A woman going into labor demonstrates a positive feedback loop. As the baby's head pushes against the cervix, receptors in the cervix send nerve impulses through neurons (nervous cells) to the brain. These neurons send a signal to the pituitary gland (in brain) to release a hormone oxytocin. This hormone acts on the effector (muscle in the uterus) to stimulate/enhance muscle contractions. The stronger the muscle contraction, the more the head of fetus pushes on the cervix continuing the loop.