Effects of stress on memory

The effects of stress on memory include interference with a person's capacity to encode memory and the ability to retrieve information. Stimuli, like stress, improved memory when it was related to learning the subject. During times of stress, the body reacts by secreting stress hormones into the bloodstream. Stress can cause acute and chronic changes in certain brain areas which can cause long-term damage. Over-secretion of stress hormones most frequently impairs long-term delayed recall memory, but can enhance short-term, immediate recall memory. This enhancement is particularly relative in emotional memory. In particular, the hippocampus, prefrontal cortex and the amygdala are affected. One class of stress hormone responsible for negatively affecting long-term, delayed recall memory is the glucocorticoids (GCs), the most notable of which is cortisol. Glucocorticoids facilitate and impair the actions of stress in the brain memory process. Cortisol is a known biomarker for stress. Under normal circumstances, the hippocampus regulates the production of cortisol through negative feedback because it has many receptors that are sensitive to these stress hormones. However, an excess of cortisol can impair the ability of the hippocampus to both encode and recall memories. These stress hormones are also hindering the hippocampus from receiving enough energy by diverting glucose levels to surrounding muscles.

Stress affects many memory functions and cognitive functioning of the brain. There are different levels of stress and the high levels can be intrinsic or extrinsic. Intrinsic stress level is triggered by a cognitive challenge whereas extrinsic can be triggered by a condition not related to a cognitive task. Intrinsic stress can be acutely and chronically experienced by a person. The varying effects of stress on performance or stress hormones are often compared to or known as "inverted-u" which induce areas in learning, memory and plasticity. Chronic stress can affect the brain structure and cognition.

Studies considered the effects of both intrinsic and extrinsic stress on memory functions, using for both of them Pavlovian conditioning and spatial learning. In regard to intrinsic memory functions, the study evaluated how stress affected memory functions that was triggered by a learning challenge. In regard to extrinsic stress, the study focused on stress that was not related to cognitive task but was elicited by other situations. The results determined that intrinsic stress was facilitated by memory consolidation process and extrinsic stress was determined to be heterogeneous in regard to memory consolidation.^{[clarification needed]} Researchers found that high stress conditions were a good representative of the effect that extrinsic stress can cause on memory functioning.^{[clarification needed]} It was also proven that extrinsic stress does affect spatial learning whereas acute extrinsic stress does not.^{[clarification needed]}

When a stressful situation is encountered, stress hormones are released into the blood stream. Adrenaline is released by the adrenal glands to begin the response in the body. Adrenaline acts as a catalyst for the fight-or-flight response, which is a response of the sympathetic nervous system to encourage the body to react to the apparent stressor. This response causes an increase in heart-rate, blood pressure, and accelerated breathing. The kidneys release glucose, providing energy to combat or flee the stressor. Blood is redirected to the brain and major muscle groups, diverted away from energy consuming bodily functions unrelated to survival at the present time. There are three important axes, the adrenocorticotropic axis, the vasopressin axis and the thyroxine axis, which are responsible for the physiologic response to stress.

When a receptor within the body senses a stressor, a signal is sent to the anterior hypothalamus. At the reception of the signal, corticotrophin-releasing factor (CRF) acts on the anterior pituitary. The anterior pituitary in turn releases adrenocorticotropic hormone (ACTH). ACTH induces the release of corticosteroids and aldosterone from the adrenal gland. These substances are the main factors responsible for the stress response in humans. Cortisol for example stimulates the mobilization of free fatty acids and proteins and the breakdown of amino acids, and increases serum glucose level and blood pressure, among other effects. On the other hand, aldosterone is responsible for water retention associated with stress. As a result of cells retaining sodium and eliminating potassium, water is retained and blood pressure is increased by increasing the blood volume.

A second physiological response in relation to stress occurs via the vasopressin axis. Vasopressin, also known as antidiuretic hormone (ADH), is synthesized by the neurons in the supraoptic nucleus of the hypothalamus and regulates fluid loss by manipulating the urinary tract. This pathway allows water reabsorption within the body and decreases the amount of water lost through perspiration. ADH has the greatest^{[greatest among what?]} effect on blood pressure within the body. Under normal circumstances, ADH will regulate the blood pressure and increase or decrease the blood volume when needed. However, when stress becomes chronic, homeostatic regulation of blood pressure is lost. Vasopressin is released and causes a static increase in blood pressure. This increase in blood pressure under stressful conditions ensures that muscles receive the oxygen that they need to be active and respond accordingly. If these stressful conditions remain elevated, muscles will become fatigued, resulting in hypertension and in extreme cases can result in death.

The third physiological response results in the release of thyrotropic hormone-release factor (TRF)^{[Where, when and how?]} which results in the release of thyrotropic hormone (TTH). TTH stimulates the release of thyroxine and triiodothyronine from the thyroid. This results in an increased basal metabolic rate (BMR).^{[What effect does that have?]} This effect is not as immediate as the other two, and can take days to weeks to become prevalent.

Chronic stress is the response to emotional pressure suffered for a prolonged period of time in which an individual perceives they have little or no control. When chronic stress is experienced, the body is in a state of continuous physiological arousal. Normally, the body activates a fight-or-flight-response, and when the perceived stress is over the body returns to a state of homeostasis. When chronic stress is perceived, however, the body is in a continuous state of fight-or-flight response and never reaches a state of homeostasis. The physiological effects of chronic stress can negatively affect memory and learning. One study used rats to show the effects of chronic stress on memory by exposing them to a cat for five weeks and being randomly assigned to a different group each day. Their stress was measured in a naturalistic setting by observing their open field behaviour, and the effect on memory was estimated using the radial arm water maze (RAWM). In the RAWM, rats are taught the place of a platform that is placed below the surface of the water. They must recall this later to discover the platform to exit the water. It was found that the rats exposed to chronic psychosocial stress could not learn to adapt to new situations and environments, and had impaired memory on the RAWM.