 Welcome back to the lecture series in animal physiology in the NPTEL lectures. So, we are through with four different topics. We started with the introduction to physiology where we talked about all the different systems which gets integrated to make a dynamic system. From there we moved on to the first unit which was membrane physiology. We talked about the structure of the membrane and the dynamics of the membrane and several other features. From there we moved on to the heart the major pump of the body which distribute the blood all over the body. And after that we talked about the cardiovascular network which regulates the supply of oxygen to all the different cells and removing carbon dioxide and other toxic materials from there. And in that context we talked about several situations and what we have studied in membrane physiology and all these things because for the fundamental basics of diffusion and filtration, absorption, osmosis, hydrostatic pressure and all those things. So, now we will switch on to the fifth section of this course that is nervous system and we will have three to four lectures in this section or maybe five in case we kind of lag behind. So, what we will be covering here? So, first thing what we are going to cover in the nervous system is the most fundamental unit the smallest structure of the nervous system which the smallest cellular structure let me be right which is the neuron the different classifications of neuron different types of neurons and the supporting cell which constitute the nervous system. And then we will be talking about the smallest functional unit of this which is in the form of electrical activity in the form of action potentials. We will talk about the action potentials we will talk about the different chemical currencies which helps in information transfer in the form of neurotransmitters. And then we will be talking about the different I should say different module or different circuits like convergent circuit, divergent circuit, or taptic circuits and all those things. So, let us start with our section five. So, this is where we are now we are in section five which is the nervous system. So, this is the first lecture of section five lecture one nervous system. So, in this class our major goal will be to start off with the neurons as I was trying to tell you neurons and the structure and classification of neuron classification of neurons. This is part one and part two will be we will be talking about a supporting cells which are called neuroglia. Neuroglial cells and their structure and classification again same line structure and classifications. And then on we will be talking about the electrical activities very basic electrical activities will be dealing under that will be talking about the chemical synapse electrical synapse. And of course, in the beginning we have to define what is the synapse of course, electrical synapse and we will talk about the different circuits. So, this will kind of give you an overall idea about how the nervous system functions at the cellular level. So, from here let us move on to the neuron and the classifications of neuron some point or other in your high school or somewhere you must have been exposed to the structure of the neuron. So, the basic structure of the neuron is something like this. This is how the basic structure of a neuron looks like and on these sides you see small processes soon we will put all the nomenclatures of all these things. And if I had to kind of blow this up little bit it will look like this it is a kind of a tube which is moving down like this that is the blow up picture kind of a three dimensional structure lot of aberrations and everything. So, what are the nomenclature of this? This is a very very distinct polarized structure it is unlike whenever we draw a cell we draw something like this like this is the nucleus and this is the cell body and everything and this is the membrane. So, in this situation it is a very very polarized structure in the sense that one specific. So, if you have to see how the neuron develops it develops like this. So, initially it is a cell like this and then it start protruding out sorry it put protrudes out in multiple directions and from those multiple directions the processes it starts coming out as of now this is very symmetrical there is no problem. But then a very interesting phenomena takes place. So, for example, this is process one this is process two this is process three this is process four this is process five this is process six. So, then out of these processes P 1 to P 6 it could be any number it does not matter just for the simplicity sake I am just giving some number there is one of the most fundamental mesmerizing thing happens out of these six one decides to elongate and what we see next is this say for example, in this case say for example, the sixth one decide sorry fifth one decides for example. So, then this process becomes like this. So, this is how it looks like. So, now the question arises what makes it decide that the fifth or the sixth or the first or the second or the third or the fourth will become a longer process. This is a fundamental problem of neurobiology and there are labs across the world who are trying to answer this question using several genetic tools, several molecular tools, several surface related surface chemistry tools they are all trying to answer these things that how possibly one of the processes can become a longer process as compared to other processes which are smaller processes. So, here we will not go in depth on that aspect of it because that is not under the purview of this course what we will do we will give them the different names. So, this longer process if I go back to my previous slide this longer process is called axon a x o n a longer one. These smaller processes are called dendrites and these dendrites form a huge mesh out here or network it is something like if you go in the microscopic details you will see it is almost like this it is kind of a branches of the tree if I had to draw it. So, this is called dendritic trees dendritic trees from you will see the word dendriomars dendriomars and all these things they all originate from there. So, here you have the nucleus here you have the cytoplasm with all the different cell organelles and this is the zone where communication occurs and we will come to that communication occurs and this long process what you see it has two possibilities either it will be covered with a specific kind of cell called which forms an insulated layer it is something like just imagine in your house you have this electrical connections these electrical connections and the wearings the wears are covered inside insulator inside a plastic casing imagine exactly something like that. So, there is here is the wear this is the cross sectional view of the wear and on top of that you see a plastic casing like this if I had to give you the cross sectional view of this it will look something like this. So, this is your wear which is your axon and here is the covering on all the side. So, this covering which is formed here is formed of there are some supporting cells in the nervous system which constitute that well just since I am repeatedly using this word nervous system I will just take a slight one step backward before I kind of you know I think I made a small error here to introduce you in the first place if I had to draw the nervous system it will look like this and we will come back and it is just I am taking a slight one step backward. So, from the top it looks like this this is the brain and this is the spinal cord you see and these are within the spinal cord what you see is. So, these are the different branches of neurons which are different pathways of neurons which are coming out from it it could it could reach your eyes, ears, heart, legs, hands and everywhere something like this and there are several circuits which are involved in it. So, these are called ganglions there are aggregation of several neurons at this place and this whole complex structure is called a nervous system it is the controlling unit which helps us to receive information, decipher information and take a decision accordingly how we are going to act it is almost the computer of our system which makes all kind of decision making and this part of the circuit which I am putting in blue now the spinal cord and the brain is called central nervous system. Whereas, anything which is outside this the nerves which are taking care of your heart or something which is taking care of your kidney or like you know so and so forth your hands and legs and everything all those neurons which are carrying information from outside this falls under peripheral nervous system there is central nervous system and there is peripheral nervous system and there are two pathways out along this spinal cord which I wish to highlight in the beginning which I missed upon. So, let me go to the next slide. So, this is what I am going to highlight now. So, if this is the brain this is the brain stem and this is the spinal cord something like this. So, now what I am trying to do is that I am magnifying this part of this spinal cord or any part of this spinal cord there are two circuits which are functioning here it is something like this on the flanks you have something called dorsal root and in the center let me put different colors that will make things easier for you people to understand. The one which I am shading with blue in the center something like this and I am shading the both the flanks with green. So, this green what you see is called the dorsal root terms now this one and this one is called the dorsal root and the center one this part from here to here is called the ventral root. So, what is dorsal root and what is ventral root next question. So, dorsal root is the one which say for example, I hit you hit somebody hits me like on the hands. So, series of signal from here is traveling all the way along these the nerves which are present here likewise if you look they are traveling all the way to the spinal cord and out there say for example, this is draw it say for example, here is my hand. So, there are some neurons sitting here. So, these neurons get the signal that somebody has hit. So, it generates an electrical signal that electrical signal is traveling all the way likewise. So, the direction of travel is like this is traveling and it enters the spinal cord through the ganglion you remember I was drawing all the ganglions out here it enters it through the ganglions and then it follows this track. Now, follow my follow the magenta color the signal it is moving like this and this track continues likewise along the side. So, this is where the information is being carried. So, there are two options this neuron means transfer the signal to another set of neuron and this will take it to the brain and telling me that telling the brain that someone has hit you out here. So, now what the brain will do. So, this information goes all the way. So, we have to follow this. So, for example, you follow this signal. So, this is going all the way to the brain likewise in the brain what happens the brain decodes the signal. So, someone has hit you. So, be be careful someone has hit you. So, you have a pain out here. So, the brain decodes this signal at some point some of its locations and from the brain this signal is being sent down by another with a different pathway. This is this pathway which tells me to remove my hand from the site where I am getting hit that pathway which brings the information back from brain likewise is called the ventral pathway or it is also called the or motor pathway. And the other one the one which carries the signal all the way up to the brain is called sensory pathway as the name indicates. Because sensory pathway and motor pathway as the name indicates sensory pathway sends the signal to the brain. And if I had to draw a cross section just imagine look at this you see the if you see the tip of the cross section. So, this tip is your ventral pathway which is bringing the information and this periphery what you see is the dorsal pathway. So, you see this tip there is a you could see that there is a bulge which is coming out from this pen. So, that is the dorsal pathway. So, imagine if something like this if this is the dorsal pathway then this is the ventral pathway which is going in the center that is how I have drawn it. So, if you have to see the cross section of it the cross section of this whole thing will look like this cross section will be like this what you see is. So, this is the cross sectional view. So, these are the dorsal pathway which are going up and this is the ventral pathway which is bringing the information back. So, this part is extremely important that is why I actually just missed upon this. So, that is why I decided to come back to it and then I will go back to the neuron. So, all the neurons where they are sitting exactly. So, all the neurons all the different neurons are at different there are thousands and thousands of them all over the place. They are sitting along the spinal cord they are sitting in the ganglion. Likewise they are sitting all over the place and these neurons have long processes which are there and what you see is the ventral pathway out here the one which is bringing the information this ventral pathway is also called the motor pathway because that is governed by the motor neurons. The neurons which are present on this pathway are called the motor neurons which brings back the signal to for us to act. So, just to give you a practical understanding of this most of the time we will see during car crash or accident or car injuries or you know any kind of this spinal cord injuries a person become paralyzed they are unable to move their hands or you know some part of the body or some side of the body. Basically what happens in that situation is this the neurons which are present on the ventral pathway they kind of get damaged a damaged takes place here out here and that is one of the challenging problem for in countries like Germany countries like US where there is very high speed driving you know especially in the freeways in Germany or in highways of United States the speed is fairly high 80 miles 100 miles 120 miles per hour. So, this is the kind of a speed with which the car moves. So, if there is a crash and if one survives most likelihood the major impact which comes it comes on the spinal cord and when it comes on the spinal cord what happens there are injuries which takes place along these and whenever there is an injury on the ventral pathway essentially that means the signal which is coming from the brain for the specific tissue is kind of the connectivity kind of gets compromised. So, the signal though the person can sense it. So, you pinch them or something they may be able to sense it, but they may not be able to respond it and very interestingly it these motor neurons are the first one to form in while we are getting developed in the mother's womb. These are the first set of neurons which are formed they form very early and they are really susceptible to these kind of injuries or damages and there are few other diseases which motor neuron diseases like those of you have heard about Stephen Hawking one of the biggest scientist of our time. He suffers from a disease called amyotropic lateral sclerosis that is a disease where most of the motor neuron which are present out here starts to die. So, that is why you see he is in a kind of a very specialized structure where only his eyebrows the motor neuron which are controlling the eyebrows can move most of the other motor neurons are unable to act. So, it is also called just forgetting the name basically it is called NALS and it is Lou Gehring disease it is also called Lou Gehring disease. So, there are whole series of diseases related to the nervous system and we will come to that there are diseases which you must have heard called Alzheimer's which takes place in the brain in a specific area of the brain called hippocampus which is involved in memory acquisition. There are diseases like Parkinson's disease where specific area of the brain which is regulating somewhere here which is regulating the movement they starts to die. So, we will be discuss all these things, but you have to keep in mind the basic overall structural architecture of the nervous system because that will be the key to understand how this disease actually affects us and mind it all this disease eventually starts with a single neuron. So, now coming back where I left you guys I told you that I just have to take a detour before I come back to the neurons. So, now I will come back to the neurons because now I am much more comfortable to talk about the neurons. Now, I showed you where is the position where what are the different circuits. So, coming back to the basic structure I have already drawn the basic structure of the neuron. So, it is a highly polarizes structure and we discussed that this polarity is a really interesting and challenging problem in the field of nervous system. So, there are several variations to this structure while talking about the variation one of the variation is that they may have these kind of and will come how these structures are formed while we will be talking about the supporting cells of the nervous system. One more thing which I just missed upon while talking about this nervous system not only consist of. So, nervous system consist of two different kind of cells broadly speaking one different cell types of nervous system. The one which is involved in all kind of communication or signal transfer are called neurons which we are discussing now. The other one which is involved in the maintenance and one of the major work is the maintenance and regulation those are called glial cells. They have enormous functions in the nervous system as more and more research are being done in glial cells. We are realizing that they are exceptionally dynamic and they play some very critical role in ensuring the signal transmission takes place properly and some people even believe that they have they are also some kind of signal generator which may have some role to play in some form of information transfer. Apart from it they help in ensuring the homeostasis they have certain components which help in strengthening the immune system and some of the diseases of these glial cells include multiple sclerosis MS which means the glial cells starts dying and the consequences are very fatal will come to those different diseases. Then apart from it there are other glial disorders which leads to some form of hyper excitability and all those things. So, coming back. So, the nervous neuron could have these kinds of insulation one option or they may be non insulated there may not be any insulation they may be just you know just like this naked that is one way one classification. Neurons could be present in the CNS or in the PNS they could be a CNS neuron and they could be a PNS neuron based on that their variations of size and everything changes. Basic structure of neuron what I drew could have whole range of variations one of the variation is something like this a single it comes out like this which is called pseudo unipolar neuron there are actually two processes one is a dendrite the other one is an axon. So, the longer one is the axon the smaller one is the dendrite they could be bipolar because this is called pseudo unipolar pseudo unipolar they could be bipolar. Bipolar means here is the cell body and here you have one process on one direction the other process another direction one is considered as dendrite the other one is considered as axon and it is bipolar instead of having multiple processes coming out and these different kinds of neurons are located in different parts this is dendrite. So, depending on the location they may have a very huge dendritic arbor it is almost like this you know some of the purkinje cells are like that the dendritic arbor is very huge as compared to. So, they have a different role to play in the information storage and information gathering. So, this is the overall classification if I have to say a very kind of a simple straight forward classification based on their structure and their functionality. Functionality depends on whether they are acting in the peripheral neuron whether they are acting for the sensory function or for the motor function. So, based on the functions we can classify the neurons as functional classification in the functional classification we have two forms of classification they could be sensory they could be motor within the sensory they could be very specialized ones you know which are involved in the special senses something like the rods cones which are sensing light and colors they could be here cells they could be the cells olfactory neurons where is the motor ones could be the one which is the higher motor neuron lower motor neuron and so on and so forth. There are whole range of such classification. So, from here we will move on to the functional unit of a neuron what it exactly does. So, this is the basic structure. So, if you remember in the membrane physiology class we talked about that these are called excitable cells. So, now today we are going to highlight that part why these are called excitable cells. So, these polarize the structure just like any other cell has a very low sodium as compared to the sodium outside which is almost 150 millimolar they have a very high potassium could see potassium they have calcium is slightly higher because here calcium is being stored inside the sarcoplasmic reticulum. So, what happens with these cells they have a series of likewise what I am putting as dots now they have series of something called voltage gated ion channels voltage gated ion channels. What does that mean if you break the word in a specific point voltage gated ion channels you know channels which promote the movement of ion like potassium sodium chloride likewise gated means there is a gate and voltage gated means it is the voltage which helps them to open or close. So, in other word what we are talking about is that within the cell there are whole bunch of ion channels whose opening and closing will depend on the voltage across the membrane this is the very core understanding of it and there may be other kind of channels which are called ligand gated ion channels. So, ligand gated ion channels are the ones where a ligand or something binds to an ion channel and then it opens. So, these 2 piece of information is exceptionally important that how this action potential is generated because these are the key players in ensuring the generation of action potential. Now, coming back to the structure of the neuron and now we will try to understand how this structure is involved in. So, this is the arborization this is where the dendritic trees are now. So, let me represent these voltage gated ion channels. Like this and we will have different color codes for them likewise they are all over scattered all over. So, the red ones are I should have done it with slightly bigger marker red ones are red and then the green ones and the blue ones. Red ones are the sodium channels which are voltage gated and the green ones are potassium channels and the blue ones which may be chloride or you know calcium. So, let us keep it chloride. So, do not mix it up chloride channels likewise maybe there may be some other which are like calcium or something else. So, but at this point we are only dealing with these 2. So, if you put an electrode inside the cell and another electrode outside the cell and try to measure the voltage across it. Given point of time you will see a neuron stands at a membrane potential with respect to inside the membrane potential is around approximately minus 90 millivolt or may be you know minus 75 millivolt. So, the inside the cell it is negative it is negative inside with respect to the outside. Now, in this situation what happens say for example, somewhere or other I could say for example, I create a situation here look at this drawing this drawing I create a situation that I change this voltage slightly towards positive say for example, I bring it down to minus 60 millivolt or say minus 40 millivolt. A very if I bring it down to at minus 60 or minus 40 millivolt most likely at minus 40 millivolt. What will happen is that there are a series of ion channels which respond at minus 40 millivolt and those are these channels voltage gated sodium channel and this sodium channels only opens to allow the sodium to enter from outside to inside is it making sense. So, the sodium can only enter like this inside the cell. So, somewhere or other the membrane becomes minus 40 millivolt. So, what will happen if it is become minus 40 millivolt immediately there will be a huge amount of sodium which is going to change. So, let us start put them in terms of graph that will be much more easy for us to understand. So, this is 0 millivolt this is sorry this is time and this is voltage in millivolt. This is 0 millivolt and we are moving in time and this is where the cell is sitting which is say minus 75 millivolt. This is where the membrane. So, cell is sitting as minus 75 millivolt. Now, somewhere or other you give some stimulus let us go back to the previous diagram. So, I told you there is one way is that you change the voltage by manually you change the voltage across it or you create a situation something some kind of a signal comes here somewhere or other what it will happen is that one such signal comes and it binds to the surface some kind of a signal may be a photon it could be something these opens up some of the pores which allows sodium to say say for example, let us this is the cell out here and which has low sodium and high potassium outside very high potassium potassium is low and potassium is very high here. So, if sodium starts gushing in likewise sodium is getting in stage one. So, what will happen is that the membrane will shift to something like minus 40 millivolt the shift will be like this membrane potential changes to minus 40, but if reaches minus 40 there is something called all or none phenomena takes place. What it does is that the sodium channels which are open. So, if I represent the sodium channel by red that promotes all the sodium channels to open. So, much so it is kind of you know 5 or 6 of them bring it to for minus 40 and then in units and all of them open it is all of all of a sudden it looks like as if the membrane became kind of collapsed membrane failed to really handle the sodium. So, all the sodium gets into the cell sodium entry is taking place. So, what you see is that you see a sharp rise from here on like this it overshoots the 0 and if it overshoots the 0. So, this is the rate limiting zone it has to reach to around minus 40 and if you recollect while I was talking about the pacemaker cells in the cardiac as telling you they always continuously remain at minus 40 from minus 40 they keep on shooting an action potential. They could shoot an action potential at minus 40 because that is the zone where the sodium channels open faster. Though they do not need a fast activating sodium channels because this is what is happening is that there are the sodium channels which are very very fast activating these are called fast activating sodium channel the open very fast and closes very fast this goes up when it starts going up. So, this these sodium channels open open also very fast and closes also very fast. So, very very fast kinetics the next step from here is once they reach certain voltage all this stop to open. But then the next thing happens once the membrane potential become positive then that potential promotes a series of another set of channels to open which opens which allows certain specific ions to move from inside the cell to outside the cell that is like this. The one which I am drawing in green now these are potassium ones potassium gushes out of the cell. So, once the potassium start going out of the cell. So, the membrane starts to come back to its original situation like this because you are trying to balance now, but this is a stage when the cell is rich in sodium and cell is lacking in potassium. So, there has to be a way by which you can regain the balance and then comes the third key player here the pumps comes into play sodium potassium ATPase pump. We have talked about this pumps in our membrane physiology section they throw away three sodium ions from inside the cell and it takes in two potassium inside the cell. These are called sodium potassium ATPase pump because inside the cell you need ATP in order for these pumps to act which brings the cell back to its normal membrane potential which is minus seventy five or minus ninety depending on the cell type. And here let me highlight this is the zone when pretty much no sodium channel can really open. This is called absolute refractory phase the sodium channels are completely inactivated and they cannot open. And then I will just shade this is the zone where sodium channels are activated, but they are not open. So, this is the phase of relative refractory phase. This is called absolute refractory phase relative refractory phase and absolute refractory phase. So, this is how the most simple currency of electrical activity is being transmitted by the neurons. There are many other things which happen which I am going to come before I move on you will come across another terminology called graded potential and what is graded potential. Graded potentials are local potentials they are very local always remember these are very very local potential which functions in a very small area local potential in a small area. Whereas, action potentials spread once it starts it spreads all the way through it is all or none. So, in order for the action potential to become all or none you have to cover that barrier of from say minus seventy five millivolt to come to minus forty millivolt or you know minus thirty five millivolt. This is the zone where all the sodium or the voltage gated sodium channels gets activated. So, this is very important for you people to understand that whenever a stimulus comes something like this is the neuron a stimulus which comes it opens up the voltage gated sodium channels initial one or few of them they bind say for example, something comes and bind they allow some of the sodium to get in this is not sufficient unless from minus seventy five you could reach to minus forty, but if it reaches minus forty you cannot stop this process. Then this will almost ensure something like a enzymatic thing or there will be lot more sodium voltage gated sodium channels in unison will get activated. Once they get activated then it overshoots all the way and then comes back because of the potassium current. So, this is the overshoot what you see here this is if this is zero millivolt. So, this is very interesting this is very important for you people to understand that this is how a cell. So, you could have a patterns you could have another one next coming and then likewise likewise this will continue and there may be you know this may change the length may change that the shape may differ there are whole bunch of things which could happen, but the threshold voltage remains the same once the threshold is there. So, these are the different kind of signals could be smooth it could be more compressed likewise there are whole series of signals which could be generated in that whole process, but then what is happening. So, along this neuron the signal is moving. So, this signal has to be transmitted to the next neuron and how that takes place that is what we are going to discuss now in the neuromuscular junction or in the neuron to neuron junction. So, for example, this is the second neuron and these are the dendritic arbors. So, here is the signal which is coming and that leads to a generation of action potential this action potential travels all the way along this longer process likewise it is traveling. Now, this signal has to be transmitted to the next neuron if this one is neuron 1 this one is neuron 2 how it does. So, how it transmit this signal from here to here. So, this particular gap is called a synaptic cleft will come in depth into this and we will talk about all the dimensions and everything of the synaptic cleft. Before I moved into the synaptic cleft and all other things I will just summarize what I have finished now. So, we talked about the neuron structures and classifications we have instilled with the glial part will be dealing with that we have talked about the basic architecture out here and the cross section. Then we talked about how the polarity things matters then we talked about the basic structure of the nervous system and then we talked about the different structures of the neurons and the their functional classification and the voltage gated ion channels and how it helps in generation of action potential. So, we will close in here in this class and the next class we will start with the synaptic cleft and the classification of glia and all other details. Thank you.