Why Zebras Don't Get Ulcers: A Guide to Stress, Stress Related Diseases, and Coping

Stress can either enhance or obstruct memory, with acute, mild-to-moderate stressors often boosting memory, while long-term or severe stressors inhibiting memory.

Memory is often discussed in terms of short-term and long-term memories. Remote memories, or childhood memories, are a subtype of long-term memory. Sapolsky also distinguishes explicit or declarative memory, which deals with factual information, and implicit or procedural memory, which deals with remembering how to do things, like riding a bike. Memories can swap between explicit and implicit, such as occurs when a musician learns a new piece of music—it is challenging at first, but with repetition, the process of playing becomes automatic. Implicit memories can also transition to being processed explicitly, which occurs when one is encouraged to pay attention to the details of how they perform an automatic task: “Imagine descending a flight of stairs in an explicit manner” (205).

Two of the primary memory regions of the brain are the cortex and the hippocampus. Sapolsky compares the hippocampus to a keyboard and the cortex to a hard drive—the hippocampus helps input or retrieve memories from the cortex memory bank. The critical role of the hippocampus in memory was fully realized in the 1950s when a patient with epilepsy, H. M., had a significant portion of his hippocampus removed and lost the ability to form long-term explicit memories.

Scientists used to think memories were stored in individual neurons, but they now propose that memories are stored in networks, or within patterns of neural activity. Memories can be strengthened through repeated activation. The repeated activation results in more neurotransmitters, like glutamate, being released, which enhances neuronal excitation. Eventually, the glutamate excitation reaches a high-enough level for learning or memorization to take place.

Short, moderate stressors enhance emotional memories, but the accuracy of the memories is not reliable. During stress, the sympathetic nervous system delivers extra energy to the brain, which enhances the ability to retrieve and to store explicit memories. Glucocorticoids help transfer memories from short-term to long-term memories, and stress boosts sensory perception. Due to activation of the amygdala, a region of the brain dealing with fear and anxiety, emotional memory is also boosted during stress.

With severe or prolonged stress, explicit memory is inhibited, particularly the memory retrieval process. Glucocorticoids interfere with explicit memory, as can be seen in Cushing’s syndrome. Individuals with Cushing’s syndrome have high levels of glucocorticoids secreted in response to tumors, and they can develop Cushingoid dementia, which impacts explicit memory. Medically-administered glucocorticoids can cause memory issues over time. Prolonged stress also disrupts the function of the hippocampus, as neural networks within the hippocampus can atrophy; the formation of new neurons, or neurogenesis, is obstructed; and some neurons may die off.

Studies, including one conducted by Sapolsky, have shown that high levels of glucocorticoid cause the hippocampus to degenerate at a faster rate than normally seen with aging, while low glucocorticoid levels appear to delay degeneration with age. Hippocampal atrophy has been documented in people with Cushing’s syndrome, post-traumatic stress disorder, major depression, and multiple bouts of jet lag, and it has been documented as a typical occurrence in aging. High levels of glucocorticoids also correlate with more neurological impairments for stroke patients.

Sapolsky notes that more research is necessary to deepen scientists’ understandings of these correlations. Some studies also demonstrate that hippocampal atrophy can reverse if glucocorticoid levels are reduced. Sapolsky suggests more research be conducted to determine the cognitive implications of natural and medical glucocorticoids, which are prescribed for an array of conditions, including autoimmune diseases and allergies. The consequences of the stress-response during prolonged stress are likely due to the human body not yet having adapted to modern chronic stress or neurological injuries.

Sapolsky reflects on a particularly difficult and sleepless night he and his wife spent attempting to soothe their newborn son. Sleep is a stress catch-22: Lack of sleep is a stressor, and stress disrupts sleep. Sleep occurs in different stages: shallow sleep, comprised of Stages 1 and 2 sleep; deep sleep, comprised of Stages 3 and 4; and rapid-eye movement, or REM sleep. Stage 4 sleep is also referred to as “slow wave sleep.” Sleep stages generally occur in repeated 90-minute cycles. Many parts of the brain slow down or are isolated during slow wave sleep, and brain activity increases during REM sleep.

However, activity in the frontal cortex, where reasoning and emotional filtering take place, reduces, which is why dreams are often erratic and illogical. The brain uses large amounts of energy, and sleep is critical as it allows the brain to flush out waste, to rebuild energy supplies, and to dream. Dreams, studies suggest, help to exercise important but underused neuronal pathways. Sleep benefits problem-solving skills and memory consolidation, although sleep deprivation boosts performance on reversal tasks—those that ask someone to do something in a reverse order, like reciting the alphabet backwards.

Lack of quality sleep is a stressor. Although the sympathetic nervous system is active during REM sleep, the parasympathetic nervous system is active during slow wave sleep, which is the predominant and most restful stage of sleep. Sleep deprivation results in more sympathetic nervous system activity and increased levels of glucocorticoids. The increased glucocorticoids are associated with declines in cognitive performance.

Modern society requires sleep deprivation from a segment of the population: “We’re accustomed to all sorts of amenities in our modern lives: overnight deliveries of packages, advice nurses who can be called at two in the morning, round-the-clock technical support staff” (234). Day workers also experience disruptions to their sleep cycle: The average person sleeps 1.5 hours less than the average person in 1910, due, in part, to 24-hour access to sources of entertainment and the intense pressure to be as financially productive as possible.

Stress often results in less or lower-quality sleep. The CRH released in response to stress disrupts sleep by exciting the brain’s anxiety and arousal pathways, and glucocorticoids can obstruct memory consolidation during sleep. However, biological sleep requirements generally overpower stress-based sleep difficulties, and sleep deprivation is a relatively minor stressor.

Stress hormones contribute to the waking process. One study showed that stress hormones rise about an hour before a person wakes, whether they sleep as long as they want or are woken up at a specified time. The unhealthiest sleeping condition is when sleep is unpredictably fragmented, such as nighttime emergency service providers experience.

Sapolsky posits that “the great secret of our species” is that “we will die and we know it” (239). The dread of aging and death does not exist in every culture. Sapolsky discusses an elderly man he met from the Masai tribe in East Africa. The man was scarred and had poor vision and a leg infection that Sapolsky treated. The man was accompanied by a woman who talked about the man’s youth and impending death without grief or anxiety. Although many in developed countries fear death, the aging process is usually smooth, with less disability and institutionalizations than one might expect. Some cognitive skills and the quality of relationships improve with aging, and levels of happiness increase.

As a person ages, they become less tolerant of stress. Sometimes their stress-response is diminished. This has both physical and cognitive implications. For instance, older people have more difficulty regulating their body temperatures under stressful conditions. They perform equal to younger individuals on intelligence tests with no time limit but poorer if a time limit is imposed. It is often more difficult for older bodies to stop the stress-response. Stress hormones are secreted after the immediate stressor has passed, and stress hormone levels, in general, are often higher than in younger individuals. This results in implications such as obstructed hippocampal neurogenesis.

Stress can also impact the aging process, and, in some species, excessive glucocorticoids cause death in elderly subjects. Salmons die as a result from the high levels of glucocorticoids secreted during the spawning process. That glucocorticoids cause the death of salmon is proven through experiments in which their adrenal glands were removed after spawning, and the fish survived for a year. However, this is not the case in humans and many other species, although the body’s ability to regulate glucocorticoid levels diminishes. This is likely due to dysfunctional hippocampal neurons that are less sensitive to glucocorticoids and, thus, are less effective at sending signals to stop glucocorticoid secretion.

Hippocampal damage is often caused by glucocorticoids, and hippocampal damage results in increased levels of glucocorticoids. Not every individual experiences these traits of aging, but Sapolsky notes that successful human aging will be discussed in Chapter 18. He also argues that scientists are likely years away from definitively knowing the full extent of how glucocorticoids impact the aging brain. Sapolsky suggests that, at this point, the reader may be “frazzled” by the imposing information presented so far, and he notes the rest of the book will shift its focus to stress management.