Humoral, Hormonal, and Neural Stimuli
The release of hormones can be triggered by changes in the blood (“humor”), by the actions of other hormones, or by neurological stimuli.
Differentiate among humeral, hormonal, and neural stimuli
- When a hormone is released in response to a change in the blood or other body fluids, such as a change in the level of a mineral or a change in temperature, this is called a humoral stimulus.
- Hormones can be released in response to the action of a second hormone; the hypothalamus often secretes hormones that trigger the release or cessation of other hormones from the pituitary gland.
- Hormone release can also be triggered directly by a stimulus from a neuron; this is often used when a rapid response is needed.
- hypothalamus: a region of the forebrain located below the thalamus that regulates body temperature, some metabolic processes and governs the autonomic nervous system
- pituitary gland: an endocrine gland, about the size of a pea, whose secretions control the other endocrine glands and influence growth, metabolism, and maturation
- humoral: of or relating to the body fluids or humours
The term “humoral” is derived from the term “humour,” which refers to bodily fluids, such as blood. A humoral stimulus refers to the control of hormone release in response to changes in extracellular fluids, such as 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. This is an example of 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. For example, the hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland. The anterior pituitary, in turn, releases hormones that regulate hormone production by other endocrine glands. The anterior pituitary releases the thyroid-stimulating hormone (TSH), which then stimulates the thyroid gland to produce the hormones T3 and T4. As blood concentrations of T3 and T4 rise, they inhibit both the pituitary and the hypothalamus in a negative feedback loop.
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.