17 Hypothalamus and Pituitary
Starting in the brain, the hypothalamus and pituitary gland form a crucial system which produces an array of hormones. The hypothalamus acts as a manager and informs the pituitary to execute orders to remain in homeostasis. You may think of the hypothalamus as a demanding parent who is always on the case of the pituitary to finish the house chores. The hypothalamus achieves this through releasing specific hormones that either stimulate or inhibit the pituitary gland, prompting it to release its hormones to regulated key processes throughout the body.
The hypothalamus, located at the base of the brain, weighs about 4g and accounts for <1% of the brain’s total mass. The hypothalamus is directly attached to the pituitary gland via the infundibulum.
The pituitary gland (or known as the hypophysis) is a pea sized organ (0.8-1.0cm) in diameter and a weight of ~500mg. The pituitary resides within the hypophyseal fossa (commonly referred to as the Turkish Saddle): a protective indentation of the sphenoid bone of the skull. The pituitary itself is located on the midline of the head just superior to the nasal cavity and directly inferior to the optic chiasm (where the two optic tracts cross paths). Because of this location, pituitary tumors can often result in visual disturbances, particularly affecting peripheral vision.
The tandem of the hypothalamus and pituitary gland account for several significant hormones which are discussed below.
Figure 27 Sagittal cross section of the brain highlighting the optic chiasm and pituitary
Figure 28 Sagittal Cross section of the brain highlighting the pituitary and hypothalamus
This organ itself can be divided into two lobes – anterior and posterior – which are derived from different embryonic tissue. The anterior pituitary (known as the adenohypophysis) originates from the oral ectoderm and the posterior pituitary (known as the neurohypophysis) originates from the neuroectoderm directly from the diencephalon.
Hypothalamus and Anterior Pituitary:
The hypothalamus interprets concentrations of various hormones in the bloodstream and then acts on the pituitary to modulate these levels to equilibrium. The hypothalamus like an demanding manager receives information from the environment and informs the pituitary to secrete a hormone to alter the original environmental stimulus.
Figure 29 Anterior pituitary and hypothalamic hormonal products
Below is a list of hormones released by the hypothalamus, their effects on the anterior pituitary and their roles in regulating various organs throughout the body:
Table 18 Anterior Pituitary and hypothalamic hormonal pathways
| Hypothalamus secretion |
Effect on the Anterior Pituitary |
Function |
Thyrotropin-releasing hormone (TRH)
|
Thyroid stimulating hormone (TSH)
|
Stimulates thyroid to produce and release thyroid hormones which regulates metabolism, energy level and growth
|
Corticotropin releasing hormone (CRH)
|
Adrenocorticotropic hormone (ACTH)
|
S