Positive feedback loops are distinct in that the end product amplifies the initial stimulus, creating a snowball effect that intensifies the process. A classic example is the role of oxytocin during childbirth. As uterine contractions begin, nerve signals stimulate the hypothalamus to prompt the posterior pituitary to release oxytocin. This hormone, in turn, strengthens uterine contractions, which further stimulates the release of more oxytocin, creating a cycle of increasing intensity. This loop continues until the baby is delivered, at which point the cycle is naturally interrupted. A common feature of positive feedback loops is that they often lead to a final, definitive outcome, such as childbirth in the case of oxytocin.
14 Hypothalamus and Pituitary
Starting in the brain, the hypothalamus and pituitary gland form a crucial system which produces an array of hormones to maintain homeostasis. The hypothalamus acts a manager which gives orders to the pituitary to execute.
Let’s start with the physical structure of this area. 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 (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). Therefore, pituitary tumors can often result in visual disturbances, particularly affecting peripheral vision. View the pituitary’s location within the brain:
This organ itself can be divided into two lobes – anterior and posterior – which are made of deriving 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.
The tandem of the hypothalamus and the pituitary gland account for several significant hormones.
Hypothalamus and anterior pituitary:
The hypothalamus acts on the anterior pituitary through signals sent through the blood via the hypophyseal portal system(more on this later). The hypothalamus interprets concentrations of various hormones in the bloodstream and then acts on the pituitary to modulate these levels to equilibrium. Below is a list of hormones released by the hypothalamus and how they act on the anterior pituitary:
| 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) | Stimulates adrenal cortex to produce cortisol to regulate stress, metabolism and control blood sugar levels |
| Gonadotropin-releasing hormone (GnRH) | Luteinizing hormone(LH)
Follicle-stimulating Hormone (FSH) |
LH triggers ovulation and estrogen production in females, and production of testosterone in males
FSH stimulates growth and maturation of ovarian follicles in females and production of sperm in males |
| Growth hormone-releasing hormone (GNRH) | Growth Hormone | Promotes growth, cell repair and metabolism through stimulation growth of bones and muscles |
| Prolactin-Inhibiting Hormone(dopamine) | Prolactin (is produced when its hypothalamic regulator is in low concentrations) | Stimulates milk production (lactation) in mammary glands after childbirth. |
Hypothalamus and posterior pituitary:
The posterior pituitary which receives direct nervous signals releases two hormones: vasopressin (antidiuretic hormone) and oxytocin.
- Vasopressin (antidiuretic hormone), a peptide hormone, acts on the kidneys to conserve water, and balance fluids and electrolytes.
- Oxytocin, a peptide hormone, stimulates uterine contractions during childbirth(milk-ejection reflex), and has several bonding/emotional regulating properties.
Feedback loops:
Each hormone follows a feedback loop: negative or positive.
Negative feedback loops are regulatory mechanisms that maintain homeostasis by responding to the concentration of a product and inhibiting the original stimulus. A good example is the role of corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH, in turn, triggers the adrenal glands to produce cortisol. When cortisol levels rise to a sufficient concentration, it binds to receptors in the hypothalamus, reducing CRH production. This decrease in CRH lowers ACTH levels, leading to a reduction in cortisol. In this way, the end product, cortisol, inhibits its own production by suppressing the initial signaling pathway, ensuring that hormone levels remain balanced and preventing overproduction.
Hypophyseal portal system:
The hypophyseal portal system (1 of 3 portal systems) is the method of communication between the hypothalamus and the anterior pituitary.
The hypophyseal portal system (1 of 3 portal systems) is a specialized network of blood vessels that directly connects the hypothalamus to the anterior pituitary gland. The reason this system is a portal system is because it embodies 2 connected capillary beds, resulting in venous blood with a very low blood pressure. The slow returning blood pressure is a result of being two capillary beds away from the heart. The hypothalamus leverages this system to send chemical messengers to the anterior pituitary to procure tropic hormones which act on other endocrine glands. Here’s how it works:
- Hypothalamic Capillary Plexus: Located in the hypothalamus, this network of capillaries picks up releasing and inhibiting hormones (e.g., TRH, CRH, GnRH) secreted by hypothalamic neurons.
- Portal Veins: These veins carry the hypothalamic hormones from the primary capillary plexus down the pituitary stalk to the anterior pituitary.
- Pituitary Capillary Plexus: In the anterior pituitary, the portal veins form another capillary bed where the hypothalamic hormones are released. These hormones act on the pituitary cells, stimulating or inhibiting the secretion of anterior pituitary hormones like TSH, ACTH, LH, and FSH.
This direct vascular link allows the hypothalamus to regulate the anterior pituitary in a highly efficient and targeted way, without the hormones needing to pass through the general circulation first. It’s critical for maintaining hormone balance and coordinating endocrine responses.
