 This video will cover the following objective from the endocrine system. Define endocrine gland hormones and target cell or target organ. Describe the factors affecting target cell activation. 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. 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. That hormone then has an effect on target cells throughout the body, such as the skeletal muscle fibers. 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. Another example of an endocrine gland 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. The thyroid gland is the target organ being stimulated by thyroid stimulating hormone, a hormone secreted from the pituitary gland. 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. In this example, where we call the message inside of the cell the second messenger, a second 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. 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. Let's look at a couple of examples of hormones and how they influence target organs and target cells. 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. 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. 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. Oxytocin and antidiuretic hormone have different target cells and will stimulate different functions in those target cells. 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. 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 secrete oxytocin. 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 then travels through the blood to the kidney where it will stimulate water reabsorption. 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.