Corpus: Hypothalamus
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from ancient Greek: ὑπό ("hypo") - under; θάλαμος ("thalamós") - room, chamber
1. Definition
The hypothalamus is a critical part of the diencephalon (also known as the interbrain). It functions as the primary regulatory center for vegetative (autonomic) and endocrine processes. It plays a key role in controlling breathing, circulation, body temperature, sexual behavior, and the intake of fluids and food.
2. Anatomy
The hypothalamus is situated directly below the thalamus. It forms the floor and part of the lateral walls of the third ventricle. On its lateral side, it is bordered by the internal capsule, and it merges caudally with the tegmentum of the midbrain.
Externally, the hypothalamus contributes to the formation of several noticeable structures:
- Mammillary body
- Tuber cinereum
- Infundibulum, which connects to the posterior pituitary gland
- Median eminence
The hypothalamus contains several important nuclei (clusters of neurons) with distinct functions, including:
- Supraoptic nucleus: Produces antidiuretic hormone (ADH)
- Paraventricular nucleus: Produces oxytocin
- Periventricular nucleus
- Dorsomedial nucleus
- Ventromedial nucleus: Regulates feelings of satiety (fullness)
- Arcuate nucleus
- Suprachiasmatic nucleus: Regulates circadian rhythms
- Preoptic area: Includes the lateral, medial, median, and periventricular preoptic nuclei
3. Physiology
The hypothalamus influences organ systems primarily by producing hormones. These hormones can be divided into two categories:
- Control hormones: These regulate the anterior pituitary gland and are classified based on their effect:
- Releasing hormones: Examples include gonadotropin-releasing hormone (GnRH), growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), and corticotropin-releasing hormone (CRH).
- Inhibiting hormones: Examples include somatostatin and dopamine.
- Effector hormones: The hypothalamus also produces prohormones for ADH and oxytocin, which are released by the posterior pituitary (neurohypophysis).
The hypothalamus is interconnected with various brain regions through afferent (incoming) and efferent (outgoing) connections:
- Afferent connections: Include inputs from the hippocampus, thalamus, striatum, limbic system, and spinal cord.
- Efferent connections: Target the thalamus, reticular formation, and other parts of the hypothalamus, including the posterior pituitary.
Hormone release is tightly regulated by negative feedback mechanisms, ensuring balance and proper physiological function.