Stress and Its Effects

Stress occurs when a person’s dynamic equilibrium known as homeostasis is threatened by the presence of physical and physiological events known as stressors.  Incoming sensory information creates an increase in vigilance, arousal, alertness, focused attention, and cognitive processing. This occurs in the limbic brain regions (hippocampus, amygdala, and prefrontal cortex).  This is accompanied by a rush of adrenaline from the adrenal cortex.

Physiological and behavioral responses known as the stress response are triggered to restore homeostasis.  During this response the sympathetic nervous system is rapidly activated which causes the release of the neurotransmitter, noradrenalin from many synapses and the release of adrenaline from the adrenal medulla.

The blood concentration of adrenal glucocorticoids (ex. cortisol) peak after 15- 30 minutes and then decline towards pre- stress levels. These are chemicals which increase the production of glucose, constrict blood vessels, and help our brains regulate stress.  They also affect memory functioning in the hippocampus.  Glucocorticoids act as structural modulators in the limbic brain regions; brief exposure to stress suppresses cell proliferation and increases cell death in the dentate gyrus of the hippocampus. (Kloet et al. 2005)




Stress and Neurogenesis



Stress inhibits adult hippocampal neurogenesis.  Several mammalian species (mice, rats, tree shrews, and marmosets) have exhibited reduced cell proliferation after being subjected to a variety of stress paradigms (Mirescu and Gould, 2006).  Some of these stress paradigms included predator odor (Figure 1a), footshock (1b), and subordination stress (1c) (Tanapat et al., 2001; Malberg and Duman, 2003; Gould et al., 1997).


How is stress mechanistically affecting adult hippocampal neurogenesis?  The answer to this question is unknown, but there are several hypotheses based on current research: Glucocoticoids have been shown to be involved in mediating the effects of stress on neurogenesis as described in the previous section (Wong & Herbert, 2005).  In addition, increased glutamate neurotransmitter release (which enhances excitatory transmission) has been shown to affect neurogenesis (by decreasing cell proliferation) (Abraham et al., 1998).

References can be found here.

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