 In this video I will cover the following objective, define endocrine gland, hormones and target cell or target organ, describe the factors affecting target cell activation, define humoral, neural and hormonal stimuli for hormone release, and be able to recognize examples of each. Here we have a figure showing the major endocrine glands or the major organs that form the endocrine system. An endocrine gland is a ductilus gland that secretes hormones, which are chemical messages that travel through the blood to regulate target cells or target organs. So the hormone is a chemical message produced by an endocrine gland. For example, the thyroid gland produces a hormone known as thyroxin, also known as T4, or the thyroid hormone T4. And that hormone then has an effect on target cells throughout the body, such as the skeletal muscle fibers, and thyroid hormone will stimulate an increase in metabolic rate. So it has a specific function in cells that have receptors. These are the target cells, which are found within organs that are responding. The target organs are the organs that are being regulated by the hormone. So another example of an endocrine gland we can see here is the pituitary gland. The pituitary gland produces a large number of hormones that regulate other endocrine glands. For example, we will see that the pituitary gland makes tropic hormones, such as the thyroid stimulating hormone, and then thyroid stimulating hormone will stimulate the thyroid gland. So the thyroid gland is the target organ being stimulated by thyroid stimulating hormone, a hormone secreted from the pituitary gland. So in order for a hormone to affect the activity of a target cell, the target cell must have receptors to detect the hormone. The hormone will bind to the receptor, and then the receptor can stimulate a variety of changes in the function of the cell. For example, a hormone could alter the permeability of the plasma membrane of the target cell, or it could stimulate the target cell to synthesize new proteins. It could stimulate the target cell to turn on or turn off enzymes to regulate metabolism, or it could stimulate the target cell to divide, to make daughter cells in order to stimulate growth. There's a variety of different receptors that are specific for the hormones, which enables the interaction where the hormone regulates target cells to be very specific. So one hormone will have an influence on certain target cells, and other cells that don't have the receptor for that hormone won't be affected by that hormone. Some hormones bind to receptors that are on the surface of the cell in the plasma membrane, as we can see in the illustration here. In this case, a message will be produced inside the cell known as a second messenger that will then have an influence on the activity of other proteins inside of the cell. And so in this example where we call the message inside of the cell the second messenger, the hormone would be a first message that is the message outside the cell that binds to the receptor on the plasma membrane. And so a receptor in the plasma membrane will be required whenever the hormone is a water-soluble hormone. However, if the hormone is a lipid, like a steroid hormone such as testosterone or estrogen, the receptor will be inside of the cytoplasm because a lipid-soluble steroid hormone is able to diffuse directly through the plasma membrane. A water-soluble hormone such as oxytocin will have to bind to a receptor in the plasma membrane of the cell because it's not able to diffuse through the plasma membrane that the receptor has to be on the surface. But either way, the receptor is specific for the hormone and that allows a hormone to influence the activity of that cell. The physiology of the cell will be regulated by that hormone. So let's look at a couple of examples of hormones and how they influence target organs and target cells. While target cell activation is dependent upon the concentration of the hormone in the blood and the availability of receptors on the target cell to bind to that hormone, a variety of factors can influence target cell activation in addition to the concentration of hormone and the affinity of the receptor for the hormone, that is how tightly the receptor binds to the hormone. Target cells can also up-regulate or down-regulate the receptors and intracellular signaling pathways in order to regulate the sensitivity to a hormone. If there's a very low concentration of a hormone, this could lead to up-regulation of the receptors and cell signaling pathways to make target cells more sensitive to that hormone. In contrast, if there's a very high concentration of hormone leading to activation of most of the receptors on a target cell, the target cell may down-regulate the receptors and intracellular signaling pathways in response to excessive hormone levels. So here we can see an illustration showing the structure of the pituitary gland and there's two distinct lobes or two regions of the pituitary gland. This is focusing on the posterior pituitary gland or the posterior lobe of the pituitary gland, which is also known as the neurohypophysis. The pituitary gland is also known as the hypophysis and the posterior lobe is also known as the neurohypophysis. The posterior pituitary gland is connected to the hypothalamus and neurons found in the hypothalamus secrete hormones from the posterior pituitary. So the cell body of these neurons is found in the hypothalamus and their axon extends down into the posterior pituitary and releases neurohormones, hormones that are released by neurons. And so these neurohormones are oxytocin and antidiuretic hormone, two hormones that are released by the posterior pituitary gland, although these hormones are actually produced and secreted by cells found in the hypothalamus. So oxytocin and antidiuretic hormone have different target cells and will stimulate different functions in those target cells. So let's take a look at the functions of oxytocin. Here we see an illustration showing the function of oxytocin in a positive feedback loop that stimulates contraction of the uterine smooth muscle during childbirth. So this starts with the head of the baby pushing against the cervix and as the cervix of the uterus is stretched a nerve impulse relays that information into the brain where it will stimulate the cells of the hypothalamus that secret oxytocin and so oxytocin will be secreted by the neurons that are having their cell body in the hypothalamus but their axons in the posterior pituitary gland and those neurons then are secreting oxytocin into the bloodstream and oxytocin then secreted by the posterior pituitary travels through the blood to reach the uterus where it binds to receptors on the smooth muscle fibers in the muscular wall of the uterus in the myometrium. Oxytocin will then activate those smooth muscle cells to contract so oxytocin stimulates contraction of the myometrium and this will push the baby further into the cervix which will stimulate more stretch receptors that relay the signal into the brain leading to even more secretion of oxytocin which activates the smooth muscle to contract in a positive feedback loop until the child is born. So here we see the example of antidiuretic hormone another hormone that's secreted from the posterior pituitary gland and produced by the hypothalamus. So the function of antidiuretic hormone is to stimulate the kidneys to reabsorb water and reabsorption of water decreases the volume of the urine makes the urine more concentrated and makes a smaller volume of highly concentrated urine. So when you're dehydrated there are cells in your hypothalamus called osmoreceptor cells which are neurons that detect the concentration of solutes and when you're dehydrated the concentration of solutes in the extracellular fluid becomes increased that's detected by these osmoreceptor cells in the hypothalamus and that stimulates those neurons to release the hormone antidiuretic hormone from their axon terminals in the posterior pituitary gland. So the posterior pituitary gland releases antidiuretic hormone in response to dehydration that then travels antidiuretic hormone and travels through the blood to the kidney where it will stimulate water reabsorption. And so the kidney is one of the target organs that's regulated by antidiuretic hormone but it's not the only one. Antidiuretic hormone also has an effect on the smooth muscle in the walls of arterioles which are small blood vessels small arteries and so it stimulates constriction of arterioles which has an effect of increasing the blood pressure. So antidiuretic hormone has multiple effects through multiple target cells in different target organs but notice the target organs and the effects of antidiuretic hormone are distinct from what we saw the effects of oxytocin so oxytocin stimulates receptors on the smooth muscle cells in the uterus to stimulate contraction during childbirth and antidiuretic hormone binds to different receptors which are found on the cells in the kidney as well as the cells in blood vessels in order to stimulate reabsorption of water in the kidneys and contraction of the smooth muscle in the arterioles and these actions work together in order to help the body deal with the stress of dehydration. There are three major mechanisms that control the release of hormones. A humoral stimulus is the concentration of a solute in the blood or extracellular fluid for example the concentration of solutes in the extracellular fluid is detected by osmo receptors which are the neurons of the hypothalamus that release antidiuretic hormone and so the release of antidiuretic hormone that we just discussed is an example of a humoral stimuli another example of humoral stimuli controlling the release of a hormone is the concentration of calcium ions in the blood controls the release of parathyroid hormone if the blood calcium concentration drops below the homeostatic set point the chief cells of the parathyroid gland will then release parathyroid hormone and then parathyroid hormone will have effects leading to an increase in blood calcium concentration as a negative feedback mechanism another example we'll see is the homeostatic concentration of blood glucose is regulated by cells in the pancreas that release hormones insulin is released in response to a high blood glucose concentration whereas glucagon is released in response to a low blood glucose concentration and these are both hormones that are released in response to humoral stimuli the concentration of a solute in the blood and that solute is not a hormone it's something like calcium or glucose or the overall solute concentration if it's a hormone in the blood we'll see that is a hormonal stimulus so we'll come back to talk about examples of hormonal stimuli in a moment but first we'll move on to neural stimuli so a neural stimulus is when a neuron releases a neurotransmitter to activate an endocrine gland to release a hormone so one example that we just discussed is the release of oxytocin in response to stretching of the cervix afferent neurons relay information into the hypothalamus and activate the hypothalamic neurons that release oxytocin and so in this positive feedback mechanism the neurons that become activated stimulate the release of a hormone and then that hormone then activates contraction of the uterus which leads to more stretching of the cervix that is then going to stimulate even stronger stimulation of the neurons in the hypothalamus that release oxytocin but this is a neural stimulus for the release of oxytocin because a neuron releases a neurotransmitter to activate the neurons in the hypothalamus that then oxytocin from the posterior pituitary gland another example is when the sympathetic nervous system becomes activated in response to stress preganglionic sympathetic fibers innervate the adrenal medulla and stimulate cells of the adrenal medulla to release the catecholamine hormones epinephrine and norepinephrine so the release of those hormones epinephrine and norepinephrine is stimulated by a neural stimulus the preganglionic sympathetic fibers release the neurotransmitter acetylcholine to stimulate the release of the hormones epinephrine and norepinephrine and so now the last example a hormonal stimulus is when a hormone binds two receptors on a target cell and that target cell is within an endocrine organ and can release a hormone in response to that hormone so we'll see lots of examples of hormonal stimuli several are shown in the illustration here where the hypothalamus secretes releasing hormones and these releasing hormones then bind to receptors on target cells in the anterior pituitary gland and stimulates the anterior pituitary gland to release other hormones so one example of this is a long-term stress response where the hypothalamus will secrete the hormone CRH CRH stands for corticotropin releasing hormone and the corticotropin releasing hormone binds to receptors on cells in the anterior pituitary and stimulates the anterior pituitary to secrete another hormone acth adrenocorticotropic hormone and so the release of acth is under a hormonal stimulus although the release of CRH is considered a neural stimulus where the the brain is sending commands into the hypothalamus to activate the release of CRH CRH is a hormone that then stimulates the release of acth so acth release is controlled by a hormonal stimulus acth itself will also stimulate the production of another hormone known as cortisol so there will be lots of examples as we move through endocrinology where we'll see one hormone stimulates the release of another hormone and so several more examples are shown right here in the illustration gonadotropin releasing hormone is a releasing hormone from the hypothalamus that stimulates the anterior pituitary to produce two hormones luteinizing hormone LH and follicle stimulating hormone FSH then TRH is a releasing hormone from the hypothalamus that stimulates the anterior pituitary to secrete TSH TRH is the thyrotropin releasing hormone and TSH is the thyroid stimulating hormone the next example here we see the prolactin releasing hormone stimulates the anterior pituitary to secrete prolactin and growth hormone releasing hormone GHRH stimulates the anterior pituitary to release growth hormone