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Imagine a thermostat for a moment. To warm the room, you bump up the temperature settings. The thermostat senses the temperature in the room, and the heat rumbles to a start. When the room gets nice and toasty (the way you like it), the heat turns off. Since the room is warm enough, there's no sense in wasting electricity to warm it.
Our bodies have internal thermostats to regulate different biological processes like neurotransmission and hormone secretion. If there's too much of a particular hormone, for example, the hormone can circulate in the body to shut off whatever pathway made it in the first place. These are called negative feedback loops.
The immune, endocrine, and nervous systems all have internal negative feedback loops to regulate how much, and what type of, activity the body actually requires. The best part? Our bodies take care of it, and we can sit around twiddling our thumbs knowing that our internal processes are in good hands.
We've already encountered one type of negative feedback loop within the immune system: suppressor T cells. These special cells get activated when the cell-mediated system gets over-activated. Perhaps there's just too much inflammation or the body is attacking something it shouldn't be (like an organ transplant). Since excessive inflammation causes unnecessary tissue damage and disease, these suppressor T cells control the activity of the immune system by releasing anti-inflammatory cytokines. These special chemicals act on other leukocytes to limit proliferation and any more inflammation, but they can also decrease the antigen fragment's visibility to T cells. Too much immune activity limits itself using a negative feedback loop involving our good friend the suppressor T cells.
Another example is the HPA axis. It's the way the endocrine system responds to stressful situations. When the hypothalamus gets activated in these situations, CRF is released, which stimulates ACTH release, which simulates the cortisol end product. Cortisol is what induces different stress responses on the body. But if the rattlesnake didn't even see you, cortisol doesn't need to be in overdrive anymore. Cortisol shuts off the HPA axis if there's too much hanging around with nothing to do. If that's the case, this negative feedback loop consists of cortisol suppressing both CRF from the hypothalamus and ACTH from pituitary gland. If that wasn't enough to shut everything down, ACTH also can block CRF release.
HPA Axis. The hypothalamus-pituitary-adrenal cortex pathway is responsible for evoking the body's stress response, but the negative feedback loops make sure everything stays in check.
A lot of the negative feedback mechanisms in the nervous system revolve around homeostasis, like body temperature. If your cat was accidentally locked out in the cold night (or maybe not so accidentally if she decided to puke in your new silver sparkly TOMs), her body temperature will plummet. Her nice fur coat just isn't doing the job, so the brain swoops in to save the day.
Within the hypothalamus are receptors that respond to extreme temperatures, and sense when the blood is too hot or too cold. If a kitty's temperature receptors sense that he's too cold, they tell the autonomic system to constrict surface blood vessels so more blood pumps to the cat's core and generate heat by activating motor movement for shivering. Once we bring the animal back inside the house, he no longer needs to be producing all that extra heat. His body temperature will rise based on his surroundings, and will actually shut off the autonomic response when those same receptors in the hypothalamus sense that things are back to normal.
We'd like to think that it's pretty common knowledge that our human minds are much different from that of a jellyfish (who has no brain) or a chameleon (whose brain has the power to make skin change colors). Humans instead have complex thought processes, memories, and language, which are a lot more practical. It's no fault of the chameleon that he can't do these things—his brain just isn't wired for it. But we'd like to see you try to turn orange without a serious spray-tan.
We can blame evolution for our inability to change the color of our skin to fit into our surroundings. Truth be told, we don't need that mode of defense. Chameleons do.
We all start out with the most primitive reptilian brain that takes care of just the basics—all they care about is stayin' alive. Between the brain stem and a cerebellum (which means "little brain"), the reptilian brain controls heart rate and balance. It directs digestion and reproduction. Because primitive animals were usually out exposed to the elements, hot and cold, their brain also manages and regulates body temperature. It does what the animal needs for survival, but not much more: it's reliable, but functionally rigid. Think of the reptilian brain like an old pickup truck with 200,000 miles on it. It may not have heated seats or Bluetooth, but it can get you from Point A to Point B.
The first mammals to come along the evolutionary tree added the limbic brain. This more advanced brain add-on includes a hippocampus, an amygdala, and a hypothalamus on top of the reptilian brain.
These brain areas allow mammals to do slightly more complex mental exercises: make memories, find experiences agreeable or disagreeable, and have emotion. These brain functions are critical to mammals' survival since they have to look for food and remember how to get back to their family. Their survival is also based on emotions; they won't seek out situations where they once were scared or frightened but seek those that made them feel all warm and cozy. Every now and then there's an oddball that prefers A Nightmare on Elm Street to My Little Pony, but we doubt he'll last too long in the wild.
The limbic brain also gives animals the ability to feel emotion towards each other, and is responsible for feelings of attachment and the need to bond. Emotions are very helpful in survival, because it provides a key motivation to finding companionship and reproducing. In essence, this brain allows mammals the ability to make those unconscious value judgments that have a huge impact on behavior.
The primate brain is the last stop in our tour of brain evolution. Here, a cerebral cortex wraps around the limbic brain, and makes up the majority of what we recognize as a human brain. The cortex gives us our imagination, the ability to use language and writing to communicate, and conscious control over emotion, activities, and thought. It allows us, and other mammals, to plan ahead for the future (whether that means leaving a few kernels of kibble left in the bowl or investing in an IRA). The primate brain is loads more flexible than the reptilian brain and is what makes us independently thinking creatures. We don't just constantly respond to outside stimuli.
The evolution of our brains is impressive, from being tiny blobs that made sure we remember to breathe to more massive organs that allow us to love, remember, and speak. What will the next brain be able to do? Time travel, probably.