Research would suggest that the type of "incessant" or "severe" stress required to precipitate a "breakdown" would the higher cognitive functions performed within the frontal lobes. As I am unsure what level of understanding you are requiring on this topic, I have included suitable quotes from a relevant study, which provides the answer to your question.
I quote Amy F. T. Arnsten's study of the effects of stress on the frontal lobe:
The prefrontal cortex (PFC)—the most evolved brain region—subserves
our highest-order cognitive abilities. However, it is also the brain
region that is most sensitive to the detrimental effects of stress
exposure. Even quite mild acute uncontrollable stress can cause a
rapid and dramatic loss of prefrontal cognitive abilities, and more
prolonged stress exposure causes architectural changes in prefrontal
dendrites. Recent research has begun to reveal the intracellular
signalling pathways that mediate the effects of stress on the PFC.
This research has provided clues as to why genetic or environmental
insults that disinhibit stress signalling pathways can lead to
symptoms of profound prefrontal cortical dysfunction in mental
illness.
Within the conclusion:
The detrimental effects of stress on PFC networks are particularly
problematic in the ‘information age’, when PFC-mediated cognitive
abilities are increasingly needed for success. understanding the
molecular mechanisms that alter PFC structure and impair PFC function
during stress exposure will help to reveal how genetic and
environmental insults can increase an individual’s susceptibility to
PFC deficits. Identifying the molecular mechanisms that alter PFC
function will provide the foundation for a new era in psychiatry, when
we will understand how genetic changes affect intracellular
signalling, neural development and neurophysiology in the circuits
that underlie neuropsychiatric symptoms. It is hoped that this
information will provide the framework for therapies aimed at
rectifying molecular errors and ameliorating the symptoms of mental
illness.
Stress signalling pathways that impair prefrontal cortex structure and function
Amy F. T. Arnsten
Nat Rev Neurosci. 2009 June; 10(6): 410–422.
doi: 10.1038/nrn2648