L19 Propagation and Synaptic Transmission

Flashcards covering action potential propagation, refractory periods, myelination, and synaptic transmission.

Refractory Period

A period after a stimulus during which another stimulus won’t have an effect. Or would need a much larger signal. This keeps the signal going DOWN the nueron.

Absolute Refractory Period (1st)

A 2nd AP cannot be generated and occurs when VG Na+ channels are already open or become inactive. During depolarising or late stage of repolarising.

Relative Refractory Period (2nd)

A 2nd AP can be generated only if the stimulus is much larger than normal and occurs when some VG Na+ channels begin to shift from an inactive to closed state. (VG Na+ channels can only reopen from closed state, not inactive)

Action Potential Propagation

The movement of the action potential down the axon to the terminal.

Initial segment of unmyelinated axon

Action potential developes at this segment, membrane potential at this site depolarises to +30mV. Na+ spreads and brings membrane at segment 2 to threshold.

Segment 2 unmyelinated axon

Action potential developes here, initial segment begins to repolarise (is now refractory)

Segment 3 unmyelinated axon

Sodium entering segment 2 spreads and brings this segments membrane to threshold. Action potential can only move foward because initial segment membrane is in absolute refractory period.

Myelin Sheath

Schwann cells (PNS) or oligodendrocytes (CNS) wrap the axon in neighbouring segments.

Myelinated axons

Have myelin sheath and nodes of ranvier, which dramatically increase AP conduction velocity.

Nodes (of Ranvier)

Narrow gaps with no myelin. High density of voltage- gated (VG) Na+ and K + channels.

Saltatory Conduction

Ion movement is restricted to the areas without myelin (nodes), so conduction appears to jump from one node to the next. Much quicker.

Presynaptic Axon Terminal (chemical synapse)

VG Ca2+ channels, Synaptic vesicles filled with neurotransmitter.

Synaptic Cleft (chemical synapse)

A space neurotransmitter diffuses across. With enzymes that inactivate neurotransmitter are present in the cleft.

Postsynaptic Cell (chemical synapse)

Chemically-gated ion channels.

Chemical synapse step 1

Axon terminal is depolarised when AP arrives because the change in voltage causes VG Ca2+ channels to open. Ca2+ moves into axon terminal driven by gradient.

Synaptic transmission step 2

Ca2+ interacts with vesicles causing them to fuse with membrane. Nuerotransmitters are released into synaptic cleft and diffuse across.

Synaptic transmission step 3

Formation of local potentials, nuerotransmitter (excitory ACh or inhibitory GABA) bind to chemically gated ion channels.

Termination of synaptic transmission

Nuerotransmitters unbind from CG channels, enzymes in synaptic cleft degrade them. Portions are recycled back into axon terminal.

Acetylcholine (ACh)

Excitatory neurotransmitter that opens Na+ channels to cause EPSPs.

GABA

Inhibitory neurotransmitter that opens Cl- or K+ channels to cause IPSPs.