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Transcript
Chapter 8: The Endocrine System Hormones...not just for teenagers •You have seen how aldosterone plays an important part in water regulation •Most of your body’s processes are regulated by hormones – they are considered to be chemical messengers •The collective set of these organs that secrete hormones important for homeostatic regulation are known as the ENDOCRINE SYSTEM Efficient or Inefficient? •If you think about the fact that the blood is the primary substance in our body that passes through EVERY organ, it makes sense to use it as a transportation vector •Therefore, chemical signals, though inefficient when compared to something like nervous signals in the brain, are useful because very distant parts of the body can communicate with each other •However, the complication is that hormones must travel through the WHOLE body before reaching its target organ Delay •This means that most systems controlled by hormones will have lag period of start up and stopping •They are also regulated by very tightly monitored feedback loops in order to stop the secretion of the hormone when it is no longer needed •Most feedback loops in the endocrine system are negative Basic set up •If you recall the first lecture on homeostasis, a given organ in the endocrine system will follow a basic feedback loop •Diagrams are often used to describe the mechanism of action for a given hormone •Others are named according to the hormone involved •There are usually at least two organs involved; a monitoring organ and a target organ, but axes may include more than one organ MONITORING ORGAN (-) (+) TARGET ORGAN (+) TARGET ORGAN CHANGE (+) Types of hormones •Hormones can be classified into two types based on their chemical structure: •Based on the chemical structures, each type of hormone has slightly different mechanisms of action and can affect cells through different methods Steroid hormones: welcome in •Recall lipids that the cellular membrane is composed of mainly •Therefore, the release and absorption of steroid hormones are less well regulated, since they have the capacity to exit and enter cells freely •If they can pass through cellular membranes, they can pass through nuclear membranes as well; they are often used to control DNA transcription for this reason http://highered.mc grawhill.com/sites/0072 507470/student_vi ew0/chapter18/ani mation__mechanis m_of_steroid_horm one_action__quiz_ 1_.html Protein hormones: knock first •Protein hormones require more regulation because they cannot easily pass through the cellular membranes to affect cells •Protein hormones must interact with cells via receptors on the surface of the cell http://highered.mcgrawhill.com/sites/dl/free/0072421975/196644/second_messeng ers.html A noteworthy gland: the pituitary gland •One particular gland that is of great importance in the endocrine system is the pituitary gland •It is located at the base of your brain, just in front of the brain stem Nervous control •The pituitary gland is a good example of how the brain directly links into the endocrine system •The brain not only helps monitor the state of the body, but it also acts to synthesize and release hormones that control other hormones •Therefore, the brain itself is an important endocrine organ Hormone axes The hormone axes we are going to study will include: Glucagon: Hypothalamic-Pituitary-Adrenal axis: Hypothalamic-Pituitary-Thyroid axis: Parathyroid: Hypothalamic-pituitary-testicular axis: Hypothalamic-pituitary-ovarian axis: 8.2 Glucose regulation Glucose regulation •Because glucose is an essential molecule for cellular survival, its regulation is important •A specific concentration of glucose must be kept in the blood in order for cells to access the glucose they need to function The pancreas •Pancreatic tissue contains many different types of cells – each different type is responsible for secreting different substances •The cluster of cells that regulate blood sugar form a structure known as the ISLETS OF LANGERHANDS Glucagon and insulin •Insulin and glucagon are antagonistic hormones – they produce opposite effects and work together to balance glucose levels •Insulin acts on cells to increase their permeability to glucose by binding to receptors on the cell surface and helping increase the activity of glucose transporters – this decreases the level of glucose in the blood Time lag •Understanding this axis helps us to understand the importance of portion sizes in healthy eating •After a meal, glucose floods your bloodstream – if there is more glucose than can be used immediately by cells, it will be put into storage by the liver, who, due to insulin secretion, will be increasing its uptake of glucose, and therefore, creating more glycogen •Between meals, your “starvation mode” – glucagon is secreted to break down the glycogen stored earlier Diabetes •Diabetes occurs when insulin production or response to insulin is compromised •There are two types of diabetes: •Type 1 Diabetes/Juvenile Diabetes/Diabetes Mellitus: •Type 2 Diabetes/Adult-Onset Diabetes: Symptoms of Diabetes •Diabetes results: •Increased urine production and therefore increased loss of water •Increased thirst •Increased glucose secretion in urine and therefore increased glucose levels in urine Long term complications of diabetes •Peripheral neuropathy – numbness in the extremeties •There is a thought that the heart is also directly affected by the inability to regulate glucose levels Treatment •Type 1 diabetes •Type 2 diabetes Gestational Diabetes •Pregnant mothers are also at risk of developing gestational diabetes •This results in an increased blood glucose level that can affect the baby 8.3 Metabolic control Metabolism •Metabolism refers to the sum total of all chemical reactions that maintain a living organism •It is most closely associated with those processes that produce and store energy in the body which would include, in simple terms, the making and breaking of chemical bonds Metabolic control •The BASAL METABOLIC RATE refers to the rate at which an organism can process energy •This rate refers to the amount of energy that the body uses when at rest •BMR is different for everyone – this explains why two people that have the same diet may have very different body masses •High BMR usually means that you burn more calories per minute than someone with low BMR Thyroid gland •The thyroid gland is a large endocrine gland on its own, but situated on top of the thyroid are 4 individual glands that collectively make up the parathyroid gland •The thyroid gland secretes very powerful steroid hormones that control metabolism Thyroid gland •Two hormones, T3 and T4, are secreted by the thyroid gland – collectively known as Thyroxine •The “3” and “4” refer to the positions of iodine which are bound to the molecule HypothalamicPituitary-Thyroid axis TRH= Thyroid releasing hormone TSH = Thyroid stimulating hormone Thyroid disorders •Because iodine is an essential part of the T3 and T4 molecule, a lack of iodine can lead to the underproduction of these hormones •If T3 and T4 are not being secreted, the hypothalamus will not receive negative feedback; therefore, it will continually secrete TRH •This will cause the anterior pituitary to continually secrete TSH •TSH stimulates the thyroid to secrete T3 and T4 – and to do meet the constant demand, the thyroid will increase in size (by increasing the number of secreting cells) •An Hyper and Hypothyroidism over or undersecretion of thyroxine can lead to a change in BMR •Hyperthyroidism refers to an oversecretion of thyroxine which can lead to rapid weight loss, heightened state of agitation and nervousness, difficulty focusing •The cause can be a tumour (for example, tumour blocks the exit duct for the hormone) and it is common with aging as cells begin to slow down Calcium balance •The parathyroid glands, along with the thyroid gland, work together to balance calcium levels in the blood •Parathyroid hormone (PTH) is secreted by the parathyroid gland and is antagonistic to Calcitonin, secreted by the thyroid gland Calcitonin and PTH •Low levels of blood Ca stimulate the release of PTH •PTH increases blood Ca by stimulating the activity of osteoclasts, bone cells that break down bone •This will lead to the release of Ca into the blood stream Problems with Ca balance •Improper Ca blood levels can affect bone and muscle •If Ca levels cannot be maintained at normal values, this can affect the amount of available Ca to bones for normal development Human Growth Hormone •HGH is an important hormone required for growth in young children •Think carefully about what happens to appetite during adolescence: growing children require large amounts of energy in order to fuel muscle, bone and neuronal development Secretion •HGH is secreted by the anterior pituitary •Its secretion is controlled by the hypothalamus that secretes GHRH (growth hormone releasing hormone) •HGH acts on the liver to secrete hormones such as IGF (Insulin like growth factor) that acts on muscles to increase hypertrophy (increase in size of cells) and hyperplasia (increase in number of cells) of muscle (-) (-) (-) (-) IGF Actions •GH will also: • Mobilize fat stores by increasing the breakdown of fat Stimulate the growth of internal organs • Stimulate immune system • Problems with GH •Lack of GH secretion has a much more severe effect during childhood than adulthood •Low GH levels during development will lead to shorter statures and smaller builds in children •This can be a cause of dwarfism – •Too much GH secretion during childhood can result in the opposite – gigantism – •Oversecretion of GH in adulthood can cause increased body mass and bone thickness which can interfere with normal skeletal movements – creating a condition called acromegaly 8.4 The stress response The stress response •Human response to stress is a very important evolutionary mechanism that has evolved to keep us alive •It is controlled by the hypothalamus •Therefore the first cues that a stress response is necessary due to a threat of danger would be processed through the brain, usually via visual information (as primates are reliant heavily on vision as a sense) If you’re in danger, then what? •To understand the mechanism better, it is important to understand what happens physiologically in times of danger •This group of symptoms is associated with what biologists refer to as the “flight or fight” response: The players: the adrenal gland •Divided •The into two main parts: medulla is innervated directly by the brain via the spinal cord, and the cortex is affected by hormones secreted by the anterior pituitary •The adrenal gland secretes three hormones to produce this group of responses: Hormone Secreted from Function Increase in: Blood glucose Heart rate Breathing rate Cellular metabolism Constriction of blood vessels to redirect blood flow to muscles and heart Increase in: Na+ and water retention in kidneys Blood volume Blood pressure Increase in blood glucose Conversion of fats and proteins into glucose Supression of inflammatory response in immune system See p. 382 from text The players: hypothalamus and the anterior pituitary gland •The brain identifies a stressful situation and signals the adrenal cortex directly; this initiates a short term stress response •This branch of the axis produces a more long term stress response SPINAL CORD (+) (+) BRAIN HYPOTHALAMUS (+) ANTERIOR PITUITARY (+) ADRENAL GLANDS CORTEX MEDULLA GLUCOCORTICOIDS ADRENALINE AND NORADRENALINE MINERALCORTICOIDS The stress response and health •It is easy to see why stressful lives can lead to long term health problems; remember that human society has changed what our “stress” is •Previously, our response to danger was physical, and so the stress response prepares our bodies for that •Now, due to changes in human society, most stress responses are emotional or neurological – the final exam, death in the family, the big deadline at work •Someone who is consistently in a state of stress will be creating physiological conditions where blood pressure is pushed past normal •This can stress the heart and lead to the increased risk of cardiovascular disease throughout one’s lifetime if coupled with poor lifestyle choices 8.5 Reproductive hormones The hormones that make teens famous •Reproductive hormones in males and females are focused on regulating reproductive processes •In males: this affects the production of sperm •In females: this affects the menstrual cycle which regulates the release of ova from the ovaraies each month The male reproductive system •The main hormone responsible for regulating the male reproductive system is testosterone •Testosterone is secreted by the interstitial cells which are found within the testicles Male reproductive anatomy •In males, the urinary and reproductive systems are linked •Both sperm and urine exit out of the urethra •The testicles contain a series of very coiled and very thin tubes known as the seminiferous tubules Male reproductive anatomy cont. •From there, sperm exits the epididymis through the vas deferens •The vas deferens is connected to 3 major glands: •Prostate gland: •Cowper’s •Seminal gland: vesicle: Male reproductive hormones •Testosterone production and secretion is controlled by a negative feedback loop that includes the hypothalamus and the anterior pituitary gland along with the testicles •GnRH (gonadotropin releasing hormone) is secreted by the hypothalamus •FSH acts on sertoli cells and LH acts on interstitial cells (AKA: Leydig cells) •LH signals interstitial cells to secrete testosterone (Interstitial cells) Hypothalamic-Pituitary-Testicular axis Testosterone’s other effects •Testosterone will increase hair growth, muscle hypertrophy (helps males increase muscle size), and increase aggression •Testosterone secretion during puberty causes the larynx to lengthen, thus deepening the male voice, maturation of the penis and the development of pubic hair •Excess testosterone secretion can lead to increased aggression Female Reproductive anatomy •In women, the reproductive system is separated from the urinary system •This is an obvious adaptation as both systems need to remain separate for the gestation of offspring •The ovaries are the main hormone producing structure (along, ofcourse, with the development and storing of ova or eggs) Horomone regulation •Once past puberty, females release one egg per month (usually – though sometimes more than one can be released) •The 27 day cycle is based on the release of the egg, the preparation of the uterine lining for pregnancy, and the shedding of the lining if pregnancy doesn’t occur Oogenesis •Unlike men, women are born with all their eggs •The eggs are stored in the ovary within structures called follicles •Therefore, older ova that have been exposed to more radiation/chemicals are at higher risk of having chromosomal anomalies that can lead to congenital birth defects The menstrual cycle •Hormone fluctuations are timed around the release of the ova •The menstrual cycle is timed from the start of menses (period) when the uterine lining is shed Follicular phase •The hypothalamus releases GnRH that stimulates the anterior pituitary to release FSH (Follicle stimulating hormone) •The developing follicle then secretes estrogen •The peak of estrogen secretion at ovulation feeds back positively on the anterior pituitary to secrete LH (Luteinizing hormone) Ovlulation •When the follicle is mature, a spike in LH (Luteinizing homone) secreted by the Anterior pituitary stimluates the follicle to rupture and release the ovum •By now, the uterine lining has thickened in case the ovum is fertilized in the fallopian tube and is ready for implantation when it reaches the uterus Luteal phase •The developing Corpus Luteum secretes some estrogen but mainly progesterone •Progesterone helps to maintain the uterine lining, keeping it ready for implantation •This prevents the maturation and release of any further follicles Pregnant or not? •If no fertilization occurs, the corpus luteum will die due to the lack of FSH and LH •Once the corpus luteum dies, production of estrogen and progesterone stops •This will remove the negative feedback on the hypothalmus and anterior pituitary leading to the secretion of FSH and LH to restart the cycle See p. 398 for Hypothalamic – pituitary – ovarian axis •If Pregnancy the ovum is fertilized, the developing placenta will secrete hormones to maintain itself •hCG (human chorionic gonadotrophin) is secreted to supress the regression of the corpus luteum – this keeps it alive to prevent levels of progesterone and estrogen from falling •The detection of hCG is the basis of the pregnancy test •All PMS men make fun of it, and some women do – but it is important to respect the effect of hormonal fluctuations in women •PMS is used to describe the effect of these hormones on the brain before menses when their concentrations are high •In some women, the brain’s response to the hormones is so severe that it can stimulate severe depression leading to suicidal tendencies or even overt aggression Menopause •Women between the ages of 46-54 will enter into Menopause (yes, the name literally tells you what is going on) where ovaries lose their sensitivity to FSH and LH •It is interesting to note that menopause is rare in most animal species Women aren’t so much from Venus and Men aren’t so much for Mars •Testosterone, progesterone and estrogen are steroid based hormones that can be converted into each other via common biochemical pathways •It is worthwhile to note that women do have testosterone and men do have estrogen – however, the levels of these hormones are very small Over or undersecretion •Under or oversecretion of any of testosterone in women or estrogen in men can result in masculinization or feminization of physical features •These hormones are very powerful and can cause these effects even after puberty