What causes an action potential? Flashcards
What are the steps of an action potential?
(1) Stimulus, (2) Depolarisation, (3) Re-polarisation, (4) Hyper-polarisation, (5) Restoring the Resting Potential.
What is depolarisation?
If an electrical current above a threshold level is applied to the membrane, it causes a massive change in potential difference. Th PD is locally reversed, making the axon positive and the outside negative.
What is the potential difference in a resting neurone?
-70mV
What does the sodium-potassium pump do?
It maintains the resting potential. The Na+/K+ pump creates conc. gradient across membrane.
What is the charge across the axon in a resting neurone? What are the concentrations of ions across the membrane?
- INSIDE: Negative, High K+ conc.
- OUTSIDE: Positive, High Na+ conc.
What is the function of sodium-potassium ion pump?
It uses active transport to move 3 Na+ ions out of the neurone for ever 2 K+ ions move in. ATP is needed to do this.
What is the function of the potassium ion channel [for a neurone at resting potential]?
They allow facilitated diffusion of K+ ions out of the neurone , down their concentration gradient.
What occurs during depolarisation?
Stimulus. Causes change in PD across membrane.Volt-dependant Na+ ion channels open. Na+ flows in. As more flows in membrane becomes more permeable and depolarisation incr. More VDNa+IC open when threshold met. Inside more positive. PD is +40mV.
What is the potential difference across the membrane of a neurone when it is polarised?
+40mV
What occurs during repolarisation?
At +40mV, v-depend. Na+ ion channels close. Voltage-dependant K+ ion channels open and K+ ions move out of axon, down electrochemical gradient. Inside of cell becomes negative.
What occurs when the neurone cell is restoring the resting potential?
Hyper-polarisation of membrane - VD K+ ion channels slow to close so more K+ move out of cell. PD more negative than normal. Resting pot. re-est. by closing of VD K+ iC and K+ diffuse into axon.
What is meant by ‘all-or-nothing’?
APs have an all-or-nothing nature (the values of the resting and action potentials are always the same for a specific neurone). A bigger stimulus increases the frequency of action potentials.
What is the threshold stimulus?
Around -50mV
What is the refractory period?
Straight after an AP, there is a refractory period when an new action potential can’t be generated because the Na+ ion channels can’t reopen.
What does the refractory period ensure?
That action potentials pass along as separate signals and that they’re unidirectional (only pass in one direction).
