Hypothalamus - subdivisions, connections and function

The hypothalamus is part of the brain (diencephalus) lying below the 3rd cerebral ventricle

structure and function

• The hypothalamus is a high-level sensory integration and motor output area that maintains homeostasis by controlling endocrine, autonomic, and somatic behavior.

• It receives internal stimuli via receptors for circulating hormones. The blood-brain barrier is particularly permeable at the subfornical organ and organum vasculosum around the hypothalamus, allowing blood osmolarity sensation. When osmolarity increases during dehydration, the antidiuretic hormone (ADH) causes renal water reabsorption.

• The hypothalamus also processes external stimuli, such as pain, through pathways like the spinothalamic tract.
• It integrates sensory information from the limbic system via structures like the 'fornix, mammillothalamic tract, and stria terminalis (which connects to the amygdala).

Sensory perceptions from the cortex are transmitted through the medial forebrain bundle. The retinohypothalamic tract conveys light information to the suprachiasmatic nucleus, which helps regulate circadian rhythms and hormonal release patterns.

• It also produces critical hormones, including oxytocin and ADH from its paraventricular and supraoptic nuclei.

These hormones are released into the posterior pituitary and function as both neurotransmitters and hormones.

Nuclei

Preoptic, Anterior, and Posterior Nuclei regulate body temperature by decreasing the sympathetic tone of skeletal muscle, increasing the sympathetic tone of the skin, dilating capillaries, and improving heat exchange with the exterior.

Suprachiasmatic Nucleus regulates hormone secretion, plays a crucial role in regulating the body’s circadian rhythms. It receives light signals from the retina via the retinohypothalamic tract and adjusts the body's internal clock to align with the day-night cycle. The SCN also influences hormone release, particularly melatonin from the pineal gland, to control sleep patterns. It ensures that various physiological processes are synchronized with external environmental cues like light.

Ventromedial Nucleus regulates hunger, satiety: It helps signal when to stop eating. Lesions in this area can lead to overeating and obesity, while activation of the VMN suppresses hunger. and regulates energy balance by responding to hormones like leptin (which signals satiety) and insulin (which regulates blood sugar).

Arcuate nucleus The arcuate nucleus in the hypothalamus releases hormones into the hypothalamohypophysial portal system to control the release of anterior pituitary hormones. Corticotropin-releasing hormone (CRH) triggers the release of ACTH, which stimulates the adrenal glands to produce cortisol, a stress-response hormone. Cortisol levels follow a daily pattern, peaking at sunrise and dropping at sunset.

The hypothalamus also controls metabolism by releasing thyrotropin-releasing hormone (TRH), which increases thyroid hormone production, and senses energy levels through leptin receptors. GnRH regulates reproductive hormones like LH and FSH, which influence sexual development and function. Growth hormone (GH) is released by GHRH and stimulates growth, while somatostatin inhibits GH release. Dopamine inhibits prolactin secretion.

mamillary nucleus The mamillary nucleus contributes to the limbic system as part of the Papez circuit. It is also involved in memory formation and controls exploratory behavior.[7] Bilateral mamillary body lesions are characteristic of Wernicke-Korsakoff syndrome, which features anterograde and possibly retrograde amnesia.