 In this video, I will define homeostasis and the components of the homeostatic control mechanisms. I will identify examples of each component and compare and contrast negative feedback and positive feedback mechanisms, providing examples of each. Then we will distinguish between intrinsic and extrinsic control mechanisms. Homeostasis is a steady state of body systems that living organisms maintain. So there is a set point for homeostatic variables, which are the factors being regulated by homeostatic control mechanisms. An example could be body temperature. For example, 37 degrees Celsius is the set point for human body temperature. 37 degrees Celsius is about 99 degrees Fahrenheit. Now, if there is a change in that variable, that is a stimulus. For example, if the body temperature exceeds the set point of 37 degrees Celsius, maybe exercise could lead to an increase in body temperature, and that would be a stimulus for the homeostatic control mechanism. A sensor or receptor detects a change in the variable. So the sensor or receptor is monitoring the variable, in this case the temperature of the body. For this example, the sensor would be the thermoreceptors, which are located in the skin, as well as in a part of the brain known as the hypothalamus. So thermoreceptors in the skin and hypothalamus are monitoring body temperature and would detect that the temperature is increasing. So they would detect the stimulus. The sensor detects the stimulus and then relays information through what we call the input or afferent pathway to the control center. So the input or the afferent pathway is a way for information to be communicated from the receptor to the control center. And the control center is a structure that analyzes information coming from the sensor and then determines the appropriate responses that are needed to maintain homeostasis. In this example, there are afferent fibers, sensory fibers of the peripheral nervous system that send information in to a thermoregulatory control center in the central nervous system within, again, the hypothalamus. And so there's a distinct input or afferent pathway of sensory nerves coming from the skin to the hypothalamus. However, I mentioned earlier there are also thermoreceptors in the hypothalamus. And so there's not always a distinct input, but if the receptors are in a different location than the control center, there will be a distinct input, as is the example where sensory nerves are carrying the information from thermoreceptors in the skin to the control center in the region of the brain within the hypothalamus. The thermoregulatory control center is an even more specific location known as the median pre-optic nucleus of the pre-optic anterior hypothalamic area. Anyway, that specific location, the control center processes information coming from the sensor and then will send commands out through an output or efferent pathway in order to regulate effectors, which are organs or cells or glands that can carry out a response. So the output or efferent pathway could be motor nerves, as is the case for this example. There are sympathetic motor neurons that can stimulate sweat glands throughout the body in the skin. And so these sweat glands are what we call the effector organs or effector glands that carry out the response. In this case, the response is sweating in order to help cool down the body. So the response is the action of the effector and because the action of the effector, the response in this homeostatic mechanism will counteract the original stimulus bringing the variable back to its normal set point, this is what we would call a negative feedback loop. So a negative feedback loop is where the response of the effector is going to counteract the initial stimulus returning us to the homeostatic set point. And so here we can see a little bit more detail of the mechanism of thermoregulation, the compensatory response that we would see to low body temperature instead of high body temperature. If the sensors in the skin and in the brain detect that the body temperature is below 37 degrees Celsius, the information will be relayed into the thermoregulatory control center in the hypothalamus that processes that sensory information from the input and sends information through the output, the efferent pathway, to the effector organs. We can see that there are superficial arteries that are going to constrict, that means there will have less blood flowing through the skin and that will reduce heat loss to the air. Then the blood vessels are one effector organ, another effector organ would be the skeletal muscles as the efferent pathway, the output from the control center can travel through motor nerves down to stimulate contraction of skeletal muscles to produce shivering, that shivering will generate heat and help warm the body. Another example would be the thyroid gland producing more hormones, and so here we have an output that is not a motor nerve, but instead the output will be a cascade of hormones. A hormone released from the hypothalamus that stimulates the pituitary gland, then the pituitary gland produces a hormone that stimulates the thyroid gland, and then the thyroid gland produces a hormone that stimulates an increase in the metabolic rate leading to increased heat production throughout the body. But together the compensatory responses of these effector organs all work to restore the homeostatic set point for the body temperature, counteracting the initial stimulus as part of a homeostatic negative feedback loop. And here we see a little more detail of the thermoregulation mechanism in response to high body temperature, when the sensors in the skin and brain detect that the temperature is above 37 degrees Celsius, that input information will be processed by the thermoregulatory control center in the hypothalamus. And then the output will be sent through motor nerves to the blood vessels to cause dilation so that there will be more blood flow through the skin helping to increase the amount of heat that is lost to the air. There will also be commands sent through motor nerves to stimulate sweat glands to help produce sweat that the watery solution that's secreted by sweat glands onto the surface of the skin will then evaporate and as it evaporates it will help to take heat from the body helping to cool the body down. And then last week we can see the thyroid gland will be secreting less of the thyroid hormones leading to a decrease in metabolic rate of cells throughout the body also helping to produce the compensatory response, lowering the body temperature and as part of a negative feedback loop maintaining the homeostatic set point for body temperature. In contrast to a negative feedback loop, a positive feedback loop is a type of homeostatic control mechanism where the response of the effector is amplified or the original stimulus is intensified so that the variable will move further and further from the initial set value. So a positive feedback mechanism will be destabilizing and it will build up to some climactic event. The example we see here is a positive feedback mechanism controlling childbirth. It starts with the sensor stretch receptors in the cervix of the uterus. So the cervix is the neck of the uterus that connects to the vagina and when the fetus is full grown the head will start to stretch the cervix. And this is detected by nervous tissue that can then send the input through an afferent pathway of sensory nerves. This information will be relayed into a control center in the brain. The control center is again found in the hypothalamus. The hypothalamus will then respond to this input and send an output, in this case the output will be a hormone known as oxytocin. And so then oxytocin will stimulate the uterus to contract. So the uterus is the effector organ that's responding to the output hormone oxytocin. Oxytocin stimulates the uterus to contract and as the uterus contracts the fetus is pushed further down. The fetus is pushed down into the cervix causing more stretching of the cervix. This will then increase the stimulus. It amplifies the stimulus so that the input pathway will relay a stronger signal that the cervix is being stretched. And then the control center in the hypothalamus will release a stronger output, more oxytocin will be released. To stimulate even more contraction of the uterus. And this positive feedback loop will build and build creating more and more stretching of the cervix until eventually the climactic ending of the positive feedback loop occurs when the infant is born. So the example of a positive feedback loop that we just described involved a control center in the brain regulating an effector organ that's a different organ, the uterus. So that's what we call extrinsic control. An extrinsic control mechanism is where the control center and the effector are two different organs. In the illustration here we have an example where the nervous system is regulating the activity of the heart. And so that is an extrinsic control mechanism. We have a negative feedback loop that helps to maintain our blood pressure. For example, if blood pressure starts to fall too low, information will be relayed from the bororeceptors that were monitoring blood pressure through an afferent pathway or input of sensory nerves that carried the information into the blood pressure. It will be sent to the brain where the control center is located in the brain stem in the medulla oblongata. And the control center will then send the output information out through motor nerves, out through the sympathetic motor nerves leading to an increased heart rate, stimulating increased heart rate to pump more blood. And this will work as a compensatory response to increase the blood pressure, helping to maintain the homeostatic set point for the mean arterial pressure, the blood pressure. We have lots of examples of extrinsic control that we will see as we go through the semester, but we've already seen another example. The thermoregulatory mechanisms we described were extrinsic control, where the control center was inside the brain in the hypothalamus and the effectors were either skeletal muscles or sweat glands or blood vessels or other organs that are distinct from the brain. So in contrast to extrinsic control, an intrinsic control mechanism has a control center and effector in the same organ. So here we have an example of intrinsic control that's regulating blood flow through an organ. If the oxygen concentration of the blood inside an organ is too low and the carbon dioxide concentration of blood in that organ is too high, there are sensors located in the blood vessels that will respond and relay that information to a local control center within the wall of the blood vessel and that will then send an output signal to the effector organs, which are in this case the effector cells, the smooth muscle cells in the wall of the blood vessel. And it will cause those smooth muscle cells to relax, leading to vasodilation. So vasodilation is a decreased contraction of the vascular smooth muscle in the wall of a blood vessel like an arterial, and this will lead to decreased resistance allowing an increase of blood flow through that vessel. And as that blood flows more quickly through the vessel, more oxygen will be carried in as more carbon dioxide is carried away. And so this is a homeostatic mechanism that functions in a negative feedback loop to help maintain a homeostatic set point for blood oxygen concentration and blood carbon dioxide concentration. And because we have a control center and effector within the same organ, within this blood vessel, it's an intrinsic control mechanism. Just to contrast intrinsic control with extrinsic control, we can see that blood vessels are also regulated by the nervous system in an extrinsic control mechanism. And so we can think about how this could work if we're looking at the example of exercise. So we already described how vasodilation would increase blood flow through a tissue that had a low oxygen concentration. If you're running and your muscles are working really hard, they're using a lot of oxygen, and vasodilation will then allow more blood to flow through those muscles to maintain the oxygen availability. It's necessary to keep those muscles working. But at the same time that's happening, there will be vasoconstriction stimulated in other parts of the body. So vasoconstriction is the opposite of vasodilation. Vasoconstriction is when the vascular smooth muscle contracts increasing resistance and decreasing the amount of blood flow. When you're exercising, the sympathetic motor neurons will send commands out to stimulate vasoconstriction in the blood vessels of other regions of your body, not the working skeletal muscles, but other organs such as the inactive organs in the digestive system like your stomach and intestines. When you're exercising, the stomach and intestines aren't working hard and they don't need as much blood flow. So this extrinsic control mechanism will stimulate vasoconstriction in the digestive organs, whereas at the same time an intrinsic control mechanism is ensuring vasodilation occurs to keep blood flowing sufficiently to meet the demands of the tissues that are working hard and have the highest metabolic demand.