3. Action Potentials Flashcards
List the main AP properties
Obey an all-or-none law
Sub-threshold stimuli do not evoke APs. Supra-threshold stimuli evoke APs. Increasing stimulus further has no effect on AP size or shape.
During AP, impossible to stimulate a further AP – cell is refractory
Absolute refractory period
no stimulus is effective
Relative refractory period
very large stimulus generates AP
APs are self-propagating
once generated, will be conducted automatically over the excitable membrane.
What are the key mechanisms of an action potential?
Transmembrane ion conc gradients
Voltage- dependent changes in permeability of membrane to Na+ and K+
What happens when the permeability to an ion increases?
The resistance decreases and conductance increases
Describe the generation of an Action Potential
At rest, membrane more permeable to K+ than Na+
Depolarizing stimulus – rapid increase in Na+ permeability
Increase in sodium conductance
Na+ diffuse into cell – driven by high [Na+] outside and drawn by negative charge inside. (electrochemical gradient)
Inward sodium current – depolarizes membrane further (positive feedback loop)
At peak of AP, membrane potential approaches ENa (high permeability to Na)
Na+ conductance falls and K+ conductance starts to rise (voltage-dependent)
Increase in outward K+ current – driven by electrochemical gradient for K+ at AP peak when membrane is very positive relative to EK
K+ current repolarizes membrane and then an hyperpolarization (not shown here, close to EK)
What are voltage gate ion channels?
Channels in the cell membrane that bring about changes in membrane permeability to Na+ and K+ during AP
Only selective to certain types of ion
Channels largely closed at rest - no ionic current can flow through- and they have a low permeability
What are voltage gate ion channels?
Channels in the cell membrane that bring about changes in membrane permeability to Na+ and K+ during AP
Only selective to certain types of ion
Channels largely closed at rest - no ionic current can flow through- and they have a low permeability
How do voltage- controlled ion channels regulate ionic conductances?
Depolarisation will change the channel conformation and the channel opens
Continued depolarisation inactivates channel - absolute refractory period
Membrane repolarised nad channel will close again
How do K+ differ from Na+ channels?
The K+ channels activate more slowly than Na+ channels
What is the relative refractory period?
K+ current repolarizes and hyperpolarises membrane, resists any depolarisation from inward current
What is the action potential peak?
When the inside is positive compared with outside
What is meant by self-propagating?
Once an AP has been initiated at one point, it will be conducted across the rest of the membrane
What does the refractory properties of the membrane ensure?
That the conduction is unidirectional
What two things does nerve conduction depend on?
Axon diameter
Myelination
How does saltatory conduction work?
AP jumps
Nodes of ranvier at regular intervals
APs generated at nodes where excitable membrane exposed
Spread of depolarising current along myelinated region is rapid
Insulation prevents local current along myelinated region is rapid
Insulation prevents local current leakage- more current available to depolarize axons at nodes
