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| Image by Slowgogostock; {http://www.flickr.com/photos/45250623@N03/} |
I’m going to take a step back from focusing on the traits of
my family to discuss what effects stress can have on our mental capacity. If
you’re like me, you have surely had many nights, studying and stressing for
hours on end, preparing for a class, a presentation or an exam. But, when the
time comes to showcase what you’ve learned, mentally you feel worse off than before
you started.
Our lives are inevitably riddled with stressful events that
require our optimum alertness and insight to handle. Once again, our biology in
that face of chronic stress seems to be more harmful than helpful. Long-term stress
can have evitable effects on our cognition and memory in negative ways.
It is helpful to understand a little background information
about how our brains allot effort in storing different kinds of memory. There
are two important regions in the brain that work in memory storage and
retrieval: the cortex and the hippocampus. Without any elaboration, the cortex
specializes in storing memories, while the hippocampus places and accesses
memories in the cortex.
Memories are stored in patterned displays of excited
neurons, called neural networks. This is why all sorts of approximate
contextual information, such as a certain smell, can trigger memory recollection.
Multiple routes of exciting a certain neural network can extract memory and
information.
The cortex and hippocampus make use of a neurotransmitter
called glutamate that is critical in creating memory. Glutamate is a uniquely
acting neurotransmitter. In glutamatergic synapses, a small amount of glutamate
released activates no response. A bit more is released and still nothing
happens. It is not until a threshold of glutamate is passed along to the second
neuron that an extensive wave of excitation is triggered. This is when we ‘learn’
something. This is when something ‘clicks.’
Once a synapse has experienced a certain number of
glutamate-driven excitations, the synapse is strengthened. It becomes
persistently more excitable and takes less of a signal the next time to reach
that excited state. The strengthened synapse has just been ‘potentiated.’
Time to fit chronic stress back into the picture.
As we move along the stress scale, from no stress, to mild
stress, to temporary moderate stress – memory improves. As we continue along to
severe and chronic stress, we see a decline in memory. High stress and
glucocorticoid levels are certainly the culprits. It could be that these things
make for a generally compromised brain altogether, affecting other areas as
well, but memory recollection is the most notable.
Interestingly, these conditions seem to more greatly disrupt
eliciting prior explicit memory rather than the formation of new memory.
Correspondingly however, stress does disrupt what’s called the ‘executive
function’ in forming and storing and retrieving new and old memory (Sapolsky,
2004). This concerns what is done with the information, whether it’s organized
strategically and how it guides judgments and decisions.
There are several ways that stress evidentially damages the
neural networks of the hippocampus. First, the hippocampus neurons do not
perform as well. Even in the absence of glucocorticoids, an overactive
sympathetic nervous system can disrupt long-term potentiation of the
hippocampus; the neurons are not firing as effectively. High and chronic levels
of glucocorticoids only elevate the problem.
The hippocampus has two types of receptors for
glucocorticoids, one of which is ten times better at accepting the hormone than
the other. The low-affinity receptors are only activated by major or
long-standing stress, while the high-affinity receptors are activated all the
time. These high-affinity receptors enhance potentiation of neural networks;
the low-affinity receptors do the opposite. (Sapolsky, 2004)
Second, long-term stress disconnects neural networks. Under
prolonged stress and excessive exposure to glucocorticoids, the neural branches
– axons and dendrites – begin to atrophy and retract. The good news: it appears
that once the stressor is over with these neurons can re-grow or re-strengthen
their connections. The memory is still there, you just have to draw from more
associative cues to extract it because the neural network is less efficient
than before. (Sapolsky, 2004)
Another happy fact: contrary to what you have probably been
told, as I was, certain areas in the brain do
make new neurons. There are two main areas of the brain that do this - one of
them being the hippocampus! And now for the bad news: the third way in which
stress detrimentally affects cognition is in inhibiting the creation of new
neurons in the hippocampus. Once again the underpinning bullies are
glucocorticoids and an overactive sympathetic nervous system. (Sapolsky, 2004)
Fourthly, the neurons in the hippocampus become more
susceptible to damage under chronic stress conditions. During a continuous
stressor, glucose levels eventually even out to their normal resting levels. As
the stressor continues, glucose to the brain is inhibited, specifically to the
hippocampus, by about 25 percent less than normal levels due to high
glucocorticoids. This would be tolerable for healthy neurons. However, in weak
neurons, like those experiencing a severe neurological stressor, the neuron is
far liker to just die off. (Sapolsky, 2004)
Moreover, stress can burden cognition simply by allocating
more energy to stress-related intrusive thoughts and avoidance behavior, which
can furthermore be exacerbated by the perception of stress. In fact, high
levels of perceived stress and diagnosed PTSD both equally elicit high
occurrences of cognitive failure – meaning forgetfulness and/or lack of
attentiveness. There is much
growing research surrounding PTSD and the notion of whether it is the traumatic
event or the subjective response to the event that more heavily tips the scale
to induce chronic stress and complications with cognition. (Boals & Banks,
2012)
Conclusively, stress isn’t just a pain in your side
(sometimes, literally), it’s a pain in your head. We often think of stress as
an abstract concept or state of being that comes and goes. In reality, stress
is rooted deeply in our biology and can leave its trace (in chronic conditions)
even when the stressor has passed.
Reference:
Boals, Adriel & Banks, Jonathan B. (2012): Effects of
traumatic stress and perceived
Stress
on everyday cognitive functioning, Cognition & Emotion, 26:7, 1335
-1343.
Doi: http://dx.doi.org/10.1080/02699931.2011.651100.
Sapolsky, Robert M. (2004). Why Zebras Don’t Get Ulcers:
The Acclaimed Guide to
Stress, Stress-Related Diseases, and Coping. New York: St. Martin’s Griffin
Press.
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