Lecture 4 Flashcards
What are other names for action potential?
spike
nerve impulse
discharge
What is action potential?
A rapid reversal of membrane potential where the inside becomes positively charged for 2-3 msec
What is synaptic transmission?
transfer of information from neuron to neuron
- frequency and pattern of action potentials constitute the code used by neurons to transfer information from one to another
How does action potential travel?
Starts at axon hillock (zone of integration) and travels down the axon to the presynaptic terminal (zone of output) = neurotransmitter release
Define depolarization
membrane voltage is more positive
Define hyperpolarization
membrane voltage is more negative
What results in depolarization?
Positive ion influx and negative ion efflux
What results in hyperpolarization?
positive ion efflux and negative ion influx
How is action potential generated?
Requires depolarization current
- occurs naturally via excitatory synaptic transmission
- artificially via current injection current into a neuron with microelectrode
What does the depolarizing current tell you about firing frequency from the generation of multiple action potentials?
Firing frequency reflects the magnitude of the depolarizing current.
Action potential firing rate increases as the depolarizing current increases
- if the current doesn’t depolarize the membrane to threshold, no action potential is generated
- if injected current depolarizes beyond threshold, action potentials will be generated
What’s the difference between electrical properties of neurons vs glia?
Neurons: electrically active - fire action potentials
Glia: electrically silent cells
What are the properties of the action potential?
- caused by depolarization of membrane beyond threshold
- All or None
- Chain reaction
What is the graph readings of an action potential?
1) Resting potential
2) Rising phase
3) Overshoot
4) Falling phase
5) Undershoot (after-hyperpolarization)
What are the necessary characteristics for action potentials?
1) ~10 mV depolarization above rest for initiation
2) below threshold = none, above threshold = all
3) Always depolarizing
4) Constant amplitude ~100 mV
5) Constant duration ~2-3 msec
6) Propagate without decrement
7) 2 part refractory period: absolute during falling phase and relative during undershoot
What is depolarization and repolarization in terms of Na+ and K+?
Depolarization: influx of Na+
Repolarization: efflux of K+
What is membrane current?
net movement of K+ across membrane
potassium current = flow and driving force
What’s the equation that links current, conductance, and driving force?
current = conductance* (membrane voltage - driving force)
What’s the ins and outs of action potential?
Rising phase: inward sodium current
Falling phase: outward potassium current
Define law of permeability.
Membrane potential is always driven toward the equilibrium potential of the ion to which the membrane is most permeable.
What’s the purpose of Hodgkin and Huxley model?
Demonstrates properties of action potentials using patch-clamp electrophysiology
- rising phase = transient increase of influx of Na+ ions
- falling phase = increase in efflux of K+ ions
What’s the structure of the voltage-gated sodium channels?
transmembrane domains and ion-selective pore
What’s the difference between voltage when the voltage-gating sodium channel when it’s open vs. closed?
Closed: -65 mV
Open: -40 mV
How does the voltage-gated sodium channel selective permeability?
Size exclusion filter
- partially hydrated Na+ ion fits
- partially hydrated K+ doesn’t fit
What is the cycle of the voltage gated sodium channel?
Closed (-65 mV) -(~10 mV Depolarization)->
Open (-55 mV) -(~1 ms delay)->
Inactivated -(channel requires -Vm reset)->
Closed
Define absolute refractory period.
Channels are inactivated
What are some diseases associated with voltage-gated sodium channels?
- Generalized epilepsy
- Febrile seizures
What are some toxins that target voltage-gated sodium channels?
1) tetrodotoxin (puffer fish) - clogs Na+ permeable pore
2) Red tide saxitoxin - Na+ channel-blocking toxin
3) Batrachotoxin (frogs) - blocks inactivation so channels stay open
4) Veratridine (lilies)
5) Aconitine (buttercups)
Compare and contrast VGSC and VGKC
Both open in response to depolarization
- Potassium opens ~1 ms after sodium
- Potassium conductance serves to rectify or rest membrane potential
When are the Na+ channels open when looking at the action potential graph?
At AP Threshold for initiation (~10 mV above baseline)
When are Na+ channels inactive when looking at the action potential graph?
At the peak of the graph
When are K+ channels open when looking at the action potential graph?
At the peak of the graph, when Na+ channels are inactive
When are K+ channels closing when looking at the action potential graph?
At the lowest point of the graph, stopping K+ efflux
When is the absolute refractory period? What happens here?
From the peak to threshold
VGSC inactivated
When is the relative refractory period? What happens here?
VGSC are reactivated but membrane potential is below resting and would require greater depolarization current to fire another action potential than at rest
What are factors influencing conduction velocity?
- Spread of action potential along membrane (dependent on axon structure)
- Path of positive charge (faster inside axon and slower across the axonal membrane)
- Axonal excitability (bigger axonal diameter = faster) also depends on number of voltage-gated channels
- Myelination
- Saltatory conduction at nodes of Ranvier
- VGSC concentrated at nodes
What does orthodromic mean?
Action potential travels in one direction (down axon to the axon terminal)
What does antidromic mean?
Backward propagation (only experimental)
What is myelin?
layers of this facilitate current flow
What is optogenetics? Where was it tested?
Introduction of foreign genes to express membrane ion channels (opens in response to light)
- tested in mouse brain
