When the autonomic nervous system ganglia are activated (for example, by low-dose nicotine), both the sympathetic and parasympathetic nervous system innervations of target organs and tissues are simultaneously stimulated. However, the “fright, fight or flight” sympathetic and “rest and digest” parasympathetic nervous systems have opposing effects in most target organs and tissues. So why do the sympathetic and parasympathetic nervous systems not just cancel each other out when activated at the same time?

It is true that in the realm of autonomic nervous system functioning, the sympathetic and parasympathetic systems often represent two sides of the same coin. These systems largely produce opposing effects on the same target organs and tissues. However, what happens when both systems are simultaneously activated? Contrary to intuitive thinking, they don’t simply cancel each other out. Instead, the dominion of activation or excitatory effects takes centre stage.

The Principle of Dominant Excitation: When both the sympathetic and parasympathetic systems are co-activated, it isn’t a zero-sum game. Rather than neutralizing each other, the excitatory effects from each system generally prevail. This principle is observed in a variety of physiological contexts.

  • Case Study 1: Cardiac Function: The sympathetic nervous system aims to prepare the body for ‘fight or flight’ situations and therefore stimulates an increased heart rate. Conversely, the parasympathetic system, often termed the ‘rest and digest’ system, aims to conserve energy and thus slows down the heart rate. In situations where both systems are engaged, the sympathetic system’s excitatory role dominates, resulting in an overall increase in heart rate.
  • Case Study 2: Gastrointestinal Motility: A similar paradigm exists in the domain of gastrointestinal function. The sympathetic system generally inhibits gastrointestinal motility to redistribute energy towards vital organs like the heart and brain. On the other hand, the parasympathetic system promotes digestive processes by enhancing gastrointestinal motility. When both systems are concurrently activated, the parasympathetic system’s excitatory influence gains the upper hand, overriding the inhibitory signals from the sympathetic system. Consequently, the net effect is an increase in gastrointestinal motility.

The dominion of excitatory effects when the sympathetic and parasympathetic systems are activated concurrently has profound implications for physiological homeostasis and has wide-reaching applications in pharmacology and medical science. In summary, when it comes to the tug-of-war between the sympathetic and parasympathetic systems, it’s not about nullifying effects but rather a dominance of excitation.