 Welcome back to the lecture series in bioelectricity in NPTEL. So, as of now we have finished 12 lectures and in the course of these 12 lectures, I have introduced you to the whole domain of bioelectricity. Then we moved on to talk about the structure of the neurons. We talked about the simple action potentials, how they are being generated, how using a Nernst equation you can calculate the balancing between the concentration gradient and the charge gradient across the membrane. And then we talked about a series of techniques. In depth we talked about patch clamp, planar patch clamp, micro electrode arrays and how these are being implanted and the intracellular recording, extracellular recording. We talked about the voltage clamp and the current clamp. So, in that whole process I tried to introduce you how the channels are being studied. Though I have not really talked about all the diversity of the channels, different kinds of channels and how they have evolved. So, I am kind of reserving that the letter half of this lecture series, where you will have a fairly good idea about the functional diversity. So, one of the thing which you will be appreciate as we will move along, annual bioelectricity is this, that all the functional variations what we see in the excitable membranes of the body like cardiac, smooth muscle, skeletal muscle, neurons. And among neurons you have rods and combs, you have the hair cells of the ear, the higher brain neurons, spinal cord, motor neurons, sensory neuron, sympathetic and parasympathetic neurons and all these different kind of neurons or even the pacemaker neurons or the pacemaker cells of the cardiac myocytes. They all have a unique signature of action potential and this unique signature of action potential is conferred to them by the wide variation, wide diversity of ion channels. And this is extremely important as to really appreciate because as we will move along you will see how the shape already you must have seen some of the action potential shapes are varying. So, this is something keeping in mind I introduce you people with the techniques which are being used especially the patch clamp, how that is being used to understand the single molecule or the single structure of ion channel. Though I plan like to introduce ion channel the detail diversity and evolution of it, but I thought that I will delay it slightly more staggered it pretty much at the end of the animal electricity because that way we will appreciate it much better. So, instead what we will do after we have talked about some of these techniques now we will move on to some of the simple basic circuits. So, one of the most basic circuits on which the body kind of relies a lot are nerve muscle circuits. These circuits which ensure that any sensation which takes place any stimuli which takes place on any kind of muscle of our body is conveyed to the processing center which could be either if it is a very rudimentary or a very primitive signal then the processing center will be the spinal cord, but if it is a higher brain signal then the processing center will be the brain. And part two of it is that after the signal is being sent there it is being computed the response signal should be sent back as efficiently as the way the impulse signal has been sent. So, these simple circuits. So, the way we will study these simple circuits will first of all talk about the circuit we will talk about how the signals are being transmitted and then we will talk about how the how these electrical signals are being measured because since now I have already introduced you to the intracellular and extracellular recording protocols. So, you will be able to appreciate how they are being recorded how multiple patch electrodes are being used and all those things. So, let us start with the nerve muscle circuits nerve muscle circuit and here we will be at this stage we will be talking about the skeletal muscle we will not talk about the cardiac we will take that up separate in a separate section among the neurons we will be talking about two kinds of neurons sensory nerve the one which is carrying the impulse and the motor nerves which are bringing back the signal. And we will be talking about two circuits and the circuits we will be talking about are stretch reflex arc circuit and this stretch reflex arc circuit has two elements actually embedded with two circuits and one of them is the input circuit and the other one is the output circuit input circuit is the sensation sensation I should say the right word will be how we are perceiving the signal. So, let us correct this this should be perceiving the signal responding to the signal responding to the signal. So, now what we will do after this before going before drawing it out let me give you a explanation of this circuit first. So, all our all our surface of our body are continuously exposed to different form of a stretch they are stretching say for example, somebody hits you there is a stretch or you are in a compressed situation they get stretch or there are some stimuli hitting there you are getting stretch. So, these stretch essentially what it does it changes the length of the muscle. So, say for example, initially muscle is say 5 millimeter it becomes say 6 millimeter fine because of the stretch. So, it cannot remain at 6 millimeter for long it has to come back to 5 millimeter otherwise the next stretch will come the 6 will become 7 and the next 7 will become 8 and eventually it will you know it is just like a rubber band it will eventually you cannot stretch it forever it will snap there will be a permanent damage in the muscle that is what you do not want. So, there has to be a mechanism by which you ensure that this muscle once from 5 centimeter or 5 millimeter it becomes 6 millimeter it comes back to 5 millimeter fine how body performs that. So, the first thing it does whenever there is a stretch and since one more just kind of for you are understanding I have just picked up the most primitive circuits there are nth number of such circuits which you can be dealt I am just giving an example for you which is the simplest of all which kind of has been conserved over millions years of evolution and it is kind of very rudimentary very primitive yet very interesting yeah coming back. So, if there is a stretch that is stretch the first step is that there has to be a mechanism by which that change in length within the muscle could be quantified some way to quantify that. So, there is this much change once it is being quantified that signal has to be sent to the spinal cord and is essentially for these kind of reflex circuit brain does not do much of a processing because there is very primitive the processing takes place within the spinal cord. So, within the spinal cord the processing takes place and a respond message is being brought back to the surrounding muscle telling them hey you know what there is a increase in muscle length you better come back to its original length essentially this is what a stretch reflex arc circuit and it is almost like an arc if you look at the map visualized the sheet. So, there is a stretch signal goes comes back. So, that is why it is called stretch reflex it is a reflex circuit arc stretch reflex arc circuit. So, if you look at if I divide this circuit into two parts one part will be the part of the sensation the stretch sensation which I was telling you if you go back if you kind of see this slide. So, this is that part which is perceiving the signal and the part 2 out here is the output response of the signal this is part 2 and these two part essentially when get integrated. So, if I integrate these two parts that what essentially makes this arc circuit that is what it is now what I will do I will draw the circuit for you and I will one by one introduce the components within the circuit which will help you to understand how the circuit works. So, say for example, this is the brain of individual and this is the spinal cord and so for example, a sensation is coming say from. So, I impulse comes let us represent the impulse by a stretch impulse some form of pressure which has been put these stretch impulse is being sensed. So, this is the muscle on the muscle stretch impulse on the muscle of the limb or it could be any other part I am just since I am trying to draw a limb. So, it is on the limb these stretch impulse are being carried by a series set of neurons called sensory neurons. Let us represent the sensory neurons by a pink color this is their end these sensory neurons are out here having their cell body sitting just outside the spinal cord. So, these sensory neurons out here let me mark it sensory neurons these sensory neurons carries the signal the electrical impulse to the spinal cord. So, the direction is this in the spinal cord they convey this signal through a set of neuron called inter neurons which I am showing in light and they convey the signal to the motor neuron which are sitting out here. These motor neuron brings back the signal and tell the muscle to act accordingly just show it something like this. So, what exactly has happened in this situation there are certain things which I have been discussed which I am going to discuss one by one. So, initially a impulse or a train of action potential is generated fine that action potential from here which I am showing you like this this train of action potential travel from the muscle all the way to the spinal cord out here there is a small interfacial zone zone which is shown here the first synapse. So, we will talk about synapse synapse is where one neuron conveys it message to its target the target just go little bit advance to show let me verbally tell you. So, a train of action potential moving from one neuron and this train has to be conveyed to the next neuron or to any other target it could be a muscle it could be any other tissue it could be a neuro endocrine or something. So, there is a very narrow zone they are discontinuous structure there is a small cleft where essentially what happens say for example, now let me graphically tell you if this is the neuron which is carrying the signal and this signal has to be conveyed to the to the muscle like this and muscle is shown by. So, this is the muscle so at this at this juncture they are there is a there is a small gap here. So, physically they are there, but they are not really physically it is not. So, the way it looks like something like this if I magnify this image it will be something like this these are the nerve ending which I am drawing now and these are the muscle membrane like this. So, you see when I am drawing this I am keeping a small gap between the two see this gap I am now just highlighting that gap with yellow you see this gap this gap is called synaptic cleft and this zone this narrow zone of few nanometers is called the synaptic zone. So, this is a synapse and since here the synapse is between nerve and nerve and muscle. So, this could this is also called neuromuscular junction or neuromuscular junction or neuromuscular you can call it a nerve muscle synapse. Similarly, I will talk come to the functionality of it just let me give you another situation where the neuron to neuron synapse is there. So, for example, this is this is one neuron and there is another neuron which is sitting in close proximity like this processes are like this. So, if you look at this zone which I am now yellowing these are the synaptic cleft these synaptic cleft are the regions. So, synaptic cleft these synaptic clefts are the regions where electrical signal from one neuron is either transmitted to another neuron or the electrical signal from one neuron is transmitted to its target tissue which could be a muscle or a neuroendocrine cell or something. Now, what exactly happens this is the anatomical part of it. So, the way it transmits is this there are two level of energy transduction which takes place. So, electrical impulse are electrical signals which are traveling. So, when they reaches synapse these electrical signals are translated into chemical signals in the form of neurotransmitters at the nerve muscle junction or at the synapse between neuron to neuron. There are neurotransmitters which are secreted out from the sender from where the signal is coming and those neurotransmitters are released into the synaptic zone or the synaptic cleft and those neurotransmitters then bind to the post or the target tissue which is also called post synaptic target tissue and on the post synaptic target tissue they bind to the series of channels and they open up the gates for the ions and then the second the target again generates another action potential. So, this is what I have told you verbally let me show you graphically what is happening. So, coming back here. So, here the electrical signal is traveling once this electrical signal reaches out here at the synaptic cleft say for example, here the electrical signal reaches out here this. So, in this situation this one is the presynaptic the one which is carrying the signal presynaptic and this one is the post synaptic presynaptic. So, it is a presynapse and post synapse. So, at the post presynaptic zone what is happening. So, let us look at it see say for example, this is the presynaptic zone and this is the post synaptic zone which I am drawing in green of course, I am here showing with two neurons. So, once the electrical signal reaches out here. So, here the electrical signal is reaching it is traveling like this these electrical impulses leads to the secretion of neurotransmitters. So, here you have the vesicles of neurotransmitters which are sitting like this actually essentially they sit like this is structure is more like this with the series of neurotransmitters all over the place. These red ones are neurotransmitters what I am drawing just let me all over the place. Now once the impulse reaches here these neurotransmitters from here are secreted out in this direction like this. So, they are into this cleft area at this cleft area the next thing happen they bind to the post synaptic neuron step 2 they bind to the post synaptic neuron like this once they bind to the post synaptic neurons. So, let me just mark it this is presynaptic this is post synaptic and this is the synaptic cleft. These are neurotransmitter vesicle and these are the binding site neurotransmitters post synaptic membrane. So, there are three events which are happening. So, the electrical signal reaches the synaptic zone at the synapse neurotransmitters are released by the presynaptic membrane those neurotransmitters diffuses into the post synaptic into the synaptic cleft in the synaptic cleft they immediately bind to the post synaptic membrane after binding to the post synaptic membrane they open up. So, the next thing what happens out here they open up a series of channels out here these are the channels which leads to the flux of sodium and of course, then as the and if it is a sufficient amount of sodium which gets in out here and this shoots another action potential like this. And this is how action potential impulses propagated from one neuron to the next neuron to the third neuron likewise and just if you try to visualize this in terms of its complexity. Now, think of it if one neuron is synapsing on 10000 other targets how this signal is going to divide out in all of this at one point of time it is a very very complex network what I am showing is the most simplistic version of a single synapse at a junction which is so clean, but really to decipher what is happening at individual synaptic level is an extremely challenging task. Now, how you could measure where electrophysiology comes into play. So, say for example, if you have a preparation like this and if you have a electrode sitting like this say for example, I have a patch electrode or some kind of any electrode it could be an extracellular it could be an intracellular electrode and I have another electrode like this or this is I am showing you the patch electrode or say for example, you have sharp electrodes like this sorry I just showed it wrong. Now, if this electrode so in a baseline situation you are not stimulating any electrode. So, if this I call this as electrode 1 or presynaptic electrode 2 electrode 1 and then I have electrode 2. So, if you are not stimulating you are just keeping the electrode intact like you know sitting there you should be able to see the spontaneous responses if at all there is if there is a spontaneous flow of impulses from the presynaptic to the post synaptic you will be able to see the change. So, for example, the way it will look like so here basically you have to do a dual channel recording. So, you have one channel so the way it will be say for example, you will have one scale showing like this another scale showing like this. So, this is the scale for E 1 this is the scale for E 2 electrode 1 and electrode 2. So, both the electrodes are kind of sitting at ease without doing anything. So, what you will see is that if there is a impulse which is there in this electrode you will see something like this deviation out here it you could either measure the current or you can measure the voltage depending on what you want to measure. And followed by this impulse with a slight delay you will see another impulse out here if this is time why there is a delay because look at the circuit. So, impulse comes here at t 1 time t 1 and this impulse travels through. So, there will be a gap out here between while it is crossing the synaptic cleft all that signal this is a t 2. So, with a delay you will see a change in the membrane voltage of this is the presynaptic cell this is the post synaptic cell. So, with a slight delay you will see a impulse get propagated or what you can do you can stimulate this cell you can do it like this you can give an stimulation out here of physical stimulation like this. And with a slight delay you will see the response in this electrode something like this say for example, I give an stimulation say at this point I give an stimulation and this slight delay I will see a response a response could be in both directions it does not matter whether up side are like which side I am showing in on the on the scale you could show the impulse like this also. So, you will see a change in the membrane potential. And this is where the electrophysiology techniques the bioelectrical measurement techniques comes so handy where you literally can quantify the biochemical changes which are taking place out here. So, what essentially happened. So, if I break it down what is trying to tell you first an electrical signal electrical signal translated into a chemical signal s is stand for signal in the form of this is in the form of neurotransmitter release followed by generation of another electrical signal. And this is in the presynaptic this is in the post synaptic this diffusion from presynaptic via synaptic cleft to the post synaptic does it make sense. So, for example, so if you have one electrical signal here with a certain time delay if this is the scale I am following the same scale of time slight delay you will see the signal out here. And same thing you can do for anything in everything as long as you can place the electrodes from a sender to the receiver you should be able to measure the electrical signal across these circuits. So, coming back here. So, this is when you do this is basically you are doing it in a dual channel similarly you could have the modern electrophysiology technique may even allow you to do these kind of recording on 4 channels if you have 4 such electrode you can really the channel is not a problem. Problem is the dimension of the cell because you are handling at a area within. So, for example, in a area of say 30 micron diameter within 30 micron you can it is really challenging to put 4 electrode like this 2 itself is a challenge 4 is even more challenge some people can do it they are really very good at it, but not very many can really do that. So, there is a physical challenge involved here you can do the similar recordings using the micro electrode arrays say for example, you have a micro electrode array like this and some I mean something like this. And again my kind request please go through all online to see the beautiful pictures of the micro electrode array in Google image that will really enhance your imagination power. So, on a micro electrode array say for example, I have this muscle cells I am growing say for example, or say a series of neurons sitting like here likewise out here they are making contacts with another set of neurons out here may be another set of neurons sitting here likewise. So, any signal you will see so with the way you will see is that at t 1 time t 2 your t 3 at t 4 say for example, a signal is generated from here something like this. So, this will travel all the way out here may be likewise. So, on a scale what you will you see say for example, you have a scale like this at if I name these as different electrodes say for example, e 1 e 2 say e 3. So, at e 1 you will see a sent signal then up with a gap of time you will see a signal on e 2 then you will see a signal on e 3 likewise. So, signal is moving like this with time. So, you are realizing that those techniques which I just taught you in previous classes how handy they become when you had to analyze these kind of circuits. So, coming back so this is all about the impulse propagation which I really did not cover a whole lot. I will talk a little bit about neuromuscular junction as I will moving through. So, where we all started we started with talking about the circuit. So, this is from where we diverged into how the impulses getting moved from here to here from here how the signal is transmitted through the synaptic clip to this neuron and by the same a this neuron picks up the signal from here and it is coming back. Now, you have understood about the impulse propagation impulse is getting propagated like this, but what we have not understood yet which you have to discuss is how the this part is taken care. How the sensory neuron out here is sensing the or rather the muscle let us propose the question how muscle sensing the change in length. This part we have not discussed yet this we have to discuss. So, we will be discussing this after this. So, we will close in here and we will initiate our discussion how the muscle senses the length and how that is transmitted to the sensory neurons. How again the electro recordings could come very helpful in understanding this and from how that signal is being picked up by the motor neurons. How again the electro physiology will come into play and now finally, the motor neuron transmit at the neuromuscular junction the signal to the muscle and how within the muscle that signals get propagated. So, we will close in here we will resume in the next class. Thank you.