The parasympathetic nervous system is one of two autonomic nervous system divisions, along with the sympathetic nervous system. The enteric nervous system, which controls the specifics of digestion, is no longer lumped together with the system-wide autonomic nervous system.
Together, they regulate our unconscious bodily activities to ensure survival and procreation, but the role of the endocannabinoid system in the parasympathetic nervous system is vital to its regulation.
What is the Parasympathetic Nervous System (PSNS)?
The parasympathetic nervous system is sometimes referred to as the “rest and digest” system, compared with the sympathetic nervous system, which is referred to as the “fight or flight” system. The PSNS regulates bodily function when an organism is at rest, or more specifically, when the organism isn’t experiencing a threat or some other stress-inducing stimuli. The parasympathetic nervous system also functions to conserve energy: it slows the heart rate, increases intestinal and glandular activity, and relaxes sphincter muscles in the gastrointestinal tract, allowing digestive peristalsis to occur.
Another simple way to explain the activity of the parasympathetic nervous system is with the acronym SLUDD: Salivation, Lacrimation (tear formation), Urination, Digestion, and Defecation. All of these functions are necessary for hour to hour survival, but are detrimental during a period of stress or attack. While the parasympathetic nervous system regulates long term well-being and procreation, the sympathetic nervous system shuts down all functions not critical to immediate survival.
How Does The PSNS work?
The parasympathetic nervous system works in concert with the sympathetic system to regulate many different mechanisms of homeostasis. This is possible because both divisions of the ANS typically innervate the same tissues, and they are both tonically active, meaning they always have a baseline activity. Tonicity allows for there to be both increases, and decreases, in the activity of a system. This is a fundamental way that homeostasis is maintained between states of rest and states of acute stress.
For this mechanism to function properly, parasympathetic nerve fibers run from the brain to the tissue they innervate, where they form a ganglion; a miniature brain inside a peripheral organ that controls the activity of that tissue to an extent. From that ganglion, the signals are refined and dampened, to be exerted over a longer period of time allowing for rest and normal body functions to occur at the proper time intervals. Sympathetic activation during a threat response rapidly inhibits parasympathetic activity. Together, they balance one another and allow for homeostasis throughout many body systems simultaneously.
What is the Role of the Endocannabinoid System in the PSNS?
The endocannabinoid system supports the basic activity of the parasympathetic nervous system in many ways. To begin, if there are changes in the environment then our endocannabinoid system may work to support the parasympathetic nervous system in supporting cell function, eliciting a healthy immune response, reducing the heart rate, stimulating hunger and more. To maintain homeostasis is a complicated task that is undertaken through the cooperation of all systems. Body temperature, blood sugar, and heart rate are three examples of systemic factors that must remain within a small range for anything to be able to work properly.
- Cannabinoid Receptors: proteins embedded in the surface of many types of cells throughout our body. When activated by a cannabinoid – either from our own body or from the cannabis plant – they reduce the activity of the cell on which they’re embedded.
- Endocannabinoids: molecules produced by cells in order to activate the cannabinoid receptors.
- Metabolic Enzymes: proteins that both synthesize endocannabinoids on-demand and degrade them after they have binded to a receptor.
When two cells or sets of cells are communicating with each other, endocannabinoid receptors are generally found on the neuron sending the communication rather than receiving it. The main communication typically is carried out by another kind of signalling molecule, but the cell on the receiving end can release endocannabinoids to help regulate the strength of the signal its receiving from the first cell. This is the basic mechanism of how the endocannabinoid system maintains homeostasis in all of the systems in which it is expressed.
Interestingly, cannabinoid CB1 receptors are found in the vagus nerve, which contains roughly 75% of all parasympathetic neurons. The ability of the endocannabinoid system to modulate hunger may be partially mediated by the presence of CB1 receptors in the vagus nerve. Specifically, they are found in afferent (body to brain) neurons in the vagus nerve, not efferent neurons (brain to body). They regulate the communication of the parasympathetic nervous system between the brain and body, ultimately regulating the regulator of our mood and bodily functions.
Can endocannabinoid deficiency affect the PSNS?
If the parasympathetic nervous system becomes deficient, huge problems will occur. This is a system that is responsible for returning the body to a relaxed state after acute stress or emergency situations: coming down from an adrenaline rush. If the endocannabinoid system, which is involved in regulating the parasympathetic nervous system, fails to do so, it can result in many different conditions from autoimmune diseases to adrenal insufficiencies to cardiovascular disease.
The immense diversity of both expression and histochemical effect of cannabinoid receptors makes elucidating their precise role in any system a complex undertaking. Moreover, the parasympathetic nervous system itself is not entirely understood, and the combined role is based on our loose understanding of their individual functions. What is clear is that the endocannabinoid system is indispensable for the regulation of the system that itself is responsible for regulating our baseline bodily functions, from heart rate to defecation.
More research needs to be conducted into the specific role of the ECS in the PSNS. A deficiency of the endocannabinoid system could devastate homeostasis throughout the entire body, but that also means that the endocannabinoid system could also represent a new therapeutic target for many diseases of the autonomic nervous system with unknown cause.