Propagation and Synaptic Transmission Flashcards
(18 cards)
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 voltage gated Ca channels to open
- Ca enters terminals, causing release of neurotransmitter into synaptic cleft to activate the post-synaptic cell
what is action potential propagation
the movement of the action potential down the axon to the terminal
what is a refractory period
a period of rest after a stimulus during which another stimulus won’t have an effect
what is the absolute refractory period
the period after a nerve or muscle cell initiates an action potential, during which it is completely unresponsive to any additional stimuli, regardless of their strength
occurs when VG Na channels are already open or become inactive
what is the relative refractory period
a period during which a second action potential can be initiated, but it requires a stronger than normal stimulus
occurs when some VG channels begin to shift from inactive to closed state
at what stage can a voltage gated ion channel open from
only ever from the closed state, not the inactive state
explain how an action potential propagates through the axon segments - step 1
in an unmyelinated axon, an influx of Na at the axon hillock leads to diffusion of Na into the initial segment, triggering depolarization towards +30mV
explain how an action potential propagates through the axon segments - step 2
as the Na entering at spread away from the open VG channels, a graded depolarization quickly brings the 2nd segment to threshold
explain how an action potential propagates through the axon segments - step 3
- as we move past threshold, an action potential is triggered in segment 2
- in the initial segment, VG Na channels are closing and VG K channels are opening (repolarizing - refractory period)
explain how an action potential propagates through the axon segments - step 4
- as the Na ions entering at segment 2 spread laterally, a graded depolarization quickly brings the membrane segment 3 to threshold
- the initial segment is well on its way to hyperpolarization
what is saltatory conduction
ion movement is restricted to the areas without myelin (nodes) so conduction jumps from one node to the next
what are the key features of the presynaptic axon terminal in the chemical synapse
- has VG Ca channels
- has synaptic vesicles filled with neurotransmitter
what are the key features of the synaptic cleft in the chemical synapse
- space neurotransmitter diffuses across
- enzymes that inactivate neurotransmitter are present in the cleft
what is the key feature of the post synaptic cleft in the chemical synapse
has chemically gated ion channels
what is the 1st step of synaptic transmission
the axon terminal is depolarized
- when the action potential arrives at the axon terminal, the charge in voltage causes VG Ca channels to open
- Ca moves down its electrochemical gradient into the axon terminal
what is the 2nd step of synaptic transmission
the neurotransmitters are released
- Ca interacts with vesicles
- causing them to fuse with the membrane and release neurotransmitter into the synaptic cleft
- neurotransmitter diffuses across the synaptic cleft
what is the 3rd step of synaptic transmission
formation of local potentials
-neurotransmitter binds to chemically gated ion channels on the postsynaptic cell
- an excitatory neurotransmitter (ACh) opens Na channels to cause EPSPs
- an inhibitory neurotransmitter (GABA) opens Cl or K channels to cause IPSPs
what are the 3 ways we can tell when synaptic transmission ends
- 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
