How Nerves Work 5

Question 1

What are most synapses?

Answer 1

Chemical

Question 2

What is the basis of a chemical synapse?

Answer 2

An electrical signal (AP) is turned into a chemical signal that is squirted onto the post synaptic cell to evoke a new electrical signal

Question 3

Where is muscle contraction triggered?

Answer 3

Triggered by AP in sacrolemma

Question 4

What occurs in the presynaptic cell at a NMJ?

Answer 4

• AP in motor neurone
• Opens voltage gated Ca channels in presynaptic terminal
• Triggers fusion of vesicles
• Acetylcholine released
• Diffuses across synaptic cleft

Question 5

What happens after the ACh diffuses across the cleft?

Answer 5

• It binds to ACh (nicotinic) receptors
• Opens ligand gated Na/K channels
• Evokes graded (local) potential (end plate potential)
• Always depolarises adjacent membrane to threshold
• Opens voltage gated Na channels which evokes a new AP
• ACh removed by acetylcholinesterase

Question 6

What does tetrodotoxin do?

Answer 6

Blocks Na channels and so blocks the AP

Question 7

What does joro spider toxin do?

Answer 7

Blocks Ca channels and so stops transmitter release

Question 8

What does botulinum toxin do?

Answer 8

Disrupts the release machinery and so blocks transmitter release

Question 9

What does curare do?

Answer 9

Blocks ACh receptors and so prevents the end plate potential

Question 10

What does anticholinesterase do?

Answer 10

Blocks ACh breakdown and so increases transmission at the NMJ

Question 11

How many neurotransmitters can be used at CNS synapses?

Answer 11

A range each with several receptors

Question 12

What postsynaptic potentials are therefor CNS synapses?

Answer 12

• Fast EPSPs (ionotropic)
• Slow EPSPs (metabotropic)
• Fast IPSPs
• Slow IPSPs

Question 13

How does the anatomical arrangement of synapses vary?

Answer 13

• NMJ does not change

- CNS synapses can be axo-somatic, ax-dendritic or axo-axonal

Question 14

What is the synaptic connectivity of NMJ?

Answer 14

Always motor neurone to muscle cell

Question 15

What is the synaptic connectivity of CNS synapses?

Answer 15

• Convergence
• Divergence
• Feedback inhibition
• Monosynaptic vs polysynaptic pathways

Question 16

Why are CNS synapses more complex than NMJ?

Answer 16

• Range of neurotransmitters
• Range of postsynaptic potentials
• Small potentials
• Variations on anatomical arrangement
• Variations on connectivity of neurones