3.6.2.2 Synaptic transmission

Question 1

Neurotransmitter in a cholinergic synapse

Answer 1

Acetylcholine

Question 2

Why are there lots of mitochondria in synapses?

Answer 2

able to produce more ATP for the release of the neurotransmitter and its re uptake (endocytosis requires ATP)

Question 3

Sequence of events during synaptic transmission

Answer 3

1. Action potential arrives at the axon terminal
2. Depolarisation of the presynaptic membrane causes it to become more permeable to Ca2+
3. Voltage gated Ca2+ channels open, Ca2+ enters by diffusion
4. Synaptic vesicles containing neurotransmitter FUSE with presynaptic membrane
5. Neurotransmitter acetylcholine released by exocytosis and diffuses across the synaptic cleft
6. Neurotransmitter binds to receptors on post synaptic membrane, causing the membrane to become more permeable to Na+
7. Na+ channels open, Na+ diffuses in and depolarises the post synaptic membrane
8. If threshold reached, action potential generated
9. Neurotransmitter hydrolysed by enzymes and reabsorbed into presynaptic neurone

Question 4

Why and how are neurotransmitters recycled?

Answer 4

• neurotransmitters swiftly removed from synaptic cleft
• this prevents reattachment to the receptors and continuous stimulation of the post synaptic cell
• re uptake into the presynaptic knob (endocytosis)
• or broken down by enzymes e.g. acetylcholine esterase

Question 5

What is a neuromuscular junction?

Answer 5

synapse between motor neurone and muscle cell - excitatory cholinergic synapse (increases change of action potentials)

Question 6

Adaptations of post synaptic membrane of neuromuscular junctions

Answer 6

• more acetylcholine receptors than other synapses (increases chance of Na+ channels opening causing depolarisation)
• sarcolemma is folded for acetylcholine esterase storage (prevents overstimulation of muscle)

Question 7

Differences between neuromuscular junctions and cholinergic synapses

Answer 7

NEUROMUSCULAR

• only excitatory
• links motor neurones to muscles
• action potential ends here
• only involves motor neurones
• ACh binds to receptors on the membrane of the muscle fibre

CHOLINERGIC

• can be excitatory or inhibitory
• links neurones to neurones or to effector
• another action potential may be generated along a different neurone
• involves motor, sensory and relay neurones
• ACh binds to receptors on the membrane of the post synaptic membrane

Question 8

How are impulses unidirectional?

Answer 8

vesicles containing neurotransmitter only secreted from presynaptic knob, and receptors are only on postsynaptic neurone

Question 9

Spatial summation

Answer 9

several impulses from multiple presynaptic neurones

Question 10

Temporal summation

Answer 10

• several impulses in quick succession to release enough acetylcholine to cause depolarisation ([NT] higher than [enzyme]
• large stimulus = higher frequency of action potential = more neurotransmitter released

Question 11

What is an agonist?

Answer 11

Complementary to receptor and stimulate

Question 12

What is an inhibitory synapse?

Answer 12

• decreases the chance of an action potential at the post synaptic membrane
• causes Cl- ions to move into postsynaptic neurone and K+ move out (hyper polarisation)

Question 13

Effects of stimulatory drugs (or neurotoxin or unknown neurotransmitter)

Answer 13

increase number of action potentials in the post synaptic membrane

1. mimic neurotransmitter by being similar shape and bind to receptor of post synaptic membrane - antagonist
2. Stimulates release of more neurotransmitter
3. Prevent the breakdown and feu-take of the neurotransmitter by inhibiting the enzyme
   e. g. SSRIs

Question 14

Effect of inhibitory drugs

Answer 14

Reduces number of action potentials in the post synaptic cells

1. Inhibits neurotransmitter release
2. Block the neurotransmitter receptors on the post synaptic membrane preventing the neurotransmitter fro binding - antagonist