Hypothalamus

In a continuation of the journey through the human brain, we arrive at the hypothalamus. The hypothalamus is an organ central to many autonomous functions of the human body, the most notable of which is the regulation of homeostasis.

Not only is it an integral part of the central nervous system but it also regulates processes of the endocrine system and is also regarded as an organ of the limbic system.

This tiny little structure (it only accounts for less than 1% of the brain’s weight!) is located just underneath the thalamus, above the pituitary gland. The role of the hypothalamus extends over many levels, regulating motor skills, emotional responses, blood pressure, and so much more.

Anatomy of the Hypothalamus

The hypothalamus is found underneath the thalamus and comprises the floor of the third ventricle (one of four open spaces in the brain through which cerebrospinal fluid flows). The hypothalamus extends downward from the brain into a stalk known as the pituitary stalk (or infundibular stalk), which connects it to the pituitary gland.

It can be divided into three main regions, each containing distinct nuclei and neuron clusters that are assigned various functions: The anterior region of the hypothalamus is known as the supraoptic region.

The supraoptic and paraventricular nuclei can be found here along with many more, smaller nuclei. The supraoptic nucleus functions as the main source of vasopressin, also known as the antidiuretic hormone (ADH), which plays a key role in the absorption of salts and glucose and maintaining the concentration of water in the extracellular fluid.

The middle region of the hypothalamus is known as the tuberal region and its primary nuclei are the ventromedial and arcuate nuclei. The ventromedial nucleus controls the appetite and the arcuate nucleus releases the growth hormone-releasing hormone (GHRH), which stimulates the pituitary gland to produce growth hormone (as its name so thoroughly explains).

Finally, the posterior region of the hypothalamus, called the mammillary region, contains the following major nuclei: the posterior hypothalamic nucleus and mammillary nuclei.

The posterior hypothalamic nucleus regulates body temperature by initiating the process of shivering and inhibiting the production of sweat. The mammillary nuclei make up part of the mammillary bodies that form a fraction of the limbic system.

The posterior region of the hypothalamus is also home to a structure called the median eminence. This structure contains many endings of neurosecretory cells (these neurons run along the pituitary stalk down into the pituitary gland).

Structures adjacent to the median eminence (apart from the third ventricle) are the mammillary bodies and the optic chiasm.

Functions of the Hypothalamus

The hypothalamus is undeniably a powerhouse, regulating processes from sleep cycles to homeostasis, endocrine functions and more. Its involvement in several autonomic processes means that its connections and roles in the central nervous system are plentiful. All this, and it’s only about the size of a pea.

Autonomous Processes Regulated by the Hypothalamus

The autonomous processes regulated by the hypothalamus are key to its ability to control homeostasis. Each of these processes must be maintained at a specific level called the set-point.

This set-point can be observed to have minimal changes over time, but overall, it will remain largely the same. The main elements that contribute to the set-point are blood pressure, body temperature, the balance of electrolytes and fluids, and body weight.

Inputs to the Hypothalamus

In order to properly manage this set-point, the hypothalamus must receive neurological inputs from multiple nervous and endocrine system sources. These sources are:

  • Nucleus of solitary tract: Also known as the nucleus tractus solitarius, this pair of cell bodies are located in the brainstem and is central to the maintenance of homeostasis. This structure has been determined to be the “primary visceral sensory relay station” of the brain, collecting information on the respiratory, cardiovascular, and gastrointestinal systems.
  • Reticular formation: This network of nerve pathways is located in the tegmentum of the brainstem and gathers information passed from the spinal cord and relays it to the hypothalamus. This network contributes to the regulation of consciousness, contains one of the largest sites of dopamine production in the brain, and more. This network contributes to the collection of sensory information, motor activity, and behavioral responses to stimuli.
  • Retina: A few fibers stemming from the optic nerve channel directly into the suprachiasmatic nucleus, a structure that takes in visual sensory information from the eyes in order to maintain the Circadian rhythm. Hormones and neuronal signals released from the suprachiasmatic nucleus influence both behavioral and physiological changes.
  • Circumventricular organs: These are areas in the brain where the blood-brain barrier is particularly weak, allowing fluids to cross into the brain more easily than in other locations. These organs include
    • Pineal body: associated with Circadian rhythm
    • Neurohypophysis: releases hormones such as oxytocin and vasopressin into the blood
    • Area postrema: controls the vomiting reflex
    • Subfornical organ: regulates bodily fluids and processes such as osmoregulation, cardiovascular regulation, and energy maintenance.
    • Vascular organ of lamina terminalis: intake of chemosensory information
    • Median eminence: a neurohemal organ that contains a “capillary bed” into which hypothalamic nerves send their neuronal transmissions
  • Limbic and olfactory systems: gathering and distribution of olfactory sensory information and the regulation of emotions.

Hormones Released by the Hypothalamus

The hypothalamus can essentially control all endocrine glands in the body and directly control blood pressure, body temperature, metabolism, and adrenaline levels through the release of endocrine signals throughout the body. Endocrine signals are hormones distributed throughout the body via the bloodstream.

There are two neurological components in the hypothalamus responsible for producing the hormones that drive autonomous nervous system functions: the parasympathetic vagal nuclei and another cluster of nerve fibers that extend all the way down to the sympathetic nervous system (the part of the autonomic nervous system that controls the fight-or-flight response), terminating in the spinal cord.

The hormones released by the hypothalamus include

  • Vasopressin, or antidiuretic hormone (ADH): causes water reabsorption in the kidneys, maintains blood pressure
  • Oxytocin: “the cuddle/love hormone,” regulates social interaction and sexual reproduction
  • Growth hormone (GH): in children, acts on several parts of the body to promote growth; in adults, maintains body structure, metabolism, and maintenance of blood glucose levels
  • Prolactin: plays a role in lactation, maintenance of the reproductive system, behavior, and regulation of the immune system
  • Corticotrophin-releasing hormone (CRH): controls the body’s response to stress
  • Growth hormone-releasing hormone (GHRH): (no, there wasn’t a typo – this is a real, redundant-sounding hormone) can you guess what this does? That’s right! This hormone stimulates the release of the growth hormone
  • Somatostatin: this hormone inhibits the secretion of pancreatic and gastrointestinal hormones
  • Gonadotrophin-releasing hormone (GnRH): this is released from the nerve cells in the brain, controlling the production of luteinizing hormone and follicle-stimulating hormone
  • Thyrotrophin-releasing hormone: regulates the production and secretion of thyroid-stimulating hormone and prolactin

Potential Problems With the Hypothalamus

Just like any other part of the body, there are potential diseases and injuries that could particularly affect the hypothalamus. The difficulty with hypothalamic injuries and diseases is that, because the hypothalamic is so far-reaching in its roles in the central nervous system, limbic system, and endocrine system, it can pose a tremendous challenge to diagnose and treat issues that may arise in connection to this organ.

One of the most well-known problems affecting the hypothalamus is hypothalamic disease (even this disease isn’t highly specific, as it can be applied to many different symptoms).

Hypothalamic disease is most commonly caused by physical trauma to the head and can span over a number of disorders or hypothalamus-related malfunctions. Symptoms can manifest as sleeping disorders, problems with appetite, growth abnormalities, and more.

Other causes include surgery, radiation, and tumors. There are even genetic links to hypothalamic diseases like Kallman and Prader-Will syndromes, for example. Diabetes insipidus and hypopituitarism are other known disorders related to the malfunctioning of the hypothalamus.

When symptoms are too difficult to decipher or multiple symptoms occur at once, the issue may be referred to as a hypothalamic-pituitary disorder. This is because the hypothalamus and pituitary gland work so closely together. But don’t worry! It’s not all a guessing game. There are tests to monitor hormone levels that narrow down the possibilities as to what may be the source of the disorder or disease.

Other symptoms that could potentially point to dysfunction of the hypothalamus include:

  • Unusually high/low blood pressure
  • Fluctuations in body temperature
  • Unintentional weight gain/loss
  • Sudden changes in appetite
  • Insomnia
  • Infertility
  • Delayed onset of puberty
  • Stunted growth
  • Excessive dehydration
  • Frequent urination

Almost every single structure in the whole of the brain (including the forebrain, midbrain, and hindbrain) has several functions that it performs simultaneously, 24/7.

Each one of these functions is essential to our survival as human beings. Even such a tiny thing as the hypothalamus plays a massive role in the regulation of many different aspects of our nervous systems, both central and peripheral.

As you’ve read, the hypothalamus is involved in many autonomic functions, maintenance of homeostasis, regulation of the Circadian rhythm, and so much more. Its essentiality to the nervous system can never be overstated.

References

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