Điều này có thể được thực hiện trong một số trường hợp bằng cách điều chỉnh điểm thiết lập và các trường hợp khác bởi một tín hiệu rằng sẽ ghi đè điểm thiết lập. Ví dụ, epinephrine được tiết ra từ tủy thượng thận trong phản ứng khẩn cấp một số ức chế sự bài tiết insulin | Regulation of Hormone Secretion 43 hormone secretion rate blood glucose concentration Figure 26 Negative feedback regulation of blood glucose concentration by insulin and glucagon. Inhibition stimulation. upward or downward deviations from some predetermined set point but changing environmental demands often require temporary deviation from constancy. This can be accomplished in some cases by adjusting the set point and in other cases by a signal that overrides the set point. For example epinephrine secreted by the adrenal medulla in response to some emergency inhibits insulin secretion and increases glucagon secretion even though the concentration of glucose in the blood may already be high. Whether the set point is changed or overridden deviation from constancy is achieved by the intervention of some additional signal from outside the negative feedback system. In most cases that additional signal originates with the nervous system. Hormones also initiate or regulate processes that are not limited to steady or constant all of these processes are self-limiting and their control resembles negative feedback but of the open-loop type. For example oxytocin is a hormone that is secreted by hypothalamic nerve cells the axons of which terminate in the posterior pituitary gland. Its secretion is necessary for the extrusion of milk from the lumen of the mammary alveolus into secretory ducts so that the infant suckling at the nipple can receive milk. In this case sensory nerve endings in the nipple detect the signal and convey afferent information to the central nervous system which in turn signals release of oxytocin from axon terminals in the pituitary gland. Oxytocin causes contraction of myoepithelial cells resulting in delivery of milk to the the infant is satisfied the suckling stimulus at the nipple ceases. Positive Feedback Positive feedback means that some consequence of hormonal secretion acts on the secretory cells to provide .
