Hypothalamus Regions and Function in the Body
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If the brain were a corporation, the hypothalamus would kind of be like the "utilities" department. While a lot of the credit and attention goes towards parts of the brain that communicate, create, and act, the hypothalamus is responsible for heating, water flow, and other basic things that keep the entire system running.
The basic function of the hypothalamus can be summarized with the word homeostasis, which means keeping the internal state of the body as constant as possible. The hypothalamus keeps us from being too hot, too cold, overfed, underfed, too thirsty, and so on.
While the hypothalamus is generally responsible for keeping us in a steady state, there are times that this state needs to change. When in an immediately life-threatening situation, you may not need to think about how hungry you are. The limbic system, which is intricately involved with emotion, communicates closely with the hypothalamus, resulting in the physical changes that are associated with particular feelings. The amygdala has reciprocal connections with the hypothalamus through at least two major pathways.
Other regions of cortex, such as the orbitofrontal cortex, insula, anterior cingulate and temporal cortices also communicate with the hypothalamus.
Regions of the Hypothalamus
Like the rest of the brain, different areas of the hypothalamus perform different functions. These areas can be distinguished by their connections to the rest of the brain. For example, the hypothalamus is divided in half by fibers of a white matter tract called the fornix, which runs from the front of the hypothalamus towards the back.
The parts of the hypothalamus closer to the inside of the brain (the medial side) communicate closely with part of the amygdala through another tract called the stria terminalis. The amygdala helps to signal fear, and the medial aspect of the hypothalamus is involved with a "fight-or-flight" response, for example by limiting appetite. There's no time to rest and digest if you're about to run for your life!
The side of the hypothalamus that is closest to the outside of the brain (the lateral side) has the opposite effect on appetite. Because this area is important in stimulating appetite, lesions in this area can lead to severely decreased body weight. This area is also important in thirst, as lesions of the more frontal part can lead to decreased water intake.
The functionality of the hypothalamus is also divided from front to back. For example, anterior parts of the hypothalamus seem more involved with cooling the body off by increasing blood flow to the skin and causing sweat to be produced. The back of the hypothalamus is more involved with keeping the body warm.
In addition, the hypothalamus is responsible for regulating our natural cycle of wakefulness and sleep. The suprachiasmatic nucleus at the front of the hypothalamus serves as our internal clock, letting us know when it's bedtime. This part of the brain is connected with light-sensitive regions that adjust our internal clock to daylight.
How Does the Hypothalamus "Talk" to the Body?
The hypothalamus modulates physical responses by communicating with the body through two routes. The first route is through the autonomic nervous system. The second is through the endocrine system, meaning the secretion of hormones into the bloodstream.
Autonomic fibers primarily come from the paraventricular nucleus of the hypothalamus, but also from the dorsomedial hypothalamic nucleus and from the lateral and posterior hypothalamus. Initially, these autonomic fibers travel in a white matter path called the medial forebrain bundle. They then pass on into the dorsolateral brainstem and periaqueductal gray matter. The fibers synapse on parasympathetic nuclei in the brainstem and intermediate zone of the sacral spinal cord, and on sympathetics in the intermediolateral cell column of the thoracolumbar spinal cord.
Many autonomic nuclei in the brainstem receive inputs from the hypothalamus, such as the nucleus solitarius, noradrenergic nuclei, raphe nucleus, and pontomedullary reticular formation.
The hypothalamus also works in conjunction with the pituitary gland to control the body's endocrine system. The pituitary has the ability to secrete hormones directly into the bloodstream. This is a rare example of a place where the blood-brain barrier normally designed to keep infections from crossing into the brain is absent from the brain's architecture.
Some hormones, such as oxytocin and vasopressin, are made directly in the hypothalamus (in the paraventricular and supraoptic nuclei, for example), and secreted near the back of the pituitary. The anterior part of the pituitary contains cells that make their own hormones. These hormones are regulated by other neurological secretions which are passed down nerve fibers into a vascular plexus, where they are released by the blood. All of these hormonal secretions are regulated by negative feedback loops, meaning that the brain is able to detect when levels of the hormone are high and decrease production as a result.
This may seem enormously complicated, and it is. But the ultimate task of homeostasis even in the face of adversity is well worth it!
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