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Limbic System - Chemicals

In parts one and two of this four-part series on the Limbic System, we discussed the major structures and their functions in the hopes of making the disorder PTSD more understandable. In this article, we will learn about the chemicals from the Limbic System that trigger reactions in the body.

The sympathetic nervous system (SNS) is located in the spinal cord and ganglia. (1) The ganglia are a collection of nerve cells that are away from the spine, but close to the organs they feed. These are acted upon by the emotional center of the brain, the Limbic System. The system releases chemicals, some of which directly affect the body and others which cause the release of hormones.

The hypothalamus is responsible for the release of hormones that increase epinephrine levels and fire up the metabolism. During the “fight or flight” response, large amounts of epinephrine are released into the body from the hypothalamus-pituitary-adrenal pathway. This binds to five different kinds of adrenergic receptor cells causing varied reactions such as blood vessel constriction, blocking norepinephrine, heart rate control, dilation of bronchial tissue, and lipolysis which provides energy. (2) Cortisol is normally released through a negative feedback loop to stop the autonomic nervous system response, but it is found to be inadequately low in PTSD patients.

Norepinephrine is released directly from the ganglia to affect cells. This chemical is responsible for mental alertness, emotional arousal, increased startle reaction, and the increase of worry/anxiety. Research on combat veterans shows that there is a higher concentration of norepinephrine in their cerebral spinal fluid and the higher the levels, the worse the PTSD symptoms. (3) Norepinephrine is related to memory consolidation, leading to the theory that over-activation of this chemical leads to memories that are more prominent and more readily accessible. This relates to the re-experiencing of trauma memories with triggers from the environment that may seem subtle to the nurse, but prominent to the patient.

Fear-conditioning is a response of the brain involving the amygdala and hippocampus. As you recall, they are involved in the emotion of fear and memory, respectively. So, the brain receives noxious stimuli that produces a fear response in the body, and this in turn, becomes a “trigger” for the PTSD client. There is no longer a threat, but the Limbic System is trained to perceive it as such. The chemicals are released activating the fight or flight mechanism. When there is not an option for either, the brain and body are at odds. This produces a freezing effect, often with dissociative symptoms. This can lead to the inability to react emotionally, causing the PTSD client problems in his/her personal life and causing feelings of isolation and guilt. They may also expend large amounts of time and energy avoiding things or places that they perceive as a trigger.

Knowing the biological basis for why the PTSD patient is experiencing their symptoms can better enable the nurse to interact with care and compassion. The organic mechanisms relate directly to the behavioral and pharmaceutical therapies for this population, and a clear understanding of the Limbic System helps the nurse understand why and how they work. Next week, we will discuss the unique challenges, therapies, and medications for the PTSD client.


References:
(1) http://thalamus.wustl.edu/course/

(2) http://www.becomehealthynow.com/article/bodynervousadvanced/822

(3) http://ajp.psychiatryonline.org/cgi/reprint/158/8/1227.pdf


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