Hypothalamus 101

Introduction

The hypothalamus is located in the brain and is part of the limbic system. It is connected to other parts of the central and autonomous nervous systems and receives many inputs from the brainstem. The hypothalamus is made up of several distinct nuclei, or groups of neurons, which all have different functions.

The hypothalamus plays many important roles in the brain – from reaction to stimuli to hormone release, to appetite, fear, and even sexual orientation [R].

Parts of the Hypothalamus

The hypothalamus is made up of three regions: the anterior, tuberal, and posterior. Each region also has different areas.

The anterior region consists of preoptic, medial and lateral areas.

  • Preoptic: This area helps regulates the body’s temperature and plays a part in parental and sexual behavior [R].
  • Medial: Releases and stimulates the release of the following hormones – GnRH,vasopressin, oxytocin, somatostatin, thyrotropin-releasing hormone, and corticotropin-releasing hormone. It is also involved in regulating body temperature, sweating, and the circadian rhythm.
  • Lateral: It is the primary source of orexin neurons. The orexin neurons project heavily to the thalamus, which is involved in the control of motivated behaviors [R].

The tuberal region consists of medial and lateral areas.

  • Medial: Controls blood pressure, heart rate, and the gastrointestinal tract stimulation. Also involved in appetite and satiety.
  • Lateral: It is the primary source of orexin neurons. The orexin neurons project heavily to the thalamus, which is involved in the control of motivated behaviors [R].

The posterior region consists of medial and lateral areas.

  • Medial: Involved in memory, blood pressure, and vasopressin release.
  • Lateral: It is the primary source of orexin neurons. The orexin neurons project heavily to the thalamus, which is involved in the control of motivated behaviors [R]. Also involved in waking and attention, energy balance, feeding, memory, and sleep.

The Roles of the Hypothalamus

The Hypothalamus Reacts to Stimuli

The hypothalamus responds to both internal and external stimuli.

  • Stress: The stressor alerts the hypothalamus, which in turn activates the pituitary gland and medulla, two other parts of the brain. These neural and endocrine systems will respond to the stressor in ways that they deem adequate. It is found that after a stressful event occurs, the synapses in the brain will undergo plasticity and change to reflect the response previously taken [R].
  • Light: The hypothalamus is in control of the body’s biological clock, the circadian rhythm. Light stimuli influences the timing of the circadian rhythm and regulates the body’s response on a daily and seasonal basis. The hypothalamus will release hormones and changes the body’s metabolism based on the timing, quality, intensity, and history of light exposure. Insomnia can be caused by hypothalamic activation and circadian disruption [R, R].
  • Steroids: The hypothalamus responds to both steroids and glucocorticoids (steroid hormones that are released from the adrenal gland) [R].
  • Body Temperature: The hypothalamus can increase or decrease body temperature in response to stimuli such as invading microorganisms [R].
  • Smell: Olfactory stimuli, such as pheromones, can be picked up by the hypothalamus and affect the way the body reacts [R]. Smells can also influence emotions and sexual arousal in humans [R, R] .
  • Input from the autonomous nervous system: The hypothalamus receives many signals from other parts of the brain and nervous systems and responds to them accordingly [R].

The Hypothalamus Stimulates Hormone Release

The hypothalamus releases several different hormones.  The hypothalamus stimulates the anterior pituitary gland. In turn, the hypothalamic neuroendocrine cells are regulated by feedback signals from the endocrine glands and other factors [R].

  • Dopamine: prompts the anterior pituitary to produce prolactin and stimulate breastmilk production
  • Oxytocin: controls some human behaviors (sleep cycle, ability to trust, etc.) and the reproductive system (lactation, orgasms)
  • Vasopressin (Antidiuretic Hormone): regulates water levels and increases water absorption into the blood by the kidneys
  • Somatostatin: inhibits growth and thyroid hormones
  • Growth Hormone-Releasing Hormone: stimulates release of growth hormones
  • Thyrotropin-Releasing Hormone: stimulates release of thyroid hormone
  • Corticotropin-Releasing Hormone: stimulates release of corticosteroids, which help regulate metabolism
  • Gonadotropin-Releasing Hormone: stimulates the anterior pituitary to release follicle stimulating hormone and luteinizing hormone which are connected to the reproductive system [R]

The Hypothalamus, Fatigue, and Stress

The lateral hypothalamus controls fatigue via orexin and inflammation. Stress can come from too much CRH, which is produced by the hypothalamus. A variety of cytokines like IL-1, TNF and IL-6 can stimulate the hypothalamus to release cortisol.

The Hypothalamus Affects Our Mood

Motivation is based on epinephrine and dopamine in the hypothalamus [R], as well as orexin and MCH [R].  Dopamine can activate orexin [R].  Read how orexin affects our mood and motivation and how to increase it.

CRH is found in the anterior region of the hypothalamus. The different hormones that are stimulated by the hypothalamus influences the level of CRH proportionately. Depressed patients have a higher level of CRH [R].

The Hypothalamus Controls Appetite

The hypothalamus is in charge of appetite and food intake, which affects an organism’s energy homeostasis [R].

Stimulation of the hypothalamus will result in food intake while damage to the area will cause food intake to stop. Inflammation of the hypothalamus can lead to obesity [R].

The hypothalamus control appetite through orexin, ghrelin, NPY, T3,  leptin, norepinephrine, serotonin, MCH, FGF21+19 and GLP-1 – all of which influence appetite and interact with each other [R, R2,  R3, R4].

Orexin, T3, ghrelin, MCH, FGF21 and NPY increase appetite, while leptin, insulin, norepinephrine, serotonin, GLP-1 and FGF19 are appetite suppressants [R].  Low hypothalamic serotonin leads to increased carb cravings.  Low orexin leads to decreased appetite.

Under normal circumstances, leptin will reduce appetite and food intake [R].

The hypothalamus is the most important factor in controlling our weight.  Inflammation from TNF causes us to lose weight as a result of decreasing hunger.

The Hypothalamus and Fear

The hypothalamus controls defensive behavior and executes fear from the amygdala. See the picture below for a walkthrough on how this works.

When exposed to a dangerous situation, the hypothalamus will be stimulated and the organism will display defensive behavior. The premammillary nucleus, located in the posterior region of the hypothalamus, plays a big role in unlearned and learned defensive responses to predators [R].

The Hypothalamus and Attention Problems

The hypothalamus is involved in attention [R] – hence why motivational and attentional issues often go together. (R)

Orexin, melanin-concentrating hormone [R], low levels of dopamine [R] and acetylcholine [R] are also involved with attention.  Inflammation can lower these neurotransmitters.

Orexin increases acetylcholine [R] and so inflammation will lead to lower levels of this neurotransmitter.  Acetylcholine also increases orexin [R].

Sexual Orientation

Differences in brain structures, including the hypothalamus, might play a part in determining a person’s sexual orientation.

In studies done on smell and the hypothalamus, it was found that the hypothalamus of heterosexual men and homosexual women both responded to estrogen while the  hypothalamus of homosexual men and heterosexual women responded totestosterone [R, R].

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