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Our bodies respond to stress with chemical and neurological changes that enable us to fight, flee, or freeze. These changes begin the moment our brain pictures a life threatening event. The neurochemical changes begin in a part of the brain known as the limbic system.

The limbic system is a group of structures in the center of the brain that are involved in the control and expression of mood and emotion, in the processing and storage of recent memory, and in the control of appetite and emotional responses to food. The limbic system affects the endocrine system and the autonomic nervous system (ANS). It is responsible for our survival instincts and reflexes and regulates the ANS responses to stress & relaxation. It consists of several structures located around the thalamus. [The thalamus is a double bulb-shaped region located in the center of the brain. The thalmus relays and modulates signals from auditory, somatic, visceral and visual regions of the peripheral nervous system to the cerebral cortex.]

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The structures in the limbic system include the hippocampus, the amygdala, and the hypothalamus. The hippocampus is the part of the brain that is involved in memory forming, organizing, and storing. It is particularly important in forming new memories (especially about personal experiences and facts) and with connecting emotions and senses, such as smell and sound, to memories. The hippocampus ceases to play a crucial role in the retention of the memory after a period of consolidation, eg. when we've mastered a new skill. Damage to the hippocampus usually results in profound difficulties in forming new memories, however, it does not affect some aspects of memory such as the ability to learn new skills (e.g., playing a musical instrument).

The amygdala is linked to responses like fear, pleasure, feelings of punishment, and awareness of behavior. The amygdla receives input from the sensory systems and attaches emotional meaning to these sensations. It signals other parts of the brain when there is danger and sends outputs to the hypothalamus for activation of the sympathetic nervous system. It is also important in the activation of dopamine, norepinephrine, and epinephrine. Conditions such as anxiety, autism, depression, narcolepsy, post-traumatic stress, and schizophrenia are suspected of being linked to the functioning of the amygdala owing to damage, developmental problems, or neurotransmitter imbalance.

The hypothalmus is also considered to be part of the limbic system. The hypothalamus mediates between the nervous system and the endocrine system. It helps to regulate blood pressure, heart rate, hunger, thirst, sexual arousal, and the sleep/wake cycle. The hypothalamus secretes releasing hormones to the nearby pituitary in response to nervous system stimuli including smell, taste, pain, and emotions. Thus, stress, cold, heat, and other stimuli cause a release of CRF, or corticotropic hormone-releasing factor, from the hypothalamus. CRF travels to the pituitary, called the "Master Gland" because it produces hormones that regulate a wide variety of bodily activities, including growth, blood pressure, pregnancy, sexual function, metabolism, and water balance in the body. CRF stimulates the pituitary to produce ACTH (adrenocorticotropic hormone) which in turn causes the adrenal cortex to produce cortisol.

Cortisol, has become known as the "stress hormone", yet it is necessary in the functioning of almost every part of the body. Cortisol is a steroid hormone made in the adrenals, which are small glands next to the kidneys. Cortisol secretion increases in response to any stress in the body, whether physical (such as illness, trauma, surgery, or temperature extremes) or psychological. It acts as an antagonist to insulin and promotes the breakdown of carbohydrates, fatty acids, and proteins to immediately increase the body's energy levels in response to a life threat and ensure that the brain receives adequate energy sources.

In the early stages of adrenal stress, cortisol levels will be too high during the day and continue rising in the evening. This is called "hyperadrenia". In the middle stages, cortisol may rise and fall unevenly as the body struggles to balance itself despite the disruptions of caffeine, carbs, and other factors, but levels are not normal and are typically too high at night. In advanced stages of prolonged stress, when the adrenals are exhausted from overwork, cortisol will never reach normal levels ("hypoadrenia"). Medically adrenal dysfunction may only show up in its extreme stages.e.g. Cushings Disease or Addisons Disease. However, sustained high cortisol destroys healthy muscle and bone, slows down healing and normal cell replacement, co-opts biochemicals needed to make other vital hormones, impairs metabolism, increases blood sugar levels, storage of abdominal fat, diabetes, heart disease, interferes with healthy endocrine function, and suppresses the immune system. By products of Cortisol depress brain activity and act as sedatives and cortisol itself blocks seratonin, contributing to feelings of depression. Adrenal dysfunction may also be a factor in many related conditions, including fibromyalgia, hypothyroidism, chronic fatigue syndrome, arthritis, and menstrual difficulties. Additional red flags of adrenal fatigue include insomnia and using caffeine and carbohydrates as "pick-me-ups." (Many of these same sequelae can also arise from long-term use of glucocorticoid drugs.) Long-term exposure to cortisol has also been found to result in damage to cells in the hippocampus. This damage results in impaired learning and mood dysregulation. However, short-term exposure of cortisol helps to create memories, called "flashbulb memories". It can also damage other parts of the brain which control emotions, impulse control, arousal, and attention.

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