First, the hypothalamus directs neural and endocrine functions. Its location in the brain connects it both anatomically and functionally to the pituitary gland (PG), also called the hypophysis. While the PG has two lobes, the anterior lobe (AL), also known as the adenohypophysis, is constructed of glandular tissue from the primitive digestive tract.
The hypothalamus and the pituitary gland function together as the control unit (think of it as a command center) for the endocrine system. While both secrete regulatory hormones, the hypothalamus contains neurosecretory neurons that directly and solely control the AL of the PG’s hormone secretion. These influential hypothalamic hormones are generally referred to as inhibiting and releasing hormones. Both types transmit to the AL via hypothalamic-hypophyseal portal veins. Then, they bind to the AL receptor cells called thyrotrophs, whereupon they signal the AL to either release or inhibit secretion of thyroid-stimulating hormone (TSH).
This process is often referred to as a negative feedback loop, where there is a set optimal hormone level. Consequently, if the hormone level is too high or too low, the body senses this and modifies secretions until circulating blood levels return to optimal levels, which is somewhat like how a thermostat in your home works to control the temperature via an air conditioning unit.
Conversely, a positive feedback loop functions with a stimulus provoking a response, which then loops back with a stronger stimulus and stronger response, thus continuing in a cyclical manner. An example of this is that during labor/childbirth, the hormone oxytocin secretes, which causes the fetus's head to move down and push against the cervix. This action in turn stimulates the release of more oxytocin. The additional oxytocin prompts the uterus to contract, then causing further stimulation, again leading to additional oxytocin secretion.
The following provides a list of the hormones and a brief description of their respective functions, as controlled by the AL:
Growth hormone (GH)
- Promotes growth of body tissues
- Promotes milk production from mammary glands
Thyroid-stimulating hormone (TSH)
- Stimulates thyroid hormone release from thyroid
Adrenocorticotropic hormone (ACTH)
- Stimulates hormone release by adrenal cortex
Follicle-stimulating hormone (FSH)
- Stimulates gamete production in gonads
Luteinizing hormone (LH)
- Stimulates androgen production by gonads
Below are a few more examples of stimuli and the effects produced:
- Humoral stimuli: Changing levels of ions/nutrients in blood
Example: parathyroid secretes parathyroid hormone when blood calcium levels drop
- Hormonal stimuli: Other endocrine glands
Example: hypothalamus stimulates pituitary, pituitary stimulates gonads.
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