PMC4316402_01
The Core Response Network (CRN). The CRN organizes immediate, instinctive response to environmental challenges, prior to extensive cortical processing. It includes the autonomic nervous system (hypothalamus), the limbic emotional system (amygdala, hippocampus, septal region), the emotional motor system (portions of the basal ganglia, red nucleus, periaqueductal gray), and the reticular arousal systems. All these systems interact strongly through multiple feed-back and feed-forward connections, forming a complex dynamical system which can enter various discrete functional and dysfunctional states.
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F1
Cortical control of the CRN. We suggest that the influence of conscious conceptual thought processes on the CRN is relatively weak and indirect, whereas the influence of those portions of the cortex mediating interoceptive, proprioceptive and kinesthetic awareness is relatively strong and direct. These areas include the insula and anterior cingulate cortex, which have been hypothesized to be involved in cortical control of the ANS; and the sensorimotor and (especially) pre-motor cortex, involved in kinesthetic and proprioceptive experience and in planning and imagining movement, as well as the parietal cortex involved in body schema, and the ventro-medial prefrontal cortex.
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All these areas have very rich and direct communication with the subcortical networks mentioned above, and SE views them as the basis for voluntary intervention on the dysregulated subcortical networks; see Figure 2.
Acute (mild) stress response. In response to a mild stressor the ANS (and the whole CRN) responds with sympathetic activation, accompanied by a reciprocal lessening of vagal (parasympathetic) tone. Usually this activation will support an appropriate response to the stressor; this response will be accompanied by proprioceptive feedback that the response has been successfully completed. Sympathetic activation then diminishes, vagal tone returns to normal, and the whole CRN resets to normal resilient functioning.
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He acknowledges Gellhorn's seminal discovery that, although under normal circumstances the sympathetic and parasympathetic (or ergotropic and trophotropic) systems maintain a reciprocal relationship and return to baseline after disturbance (see Figure 3) following even moderately intense disturbance they can become "tuned" (Gellhorn,), chronically biased in one direction, and can fail to return to baseline; see Figure 4.
Chronic stress response. If the stressor is above a certain intensity or duration, the sympathetic response is more intense; if there is an inadequate defensive response, the system as a whole may fail to reset to normal functioning, remaining "tuned" to excess sympathetic and deficient parasympathetic activation. This state may persist indefinitely, giving rise to a state of "chronic stress," where the system responds inappropriately to environmental challenge with excess activation. Note that this is not "allostatic wear and tear," but an altered (dys-)functional state; such a chronic state is a major contributor to allostatic over-load. Through appropriate intervention, the system can be returned to a normalized, fully functional state; but without such intervention the dysfunctional state may last indefinitely.
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He acknowledges Gellhorn's seminal discovery that, although under normal circumstances the sympathetic and parasympathetic (or ergotropic and trophotropic) systems maintain a reciprocal relationship and return to baseline after disturbance (see Figure 3) following even moderately intense disturbance they can become "tuned" (Gellhorn,), chronically biased in one direction, and can fail to return to baseline; see Figure 4.
Traumatic stress response. In the face of extreme challenge, when either the situation is extremely threatening and overwhelms the capacity of the organism to respond effectively, or if the response is prevented in some way (restraint), there is first an extreme sympathetic (ergotropic) activation with loss of vagal tone. With continued challenge, there is a sudden intense co-activation of the parasympathetic (dorsal vagal) system along with the sympathetic, leading to freeze, collapse or dissociation. The ANS (and whole CRN) becomes locked into a dysfunctional state of extremely high activation of both the sympathetic and parasympathetic systems, and may oscillate erratically between extremes. This may manifest as alternating depressive shutdown and extreme anxiety or rage. This is not the result of wear and tear, but is a specific dysfunctional state of operation of the complex dynamical system, which through appropriate intervention can be returned to normal resilient functioning.
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F5
At precisely what point the stress should be regarded as "traumatic" is less important than the understanding of the nature of the dysregulation of the nervous system; however, the phenomenon (demonstrated in cats by Gellhorn,) of extreme co-activation of sympathetic and parasympathetic systems under life-threatening conditions offers a compelling model for the freeze, collapse, and dissociation often observed in PTSD (Nijenhuis et al.,; Halvorsen,); see Figure 5.