The Theme of Regulation in Animal Systems
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.