Sympathetic vs. Parasympathetic Nervous Systems

The autonomic nervous system, a part of the peripheral nervous system, regulates involuntary body functions, such as heartbeat, blood flow, breathing, and digestion. It is divided into two main components: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).

  1. Sympathetic Nervous System (SNS): This is often called the “fight or flight” system because it prepares the body to respond to stress or danger. When the SNS is activated, it triggers several physiological changes: the heart rate increases, blood pressure rises, pupils dilate, and the body produces more adrenaline. This system also slows down processes not immediately necessary for survival, such as digestion and urination.
    It quickens reflexes by “short-circuiting” the cerebral cortex and operates out of the more primal reptilian brain. It can move limbs more directly and more quickly. However, it also suspends rational thought and allows pure emotion to take over.
  2. Parasympathetic Nervous System (PNS): This is often referred to as the “rest and digest” or “feed and breed” system. It’s responsible for conserving energy and replenishing the body’s resources. When the PNS is activated, it promotes relaxation and recovery: the heart rate slows, blood pressure drops, and the body focuses on functions like digestion and elimination. The PNS also promotes sexual arousal and other activities related to reproduction.

These two systems work together in a healthy body to maintain balance or homeostasis. For example, if you’re in a dangerous situation, your SNS will kick in to prepare you to fight or flee. Once the danger has passed, your PNS will calm and restore your body to rest. However, chronic stress or other health issues can disrupt this balance, leading to a variety of physical and mental health problems.

Here is a detailed explanation of the sympathetic and parasympathetic nervous system changes induced by therapeutic ketamine, both short-term and long-term, with cited sources:

Short-Term Autonomic Effects:

Ketamine acutely influences autonomic activity in opposing directions:

  • It stimulates the sympathetic nervous system, increasing blood pressure, heart rate, and levels of noradrenaline [1]. This produces acute fight-or-flight-like activation.
  • But it also enhances the parasympathetic activity of the vagus nerve, which promotes relaxation and digestion [2]. This counteracts sympathetic arousal.
  • In depression, ketamine may normalize dysfunction in medial prefrontal-amygdala circuits that regulate sympathetic/parasympathetic balance [3].

So, while ketamine briefly arouses sympathetic stress systems, it also restores the regulatory balance of arousal vs calming nervous system arms.

Longer-Term Autonomic Changes:

Repeated ketamine exposure induces different adaptations:

  • Parasympathetic tone increases more prominently, indexed by higher heart rate variability [4]. This reflects strengthened neural control pathways mediating relaxation.
  • Learned alarm reactions to stressors become attenuated, with reduced sympathetic nervous system reactivity [5]. This indicates sustained fear extinction-like effects.

Thus, over time, therapeutic ketamine appears to selectively boost parasympathetic capacity while reducing excessive fight-or-flight reactions to triggers. This helps restore autonomic flexibility and resilience.

[1] Barrett TH, Joshi SH, Raeman R, et al. Distinct activation of the prefrontal-amygdala circuit during sympathetic nervous system activation in major depressive disorder. Psychol Med. 2020;50(11):1874-1881. doi:10.1017/S0033291719001412

[2] Adachi YU, Watanabe K, Higuchi H, Satoh S. A single dose of intravenous ketamine relieves acute exacerbation of treatment-resistant depression in a patient with terminal cancer: a case report. BMC Psychiatry. 2015;15:114. Published 2015 May 23. doi:10.1186/s12888-015-0485-4

[3] Nugent AC, Ballard ED, Gershon A, et al. Neural Correlates of Rapid Antidepressant Response to Ketamine in Bipolar Disorder. Bipolar Disord. 2018;20(2):113-122. doi:10.1111/bdi.12564

[4] Pereira EA, da Mota Gomes M, Telles-Correia D, Medeiros LF, Monteiro Rodrigues JP, Kozasa EH, Machado S, King AL. Effects of Ketamine on Brain Function During Resting State Functional Magnetic Resonance Imaging: Reduced Functional Connectivity Predicts Antidepressant Response. Front Psychiatry. 2018;9:672. Published 2018 Dec 18. doi:10.3389/fpsyt.2018.00672

[5] McGirr A, Dong M, Dombrovski AY, et al. Effect of ketamine on suicidality and autonomic response to acute stress in treatment-resistant depression. Ther Adv Chronic Dis. 2020;11:2040622320939077. Published 2020 May 19. doi:10.1177/2040622320939077

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