The Hypothalamus and Homeostasis

The hypothalamus is a central almond-sized portion of the brain containing various nuclei that play roles in regulating the fundamental state of our physiology: temperature, hunger, satiety, thirst, sexual development, fight or flight stress response, and sleep.

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The Pituitary Gland

The pituitary is the ‘master’ endocrine gland (about the size of a pea) which is controlled by the hypothalamus to release a variety of hormones which modify the state of physiology and our basic motivations.

It's not a part of the brain, but instead is a protrusion off the hypothalamus that sits in a cavity at the base of the brain. The hypothalamus and pituitary are the interface between the brain and the endocrine system controlling the basic functions of the body.

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Circle of Willis

The Circle of Willis is a circle of blood vessels that supply blood to the brain and surrounding arteries; it surrounds the pituitary and hypothalamus, and enables them to release hormones into the blood stream for rapid distribution throughout the brain+body.

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Or, for reference on the brain:

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Nine Hormones of the Pituitary

The anterior pituitary secretes 9 different important hormones:

  • Growth hormone (also referred to as 'Human Growth Hormone', 'HGH' or 'GH' or somatotropin)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Vasopressin
  • Oxytocin
  • Luteinizing hormone (LH)
  • Follicle-stimulating hormone (FSH)
  • Melanocyte–stimulating hormones (MSHs) or "intermedins,"

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Hypothalamus Nuclei

The hypothalamus can be broken down into different nuclei, each of which has been implicated in a different aspect of physiological regulation.

  • Medial Preoptic
  • Anterior Hypothalamic Area
  • Paraventicular Nucleus
  • Supra-optic Nucleus
  • Suprachiasmatic Nucleus
  • Ventromedial Nucleus
  • Mammillary Body
  • Dorsomedial Hypothalamic Nucleus

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Hunger and Satiety

Eating behaviour such that an optimal ration of energy molecules, between stored (triglycerides/body fat) and short term (glycogen), is maintained.

After eating, leptin increases from the fat cells (satiety signal) (and insulin from the pancreas, as well) and ghrelin (hunger signal) decreases from the stomach; the two molecules act in opposing ways upon the two relevant hypothalamus nuclei.

The lateral hypothalamus ('start eating') and the ventromedial hypothalamus ('stop eating') centre maintain an energy balance by dictating how much food is required.

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Obesity and Anorexia

Whist lesions in the two nuclei do cause weight problems, the global increase in obesity is likely to reflect a changed ‘top-down’ motivation of feeding, caused by the high availability of easy to obtain, palatable, salient (noticeable/desirable) foods that override the regulatory functions of the hypothalamus.

Similarly, anorexia is linked to low levels of BDNF (a protein that regulates food intake and energy expenditure). However given that anorexia is more common in pubescent females, the disorder probably arises from broader social and biological mechanisms underpinning female sexual development.

Thirst

There are two mechanisms for maintaining the optimal set fluid point within the body:

  1. Osmoreceptor cells in the supraoptic nuclei transduce (convert) dehydration (marked by cell shrinkage due to fluid loss) into action potentials to increase the production of ADH, which is transported and released into the bloodstream by the pituitary. This binds to receptors in the kidney and reduces water extraction from the body into the urine, hence reducing urine production.
  2. The subfornical organ lies outside the blood-brain barrier within the cerebrospinal fluid atop the third ventricle - this allows it to transduce dehydration within the blood supply of the blood, into action potentials. These then are carried to the supraoptic nucleus, whereby the above happens.

Lesions to the supraoptic nucleus cause a reduction in fluid intake, suggesting a link between the hypothalamus and mediation of actual drinking behaviour as a response to thirst.

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Sexual Dimorphism Nucleus

The Sexually dimorphic nucleus (SDN) is a cluter of cells located in the preoptic area of the hypothalamus. It's approximately twice the volume in males than in females, and is correlated with testosterone concentration. It's also an indicator for sexual preference in males - it's about 1.7 times larger in heterosexual than homosexual males - and is more activated by estrogen (female pheremone) in heterosexual males and homosexual females than homosexual males and heterosexual females.

Fight or Flight Response

The Fight or flight or freeze response is the bodily reaction to conditions of predatory or defensive stress; an acute cascade of physiological changes enabling the extreme physical exertion required.

The stressful stimulus is detected through sensory systems and communicated to the hypothalamus, which releases the hormone CRF (corticotrophin-releasing factor), causing the pituitary to produce ACTH.
ACTH then diffuses through the blood to the adrenal gland (endocrine glands) on top of the kidneys, which release cortisol (steroid hormone with primary function to increase glucose availability in the blood), enabling greater muscular output.

At the same time a cascade of hypothalamic mediated changes in the autonomic nervous system prepare physiology for action:

  • Acceleration of heart and lung action
  • Constriction of blood vessels to reduce blood flow to periphery (skin) and increase blood flow to heart
  • Lungs and muscles aiding oxygen transfer and power output
  • Reduced digestion
  • Reduced tear production and salivation
  • Dilation of pupil to increase light entry
  • Relaxation of bladder
  • Inhibition of erection
  • Loss of hearing
  • Loss of peripheral vision
  • Disinhibition of spinal withdrawal reflexes
  • Shaking
  • Sweating

The reduction of stress following the fight or flight response then switches off this reaction through the absence of threat information into the hypothalamus thus enabling the physiological state to return to set point.

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Sleep

The suprachiasmatic nucleus is about the size of a grain of rice, and is located directly above the optic chiasm (were the optic nerves from the two eyes merge and cross), receiving information about the light-dark cycle.

This nucleus is called the biological clock because it plays a role in the regulation of our sleep wake cycle (think jet lag). The suprachiasmatic nucleus is connected to the pineal gland (small endocrine gland), and governs its secretion of melatonin (produced when dark), which enters the blood and reduces the activity of the SNS, reducing physiological activity.
SUprachiasmatic neurons are also strongly activated during sleep, and express GABA (inhibitory), inhibiting the reticular activating systems (Dopamine, noradrenaline, seratonin and acetylcholinergic) which collectively determine the level of activity across the rest of the brain.

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Homeostasis

Homeostasis is the concept of self-regulating system that maintains its own inner environment, reasonably separate from the external environment.

Claude Bernard first pointed to human's homeostasis, arguing that our independence as an organism derives from the fact that the human tissues are withdrawn from direct external influence (being protected by a veritable internal environment - partly by the fluids circulating the body).

Walter Cannon built upon the concept, saying that physiological constancy requires a means of detecting the current state and moving towards an optimal one (e.g. optimal temperature of 37 degrees).

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Feedback Loop

The multitude of signalling between sites (the brain, hypothalamus, hormones between organs and endocrine glands) initiated by the hypothalamus are commonly called cascades.

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