The autonomic nervous system consists of the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The sympathetic branch of the ANS increases heart rate and is active during activity and stress, while the parasympathetic branch slows heart rate and is active during rest and recovery and facilitates digestion. The intrinsic heart rate in healthy humans ranges from 107 beats/min at 20 years old, to 90 beats/min at 50, regulated by the sinus node. However, the resting heart rate is typically 60-70 beats/min, due to the effects of the sympathetic and parasympathetic nervous system, hormones and breathing.
Heart rate variability (HRV) — the variation in timing between successive heart beats — has been confirmed to be a reliable indicator of the state of the autonomic nervous system, and in recent years has been shown to be a useful indicator of the body's adaptation to chronic stress. Higher HRV indicates parasympathetic predominance, whereas low HRV indicates sympathetic predominance. HRV tends to decrease during illness, periods of chronic stress, disrupted sleep and physical overtraining when the sympathetic branch of the ANS is dominant. Deconditioning, due to prolonged inactivity and bed rest, also reduces HRV.
Neuroimaging studies have shown that the medial prefrontal cortex (mPFC) seems to be important in regulating autonomic responses to stress, with the dorsal mPFC being linked to sympathetic outflow, and the ventromedial PFC (vmPFC) being associated with parasympathetic outflow. A reduction in vmPFC output is associated with increased heart rate and sympathetic activation. The vmPFC appears to play an important role in regulating the suppression of affective responses to negative emotional stimuli by regulating amygdala activity, and becomes activated when a situation is perceived as controllable. Patients with depressive disorders show a reduced suppression of the amygdala by the vmPFC.
HRV tends to be reduced in IBS, CFS, overtraining and burnout, and it seems to be due to reduced parasympathetic activation. This then leads to sympathetic predominance, resulting in higher resting heart rate.
Many symptoms of CFS are likely to be caused by the reduced parasympathetic nervous system activation and sympathetic predominance, such as: poor digestion and gastroparesis, anxiety, dizziness, IBS, and postural tachycardia (POTS).
The ultimate cause of this ANS disruption is likely to be chronic stress, which has been shown to cause reductions in HRV and sympathetic predominance, and is known to be a precipitating factor in CFS.
The ventromedial prefrontal cortex seems to be an important regulator of the parasympathetic nervous system: reduced activation of the vmPFC causes a reduction in suppression of the amygdala in response to negative stimuli, which then results in reduced parasympathetic output. The vmPFC is more active when stressful situations are seen as controllable, which points towards a potential reason why CFS can persist due to the patient being in what seems to be an uncontrollable illness state.
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DISCLAIMER: Mind-Body-Health.net is an educational resource for chronic fatigue syndrome (CFS), myalgic encephalomyelitis (ME), burnout and related disorders, and is not giving medical advice. Seek advice from a medical practitioner before making any changes to your life, or if you experience worsening symptoms. CFS is a diagnosis of exclusion, so it is important to rule out other causes for illness.