Lecture 19: Propagation and Synaptic Transmission Flashcards
How does an electrical signal (action potential propagation) trigger a chemical signal (neurotransmitter release)?
Na+ diffuses from the axon hillock to initiate an AP in the initial segment of the axon
AP propagates to each neighboring axon segment (unmyelinated axon) or node (myelinated axon) in one direction.
The AP arrives at the axon terminals, causing VG Ca2+ channels to open
Ca2+ enters terminals, causing release of neurotransmitter into synaptic cleft to
activate the post-synaptic cell
What is action potential propagation
Action potential propagation is the movement of the action potential down the axon to the terminal
What is a refractory period
A refractory period is a period of rest after a stimulus during which another stimulus won’t have an effect
What is the absolute refractory period
A 2nd AP cannot be generated
Occurs when VG Na+ channels are already open or become inactive.
What is a Relative refractory period
A 2nd AP can be generated only if the stimulus is much larger than normal
Occurs when some VG Na+ channels begin to shift from an inactive to closed state.
What triggers depolerization in an unmyelinated axon
In an unmyelinated axon, an influx of Na+ at the axon hillock leads to diffusion of Na+ into the initial segment, triggering depolarization
How long can some axons be
1m long (lower limbs)
What is saltatory conduction
Ion movement is restricted to the areas without myelin (nodes), so conduction appears to jump from one node to the next.
What is found in the presynaptic axon terminal
VG Ca+ channels
Synaptic vesicles filled with neurotransmitter
What is the synaptic cleft
A space neurotransmitter
diffuses across
Enzymes that inactivate neurotransmitter are present in the cleft
What is in the post synaptic cell
Chemically-gated ion channels
Describe synaptic transmission
Ca2+ moves down electrochemical gradient to axon terminal
Interacts with vesicles causing them to fuse with membrane and release neurotransmitter into synaptic cleft
Neurotransmitter diffuses across synaptic cleft and binds to CG ion channels on post synaptic cell
When does synaptic transmission end
when Neurotransmitter unbinds
from chemically-gated channels
Enzymes in the synaptic cleft degrade neurotransmitter
Portions of the degraded neurotransmitter are recycled back into the axon terminal
