Triggers for Hormone Release :

When and why are hormones released ?

Control of Hormone Secretions

There are two aspects to how the body controls secretion of hormones.

  • Immediate Stimuli lead to the release of hormones - see list of 3 possibilities (left), and
  • Hormone Regulation happen by feedback mechanisms

Hormones are produced ('synthesized') and released ('secreted') into the bloodstream by endocrine glands and certain other tissues in the body.

Depending on the specific hormone and the physiological conditions, the immediate stimulus for the release of hormones is one of : Endocrine glands release hormones in response to one (or more) of the following stimuli:

  1. Specific molecules in the blood, e.g. certain minerals or nutrients (as regulated by feedback mechanisms that maintain homeostasis).
  2. Stimulation by other (specific) hormones
    (leading to the rhythmic release of hormones, i.e. hormone levels rising and falling in a predictable pattern,
    also regulated by feedback mechanisms)
  3. Stimulation by signals from the nervous system
    (leading to hormone release in short bursts or spurts as required - as opposed to in a steady rhythm,
    also regulated by feedback mechanisms)

The control of hormone secretions in the form of their immediate secretion being stimulated (or 'triggered') by these 3 mechanisms is described in more detail below.

1.

Specific molecules in the blood

The presence or level (concentration) of specific molecules in the blood, e.g. certain minerals or nutrients - sometimes referred to by more technical terms that have specific meanings in biochemistry such as electrolyte and metabolite.

Examples:

  • Low levels of calcium (Ca2+) ions in the blood stimulate the parathyroid glands to release parathyroid hormone (also known as parathormone and parathyrin).
  • Increasing levels of blood glucose contribute to stimulation of the secretion of the hormone insulin from the pancreas.
    See also Section 3., below. See diabetes for more about the cycle of regulation of blood sugar levels.

2.

Stimulation by other (specific) hormones (Hormonal Regulation of Hormone Release)

A hormone that stimulates hormone secretion (i.e. that causes endocrine cells to release hormones) is called a tropic hormone ( sometimes written trophic hormone), or a tropin (pl. tropins).

Stimulation of hormone secretion isn't necessarily the only function of tropic hormones - they may also stimulate the development of more endocrine cells by cell division - i.e. the proliferation of endocrine cells.

Examples:

3.

Stimulation by signals from the nervous system (Neural Regulation of Hormone Release)

Neural regulation of hormone release applies when a neuronal input, i.e. a signal from a nerve cell, to an endocrine cell increases or decreases the rate of release of hormones from an endocrine cell.

Examples:

  • Sympathetic preganglionic neurons (part of the sympathetic nervous system) stimulate the adrenal glands to release the hormone adrenaline.
  • Neurosecretory cells in the hypothalamus have the usual features of nerve cells (e.g. dendrites, axons, terminals etc.) and act as endocrine cells by secreting hormones into the bloodstream via capillaries adjacent to the neurosecretory cells. Release of hormones from the neurosecretory cells of the hypothalamus is regulated by the nerve cells that form synapses with the dendrites of the individual neurosecretory cells. In this way, the nervous system controls the release of hormones from the hypothalamus
  • Secretion of insulin from the beta cells of the pancreas is regulated by both the sympathetic and the parasympathetic divisions of the autonomic (i.e. involuntary) nervous system.
    • Parasympathetic input to the pancreatic beta cells stimulates insulin secretion.
    • Sympathetic input to the pancreatic beta cells inhibits insulin secretion.

Hormone Regulation

The above 3 immediate stimuli for release of hormones into the body (via the blood system) occur within the body's overall homeostasis mechanisms, i.e. processes to regulate its internal environment in order to maintain the stable conditions necessary for the body to operate.

These three immediate stimuli for hormone release generally occur as part of hormone regulation via feedback mechanisms.


Recap Question:

Which stimuli might lead to the release of hormones into the human body ?

Endocrine glands (and some other tissues in the human body) release hormones in response to one or more of:

  • Specific molecules in the blood - i.e. the chemical composition of blood - not just hormones in the blood
  • Stimulation by other (specific) hormones
  • Signals from the nervous system

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