Lecture 19

Question 1

How does an electrical signal (AP propogation) trigger a chemical signal (neurotransmitter release)?

Answer 1

• Na+ diffuses from axon hillock to initiate AP in initial segment of axon
• AP spreads/propagates to each neighbouring segment (unmyelinated axon), or node (unmyelinated axon) in one direction
• AP arrives at axon terminals, cause VG Ca2+ channels to open
• Ca2+ enters terminals, releases neurotransmitter into synaptic cleft to active post-synaptic cell

Question 2

What is the difference between an absolute and relative refractory period?

Answer 2

• if absolute, a 2nd AP can not be generated, but in refractory, a 2nd AP can be generated with a larger stimulus
• absolute refractory period occurs when VG Na+ channels open or inactive, relative refractory period occurs when some VG Na+ channels are closed

Question 3

A VG Na+ channel can not be opened if the channel is…

Answer 3

Inactive

Question 4

How does an Ap flow through an unmyelinated axon?

Answer 4

• influx of Na+ at axon hillock leads to diffusion of Na+ into initial segment, triggering depolarization
• Na+ in first segment continues to spread to second segment, moving it to threshold for AP
• in initial segment, VG Ca+ channels close and VG K+ channels open to repolarize cell
• segment three reaches threshold, segment two starts depolarizing, initial segment is in hyperpolarization

Question 5

What are the key features of the pre-synaptic axon terminal?

Answer 5

• VG Ca2+ channels
• synaptic vesicles filled with neurotransmitter

Question 6

What are the key features of the synaptic cleft?

Answer 6

• space neurotransmitter diffuses across
• enzymes that inactivate neurotransmitters present

Question 7

What is the key feature of the post-synaptic cell?

Answer 7

Has chemically gated ion channels

Question 8

How does the depolarization of the axon terminal work in synaptic transmission?

Answer 8

• AP arrives at axon terminal, change in voltage causes VG Ca2+ channels to open
• Ca2+ moves down electrochemical gradient into axon terminal

Question 9

How are neurotransmitters released in synaptic transmission?

Answer 9

• Ca2+ interacts with vesicles
• fuses with membrane and releases neurotransmitter to synaptic cleft
• neurotransmitter diffuses across synaptic cleft

Question 10

How does the formation of local potentials work in synaptic transmission?

Answer 10

• neurotransmitter binds to chemically-gated ion channels on post-synaptic cell
• excitatory neurotransmitter (e.g ACh) opens Na+ ion channels for an excitatory post-synaptic potential
• Inhibitory neurotransmitter (e.g GABA) opens Cl- or K+ ion channels to cause inhibitory post-synaptic potential

Question 11

How is a synaptic transmission terminated?

Answer 11

• neurotransmitter unbinds from chemically gated ion channels
• enzymes in synaptic cleft degrade neurotransmitter
• portions of degraded neurotransmitter are recycled back into axon terminal