Action Potential Flashcards
Review of the graded and action potential
What are the two types of membrane potentials?
Gradee potentials and action potentials
What are graded potentials?
Short distance signals, where ions flow rapidly across membrane in ion channels, causing depolarisation
Outline the process of local current loops in graded potentials
Na+ ions into cell through open Na+ channels due to both concentration and electrical gradients (+ve attracted to less +ve regions); causes depolarisation at this point; once Na+ ions are inside cell, move away from depolarisation towards less +ve regions; Na+ inside flow increasingly further from open channel; Na+ outside flow consistently towards open channel; signal moves along membrane
How do graded potentials contribute to initiation of action potentials?
Single graded potential too small to reach threshold, but though summation can trigger action potential
How does summation of graded potentials occur?
Stimuli occur and trigger graded potentials before the previous graded potential dies away
What are the two types of graded potentials, and their implications?
Excitatory and inhibitory. Excitatory causes depolarisation and is more likely to create an action potential. Inhibitory causes hyperpolarisation and means an action potential is less likely to form.
How are graded and action potentials in their relationship to size of stimulus?
Graded potentials’ size is proportional to size of stimulus, whilst size of action potentials is not
Why are graded potentials only short distance?
They decay as they move over distance due to leakage of charge across plasmalemma reducing current at sites further along membrane (away from open channel). Thus depolarisation is confined to a small region of the membrane
What are action potentials?
Long distance signals
How fast are action potentials?
1-4ms
What is the threshold?
-55mV
What is the membrane depolarised from and to?
-70mV to +30mV
How are action potentials self-propagating over long distances?
Through positive feedback: Na+ entry causes depolarisation, which causes more Na+ v-gated channels to open (as v-gated ion channels always open when membrane is depolarised)
What is the initial depolarisation caused by?
Graded potentials (entry of Na+)
In afferent neurons, what is a graded potential called?
Receptor potential, as is generated by sensory receptors at peripheral nerve ends
In efferent and inter-neurons, what is a graded potential called?
Synaptic potential, as is generated by synaptic input to neuron
What are the differences between Na+ and K+ v-gated channels?
Na+ open and inactivate very rapidly, whilst K+ open and close slowly; Na+ has to inactivate before returning to “rest” (close) and opening again, whilst K+ simply open and shut
How does Na+ v-gated channel go from open to open again?
Gate enters pore to inactivate, breaking positive feedback; repolarisation pops inactivation gate out of pore, closing the channel; Na+ channel opens with depolarisation
What is the absolute refractory period?
From start of depolarisation to start of hyperpolarisation; another stimulus cannot produce an action potential; caused by Na+ v-gated channels being open or inactivated (and thus unable to open further, or at all)
What is the purpose of the absolute refractory period?
To limit number of action potentials produced by membrane in a period of time; separate action potentials; determine direction of action potential, as cannot propagate in direction from which action potential came as membrane is still in refractory period
What is the relative refractory period?
During hyperpolarisation; only a very large stimulus can produce an action potential; caused by some K+ v-gated channels still being open, and some Na+ channels being returned to resting state
Which movement of ions is controlled by positive feedback?
Movement of Na+ in
Which movement of ions is controlled by negative feedback?
Movement of K+ out
Outline the process of an action potential
At rest, Na+ and K+ gated channels are closed; a graded potential causes depolarisation of the membrane, opening Na+ and K+ channels; due to the rapid opening of Na+ channels, rapid depolarisation occurs as Na+ enter down concentration and electrical gradients; depolarisation of membrane causes further Na+ channels to open; membrane almost reaches peak and Na+ channels are inactivated, breaking the positive feedback, stopping depolarisation; due to K+ v-gated channels opening slowly in reaction to the depolarisation created, repolarisation slowly occurs as K+ moves out of cell due to concentration gradient; repolarisation causes Na+ channels to return to rest, and K+ v-gated to close (as repolarisation causes v-gated channels to close); due to K+ v-gated channels closing slowly, there is an overshoot to hyperpolarisation as K+ keeps leaving the cell after resting membrane potential is returned; full closure of K+ v-gated channels allows the Na+/K+ pumps to return the membrane to resting potentials
Why are K+ v-gated channels controlled by negative feedback?
K+ v-gated channels opening allows K+ to leave cell, causing repolarisation, but repolarisation causes v-gated channels to close
What factors influence speed of an action potential?
Myelination, fibre diameter
How does myelination increase speed of action potential?
Saltatory conduction along nodes of Ranvier; less charge leakage means action potentials do not become weaker down axon, so more charge arrives at next mode, thus pumps do less work, and so is more efficient
How does increased diameter of fibre increase speed of action potential?
Less resistance to current
How fast does an action potential travel in a unmyelinated neuron with small diameter?
0.5m/s
How fast does an action potential travel in a myelinated neuron with large diameter?
100m/s
