21.7: Regulation of Hormone Secretion
Regulation of hormone secretion is a finely tuned orchestration driven by various types of stimuli, encompassing neural, humoral, and hormonal signals. Environmental cues instigate neural stimuli, where action potentials traverse nerve fibers to reach their designated targets. An illustrative scenario is the body's response to stress, wherein the sympathetic nervous system releases epinephrine from the adrenal glands, inducing the well-known 'fight or flight' reaction.
Humoral stimuli, conversely, involve concentration fluctuations of specific ions or nutrients in the bloodstream. A case in point is the response to diminished blood calcium levels, triggering the secretion of parathyroid hormones to bolster circulating calcium concentrations.
Hormonal stimuli manifest when one hormone induces the secretion of another hormone from a distinct endocrine organ. For instance, the hypothalamus secretes the thyrotropin-releasing hormone, instigating the anterior pituitary lobe to release the thyroid-stimulating hormone (TSH). TSH, in turn, stimulates the thyroid glands to discharge thyroid hormones like T3. As circulating T3 levels rise, a sophisticated negative feedback loop comes into play. This loop inhibits the continuous production of T3 by dispatching inhibitory signals to both the pituitary gland and hypothalamus. The endocrine system maintains a delicate balance in this intricate interplay of signals and responses, ensuring precise regulation and harmony in hormone secretion.
