Understanding Stress and Autonomic Nervous System
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The central nervous system (CNS) mediates the distribution of resources to deal with internal and external demands. Perceptions and assumed threats to survival may promote a massive withdrawal of PNS tone and a reciprocal excitation of SNS tone. The trade-off between internal and external needs may be used in developing definitions of stress and homeostasis. In this model stress and homeostasis are interdependent. Homeostasis reflects the regulation of the internal organs and stress reflects the subjugation of internal needs in response to external needs. This is why measuring PNS tone may provide the indexing variable for defining stress and stress vulnerability. Stress and stress vulnerability can therefore be defined in the absence of major shifts in SNS tone. In research assessing stress in neonates in healthy children, withdrawal of PNS tone to a stressor is paralleled by an increased expression of SNS tone. However, in severely compromised children they may not exhibit SNS reactivity and SNS tone might be low. These children generally have low PNS tone and very little PNS reactivity. Clinically, they would be described as chronically stressed and physiologically unstable. Thus PNS tone withdrawal in relation to SNS tone may define stress and high PNS tone prior to the stressor would represent low stress vulnerability, whilst low PNS tone would represent high stress vulnerability. Individuals therefore exhibiting problems of homeostasis will have the greatest stress vulnerability. |
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In many physiological systems efficient neural control is manifested as rhythmic physiological variability, and within normal parameters the greater the amplitude of oscillation, the healthier the individual.
The greater the amplitude of organized rhythmic physiological variability, the greater the response potential or possible range of behavior. Individuals with attenuated physiological variability would then exhibit a lack of physiological and behavioral flexibility in response to environmental demands. This was the situation observed with severely ill infants.
Stimulation of other PNS afferents seems to give a reflex increase in cardiac vagal tone and therefore the latter seems to reflect the general PNS input to the viscera.
The most readily available measure of PNS activity is derived from heart rate pattern in response to breathing i.e. respiratory sinus arrhythmia. The heart rate increases with inspiration and decreases during expiration under the control of efferent parasympathetic impulses along the vagus nerve. Heart rate patterns, like behavioral processes, are dependent on the status of the nervous system and the quality of neural feedback. Stress results in a disorganization of the rhythmic structure of both behavior and autonomic state. Thus, measures of cardiac vagal tone provide a window into the central processes necessary for organized behavior. If vagal tone is a sensitive index of the functional status of the nervous system, then we would predict that individuals with greater vagal tone would exhibit a greater range of competent behaviors.
The pattern of heart rate reflects the continuous feedback between the CNS and the peripheral autonomic receptors.
The primary source of HRV is mediated by phasic increases and decreases in neural efferent output via the vagus nerve to the heart. The greater the range of the phasic increases and decreases, the “healthier” the individual. An attenuation in the range of homeostatic function is paralleled by a reduction in vagal tone.
HRV is a marker of the efficiency of neural feedback mechanisms and may index health status or the individual’s capacity to organize physiological resources to respond appropriately.
hus, the better the “organised” physiological variability, the greater the range of behavior. States characterized by attenuated vagal influences should be paralleled by reduced behavioral flexibility in response to environmental demands. So, not only the basal level of vagal tone (measurable during sleep) is important but also the vagal responsivity during sensory and cognitive challenge. Individuals with greater vagal responsivity as exemplified in larger heart rate acceleration also exhibited fewer signs of distress.
The greater the amplitude of organized rhythmic physiological variability, the greater the response potential or possible range of behavior. Individuals with attenuated physiological variability would then exhibit a lack of physiological and behavioral flexibility in response to environmental demands. This was the situation observed with severely ill infants.
Stimulation of other PNS afferents seems to give a reflex increase in cardiac vagal tone and therefore the latter seems to reflect the general PNS input to the viscera.
The most readily available measure of PNS activity is derived from heart rate pattern in response to breathing i.e. respiratory sinus arrhythmia. The heart rate increases with inspiration and decreases during expiration under the control of efferent parasympathetic impulses along the vagus nerve. Heart rate patterns, like behavioral processes, are dependent on the status of the nervous system and the quality of neural feedback. Stress results in a disorganization of the rhythmic structure of both behavior and autonomic state. Thus, measures of cardiac vagal tone provide a window into the central processes necessary for organized behavior. If vagal tone is a sensitive index of the functional status of the nervous system, then we would predict that individuals with greater vagal tone would exhibit a greater range of competent behaviors.
The pattern of heart rate reflects the continuous feedback between the CNS and the peripheral autonomic receptors.
The primary source of HRV is mediated by phasic increases and decreases in neural efferent output via the vagus nerve to the heart. The greater the range of the phasic increases and decreases, the “healthier” the individual. An attenuation in the range of homeostatic function is paralleled by a reduction in vagal tone.
HRV is a marker of the efficiency of neural feedback mechanisms and may index health status or the individual’s capacity to organize physiological resources to respond appropriately.
hus, the better the “organised” physiological variability, the greater the range of behavior. States characterized by attenuated vagal influences should be paralleled by reduced behavioral flexibility in response to environmental demands. So, not only the basal level of vagal tone (measurable during sleep) is important but also the vagal responsivity during sensory and cognitive challenge. Individuals with greater vagal responsivity as exemplified in larger heart rate acceleration also exhibited fewer signs of distress.
