The Endocrine System, Control of Hormone release

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Hormones can be classified by their reflex pathways.


Endocrine Cell Is the sensor in the Simplest Endocrine Response
The endocrine gland acts as both the sensor (receptor) and the integrating sensor. PTH is an example, so is insulin and glucagon.

Parathyroid endocrine cells monitor blood calcium levels, from its four clusters behind the thyroid. Calcium concentration is monitored with the aid of G-Protein-coupled calcium receptors on their cell membrane. When certain number of Receptors are bound to Calcium, PTH is inhibited. If the plasma calcium falls below a certain level and fewer receptors are bound to Calcium, inhibition ceases and the parathyroid cells secrete PTH. PTH released in the blood, effect on increased bone resorption, increased kidney reabsorption of calcium, and production of calcitrol leads to increased intestinal absorption of calcium.
Together this increases the plasma calcium concentration.

Many endocrine Reflexes involve the Nervous System
Two endocrine glands are incorporated into the nervous system - the pineal gland and the pituitary gland.

Neurohormones are Secreted into the blood by Neurons
Catcholamines are made by modified neurons in the adrenal medulla
Hypothalamic neurohormones secreted from the posterior pituitary
Hypothalamic neurohormones that control hormone release from the anterior pituitary.



The Pituitary Gland Is actually Two Fused Glands



The Posterior Pituitary Stores and Releases Two Hormones
Oxytocin and Vasopressin

Follow the standard pattern of peptide synthesis,  storage, and release. The secretory vesicles are transported down long extensions of neurons into the posterior pituitary, where they are stored.  When the stimulus reaches the hypothalamus,the electrical signal passes from neuron to cell body to distal end of the posterior pituitary, and the vesicle contents are released into circulation.

Vasopressin (Antidiuretic hormone) regulates water balance in the body.
Oxytocin released from the posterior pituitary controls milk excretion and contracts the uterus during labour and delivery. Plays an important role in social, sexual and maternal behaviors. Autism, a developmental disorder in which patients are unable to form normal social relationships, may be related to defects in the normal oxytocin-modulated pathways of the brain.


Microtubules are the intracellular mechanism used in the transport of secretory vesicles within the cytoplasm.

The Anterior Pituitary Secretes Six Hormones





A hormone that controls the secretion of another hormone is a trophic hormone. (suffix - trophin)
Feedback Loops are different in the Hypothalamic-Pituitary Pathway

  1. Long loop negative feedback
  2. Short loop negative feedback

The Hypothalamic-Hypophyseal Portal system Directs Trophic Hormone Delivery
The hypothalamic trophic hormones that regulate the secretion of anterior pituitary hormones are transported directly to the pituitary through a special set of blood vessels known as the Hypothalamic-hypophyseal portal system.
The hormones secreted into the portal system require less concentration of hormone, than general circulation to elicit the desired response.











Anterior Pituitary Hormones Control Growth, Metabolism and Reproduction 

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