The hypothalamus is an organ that serves as an important link, along the hypothalamic-hypophyseal axis, between the nervous system and the endocrine system. It is located within the cranial cavity, in the cerebrum, right below the thalamus. It also forms the floor of the third ventricle in the brain. It contains neural pathways, blood vessels, glial cells, and secretory cells—all of which work together to control things like body temperature, hunger, thirst, sleep, and hormonal and behavioral circadian cycles along with other metabolic processes.
The hypothalamus contains cells that produce various hormones, such as thyrotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, corticotropin-releasing hormone, somatostatin, and dopamine, as well as vasopressin and oxytocin. These assorted hormones are released into the blood stream and target other organ systems. Every hormone that is secreted by the hypothalamus is also produced from the hypothalamus. For example, Thyrotropin-releasing hormone is produced by parvocellular neurosecretory neurons and stimulates the secretion of thyroid-stimulating hormone from the anterior pituitary gland.
When a hormone is secreted, it is released into the capillaries of the Hypophyseal Portal System, which is a network of capillaries that surrounds the hypothalamus as well as the pituitary gland. Therefore, a good majority of the hormones that are secreted by the hypothalamus target the pituitary gland. There are two classification types of these hormones: releasing hormones, which stimulate the anterior pituitary, such as Thyrotropin-Releasing Hormone and Corticotropin-Releasing Hormone, and inhibitory hormones, which inhibit the anterior pituitary. The hypothalamus also contains ANS centers that control the adrenal medulla and stimulate it to release epinephrine and norepinephrine during times of stress.
In addition, the hypothalamus contains many neural cells that are sensitive to steroids and glucocorticoids, which are secreted by the adrenal glands. It also contains receptors specific for appetite, hearing, sexual drive, and preoptic vision, therefore putting it in control of emotional behavior, such as anger and sexual activity, and food intake.
There are a few unique circumstances in which other molecules directly influence the hypothalamus. Peptide hormones have important influences upon the hypothalamus because they can bypass the blood–brain barrier. Parts of the brain tissue surrounding the hypothalamus lack an effective blood–brain barrier, so the capillary tissue layer in these parts allows free passage of large protein molecules. Other peptides gain access to the hypothalamus by way of CSF. For example, prolactin and leptin, most of the active uptake from the blood occurs at the choroid plexus. The hypothalamus also takes up thyroid hormone through its glial cells in the infundibular nucleus. Here the hormone meets with type 2 deiodinase and is converted into T3. The hypothalamus receives many inputs from the brainstem. For example, suckling or vaginocervical stimulation causes the hypothalamus to secrete oxytocin, a hormone that causes contractions and lactation in women. In response to cardiovascular stimuli, it will secrete vasopressin, an antidiuretic hormone that functions by allowing more molecules to enter into kidney cells to retain and reabsorb water.
The hypothalamus also functions to maintain homeostasis, including blood pressure, heart rate, and as the body’s internal thermostat. It sets a desired body temperature and, as a response, stimulates either heat production and retention or sweating and vasodilation to increase or decrease in temperature. For example, when a person has a fever, it means that the setting in the hypothalamus has increased.
Image Caption: Hypothalamus. Credit: Wikipedia