Page 179 - Biobehavioral Resilence to Stress

156 Biobehavioral Resilience to Stress

that the size of the hippocampus itself may be reduced (Bremner et al., 2000).
Recent work with magnetic resonance imaging indicates that biochemical
changes (e.g., reduction in N-acetylaspartate) may occur even without ana-
tomical changes (Schuff et al., 2001).

Physiological Limits to Risky Behavior

Were it not for neural mechanisms that limit behavior through fatigue, pain,

and other afferent processes, human beings would frequently behave in ways

that exceed their physiologically tolerable limits. These processes are adap-
tive to the extent that they limit voluntary risky behavior under ordinary
circumstances. However, their related effects are not necessarily adaptive

in extreme environments where fatigue, loss of motivation, disorientation,

confusion, and impaired judgment may make it difficult or impossible to

survive. Th us, effective psychological resilience to stress in extreme environ-

ments should ideally involve knowledge and awareness of one’s own limits,

and an ability to push to the limits. This is the rationale for some types of
brutally intensive military training programs that restrict food and sleep
under harsh environmental conditions, such as the Army’s Ranger course.
By such training, personnel become more aware of how much physical and
psychological stress they can tolerate. Nonetheless, highly motivated indi-
viduals can exceed their own limits by pushing themselves beyond rational
indicators of extreme stress, injury, illness, or performance decrement. Th is
is illustrated by the many documented cases of heat and musculoskeletal
overuse injuries that occur among athletes, soldiers, and public safety per-
sonnel during training, in real operational environments, and in emergency
situations where soldiers and public servants sometimes feel compelled to
“do or die” for mission success or survival.
Brain physiological mechanisms support key behavioral limiters of max-
imal and excessive physical and mental engagement. Central limiters include
fatigue, loss of motivation, pain, discomfort, and conditioned avoidance.

Physiological effectors include hypoxia, excessive or inadequate glucose

levels, fluid compartment shifts, changes in acid–base balance, and stress-
induced neurotransmitter release or imbalance. Performance at high altitude
is affected by hypoxemia, but hyperventilation and consequent respiratory

alkalosis may have central neural effects as well. Dehydration and heat strain

affect cardiovascular mechanisms, leading eventually to a reduction in oxy-

gen and glucose delivery to the brain (Cade et al., 1992). During prolonged

work effort, glucose delivery to the brain may be the key fatiguing limiter of
behavior (Frier, 2001) and sets a lower ceiling by limiting voluntary energy
expenditure (Spurr & Reina, 1988). When adequate energy is provided in a
readily accessible form (e.g., glucose), very high levels of human sustained

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