Hormone Release System

Endocrine glands release hormones in response to three main types of stimulus.1. A change in the concentration of a substance in the blood. Some gland cells detect a variable directly. For example, β cells in the islets of Langerhans detect a rise in blood glucose and secrete insulin.2. Nerve impulses. Some glands are stimulated by the nervous system. For example, during stress, sympathetic neurones stimulate the adrenal medulla to release adrenaline. The hypothalamus also sends nerve impulses to the posterior pituitary to release ADH and oxytocin.3. Another hormone. Many hormones control the release of other hormones in a chain, controlled by the hypothalamus and pituitary.The hypothalamus-pituitary chainThe hypothalamus secretes a releasing hormone into blood vessels leading to the anterior pituitary. The anterior pituitary then secretes a stimulating (tropic) hormone, which travels in the blood to a target gland. The target gland secretes its own hormone.For example: hypothalamus → TRH → anterior pituitary → TSH → thyroid gland → thyroxine.This chain is controlled by negative feedback. When the thyroxine concentration in the blood rises, it inhibits the release of TRH by the hypothalamus and of TSH by the anterior pituitary, so less thyroxine is produced. When thyroxine falls, the inhibition is removed and more TRH and TSH are released.A chain like this lets a small signal from the hypothalamus be amplified at each step, and it allows fine control of hormone concentrations.