Action Potentials Flashcards
What is the pattern of communication in the nervous system?
Sensory stimulus -> neural code -> Interpretation/Action
How long can action potentials travel?
0.1mm to 1m
What are the 4 phases of an action potential?
- Rising Phase (depolarization)
- Overshoot
- Falling phase (repolarization)
- Hyperpolarization
What is the difference between an absolute and relative refractory period?
An absolute refractory period means no other action potentials can be fired at this time, a relative one means that it is just much more difficult to fire an action potential
Who was responsible for the discovery of in squid?
Hodgkin + Huxley
What can we say mathematically about different ion channels at rest?
Gk «< Gna
What happens to channels when depolarization happens?
Rapid opening of Na channels
What happens to channels during the repolarization phase?
The rapid closing of Na channels and slow opening of K channels
What did Hodgkin and Huxley conclude about action potentials?
It requires that certain channels become inactivated independent of membrane potential.
What is the variable of interest in a voltage-clamp experiment?
Current
How can we verbalize the results of a current clamp experiment?
How can we measure Vm by injecting currents through an electrode?
How can we verbalize the results of a voltage clamp experiment?
To measure Im at a desired Vm, we fix Vm at a value and determine the current needed to keep Vm equal to Vclamp
What can we learn about the neuron based on the voltage-clamp experiments?
We can measure the Na+ and K+ currents activated when Vm is at different points in an action potential cycle
What ionic current is activated by the depolarization and is sustained throughout the depolarization?
The K+ current
What happens to the Na+ current when a depolarization happens?
The current is activated by a depolarization but is not sustained - it deactivates itself
What is patch clamping?
It is a way of determining the permeability of a single channel of a cell membrane. It involves extracting a portion of membrane and imposing a voltage on it
What are the three types of patch clamping?
- Cell attached
- Inside-Out mode
- Whole-cell mode
How can we tell if a channel is voltage gated?
When the voltage across the membrane is higher or lower with the same ion concentration, the frequency of action potentials change.
What are the two gates that are responsible for the status of channels?
The activation and inactivation gate
What is the gate cycle of channel cells?
Closed (activation gate closed, inactivation gate open)->
Open (activation gate open, inactivation gate open) ->
Inactivated (activation gate open, inactivation gate closed)
What gate cycle phases are associated with which neuronal action potential cycle phases?
Activation = Polarization
Inactivation = maintained depolarization
Recovery from inactivation = repolarization
Would a voltage gated cell be more likely to be open at -80 or -40mv?
-40
What can be said about delay times and how long Na+ channels remain open?
They open very rapidly and remain open for about 1 ms
What is the mechanism responsible for the quick closure of Na+ channels?
The ball and chain mechanism
What kind of refractory periods do Na+ channels experinece?
absolute refractory periods
What does patch clamping tell us about Na+ channel currents?
The current only rises during the rising phase of the action potential
At what currents do potassium channels activate and inactivate?
Inactivated at -100mV and channels open fully (after a delay) at 50mV
What does patch clamping tell us about the current of K+ in a neuron?
The current enters at the end of the rising phase and peaks at the start of the falling phase
What are the three comparisons made between Na+ and K+ voltage gated channels?
- They both open in response to depolarization
- K+ channels open much more slowly
- Only Na+ channels inactivate
How does the K+ channel act as a “delayed rectifier”?
It serves to reset the Vm, it is necessary to end Na+ channel inactivation
What is the difference in structure between Na+ channels and K+ Channels?
Na+ channels have 4 domains, and K+ channels have 4 subunits
What are the two potential directions of an action potential?
Orthodromic = towards the nerve terminals
Antidromic = from the nerve terminals to the dendrites
What is the typical duration of an action potential?
2ms
What is the typical velocity of an action potential?
10 m/s
What is the described role of myelin sheaths?
It prevents the leakage of ions
What can we say about channel density in the nodes of Ranvier?
There is a very high Na+ channel density
What is the difference between conduction at the Nodes of Ranvier and under the myelinated sheath?
Nodes of Ranvier = active conduction with action potentials and Na+ channels
Myelinated Sheaths: Electronic conduction - passive diffusion of Na+ ions
In what two areas do actions potentials generate?
In the nerve terminals of sensory neurons, and in the axon hillock of other neurons
