Parasympathetic nervous system

The parasympathetic nervous system (PNS) is the part of the autonomic nervous system that prepares the “relaxation response” in the body. It is mainly formed by cholinergic fibers and tends to decrease contractility of cardiac muscle. But in smooth muscle, parasympathetic stimulation will increase tone and contractility.

This “rest and digest” response has various possible effects on your body:

• Decrease of stress hormone levels
• The resting heart rate decreases, so less blood is being pumped into the muscles
• Respiration slows down due to low oxygen demand
• The digestive system is prioritized, so the blood flow to the gastrointestinal tract increases
• Pupils shrink
• Organs perform essential day-to-day functions

It consists of preganglionic and postganglionic fibers, both connected in the ganglia. The preganglionic fibers are long, while the latter is short (Figure 1).

Figure 1. Schematic representation of smooth or cardiac muscle parasympathetic control. There is only one neurotransmitter released in the PNS: acetylcholine (ACh) from both preganglionic and postganglionic fiber to the end organ. However, ACh binds to different receptors in this case: nicotinic receptors in the postganglionic fiber and muscarinic receptors in the end tissue.