An article on thinkquest.org provides a good summary of how cortisol kills brain cells:

"Cortisol harms brain in three ways: it interferes with brain's supply of glucose; it also interferes with function of neurotransmitters; and it causes influx of calcium into brain cells. Calcium creates free-radical molecules; therefore, directly killing and damaging brain cells. When the brain is damaged by cortisol, the patient has difficulty paying attention, which prevents adequate storage of memory; therefore, interferes with general intelligence."

"Cortisol killed brain cells by the millions."  These were the words spoken by Dr. Dharma Singh Khalsa when he gave a talk about brain longevity at the Chautauqua Institution in New York state.  View video of his full talk here.

In his article, "Stress, Aging, and the Brain", Dr. Khalsa writes ...

"This rise in cortisol is a normal response to stress to help us cope, but when the stress is over, cortisol levels should return to normal. With chronic stress, however, this does not happen and cortisol levels stay high with disastrous consequences for the brain. Cortisol affects the hippocampus, which is the part of our brain that helps sort and store memories. It prevents it from taking up glucose; it also slows nerve impulse transmission and eventually can lead to death of brain cells. ... Altogether high levels of cortisol are lethal to brain cells and seriously impair our brain function."

Dr. Khalsa refers to the relationship between cortisol and memory as the "Cortisol Connection" in his book "Brain Longevity" where he writes ...

"In moderate amounts, cortisol is not harmful.  But when produced in excess, day after day -- as a result of chronic, unrelenting stress -- this hormone is so toxic to the brain that it kills and injures brain cells by the billions.  I am now certain that chronic exposure of the brain to toxic levels of cortisol is a primary cause of brain degeneration during the aging process."

Dr. Robert Sapolsky is a neuro-biologist at Stanford University and author of the book Stress, the Aging Brain, and the Mechanisms of Neuron Death in which he writes about "how regulation of glucocorticoid secretion fails during aging, producing excessive secretion of these hormones, and how an only recently recognized consequence of that glucocorticoid excess is damage to the brain."^{[page 8]}

Glucocorticoids are a class of steroid hormones in the body and cortisol is a main player among them. The book Stress, the Aging Brain, and the Mechanisms of Neuron Death is actually quite technical. Dr. Sapolsky wrote another book that is more geared for the general public that goes by the fun title Why Zebras Don't Get Ulcers.

An article in "The Franklin Institute" says ...

"The renowned brain researcher, Robert M. Sapolsky, has shown that sustained stress can damage the hippocampus, the part of the limbic brain which is central to learning and memory. The culprits are "glucocorticoids," a class of steroid hormones secreted from the adrenal glands during stress. They are more commonly know as corticosteroids or cortisol."

In the video on the right, you can see what Dr. Sapolsky says about stress in regards to the average professional.

Dr. Sapolsky is also featured in National Geograhic's documentary DVD "Stress: Portrait of a Killer".

Other doctors has also confirms that stress and cortisol is hazardous to the brain.

A book by Shawn Talbott titled Cortisol Connection says on page 82: "Researchers at the Neurological Institute at the University of California at San Francisco (UCSF) have linked excessive cortisol levels to depression, anxiety, and Alzheimer's disease, as well as to direct changes in brain structure (atrophy) leading to cognitive defects (meaning that cortisol can shrink and kill brain cells)."

Dr. Andrew Weil writes in a Q&A, "Cortisol is directly toxic to neurons, it actually destroys hippocampal cells resulting in loss of the ability to voluntarily recall previously learned information. You can lower cortisol levels with meditation and other relaxation techniques."

An article in youramazingbrain.org asks "does stress kill brain cells?"   In conclusion, the article says ...

"The answer seems to be yes.  Stress causes the release of a hormone called cortisol.  Giving rats daily injections of corticosterone (rat cortisol) for several weeks kills certain brain cells. Stressing the rats each day for the same amount of time has an identical effect.  Cortisol has been shown to damage and kill cells in the hippocampus (the brain area responsible for your episodic memory) and there is robust evidence that chronic stress causes premature brain aging."