The action potential Flashcards
What is an AP?
Transient impulse triggered by a depolarisation beyond threshold
Length of AP?
1-2 Ms
What three things can trigger an AP?
Electrode
Synaptic activity
Propagation of AP
How much is the Em displaced in an action potential?
Up to 100mv
What underpins an action potential?
Changes in membrane conductance to different ions
VGIC
Describe how pd changes in AP?
RMP at -70mV
Depolarisation til threshold of -55mV
Rapid upstroke to 40mV
Hyperpolarisation below RMP (-80mv)
What happens in initial depolarisation of AP?
Gentle increase in Em
Cation (Na+) channels open by chemical/physical stimulus
Or current has spread from neighbouring already excited tissues
What happens in upstroke?
Threshold reached, VGNC open and Na+ goes down electrochemical gradient
Once some open, the depolarisation initiates opening of others causing propagation (+ve feedback)
What happens in repolarisation?
Na+ channels inactivated
VGKC open, leads to K+ efflux down their electrochemical gradient
When do VGKC get stimulated to open and when do they open, why?
Stimulated at the same time as VGNC, but slow to open so open later
Why does hyperpolarisation occur?
VGKC slow to close
What occurs in the hyperpolarisation stage?
Hyperpolarisation of the membrane - raised K+ permeability takes the Em closer to Ek.
The Na+ channels are restored from inactivated to closed.
How is the hyperpolarised membrane returned to RMP?
Mainly action of Na/KATPase
What are the K+ channels called?
Delayed rectifier channels
What is the difference between absolute and relative refractory period?
Absolute: no upstroke can be initiated regardless of depolarisation. VGNC recover from activation (time and voltage dependent)
Relative refractory period: large stimulus required to elicit AP. Slightly after closure of K+ channels
What is the refractory period?
Neurone cannot fire another action potential
When the voltage-gated sodium ion channels are in their inactivated state the
What can the cell membrane be imagined as and why (physics)?
A capacitor, it separates charge
How is AP propagated in unmyelinated neurones?
Na+ influx generates local depolarisation
Negative region draws +ve charge from areas behind and ahead
Depol in one region is stimulus for depol and opening of VGNC in the other
What does the refractory period provide?
Unidirectional flow
What is the term for one way AP propagation?
Orthrodromic
How can nerve propagation be sped up (overall)?
Capacitance or resistance must be reduced, to decrease the time period of depolarisation.
What are gaps between myelination called?
Nodes of ranvier
Effect of myelination on length and time constant
Decrease capacitance - decrease time constant
Increase membrane resistance - increase length constant
Describe AP propagation in myelinated neurones
Saltatory conduction
Depolarisation wave jumps from node to node as the Na+ channels are packed into the nodes.
