Flashcards in M&R 4.1 - The Action Potential and its Properties Deck (11):
What is an action potential?
The change in voltage across a membrane depending on both ionic gradients and relative permeability of the membrane
Why is the 'all or nothing' principle of action potentials significant?
Ensures there are no half or double action potentials i.e. they are propagated without getting smaller
What is the significance of positive feedback in opening VG Na+ channels during the action potential?
- Positive feedback causes more channels to open so causes more depolarisation
- Ensures depolarisation doesn't stop half way through
What is the absolute refractory period?
The period of time at which nearly all the Na+ channels are inactivated meaning the excitability is at 0
What is the relative refractory period?
- The period of time at which the Na+ channels are recovering from inactivation so the excitability is returning to normal
- Number of inactivated channels decreases
Describe what happens during repolarisation
- Inactivation of Na+ channels = potential returns to RMP
- VG K+ channels open = potential moves towards Ek
- Longer stimulus = larger depolarisation needed to generate an action potential due to inactivation
- Threshold becomes more positive
- Decreases intensity as initial action potential doesn't reach threshold as more Na+ channels are inactive
Describe the structure of Na+/Ca2+ channels
- One peptide of FOUR homologous repeats
- Each repeat has SIX transmembrane domains
- One transmembrane domain is sensitive to changes in voltage (positive amino acid residue)
- ONE pore formed by a dipped Beta sheet in which the inactive particle sits
Describe the structure of a VG K+ channel
- FOUR subunits
- Each subunit has SIX transmembrane domains
- One transmembrane domain is sensitive to changes in voltage
- Pore is formed by FOUR subunits
Describe the effects of local anaesthetics e.g. Procaine
How are these effects caused?
- Bind to and block Na+ channels = stops AP generation
- Cross membrane in unionised form and block Na+ channels when open
- Have a higher affinity to the inactive state of Na+ channels