmission

Question 1

List the main stages involved in transmission across a cholinergic synapse

Answer 1

• Arrival of an action potential
• Fusion of the vesicles
• Diffusion of ACh
• Removal of ACh

Question 2

Describe what happens during the arrival of an action potential

Answer 2

• Action potential arrives at the synaptic knob of presynaptic neurone
• Action potential stimulates voltage-gated calcium ion channels in presynaptic neurone to open
• Calcium ions (Ca²⁺) diffuse into synaptic knob

Question 3

Describe what happens during the fusion of the vesicles

Answer 3

• The influx of calcium ions (Ca²⁺) causes the synaptic vesicles to fuse with presynaptic membrane
• Vesicles release acetylcholine (ACh) into synaptic cleft by exocytosis

Question 4

Define the term exocytosis

Answer 4

• Process by which a vesicle in a cell moves to cell-surface membrane, fuses with membrane and releases its contents outside the cell

Question 5

Describe what happens during the diffusion of acetylcholine (ACh)

Answer 5

• ACh diffuses across synaptic cleft
• ACh binds to specific cholinergic receptors on postsynaptic membrane
• This causes sodium ion (Na⁺) channels in postsynaptic neurone to open
• Influx of sodium ions into postsynaptic membrane causes depolarisation
• Action potential on postsynaptic generated if threshold is reached

Question 6

Describe what happens during the removal of acetylcholine (ACh)

Answer 6

• ACh removed from synaptic cleft so response doesn’t keep happening
• ACh hydrolysed by the enzyme acetylcholinesterase (AChE)
• Products are re-absorbed by presynaptic neurone to make more ACh

Question 7

Define the term unidirectional in the context of synapses

Answer 7

• Movement of impulses in one direction

Question 8

Explain how cholinergic synapses show unidirectionality

Answer 8

• Synapses only pass impulses in one direction, from presynaptic to postsynaptic neurone
• Only presynaptic knob has vesicles of ACh and can only release ACh into synaptic cleft
• Only postsynaptic membrane contains specific receptors for ACh to bind to

Question 9

What are the different types of neurotransmitters?

Answer 9

• Excitatory
• Inhibitory
• Or both

Question 10

What do excitatory neurotransmitters do?

Answer 10

• Depolarise the postsynaptic membrane, causing it to send an action potential if threshold is reached

Question 11

Define the term depolarise

Answer 11

• Makes potential difference across a neurone membrane more positive

Question 12

What do inhibitory neurotransmitters do?

Answer 12

• Hyperpolarise the postsynaptic membrane, preventing it from sending an action potential

Question 13

Define the term hyperpolarise

Answer 13

• Making the potential difference across the neurone membrane more negative

Question 14

Define the term summation

Answer 14

• Process where the sum of lots of smaller impulses triggers an action potential

Question 15

What are the different types of summation?

Answer 15

• Spatial
• Temporal

Question 16

What is the difference between spatial and temporal summation?

Answer 16

• Spatial

• Multiple presynaptic neurones release neurotransmitter

• Temporal

• One presynaptic neurone releases neurotransmitter in quick succession