 So, welcome back to the NPTEL lecture series on bioelectricity. So, let us do a bit of a recap. So, what we did we started with the stress reflex arc circuit and we talked about muscle spindle and we talked about how the muscle spindle can anticipate the change in length of a muscle and it is innervated by the sensory neuron and the sensory neuron takes the message to the spinal cord and in the spinal cord this message is transmitted to the motor neuron through an inter neuron and then the motor neuron brings back the message telling the muscle to come back to its original position regaining its original length and simultaneously there is another set of motor neuron which are the gamma motor neuron which tells the muscle spindle to come back to its original shape. So, in this process today we will be briefly discussing how the signal from the muscle spindle is transmitted to the sensory neuron and how that could be measured. So, coming back to the slides so this is lecture 15. So, essentially this is the situation here you have the muscle with all the myotubes and everything likewise striations and all those things you see in between you have having these muscle spindle sitting there. These muscle spindles are innervated by that means a contrasting color something like this they are innervated by the sensory neuron ending. So, our pertinent question is how this signal from here is being transmitted from this from this muscle spindle to sensory neuron and what we know about it in terms of its electrical signature. How this electrical signal transfer is taking place? So, it is believed that there may be a series of sodium channels which are either present in the muscle spindle or they are present in the sensory neuron. These channels are again this is all currently under speculation these are believed to be mechanosensitive ion channels. So, these kind of sodium channels are considered as mechanosensitive ion channels. What we meant by mechanosensitive ion channel as the name itself indicates because of mechanical stimulus the ion gates open up. So, it has a very unique way of opening up. So, this is a classic case of mechanotransduction or in other word mechanical energy leading to the generation of an electrical energy or mechanical energy is converting into electrical energy which is over where you want to put it. So, essentially this is the first level of energy transduction which is taking place and so essentially what is happening is that this muscle spindle is getting a mechanical force and this or this this is not yet clear opens up a series of sodium or cationic channels it may be even calcium which is not really known this leads to a generation of an action potential. This is how far we know about how the interfusal fibers are communicating the signals to the sensory neurons things are under intensive study, but still we have a long way to go before we really understand how this first level of information transfer is being executed. So, this is the first level what is the second level second level. So, this is here is the train of action potential which is travelling through fine now that the level. So, let us put it at level one level one what is happening in level two coming back to level two. So, a signal has arrived from the sensory neuron this is sensory neuron through the inter neurons this signal is being getting a split up into two different components. So, one component is going for once again. So, this is the train of action potential which is travelling. Now, here this signal is getting a split up into two component one component say I am representing by green the other component I am representing by red. So, one component of this signal if this signal I represent by some say function say f x is equal to s then here we will have two signal f x 1 and f x 2 fine or you can represent it by anyway and essentially signal s. So, f x is equal to f x 1 plus f x 2 this is how the mathematically this could be defined. Now, from here this signal is transmitted to two different kind of neurons one is a smaller one which is called a gamma motor neuron and the other one is the bigger one which is called an alpha motor neuron just for the standing sake and here you have all the synapses and everything and. So, this is alpha motor neuron now this alpha motor neuron is the one which is responsible for contraction of the extra fusal fiber or the bulk of the muscle and the gamma is the one which ensures that the muscle spindle comes back to its original shape. So, this is if this is level two and at this stage there are three levels which are starting from here we are initiating level three. So, here this is the train of action potential which is travelling to ensure that the muscle spindle regain its original shape or length. Again this part of the circuit how the gamma and motor neuron is executing this function is still not very clear we know the end result that is how it does, but the real mechanistic details are still not very clear. Instead the other part of the circuit which where the alpha motor neuron is regulating the muscle length is much more clear. So, now what we will do we will talk in bit of a bigger details on a greater details about this particular part of the circuit where the alpha motor neuron is communicating its information to the extra fusal fiber and this it does at neuromuscular junction. So, this is now where we are moving we are moving into the level four which is of this circuit level four which is neuromuscular junction. In short it is also called NNJ. So, neuromuscular junction pretty much all of the vertebrates are regulated by a specific kind of neurotransmitter called acetylcholine barring aside some of the neuromuscular junctions which are present in the drosophila which are driven by glutamate kind of neurotransmitters. So, we will not take those exceptional case into account we will only talk about the acetylcholine based nerve muscle junction where your alpha motor neurons are coming and represent the alpha motor neuron by red these are the huge neurons they are the one which are coming and inner weighting the muscle here you have the here you have the muscle. So, and the input it is receiving from the inter neuron or directly from the sensory neuron. So, here is the input which is arriving at its different terminals these are the dendritic terminals where the input it is receiving all the inputs. So, the direction of flow of electrical stimulus is like this. So, there is a form of spatial and temporal summation of information spatial in the sense because if you look at this image out here if you look at this whole image signals are coming from different location in a space it is coming from here it is coming from here it is coming from here it is coming from here it is coming from here. So, that is why it is spatial it is also temporal because the signal may come at time t 1 time t 2 time t 3 time t 4 and one signal may overlap over another signal. So, say for example, like this and another signal coming like this and another signal coming like this if this one is t 1 this is t 2 this is t 3 this is t 4. So, this is also called temporal. So, in other word electrical signals which are arriving at the motor neuron are electrical signal or we can call it or train of action potentials are spatio temporal they are spatial as well as temporal in nature. And this motor neuron what is the what it does it is just like a computer out here it integrate all this signal and this integrated signal is now is the master signal which starts to travel down like this along the axon without allowing any kind of back propagation of information in this direction it is moving. So, what essentially is moving is a train of action potential which is coded with the information that how much length the muscle has to come back. Now, what happens here? So, this at this stage there are two things which I wish to bring to you notice first of all these neurons are myelinated and if you look at these neurons they are. So, if this is your motor neuron this is how the process is. So, now and here you have the nucleus. So, these are all myelinated structures myelinated in the sense they are something like a like this. So, these are very similar to you have seen the wires on the walls they are very similar to the wires on the walls you cannot have a naked wire in the on the walls. Because if you have a naked wire then if you touch it you will get a shock. So, in order to prevent that the same thing holds true for these neurons because they are conducting electrical impulses. So, the current should not be lost the information should not be lost. So, that is why it is put inside an insulator and these insulating cells are in the case of motor neuron when the motor neuron. So, as long as this motor neuron is inside the spinal cord it is myelinated or the insulation is done with a specific kind of cells called oligodendrocytes and when it is outside the central nervous system it is insulated by another kind of neuron another kind of glial cells which are the supporting cells of the nervous system those are called Schwann cells. We will come in depth on those we will be talking about some of the diseases and how to really handle it, but at this time just remember these green covering what I have drawn they do not allow the electrical signal to be lost from the axon to the another one there should not be any short circuit. So, these are your they could be Schwann cells or an oligodendrocytes they ensure insulating covering of the axon. So, now this zone is a very important this zone where I am circling now is called the axon halock. These small gaps what you see out here what I am labeling now are called nodes of Ranvier and at the nodes of Ranvier you have a large population of sodium channels they are concentrated out here like this what you see in green are high population of voltage gated sodium channels. And the current which is now it is obtaining from all the sources and it is doing the summation this current actually hops like this it is called saltatory conduction mode. So, at this stage what is happening now once you are at the zone where it is in contact with. So, this is the zone where this whole structure is in contact with the muscle. So, now at this stage this electrical signals at to lead is to translate in information to the muscle and it does at these junctions which are very very specialized junctions it is these are the junctions where this information is being transmitted and how it does. Now, what we will do we have talked about the movement of the electrical impulse out here what is happening out here. So, we will magnify these images of neuromuscular junction and we will talk about them in the next slide. So, let us redraw the neuromuscular junction now. So, I to redraw the neuromuscular junction it will be something like this something like very similar to the way I am drawing it will do the nomenclature very soon. So, this is the neuronal terminal and here you have the muscle. So, at this stage what you see out here essentially these are filled with these are filled with something like vesicles or small packets of neurotransmitters. These neurotransmitters are the molecules which translate the electrical impulses into chemical impulses come to that. So, these are the small vesicles these are filled with you know in this situation when we are discussing about neuromuscular junction in mammals and everything they are filled with a chemical called acetylcholine like this. And these neurotransmitters are actually synthesized by the neuron and they are transmitted at these different terminals. And the by-product of it is actually also carried back to the cell body and again re-synthesized and brought it back. So, there are five different steps what is happening out here. The first step is so let us start enumerating the steps now. So, the step one of this game is step one of this game is step one. Then arrival of action potential, arrival of I am just putting P for potential and which basically depolarizes the synaptic knobs. So, just let me introduce two more terminologies. The one which is sending the signal it is called the presynaptic and the one which is receiving the signal is called postsynaptic. In this situation the presynaptic membrane is the neuron and the postsynaptic membrane is the muscle. So, this is postsynaptic and this is presynaptic and these terminals these are called synaptic knobs and these small gap physical gap this is called nerve muscle synapse. So, as a step one with the arrival of action potential it depolarizes the synaptic knobs. So, what essentially that mean? That mean coming back to the slide that essentially mean that the step what is happening is that this is the muscle. So, out here when the action potential arrives this leads to the entry of they are depolarizing in other word they are making trying to make the membrane much more positive which was at minus 90 or minus 80. So, now with the entry of the calcium what will happen there will be this membrane potential will start to depolarize from the negatively polarized state of minus 80 to minus 90 with the entry of the calcium it will become more positive. As it becomes more positive the next thing what will happen is this. So, coming back so there is a entry of calcium which is the step one this followed by this this calcium goes coming back to this. So, with the entry of the calcium this calcium goes and bind to these synaptic vesicles and as soon as it binds to this synaptic vesicles they open up they started secreting the neurotransmitter in this zone. So, these are the acetylcholine which are being secreted by the nerve terminal this acetylcholine immediately goes and binds to the binds to the post synaptic membrane is coming back to the post synaptic on the post synaptic there are receptors for the acetylcholine and they bind on top of them. As soon as the they bind on top of these post synaptic membranes this opens up a series of cation channels. So, what is happening utilize this slide so these are the terminals. So, this is where the synapses are fine this is the muscle. So, this muscle has a series of receptors which binds to acetylcholine. So, the green one let represent them by let us pick up a color which is much more prominent which would be seen out here something like this. So, make it thicker so the dots what I am drawing now are the receptor for acetylcholine on the post synaptic or on the muscle membrane. So, as soon as the acetylcholine is released into the synaptic cleft or the space between pre synaptic and post synaptic membrane that acetylcholine goes and binds to the post synaptic membrane. Once it binds to the post synaptic membrane it leads to the influx of sodium into the muscle and that sodium which is generated into the muscle leads to the generation of an action potential by the muscle this leads to a generation of action potential. So, if we have to put it in perspective think what all happened let us go back to the first slide where where we started. So, this is how the circuit looks like from muscle spindle because of mechanical presence of mechanical sensitive ion channel the muscle spindle kind of stretches and this mechanical signal is translated into electrical signal. Then this electrical signal travels through through and through to the stage two to the interneurons. In the interneuron the signal is divided into two parts one goes to gamma one goes to alpha the gamma electrical information comes to the muscle spindle and gets translated it into mechanical signal to tell the muscle spindle to come back to its original position. Whereas on the other hand in the case of alpha these electrical signals travel along the alpha motor neurons as I was showing you out here without getting lost they travel out here and after reaching here they depolarize the membrane by allowing the entry of calcium into the cytoplasm with the entry of calcium the membrane we got depolarized once the membrane got depolarized the calcium in the meantime goes and binds to the synaptic knobs which are filled with vessels of acetylcholine and once the acetylcholine is released into the synaptic cleft or in the neuromuscular junction the acetylcholine goes and binds on the postsynaptic receptors which are present on the postsynaptic membrane and leads to the entry of the sodium ions into the muscle and then the muscle generates an action potential. So, electrical chemical again electrical and then what we have not discussed how this action potential within the muscle leads to a generation of a muscle contraction we will come to that, but before we come to that now the way these are being studied is known. So, if you could put an electrode here and if you could put an electrode here and if you stimulate it if you stimulate the presynaptic membrane and then you could make the recording from the postsynaptic membrane. So, you can put an electrode here like this you can put an electrode like this is how all these studies over last 70, 80 years has been characterized all the muscle contraction all the in vivo studies there are in vitro preparation where outside the body you have neuromuscular junctions you have live animal preparation there are whole series of there are slice preparations where all these electrical measurements are being done all how the all the quantas are being released as a matter of fact, studying the electrical properties of neuromuscular junction has opened up a wide window of understanding the synapse it is synapse itself because this is one of the very well accessible synapse where you really can see how the ultra structure of synapse looks like because all the synapses are not that big this is huge you really can see in on in any muscle of your body you really can see a neuromuscular junction. So, if there is a cross section from an experimental animal they actually can do a electron micrograph and everything and you can really figure out what is happening and this whole site what you see here is one of the hot site for pharmacological intervention there are several diseases which are related to it. So, one of the thing what happens when acetylcholine is being secreted. So, acetylcholine binds but very soon very immediately at pretty much at the same time the acetylcholine has to be degraded out. So, what has to be done essentially is so the acetylcholine which is secreted is being degraded out as choline and acetate and this choline is again taken up by the by the neuron in order to make new acetylcholine and acetate is further degraded and this is being done by an enzyme called acetylcholine esterase. So, if you have a blocker for acetylcholine esterase then the synaptic transmission will be disbalanced there would not be any control root because then they will keep the channels open and the individual muscle will start shaking and there are several poisons which actually does that they ensure that acetylcholine esterase does not break down or something very similar to acetylcholine or something which blocks acetylcholine esterase or something of that kind. So, this is another feedback mechanism by which it is being ensured at the synapse the neurotransmitter which is translating the electrical signal or transferring the electrical signal from the neuron or from the presynaptic terminal to the postsynaptics terminal in this situation the muscle that neurotransmitter should have a very very limited lifetime. If it is not so and there will be problem we will have serious issues if this biochemical pathway is not being regulated the way it is. There is another thing which is worth mentioning here as I was trying to tell you though this is not part of this, but just for your understanding sake the way it works is. So, all the if I have to show in a three dimension like this that will be easy for you to understand. So, all these neurotransmitters are actually synthesized out in the nucleus and they are transported out into the nerve terminals and they are byproducts like colline or something they are transported back out here. And this whole transport mechanism is very very essential because at times in several disease several neurodegenerative diseases it has been observed that this transport phenomena is obstructed. So, this is called a trafficking those who are not from a very well versed biology background for them. So, this is what is the traffic trafficking process when neurotransmitters are being put in vesicles come back and they adhere and form the synaptic knobs out here the neuromuscular junction and then they are being again taken back after they are being used. So, this is a continuous trafficking trafficking process and this is called anterograde and retrograde transport retrograde transport of vesicle. As I have already told you that you can study them by putting an electrode out here and putting an electrode at the other terminals. So, if you see if you give a stimulus at E 1 if this is E 1 and this is E 2 just with a slide gap you will see a signal at electrode at E 2 that gap is the time what it takes from here to travel to the whole synaptic membrane. And if you do not see such signal it means they may be physically in close to each other, but there is no functional connection these are acid test of figuring out whether your motor neuron is sending a signal or not. But this takes us to another level of complexity which I am going to delta in the next class that is