The physiological activity of a hormone depends largely on its concentration within the circulatory system. The effects of too high or too low a concentration of hormones can be damaging—this level must be tightly controlled. The endocrine system relies on feedback systems to regulate hormone production and secretion.
Most endocrine glands are under negative feedback control that acts to maintain homeostasis, i.e., prevent deviation from an ideal value. A key example of a negative feedback system is the regulation of the thyroid hormone thyroxine, which regulates numerous key metabolic processes. Briefly, neurons in the hypothalamus secrete a thyroid-releasing hormone that stimulates cells in the anterior pituitary gland to secrete thyroid-stimulating hormone. The thyroid-stimulating hormone then stimulates the release of thyroxine from the thyroid gland. When the blood concentration of thyroxine rises above the ideal value as detected by sensory neurons, the hypothalamus is signaled to stop thyroid-releasing hormone production, which eventually lowers the levels of thyroxine in the blood. When these drop below the ideal value the hypothalamus is signaled to begin secreting thyroid-releasing hormone again.
Positive feedback systems are much less common, although they do exist. A key example occurs during childbirth. The hormone oxytocin is produced by the posterior pituitary that stimulates and enhances contractions during labor. During birth, as the baby moves through the birth canal, pressure receptors within the cervix signal the hypothalamus to stimulate the pituitary to secrete oxytocin. Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract, which in turn increases the activation of the pressure receptors and stimulates the further release of oxytocin. The strength of muscle contractions intensifies until the baby is born, and the stimulation of the pressure receptors is removed, which stops the release of oxytocin.
Whilst the endocrine glands are controlled by negative and positive feedback; there are three mechanisms by which endocrine glands are stimulated to synthesize and release hormones: humoral stimuli, hormonal stimuli, and neural stimuli.
Humoral stimuli refers to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. For example, a rise in blood glucose levels triggers the pancreatic release of insulin. Insulin causes blood glucose levels to drop, which signals the pancreas to stop producing insulin in a negative feedback loop.
Hormonal stimuli refers to the release of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by hormones released by other endocrine glands.
In some cases, the nervous system directly stimulates endocrine glands to release hormones, which is referred to as neural stimuli. Recall that in a short-term stress response, the hormones epinephrine and norepinephrine are important for providing the bursts of energy required for the body to respond. Here, neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.
Practice Questions
MCAT Official Prep (AAMC)
Biology Question Pack, Vol. 1 Passage 15 Question 95
Practice Exam 1 B/B Section Passage 9 Question 49
Practice Exam 4 B/B Section Passage 7 Question 36
Key Points
• The endocrine system relies on feedback mechanisms to control the hormone levels in the circulatory system.
• Negative feedback systems prevent deviation from an ideal mean to maintain homeostasis.
• Positive feedback systems facilitate deviation from the mean.
• Humoral stimuli refers to the control of hormone release in response to changes in extracellular fluids such as blood or the ion concentration in the blood.
• Hormonal stimuli refers to the release of a hormone in response to another hormone.
• Neural stimuli is when the nervous system directly stimulates endocrine glands to release hormones.
Key Terms
Hypothalamus: A region of the forebrain located below the thalamus, forming the basal portion of the diencephalon, and functioning to regulate body temperature, some metabolic processes, and governing the autonomic nervous system
Hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism
Oxytocin: A hormone that stimulates contractions during labor
Negative feedback: A system that prevents deviation from a mean value
Positive feedback: A system that promotes deviation from a mean value
Endocrine: glands which release secretion directly into the blood
Pituitary gland: the major endocrine gland is important in controlling growth and development