Chapter 3 - The Action Potential Flashcards
Define hyperpolarization and depolarization
Hyp : membrane potential is < Resting potential
Depolarization : membrane potential is > resting potential
What causes an action potential?
… a depolarization of the membrane beyond the threshold
What constitutes the neural code?
Frequency and pattern of APs
What are the 4 phases of the AP
Rising phase, overshoot, falling phase and undershoot
What can cause a change in membrane potential?
- naturally : — adequate stimulus to a sensory receptors
— chemical neurotransmitters - experimentally : current injection
- light : optogenetics
What are the three types of protein molecules present on the membrane?
Sodium/potassium pump
Sodium channel
Potassium channel
What is the net current flow formula?
NCF = conductance x ionic driving force
NCF = g(ion) x (Vm - E(ion))
What are the 6 phases of the action potential in an ideal neuron
1) Membrane potential is depolarized to threshold.
2) transient increase in g(Na)
3) Entey of Na+ into the cell dépolarises further the cell
4) More voltage-gated Na+ channels are opened (positive feedback)
5) the time during which these channels are open is short
6) during falling phase, the Na+ gates close whereas the K+ channels stay open. Hyperpolarization of cell follows.
What does the voltage clamp do?
How does it work?
It allows to control the membrane potential.
See slide/ book
What is the Hodgkin and Huxley formula that calculates the conductance changes?
g(ion) = I(ion) / Vm - E(ion)
What is the structure of a voltage-gated Na channel?
Large protein, 4 domains with 6 transmembrane alpha helices
What are the two types of components for a domain (sodium channels)
- Voltage sensor S4
- Selectivity filter (for Na+)
What are the 4 steps to the activation and deactivation of a sodium channel?
1) Closed channel
2) Open channel upon depolarization
3) Inactivation of gate
4) Deinactivation —> ball removes itself
What are the different phases of the AP?
- Threshold : Vm where enough voltage gated Na+ channels are open : PNa > PK
- Rising phase : large driving force on Na+. Na+ rushes into cell
- Overshoot : Vm approaches ENa
- Falling phase : a) voltage gated Na+ channels inactivate
b) (this starts before falling phase, during rising phase) the v-gated K+ channels finally open, delayed colored to the Na ones, K+ rushes out of cell - Undershoot : Vm approaches EK as K+ channels remain open.
What is the absolute refractory period?
Na* channels remain inactivated until membrane potential goes negative enough to deinactivate the Na+ channels.
What is the relative refractory period?
Membrane potential remains hyperpolarized until K+ channels close.
(If these K+ channels didn’t close, more depolarization would be needed to reach threshold, and initiate AP)
What are two factors influencing speed of AP
1) Diameter of axon : big diameter entails less contact between ions and membrane, less current leakage, more current down axon, faster propagation
2) Myelination : the specialized glia cells wrap the axon making a very efficient insulator that allows no current leakage
What is saltatory conduction?
The AP jumps from node to node. This leads to faster conduction.
At each node, there is an action potential reaction (Na enters, K exits)
What are two demyelination guys diseases :
- Multiple sclerosis (CNS)
—> MS attacks myelin sheath in brain, spinal cord, optic nerves - Guillain-Barré syndrome (PNS)
—> immune response against one’s own myelin
Attacks myelin of nerves
