Passage of an action potential Flashcards
(3 cards)
what is an action potential and how does it work?
- once it has been created, an action potential moves rapidly along an axon
- the size of the action potential remains the same from one end of the axon to the other
- strictly speaking nothing physically moves from place to place along the axon of the neurone
- as one region of the axon produces an action and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon
- in this manner action potentials are generated along each small region of the axon membrane
- the action potential is therefore a region of the axon membrane
- the action potential is therefore a travelling wave of depolarisation
- in the meantime, the previous region of the membrane returns to its resting potential that is it undergoes repolarisation
What is the process of the passage of an action potential along a myelinated axon?
- in myelinated axons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming
- at intervals of 1-3mm there are breaks in this myelin insulator, called nodes of Rankers
- action potentials can occur at these points
- the localised circuits therefore arise between adjacent nodes of Ranvier
- action potentials can occur at these points
- the localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials in effect jumps from node to node in a process known as saltatory conduction
- as a result an action potential passes along a myelinated neurone faster than along the axon of an unmyelinated one of the same diameter
- this is because in an unmyelinated neurone, the events of depolarisation have to take place all the way along an axon and this takes more time
- in our Mexican wave analogy, this is equivalent to a whole block of spectators leaping up simultaneously, followed by the next block and so on
- instead of the wave passing around the stadium in hundreds of small stages, it passes around in 20 or so large ones and is consequently more rapid
What is the process of the passage of an impulse along the axon of an unmyelinated neurone?
1-at resting potential the concentration of sodium ions outside the axon membrane is high relative to the inside, whereas that of the potassium ions is high inside the membrane relative to the outside
-the overall concentration of positive ions is however, greater on the outside, making this positive compared with the inside
-the axon membrane is polarised
2-a stimulus causes a sudden influx of sodium ions and hence a reversal or charge on the axon membrane
-this is the action potential and the membrane is depolarised
-in our analogy a prompt leads a verticals line of people to stand and wave their arms, that is, they are stimulated into action
3-the localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gated channels a little further along the axon
-the resulting influx of sodium ions in this region causes depolarisation
-behind this new region of depolarisation, the sodium-voltage gated channels close and the potassium ones open
-potassium ions begin to leave the axon along their electrochemical gradient
-so once initiated, the depolarisation moves along the membrane
-the sight of the person next to them standing and waving prompts the person in the adjacent seat to stand and wave
-a new verticals line of people stands and waves while the original line of people are seated again
4-the action potential (depolarisation) is propagated in the same way further along the axon
-the outward movement of the potassium ions has continued to the extent that the axon membrane behind the action potential has returned behind the action potential has returned to its original state (positive outside, negative inside) that is it has been repolarised
5-repolarisation of the axon allows sodium ions to be actively transported out once again returning the axon to its resting potential in readiness for a new stimulus if it comes
