Hypothalamic-Pituitary-Adrenal (HPA) Axis
Yesterday we focused on the Sympathetic-Adrenal-Medullary axis, the brains direct pathway. Today we shall focus on the purpose of the indirect, the second mind-body communication pathway. The hypothalamus is the consolidation center of the ANS-
the circuit board combining the deep feeling, emotional, and interpretive responses. The hypothalamus also contains the cores that control heart activity, body temperature, blood pressure, and endocrine activity based on our needs. In addition, the hypothalamus harbors centers controlling emotional states like rage, pleasure, and the most biologic drives like sex, thirst, and hunger concurrently.
The Emotional Brain
The limbic system (the emotional part of the brain),
contains a formation called the amygdala that connects our emotional reaction to our memories and to the situation we are exposed to at the moment. It should come as no surprise that the limbic system has a strong bond to the hypothalamus. When the amygdala in the limbic system reacts to danger or stress, it cues the hypothalamus to resort to a fight-or-flight reaction from the SNS (sympathetic nervous system). Hence the hypothalamus, as a consequence, serves as the hub between emotion and visceral reactions, and the hypothalamic center all to often directs our behaviors. (2) However, this second, indirect pathway allows us some conscious control over ANS happenings. This very control allows us to influence hormonal reactions that affect physiology, biochemistry, immune function, and most importantly, stress related health outcomes.
A short explanation how the emotional brain works:
Stressors (fear, anxiety, exercise etc.) cause nerve impulses to be relayed from the outer limits of the body through, until now, still unknown higher centers of the brain to the hypothalamus. When the hypothalamus receives a stress or fear response from the limbic system, the hypothalamus responds by discharging a corticotropin-releasing hormone (CRH) that then arouses the pituitary gland to release and adrenocorticotropic hormone (ACTH). The ACTH, in turn, stimulates the adrenal cortex into liberating cortisol, a mondo stress hormone. This flow of arousing activity with its related hormones is achieved by the HPA axis. (3)
The known effects of cortisol include mobilizing energy stores for immediate energy needs, boost tissue sensitivity to other stress related neuro-hormones, and hinder immune and inflammatory responses.
These effects provide the organism with the ability to respond rapidly to stressors. Once the immediate threat is removed, negative feedback mechanisms are turned off, the release of cortisol diminished, and cortisol returns to pre-stress levels.
Endorphins or Enkephalins
The hypothalamus also releases hormones responsible for the regulation of emotion and more pleasurable behavior. If the hypothalamus receives messages of pleasure, it is enkephalins or endorphins (our "happy" hormones) which are released. When we are sexually aroused, the hypothalamus, by way of anterior pituitary gland, can prompt an increase in sex hormones. It is shown that addiction to certain drugs can be related to the hypothalamic pleasure centers.
Great research was conducted where immune cells (natural killer [NK] cells) were shaped or trained to respond in a certain way when a subject was exposed to camphor odor. The researchers recognized that when the odor (conditioned stimulus) was present, a direct input occurred in the hypothalamus, with the effect of redirecting the activity of NK cells by releasing certain mediator chemicals.(6) It is this action that suggested that a hypothalamic interpretation of events affects how the body responds to life events.
How we interpret these events can determine what informational substances are released, thereby affecting our response to these events and, indirectly, our biochemical reactions.
The hypothalamus is also engaged in the learning of conditioned immunologic responses. This very discovery has great implications concerning to our vulnerability to acute and chronic diseases and illnesses.
Next: The stress response and how it affects the body.
2. Felten D., et al. Central Neural Circuits Involved in Neural-immune Interactions. In Ader R., Felten D., Cohen N., editors: Psychoneuroimmunology. New York. (1991) Academic Press.
3. Besedovsky, del Rey, Sorkin. 1979
5. Klotho : Biochemical Compounds Declarative Database Bryan Cannon, Sumit Sadekar, Demetrius Taylor, Brian H. Dunford-Shore, Waheeda Sulaman, Brian Feng, Francis Fabrizio, Jason Holcomb, William Wise and Toni Kazic, 1994–present. Klotho: Biochemical Compounds Declarative Database http://www.biocheminfo.org/klotho/.2.
6. Hiramoto et al. 1993