Action Potential Flashcards
How is an action potential created?
Energy of stimulus opens Na+ channels- membrane more permeable to Na+ ions- more Na+ ions diffuse in- inside of axon becomes less negative- Positive feedback- change in charge opens Na+ voltage gated channels- +40mV.
How does the axon membrane return to resting potential?
Na+ channels close- Voltage gated K+ channels open- K+ efflux- inside becomes more negative than outside- Repolarisation- Initially, lots of K+ diffuse out- membrane potential more negative than the resting potential- Hyperpolarisation- Na/K pump restarts- Repolarisation- membrane returns to resting potentials.
Basics of action potential propagation
Initial stimulus creates action potential in the sensory receptor-first region of axon membrane depolarised- this acts as a stimulus for the next region which is then depolarised.
How is the action potential propagated?
Na+ ions are attracted by the negative charge ahead and diffuse down their concentration down the axon.
What happens to the regions behind the new region of depolarisation?
K+ channels open- K+ efflux- region is repolarised and returns to the resting potential ready for the arrival of a new stimulus.
What is the refactory period?
Short period of time where the axon membrane cannot be excited again after the transmission of an action potential- the voltage gated Na+ channels remain closed.
What is the purpose of the refactory period?
Ensures that the action potential is uni-directional.
It means that the action potentials do not overlap and that they occur as discrete impulses.
Advantages of saltatory conduction?
The Myelin sheath is impermeable to Na+ ions- channels opening and closing takes time- hence reducing the number of times this occurs speeds up the transmission of the impulse.
it is also more energy-efficient- repolarisation requires ATP for the Na/K pump.
Factors that effect the speed of transmission of the electrical impulse.
Axon Diameter: The greater the axon diameter, the faster the speed of the impulse- due to less resistance to the flow of ions in the cytoplasm.
Temperature: Ions diffuse faster at higher temperatures- up to 40 degrees celcius- channel proteins start to denature.
What is the all or nothing principle?
If the threshold potential is reached- an action potential is triggered- if not- no action potential is triggered. Same size action potential is reached irrespective of the size of the stimulus.
What determines the strength of the stimulus?
The frequency of action potentials- higher frequency=stronger stimulus.
What is the threshold potential?
The p.d at which voltage gated Na+ channels open.
Types of neurotransmitter:
Excitatory: Results in depolarisation of the post-synaptic neurone- acetylcholine.
Inhibitory: Results in hyperpolarisation of the post-synaptic neurone- prevents the action potential- Gamma-aminobutyric acid (GAMA)
Transmission across synapses.
Action potential reaches the pre-synaptic neurone-depolarisation- Ca2+ channels open and diffuse into the pre-synaptic knob.
Ca2+ ions fuse with synaptic vesicles containing neurotransmitters, these vesicles then fuse with the pre-synaptic membrane- releasing the neurotransmitter by exocytosis into the synaptic cleft.
NT diffuses across the cleft- binding with the specific receptor on the post-synaptic membrane.
Na+ channels open-diffuse into post-synaptic neurone- action potential triggered.
How is the neurotransmitter recycled?
The neurotransmitter left in the synaptic cleft is removed so that another stimulus can arrive- acetylcholinesterase hydrolyses acetylcholine into ethanoic acid and choline- which diffuse back to the pre-synaptic neurone- recycling the acetylcholine.
