Did you know that the hypothalamus is called the head ganglion? This is because of its ability to control several visceral and metabolic activities, from controlling virtually every endocrine gland to regulating appetite, temperature, and the autonomic nervous system.
In this article, we’ll try to understand how the hypothalamus executes this seemingly complex task.
Anatomy of the Hypothalamus
The hypothalamus lies in the floor and lateral wall of the third ventricle.
Author: OpenStax College
Author: BodyParts3D by DBCLS
To simplify the study of its structure, the hypothalamus is subdivided into the optic, tuberal, and mamillary.
Each part has nuclei and specific functions:
- The optic part consists of the supraoptic and paraventricular nuclei.
- The tuberal part consists of the ventromedial, dorsomedial, and tuberal nuclei.
- The mamillary part consists of posterior and lateral nuclei.
Evolutionary Significance of the Hypothalamus
Hypothalamic control of the body temperature is a mechanism better observed and well developed in the vertebrates. Specifically, they are quite prominent in homeothermic (warm-blooded) mammals.
You see, they have a cluster of reflex responses for the regulations of their body temperature.
This differentiates them from those of invertebrates, and poikilothermic (cold-blooded) animals as these animals are dependent on the external environment for maintaining their body temperature.
Temperature Regulation by Hypothalamus
As the head ganglion, the hypothalamus acts as an integration center for sensory system and response (effector) systems responsible for the balance of the optimum body temperature.
The temperature regulating process is grossly classified into two aspects. The posterior hypothalamus controls the reflexes that occur in response to exposure to cold.
In contrast, the anterior hypothalamus controls the reflexes that occur in response to exposure to warm environments.
To understand the autonomic-vascular regulation of the body temperature, we need to realize that the surface area that comes in contact with the environment is directly proportional to the heat loss.
For the responses to get triggered, there is a threshold temperature level. The threshold levels are:
Threshold temperature | Reflex related |
37.0 ℃ | Sweating and vasodilation |
36.8 ℃ | Vasoconstriction |
36.0 ℃ | non-shivering thermogenesis |
35.5 ℃ | Shivering |
The shivering center is located on the dorsomedial portion of the hypothalamus.
The temperature above which sweating begins and below which shivering begins is called the hypothalamic set-point temperature.
Let us now see how the body responds to a typical fever under the control of the hypothalamus.
Pyrogens are chemical substances that can give rise to a fever. They can be of two varieties, exogenous and endogenous.
The pyrogens increase the hypothalamic set-point temperature. In response to this increase, the hypothalamus promotes heat production via shivering and heat conservation via vasoconstriction.
The heat generated increases the core body temperature to the new set-point temperature, thus giving rise to fever, as shown below.
Author: DrV
However, this is a temporary phenomenon. As time passes, the set-point temperature comes back to normal. Realizing its mistake, the hypothalamus brings the core body temperature back down. It achieves this by promoting heat loss through sweating and vasodilatation.
Temperature Regulation by the Integration of Systems
The temperature of the blood entering the hypothalamus acts as a sample for the thermoregulatory centers of the brain.
The neuronal sensors (hot and cold sensors) present in the hypothalamus fires at a rate of 2 to 10 times more for every 10℃ change in the body temperature.
Wisdom from Grand Wizard Zeo
“Talking about the significance of the number 10 concerning temperature, here’s another - every 1℃ increase in core body temperature increases the heart rate by 10 bpm.
Clinically, this particular fact helps us to identify relative bradycardia! If the pulse rate does not rise 10 bpm for every 1℃, then we call it relative bradycardia. Simply put - the heart rate is slow relative to the temperature.
There are many causes for relative bradycardia, including infections like typhoid fever, legionella, Q fever, malaria, leptospirosis, and rocky mounted spotted fever.”
Reflexes on Cold Exposure
Ever wonder why you feel hungrier than usual in hill stations? Or the occasional increased craving for yummy samosas on a cold rainy day? The answer lies literally inside your head.
The hypothalamus increases food intake when a person is exposed to cold. This increases the metabolic rate and increases heat production. Increased food intake also facilitates fat storage to act as insulation to prevent heat loss.
So yes, cold exposure does increase your appetite!
This phenomenon occurs by the coordination between the temperature regulation center and the food intake center within the hypothalamus.
The thermogenesis is either stimulated or inhibited based on the temperature stimulus to the hypothalamus.
Food intake and increased adipose tissue levels lead to increased production of a hormone called leptin. When released into the bloodstream, leptin crosses the blood-brain-barrier to act on its receptors in the hypothalamus. Ultimately, leptin leads to a decrease in appetite and energy storage.
Another interesting reflex to cold exposure is goosebumps! The neural stimulation of the arrector pili muscles in the skin following hypothalamic stimuli gives rise to a bumpy, goose-flesh like appearance of skin.
Interestingly, goosebumps can also occur when a person experiences fear. This explains the title for the series of popular horror fiction novels by R.L Stine!
Reflexes on Exposure to Hot Temperature
Exposure to heat causes the stimulation of the preoptic nucleus in the anterior hypothalamus. This, in turn, sends signals to the cholinergic nerves all over the body, thus increasing sweat production and vasodilatation.
Sweat evaporates on the surface of the skin producing a cooling effect that reduces the core body temperature.
Loss of heat from the body is also facilitated by increased blood flow to the cutaneous blood vessels. This reflex vasodilatation of the cutaneous blood vessels is also mediated by the hypothalamic signals.
Wisdom from Grand Wizard Zeo
“Ever wonder why dogs pant? The hypothalamus and the mechanism of temperature regulation are present but underdeveloped in lower animals. Thus they lose excess heat by panting.”
References:
- Ganong’s Review of Medical Physiology, 23rd Edition, Page no:282-286
- Guyton and Hall Textbook of Medical Physiology, 11th edition, Page no: 894- 896
- https://www.ncbi.nlm.nih.gov/books/NBK535380/
- https://journals.lww.com/nursing/fulltext/2008/08000/understanding_the_pathophysiology_of_fever.45.aspx
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2367118/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3717527/
- https://www.ncbi.nlm.nih.gov/books/NBK236229/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6071615/
Author’s footnote
Hypothalamic regulation and the course of a typical fever have been tested in the competitive exams of the past, making them potential repeat concepts for future years.
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