5: Action potentials Flashcards
What is depolarisation?
Membrane potential becomes LESS NEGATIVE or even positive
What is repolarisation?
Membrane potential becomes MORE NEGATIVE
The movement of ___ across membranes causes changes in membrane potential
ions
In which direction do Na+ ions move?
Into the cell
In which direction do K+ ions move?
Out of the cell
When sodium channels open, the membrane potential (repolarises / depolarises).
depolarises
As ENa+ = +60 mV
When potassium channels open, the membrane potential (repolarises / depolarises).
repolarises
As EK+ = -90mV, membrane potential tends towards -90mV - membrane may even hyperpolarise beyond its normal value of -70mV
What are two common gated ion channels?
Voltage-gated ion channels - open depending on membrane voltage
Ligand-gated ion channels - open upon binding of certain ligands
If a membrane depolarises, it’s probably due to the influx of which ion?
Na+
If a membrane repolarises or hyperpolarises, it’s probably due to the efflux of which ion?
K+
What is an action potential?
Short-term reversal of membrane potential
Which cells make use of action potentials to communicate?
Nerve cells
Action potentials are only generated when a ___ is reached and cannot be stopped after they are triggered. What kind of reaction is this?
threshold
all-or-nothing reaction (like swallowing)
The initial activation of Na+ channels in an action potential causes more Na+ channels to open. Which kind of feedback mechanism is this?
Positive feedback
The activation of K+ channels causes repolarisation, which in turn switches K+ channels off. What kind of feedback mechanism is this?
Negative feedback
In an action potential, ___ channels open first, then ___ channels open with a slight delay.
Na+ first
K+ after a delay
Na+ channels open in response to depolarisation by positive feedback.
What happens to them during sustained depolarisation?
They become inactivated (NOT closed) and are unable to provide passage for Na+
What closes Na+ channels following their inactivation?
Repolarisation by K+ efflux
What are the three states of Na+ channels, and during which stages of an action potential do they present?
Open state during depolarisation
Inactivated state after maintained depolarisation
Closed state after repolarisation
Which state of Na+ channel is responsible for the refractory period?
Inactivated state
What is the refractory period?
Time between action potentials
What material increases the resistance of axons to current leak and therefore increases conduction speed?
Myelin
How does current spread in unmyelinated axons?
Passively
