This is the first essay in a four-part series about the Limbic System. This system is responsible for so much of how we perceive and respond to day-to-day life. It is our “emotional brain” and is found in all mammals. We spend large amounts of time and energy acting and reacting dependent on the function or dysfunction of these complex brain structures. The reason for this series is to lead up to a discussion of PTSD, as understanding of this tragic disorder requires this knowledge. This is a mental disorder we will see more and more frequently as our brave soldiers return home from war.

There are many structures that physically comprise this system, and we will start by discussing the hypothalamus. The hypothalamus is below the thalamus, as the name implies, and is connected physically to the pituitary gland. It receives input from the hippocampus (responsible for memory function) and the amygdala (responsible for mediating emotions). It also takes in information from the body such as smell, sight, and “gut” or viscera via the vagus nerve. (2) The hypothalamus also has internal sensors that allow it to help the body maintain homeostasis. These include temperature, osmolarity, glucose, sodium, and hormones such as leptin that affects appetite. (2) It is the director of all endocrine function in the body.

Our circadian rhythms start with the light entering directly into the hypothalamus through off-shoots of the optic nerve. Many mammalian body tissues have regulatory “clocks”, but they are all controlled by the hypothalamus through hormones such as vasopressin and cortisol. The area of the brain called the circumventricular organs do not have a blood-brain barrier and are able to feed information about toxins in the blood stream to the hypothalamus, which can then release hormones to induce vomiting.(3) With the information it receives internally and externally, it uses neurotransmitters, neuropeptides, and neurohormones to alter and adjust, regulating appetite, blood pressure, skin temperature, sexual function, and thirst.

Ever have a feeling of dread? That sense in your gut that something is not quite right? Your limbic system is responsible for that sensation. Your body is preparing for “fight or flight”, taking in sensory information and feeding it to the hypothalamus and thalamus. Think of them as the phone operators of long ago who directed the important calls to everyone. The hypothalamus directs them to the endocrine system, where hormones are released. It even controls how urgently those signals are perceived using neuropeptides. The thalamus directs those signals to the cerebral cortex, where decision making occurs.

Disorders of the hypothalamus are most often caused by tumors, but can also be seen with genetic factors, malnutrition, and head trauma. Symptoms relate to the hormones that are missing and can involve growth, vision, and metabolism. Treatment is cause-specific, meaning surgery for tumors or hormone replacement for deficiencies.(4)

References:
(1) https://www.ragebehavior.com/limbic.htm
(2) https://www.becomehealthynow.com/article/bodynervousadvanced/956/
(3) https://thalamus.wustl.edu/course/hypoANS.html
(4) https://www.lifespan.org/adam/healthillustratedencyclopedia/1/001202.html