 So, let us resume the lectures on animal physiology in the NPTEL series. So, now we will be starting with that thyroid endocrine gland. Thyroid in general is one of the very active gland and thyroid hormone has significant physiological implications in our body. And at the national level there are there is a huge problem related to thyroid gland disorder across Indian female and male population. And this is a really challenging problem because it has to be cured by administering in a controlled way the specific hormone the thyroxin which is secreted by the and thyroid gland. So, let us start with the thyroid. So, if you recollect back. So, basically it is the hypothalamus from where the story starts. Hypothalamus secretes thyroid releasing hormone then from the pituitary it receives the signal in the anterior pituitary anterior pituitary sends the signal of thyroid stimulating hormone. And this thyroid stimulating hormone goes to the thyroid gland and after reaching the thyroid gland it stimulates the secretion of thyroxin. So, overall it is like this start with the thyroid. So, basically at the level of hypothalamus three level of control hypothalamus anterior pituitary and thyroid. So, in the hypothalamus there is the TRH which is secreted thyroid releasing hormone this stimulates thyroid stimulating hormone thyroid stimulating hormone eventually led to the secretion of thyroid hormone which is basically in two format comes and we will discuss about that T 3 and T 4. Interestingly, the thyroid hormone secretion is regulated by several environmental factor one such example of environmental factor is that when you are exposed to extreme cold weather extreme cold weather. So, if you think of the situation whenever there is an exposure to extreme cold weather. So, there are cold receptors which sense that fluctuation in temperature and this through the spinal cord sends signal to the higher part of the brain and from there a signal is transmitted to the hypothalamus and the hypothalamus releases TRH then along with the portal blood vessel reaches pituitary from pituitary again through the blood vessel the signal reaches the thyroid and thyroid secretes the thyroxin. So, this is how this whole cascade goes and any kind of temperature fluctuation effects most of these hormonal milieu but thyroid is the one which is very pronounced affected by the change in the weather conditions specially when you are in a very cold or you move to a very cold weather there is always a change in the concentration of this particular hormone and it takes certain time to get back to its original state. So, and that leads to a whole lot of physiological changes in the body. So, now what we will do straight away we will talk about the what are the functions of the thyroid. So, that will help us to appreciate that where it is all involved what all the different functions it is kind of taking care of. So, basically the action of thyroid hormone I have to enumerate them action of TH first of all it is involved in growth and development after birth after birth this is one of the major thing. So, thyroid deficiency leads to a lot of developmental abnormalities parental deficiency of T 3 or T 4 causes retardation in growth retardation of physical and mental development another key point what thyroid does it increases the metabolic rate. So, many people who suffers from thyroid suffers from weight gain because especially what it does in terms of the metabolic rate it influences the size of the mitochondria within the cell size and activity of mitochondria within the cell of the mitochondria in the cell this is one of the major thing it does and the abnormalities of thyroid once again sorry for the inconvenience abnormalities leads to either when there is an excess of thyroid it is called hyper thyroidism or if there is a lesser quantity within the blood of thyroid hormone it is called hypothyroidism and if you recollect how the second again there is a small and if you recollect back how the thyroid gland looks like while I drew the thyroid gland it is more like this along just behind the tray can you see 4 para thyroid gland on the flanking the heads. So, if you take a cross section of this it looks like something like this this is how the thyroid gland look like. So, you have the blood vessel in between the blood vessels in red and this is how the matrix look like this kind of secretory structure something like this these are the individual cells they form a secretory channel and this part is called the matrix similarly here also it is like that tightly coupled like this something like this and these cells are called the thyroid cells these are the ones which are responsible for secretion of thyroid hormone and these are the capillaries. So, now this is the basic structure then from here we will come on to the chemical nature of the thyroid hormone chemical nature of thyroid hormone has a direct link with an amino acid called chemical nature thyroid hormone as I told you that it form comes in two form T 3 T 4 the amino acid which is the precursor for this is tyrosine which is something residue this and I just request you people kindly look through the structure of the amino acids that will be helpful for people to understand the structure. So, there is an hydroxyl group here and then you have the COO group and if you revise through the initial lectures then you will realize that how this is the structure is stopped. So, basically what happens this tyrosine is modified with iodine. So, there are iodine which are attaching at these different zones. So, if there are three iodine which are attached it from tri-iodothyrene three iodine if there are four then the four and essentially what happens is this is part of a big protein. So, basically that is called thyroglobulin and within thyroglobulin you have this tyrosine residues. So, there are structure is something like this look at it. So, you may have four sites which have iodine modified by iodine or you may have three sites which is modified by iodine based on the number of iodine molecule at the side whether it is tri-iodothyrene tri-iodothyronene or thyroxine which is 34. So, this is basically how the thyroid hormone looks like. So, while seeing this structure it must have a strike you there is a role of iodine. So, essentially one of the problem people who suffer from thyroid has something to do at times something to do with lack of iodine in the diet. So, it is recommended in India that you should take iodinated salt because that helps in the synthesis of thyroid hormone and whenever there is a deficiency in the diet of iodine salt. So, that thyroid hormone synthesis gets hampered and on all likelihood there is a possibility that there may be a persons suffering from hypothyroidism like disorders where thyroid hormone synthesis is below normal. So, coming back where we were. So, this is mostly about the chemical nature of the thyroid hormone and the gland which is secreting it, but then there is another while I was drawing the structure I told you that there is along the thyroid there is four other structure which are there which is called parathyroid and they are all pretty much fairly link in their functions. So, this parathyroid and thyroid has certain role in terms of calcium regulation. So, this particular parathyroid has. So, now what we will do we will talk about how calcium level is being regulated through this parathyroid hormone parathyroid gland. So, the way it happens is this whenever the body suffers from calcium goes down in the body. So, in the blood serum there is falls below the normal level of calcium which is basically the normal level of calcium within the blood is 2.5 millimolar per liter this is the level of calcium normal level of calcium, but when it goes down this normal level of calcium it is immediately those four glands what you see they gets activated. Once they get activated they secrete something called parathyroid hormone PTH parathyroid hormone. This parathyroid hormone does what it does it triggers it sends its signal to the bones because bone is a large deposit of calcium which has rich source of calcium from the bone there is a process called demeneralization taking place because of the action of the parathyroid hormone that demeneralization leads to raising the level of calcium within the blood. This is one root by which parathyroid functions by enhancing the demeneralization within the bone there is another root how it works the second root is that whenever it there is a calcium level goes down a signal is sent to the kidney in the kidney this leads to the secretion of something called de hormone or calcitronin this there are two terms in the literature de hormone or calcitrol or calcitronin like we show we want to put it. Then this de hormone sends a signal to the intestine to absorb as much calcium as possible this is the second way by which the calcium pool is being maintained. There is a third root which is the root followed by parathyroid hormone this time it again sends a signal to the kidney but it sends to the tubules of the kidney and ask it not to lose calcium. So, to summarize how calcium is being regulated in our body there are three mechanism which are being followed the first is whenever there is. So, I told you 2.5 millimolar per liter is the average calcium concentration in our blood. So, whenever it goes down the signal is being sent to the brain and from there the signal is received to the parathyroid once parathyroid receives the signal parathyroid leads to the secretion of parathyroid hormone. Parathyroid hormone immediately triggers a series of signal which reaches the bones once it reaches the bone there it triggers another set of signal which leads to the demaneralization of the calcium present in the bone this is root number 1. Simultaneously parathyroid sends signal to the kidney and ask the kidney that do not lose calcium during the process of filtration. So, restore as much calcium as possible this is the second root. There is a third root the same parathyroid the calcium lowering of the calcium level triggers the kidney to secrete another hormone called de-hormone or you can call it calcitonin or calcitrol. What it does is that this sends a signal to the intestine and request the intestine absorb as much calcium as possible the same way parathyroid hormone does in the kidney it request the kidney reabsorb as much calcium as possible. So, if you look at this whole thing while I am explaining all these three systems see you always have to whenever you study endocrinology you have to look it from a very holistic or as a matter of fact the whole physiology it is very holistic it is not a standalone system it is a very integrated function and each one of these system is a backup for another system. So, if you one fails someone will have more stress because someone has to back it up for the failure of the one system. So, body does not depend on one root or one circuit it always have multiple circuit to ensure that we make the best use of all the resources available around us. So, this is extremely important for if people to appreciate that it is not a standalone system this goes bad fine you know this is how it is going to work. So, there are multiple channels multiple roots multiple level of clearance and multiple level of rectifying the system. So, now we will talk about the reverse situation when the calcium level goes up in the body it goes more than 2.5 millimolar. So, then what happens calcium level is up into the system. So, when the calcium level goes up in the system again the parathyroid gets activated and parathyroid secretes calcitonin this calcitonin goes to the kidney this time it is following a different rule goes to the kidney and promote more secretion of calcium because we have to get rid of the calcium. Second thing calcitonin of course does is that it promotes it goes to the bone request the bone to accept more calcium because bone is a bone could really store a lot of calcium. So, there is a mobilization of calcium towards the bone and in this process the parathyroid hormone does not play any role it is these two root which significantly rule the whole thing. So, to summarize it. So, whenever the calcium level goes up parathyroid again gets activated. So, then parathyroid secretes calcitonin calcitonin goes to the kidney and tell the kidney to body has excess calcium. So, we need to get rid of the calcium because if there is an excess calcium there will be if you remember while we were talking about the nervous system and the cardiac system. Calcium plays a very vital role in excitability of the nervous transmission and cardiac transmission. So, under hyper calcium condition those transmission gets affected. So, kidney has to throw away that excess calcium which is present and simultaneously that same calcitonin send some message to the bone telling it you pull out as much calcium as much calcium as possible from the blood stream and store it in the bone or transform it in the bone. So, that the bone has significantly higher reserve of calcium reposition. So, summarizing what happens in just before this when calcium was low you could see parathyroid hormone. So, calcium level goes down. So, kidney gets activated it secretes de hormone then from the or calcitrol the de hormone tell the intestine to you know absorb as much calcium as possible simultaneously parathyroid secretes parathyroid hormone that parathyroid hormone goes to the bone and ask it to you know demeneralize as much calcium as possible which is needed by the body and simultaneously tells the kidney that you know reabsorb calcium and thereby increase the extra cellular fluid concentration of calcium. This is one root and the second root is what we talked about when calcium level goes up parathyroid gets activated again, but this time it secretes calcitonin calcitonin goes to the bone and tell the bone to absorb as much calcium as possible. So, that from the extra cellular fluid the excess calcium goes down and whereas calcitonin also regulates the kidney and tells the kidney that throw away more calcium because excess cellular fluid has more calcium. So, with this we covered pretty much all the major hormones the one which are secreted by the pancreas I will be dealing with that in the digestive system or what I will do now there is one tail piece which is left we will be talking about the hormone secreted by the pituitary we directly affects our gonads. So, here we will be talking about the gonadotrophins. So, if you remember I told you there are two hormones which are secreted by the first hormone which are secreted was GNRH gonadotrophin releasing hormone by the hypothalamus and which leads to the secretion of follicle stimulating hormone and the luteinizing hormone FSH and LH if I take you back to the some of the initial chart while I was trying to explain go back. So, that will help you to coming back to this chart. So, if you look at it GNRH secreted by the hypothalamus and then anterior pituitary secreting FSH and LH and this leads to the secretion of the gonads secretion of the hormones by the gonads. So, here there are there will be classification one will be talking about the male other over the female. So, let us talk about the what happens in the males. So, in the males there is a continuous once we attain puberty or adolescent or sexual maturity our testes could produce sperm continuously how it does so. So, the hormone which is involved in it is FSH. FSH is also the full form of FSH is follicle stimulating hormone. So, here let me remind you we talked about the androgen we talked about that androgen is responsible not in the really the development of the sexual part, but it is mostly involved in the sexual behavior. Here is follicle stimulating hormone is involved in the production of the sperm and there is another hormone which we talked about LH luteinizing hormone. Luteinizing hormone what it does within the testes there are specific cells. So, your testes are something like this within the testes you have a specific cells called ledic cells l e y ledic cells. So, whenever there is luteinizing hormone goes there it tells a ledic cell to secrete one of the sex hormone called the testosterone. This testosterone is involved in proper maturity and growth of the testes of the of the sperm. Here since I have gone directly into it I will tell you couple of other things about the production which is very essential for you to appreciate production. So, this part I have not touched really. So, I will just give you a brief introduction all our cells of the body contains two pairs of chromosome call it as two x. So, something like this one pair of chromosome we receive from our father the other pair we receive from our mother. So, all our cells have two x or like pairs of chromosomes of course, we have 23 pairs of chromosomes human. So, it is like 23 pairs. So, out of 23 pairs 22 pairs of normal chromosome and then you have x y which makes us male or. So, this is the 23rd pair or 22 pairs plus x x which makes someone female. So, this is the only difference in that y and there is an x rest is all fairly the same. So, now when we reproduce the reproductive cells. So, for example, I have two x and if the two x and two x. So, the next will be a four x. So, if that happens then we will become continuously bigger and bigger because our size of the genome size of the nucleus has to accommodate more and more genes and more and more chromosomes and all the time. So, it will keep on doubling. So, if somebody is four x. So, then it has to have a sex with another four x then it will become you know eight x likewise we will move on, but that does not happen in nature essentially what happens is that. So, we all ourselves. So, this two x is called a diploid diploid means the it is called ploidy the set of chromosomes ploidy and diploid means something like that. So, one pair. So, this is diploid or x is a single ploid. So, we are having a diploid cells on our body, but the sperms what we produce is not diploid. So, this is haploid haploid means if this is two x which is diploid haploid is having only x. So, in case of female it is they produce and these haploid cells are called gametes x y. So, this is in the case of male this is in the case of female. So, a female so what is a. So, these haploid cells are called gametes. So, I will not go in depth how the cell division takes place you can pick up any book or any online resource material given google cell division it will show you that how this division taking place when a cell maintains its ploidy. So, two x produces another two x and the two x produces two x this is called mitosis and when a two x makes two x or x y making x y becoming x and y it is ploidy splits up. So, diploid become haploid two x become one x that process is called meiosis. So, I request you please go through any online resources it is available all over online there are beautiful if you go to google images meiosis and mitosis that will tell you exactly how this meiosis and mitosis takes place you can go through that. What is essential is that for you people to understand the reproductive process what regulates this meiosis process. So, this process of formation is called gametogenesis or the formation of gametes is called gametogenesis and gametogenesis falls under two level one is called spermetogenesis the other one is called oogenesis what does all these words mean. So, we attain reproductive maturity say at the age of 14 15 or 16 or 17 when our body in the case of males we start producing the sperms or in other word we are in a position to reproduce. So, if you have a sex with a female and we it could lead to a production of a baby vice versa a female attain to her maturity at the age of 14 15 when the menstrual cycle starts. So, what does this mean this is the time when the gonads of the body starts producing the sex cells this process is very very tightly regulated and it is a very time bound process is very tightly regulated by the hormonal system and this whole thing is under the neuro endocrine axis of the gonadotrophin releasing hormone GNRH, FSH, LH and androgen which is the secondary sexual behavior. And in this section what we will be studying is the interplay of these different hormones which leads to the formation of the gametes in the case of males in the case of females. And what happens when that cyclicity especially in the case of female is being disturbed or perturbed by some foreign agents which you can call them endocrine disruptors or something. And why this process in the case of male is once it starts at puberty say at the age of 15 it goes pretty much lifelong in the case of males, but in the case of female this last only up to a certain age like at the age of 50 or 45 50 it last and after that the production of gametes by women stops. So, that phase is called menopause. So, what is the what is menstrual cycle and what is menopause and how the hormones regulate this process is exceptionally important for us to understand. And what is reproductive age and what is the exact time when a male gamete could fertilize a female gamete. So, as to make another diploid cell which we call at zygote is under a very tight regulation on the endocrine system. So, what will we doing after this is that we will talk about those cycles and we will finish off this part. So, I will close in here and the next class I will resume from here talking about the spermetogenesis and oogenesis and how the different level of FSH, LH and other secondary sexual hormones are regulating this whole process. Thank you.