Neuro 4 - propagation and synaptic transmission

action potential propagation

movement of the action potential down the axon to the terminal

absolute refractory period

2nd AP CANNOT be generated. VG Na channels inactive/already open and cannot be opened again

relative refractory period

2nd AP can be generated — VG Na channels have begun to close, stimulus must be large enough to counteract K+ leaving and bring membrane to threshold

refractory period

period of rest following a stimulus in which another stimulus won’t have any effect

propagation in unmyelinated axon step 1

Na diffuses into initial segment from influx of Na at axon hillock triggering depolarization

propagation in unmyelinated axon step 2

Na diffuses away from open VG channel bringing segment 2 to threshold

propogation in unmyelinated axon step 3

action potential triggered in segment 2 while initial segment starts to repolarize (absolute refractory period)

propogation in unmyelinated axon step 4

Na diffuses away from open VG channel, bringing segment 3 to threshold while intial segment approaches hyperpolarization (remains in absolute refractory period)

why can AP only move forward along axon?

while next segment depolarizes, previous segment is in absolute refractory period

how do myelinated axons/nodes of ranvier increase conduction velocity

ion movement restricted to areas without myelin where there is a high density of VG Na and K channels

saltatory conduction

in myelinated axons, where APs jump from node to node

synaptic transmission step 1

axon terminal depolarized — AP change in voltage causes VG Ca channels to open

synaptic transmission step 2

neurotransmitters released — Ca interacts with synaptic vesicles causing the release of neurotransmitters which diffuse across synaptic cleft.

synaptic transmission step 3

Formation of local potentials — neurotransmitter binds to chemically gated channels on postsynaptic cell causing excitatory or inhibitory LPs

synaptic transmission step 4

termination — neurotransmitters unbind, enzymes in synaptic cleft degrade and recycle neurotransmitter back into axon terminal