Parasympathetic Nervous System: Structure, Function & Diseases

The parasympathetic nervous system is the part of the autonomic nervous system that provides rest and relaxation in the body. It influences a variety of internal organs. The parasympathetic nervous system coordinates the functions of the organs so that the entire body can slide into a state of rest.

What is the parasympathetic nervous system?

Schematic diagram of the parasympathetic nervous system Click to enlarge. The parasympathetic nervous system is the resting nerve of the body. Together with the sympathetic nervous system, it forms the autonomic nervous system. As part of the autonomic nervous system, the parasympathetic nervous system cannot be influenced voluntarily. It independently controls those processes in the body that are associated with recovery, rest and relaxation. Metabolism and heart rate slow down under the influence of the resting nerve. At the same time, it promotes activities that the body performs in a state of rest, such as digestion and a relaxed view of the environment. Nerve fibers of the resting nerve connect the brainstem and the sacral part of the spinal cord with various internal organs. Communication takes place in both directions: Nerve fibers send information from the organs to the brain and spinal cord. The brain and spinal cord send information to the organs. Thus, both internal and external impulses can lead to activation or deactivation of the parasympathetic nervous system. The parasympathetic nervous system is the antagonist of the sympathetic nervous system, which provides increased activity and performance in the body.

Anatomy and structure

The nerves of the parasympathetic nervous system emerge from the brainstem (midbrain and medulla oblongata) and the sacral portion of the spinal cord. They are therefore also grouped under the term craniosacral system. The nerve fibers of the brainstem influence the internal muscles of the eyes as well as the salivary and lacrimal glands. The largest nerve of the parasympathetic nervous system, the tenth cranial nerve, continues to originate from the brainstem. It is very long and influences the function of most internal organs. The area of influence of the tenth cranial nerve is very extensive. It extends to the last third of the colon. From the so-called Cannon-Böhm point, nerve fibers originating from the sacral part of the spinal cord supply the colon. They continue to supply nerves to the urinary bladder and genitals. The Cannon-Böhm point is not an exact point. It is a transition zone where nerves from both the brainstem and the sacral portion of the spinal cord supply the colon.

Function and tasks

The parasympathetic nervous system ensures that the body recovers. It does this through a series of processes initiated by the parasympathetic nervous system. The resting nerve stimulates the production of secretions and fluids in the lacrimal and salivary glands, as well as in the bronchi and digestive tract. The body excretes toxins through the lacrimal fluid. The bronchi constrict and mucus production increases as oxygen demand decreases. Increased saliva production prepares the body for food intake. Further digestion of food is favored as the parasympathetic nervous system stimulates intestinal activity. Increased intestinal movements aid digestion, as does increased glandular secretion in the digestive organs. The resting nerve causes the urinary bladder to empty and promotes defecation. The heartbeat slows down under the influence of the parasympathetic nervous system. Blood pressure also decreases. In the genital area, the vessels dilate. The resting nerve continues to cause the pupils to constrict. This causes the depth of field to increase. Good all-around vision allows for an accurate view of the environment, which allows for the recognition of details.

Diseases and disorders

Normally, there is a balance between the sympathetic and parasympathetic nervous systems in the body. Both states of the autonomic nervous system usually occur in succession and inhibit the other state. This means that an active phase (sympathetic nervous system) is followed by a passive, relaxed phase (parasympathetic nervous system). If the balance is disturbed, vegetative dystonia can occur. The communication between the brain and the autonomic nervous system is disturbed, which is why the sympathetic and parasympathetic nervous systems cannot establish a balance between activity and relaxation in the body. Since the autonomic nervous system affects all organs of the body, disturbances are the result, which can occur in all organs and regions of the body.Symptoms include insomnia, muscle cramps, headaches, discomfort throughout the digestive system, heart trouble, nervousness, and respiratory problems. The complaints are functional in nature and not rooted in organic disorders. This may make it difficult to make a diagnosis. Triggers are usually long-lasting stress and mental strain. A shift of the balance only in the direction of the parasympathetic nervous system can lead to vagotonia (parasympathiconia). The blood pressure is permanently very low, the pulse slow, the pupils constricted and the feet and hands cold. The affected person suffers from a general lack of drive and has problems with actively organizing his or her life. People who frequently engage in endurance training also tend toward vagotonia. Vagotonia caused by exercise is not pathological. It does not include the symptom of listlessness, unlike pathological vagotonia.