Stress takes many forms, from the anxiety before a big test to the trauma experienced during combat. In the face of these situations our bodies mobilize a host of biological changes to prepare us for the challenges ahead. The stress response affects many aspects of how we think and feel, including how we learn and form memories. Evidence suggests that in some situations stress can improve memory formation, while too much stress can have harmful effects.
The stress response
Imagine you are crossing a busy street on your way to school as you do every morning. A car that has been waiting to turn left starts making its move, not realizing you are directly in its path. Before you have fully registered what is happening, a host of reactions within your nervous system have already mobilized to prepare you for this threat[1]. First, a system known as the the sympathetic nervous system is triggered, causing the rapid release of the stress hormones adrenaline and noradrenaline into the bloodstream. Within seconds you experience the classic “fight or flight” response such as a pounding heart, sweating and dilated pupils. This process is so quick you might have jumped out of the way before you were fully aware that it was coming towards you.
Your body will also mobilize a system called the hypothalamus-pituitary-adrenal axis (HPA axis). This system involves the cascading release of chemical signals that ultimately triggers the release of stress hormones known as glucocorticoids from the adrenal gland (primarily cortisol in humans and corticosterone in many non-humans such as rodents). This second system is much slower, acting over the course of minutes rather than seconds. However, both systems produce long-lasting changes in the brain that profoundly impact not only how we respond to present threats, but also alter how we learn and form new memories.
How stress impacts memory
You may not typically remember much of your daily commute, but you will vividly remember the car that almost hit you-the sight of it coming towards you, and the surprise on the driver’s face as they hit the brakes. Understanding why this happens requires some understanding of the underlying brain circuitry. The hippocampus is a curved structure located within the medial temporal lobe that plays a key role in many types of memory including remembering events that happened to you or storing facts and information. As you learn, specific sets of neurons are activated and the connections between these neurons grows stronger. This process is known as long-term potentiation or LTP and is believed to be the biological basis of memory[2].
The hippocampus is heavily interconnected with the amygdala, an adjacent brain structure that is particularly important for encoding emotionally significant information. Both regions are densely populated with glucocorticoid receptors, specialized protein structures on the surfaces of neurons that trigger a host of changes within the cell when they come into contact with a glucocorticoid molecule[3]. Evidence suggests that the amygdala integrates input from stress hormones and in turn boosts memory formation in the hippocampus. For example, mimicking the activity of noradrenaline in the amygdala has been shown to trigger LTP in the hippocampus, while inactivating the amygdala impairs memory formation[4].
While moderate stress can improve memory formation, severe or chronic stress can produce the opposite effect. Chronic childhood stress is associated with memory impairments in adulthood[5], while chronic exposure to stress or glucocorticoids has been shown to produce spatial memory impairments in laboratory settings[6,7]. These effects may be due to the damaging effects of stress on the hippocampus. Chronic stress and exposure to high amounts of glucocorticoids can reduce LTP in the hippocampus and cause atrophy of the dendritic spines that these neurons use to receive signals from each other[8].
Stress and fear learning
Not all types of memory depend on the hippocampus. The amygdala is crucial for learning about which things in the environment signal threat, a process known as fear learning[9]. You may learn to associate the sight of a car or a busy intersection with the experience of nearly being hit, causing you to be more cautious when crossing the road in future. While the ability to recognize and avoid threats is crucial for survival, chronic or severe stress can cause exaggerated fear learning that has a harmful impact on mental health and well-being.
Chronic stress has a very different effect on the amygdala than it does on the hippocampus. The same chronic stress that impairs spatial memory can actually promote the growth of new dendritic spines on amygdala neurons and enhance fear memory[10]. Similar increases in fear learning have been shown following exposure to a brief but intense stress[11]. These effects depend on stress hormones as blocking the effects of glucocorticoids prevents enhancements in fear learning.
This increased amygdala activity following chronic or severe stress is thought to play an important role in conditions such as post-traumatic stress disorder (PTSD) that involve high levels of fear. Individuals with PTSD often experience intrusive memories of the traumatic event and heightened fear to events that are reminiscent of the original trauma. These symptoms appear to partially driven by a hyperactive amygdala, as imaging studies have shown increased amygdala activity in PTSD patients when observing both emotionally-charged and trauma-related images[12].
Ways to prevent the effects of stress
The past year has been a uniquely difficult time for many of us. Fortunately, there are ways we can combat the effects of stress. Putting away the phone before bed and getting plenty of sleep can go a long way towards preventing the effects of stress[13]. Meditation and mindfulness practices have also been shown to effectively reduce stress[14]. Lastly, making sure to get enough exercise has a host of beneficial effects and may help prevent the harmful effects of stress on memory[15].
References
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2. Bliss, T.V. and G.L. Collingridge, A synaptic model of memory: long-term potentiation in the hippocampus. Nature, 1993. 361(6407): p. 31-9.
3. de Kloet, E.R., et al., Stress and the brain: from adaptation to disease. Nat Rev Neurosci, 2005. 6(6): p. 463-75.
4. McIntyre, C.K., et al., Memory-influencing intra-basolateral amygdala drug infusions modulate expression of Arc protein in the hippocampus. Proc Natl Acad Sci U S A, 2005. 102(30): p. 10718-23.
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12. Shin, L.M., et al., Amygdala, medial prefrontal cortex, and hippocampal function in PTSD. Ann N Y Acad Sci, 2006. 1071: p. 67-79.
13. Thomée, S., et al., Mobile phone use and stress, sleep disturbances, and symptoms of depression among young adults–a prospective cohort study. BMC Public Health, 2011. 11: p. 66.
14. Miller, J.J., et al, Three-year follow-up and clinical implications of a mindfulness meditation-based stress reduction intervention in the treatment of anxiety disorders. Gen Hosp Psychiatry, 1995. 17(3): p. 192-200.
15. Head, D., et al., The moderating role of exercise on stress-related effects on the hippocampus and memory in later adulthood. Neuropsychology, 2012. 26(2): p. 133-43.