biologists in this session we're gonna be taking a look at a myelinated neuron and the significance on the frequency of impulse transmission this is part three of this specification point for the other parts please take a look at the previous videos all right so first of all gonna have a look at um this all or nothing law which is very very popular on the exam now as you can see here um i have some kind of a stimuli a low stimuli that's caused some of the sodium body gait channels to open now as a result of some of the volatility channels to open some sodium has rushed in along the electrochemical gradient and it's caused a tiny increase in the membrane's potential however because this is only a low stimuli and it doesn't open enough sodium volatile channels this increase does not reach the threshold value of approximately minus 50 minus 55 millivolts therefore an action potential is not generated in this particular instance now you can see here in this second one here the stimuli is slightly larger than the first one but again this increase in membrane potential does not go above that threshold value so therefore i do not get an action potential generated in this one however in the sec in the third and the fourth stimuli as you can see these are much larger stimuli and i get therefore an action potential generated because they open up enough sodium variegate channels to go above the threshold value and therefore establish an action potential now as you can see in both these instances the action potential is exactly the same magnitude this last one here is a stronger stimulus but the action potential is exactly the same now this is the all or nothing law it basically means you either have an action potential or you don't you don't have a bigger action potential for a larger stimulus and you don't have a small action potential for a small stimuli you either have an action potential which is the same magnitude or you have nothing at all because it doesn't reach that threshold value and it's really important that we recognize it's the all or nothing law the next thing that we need to be aware of and which i have seen in the matter scheme a couple of times is this word generator potential now i generate a potential this is generated when i've got a sensory receptor that results in an appropriate stimulus which either excites or inhibits an action potential now we're going to look a little bit more about inhibitory action potentials a bit later on but a generated potential can be triggered by that stimulus at the beginning and it's either going to go above the threshold value or it won't now um in regards to a strong and a weak stimuli as you can see in this image here if i have a weak stimulus that yes it um it triggers enough sodium channels to open to go above the threshold value i will get an action potential however in a week stimulus i get very few action potentials whereas as you can see here in b um it's a stronger stimulus so therefore i have a lot more frequent um action potentials now as you can see in both the weak and the strong stimuli the magnitude or the height if you like it's really important to use the word magnitude though the magnitude each of the action potentials is exactly the same in the weak and the strong but in the strong stimulus i have a lot more of a higher frequency of the action potentials generated due to this stronger stimulus the last thing we need to be aware of is this myelination of the neuron so anything in the red box here is taken directly from the mast scheme so myelination increases the rate of impulse transmission and this is because it's solitary conduction and what happens here is the schwann cell which wraps around the neuron it creates an insulating barrier which means i cannot get movement of ions across the membrane where i've got my schwann cell which are the blue bits here wrapped around my my neuron the gaps in between these schwann cells are called the nodes of ranvier and that is the only place on the axon or dendron where the movement of ions can occur so therefore the membrane can only be depolarized at the nodes of ranvier um it creates a longer localized circuit which means it the movement of ions can only occur in this kind of where the arrows are if the if the schwann cell wasn't there it'd be occurring all the way along in shorter circuits and therefore it increases the rate of impulse transmission really important here you do not say that the impulse jumps from node to node because they do not like on the exam we need to use the term the membrane and it's really important we use membrane and not axon and not nerve the membrane is only depolarized at the nose of mva guys good luck with your exams i hope this has helped and all the best with your studies