PNS, also called the parasympathetic nervous system, is a part of the autonomic nervous system. It controls the various functions of the body organs and also blood vessels and the smooth muscles. The parasympathetic nervous system controls some voluntary and involuntary actions like breathing, or which takes the working of the conscious mind. When the conditions in the external environment are calm and normal, the PNS takes over most of the functions. For example, the PNS causes slower heart beat, slows down the respiratory rate, increases salivation and sweatiness, keeps the pupil normal and increases waste disposal and also sexual interest.
The sympathetic nervous system, unlike the other subdivisions of the autonomic nervous system, causes something called the flight or fight response in the body. The PNS does not require acting immediately when the body is faced with a sudden kind of stress. At this time the SNS takes over and takes the necessary steps to protect the body. The functions of the PNS system are known as digest and rest. They are equally opposite to the functions of the sympathetic nervous system. Both the functions of the nervous systems are equally important for the functioning of the body and maintaining the balance.
The parasympathetic nervous system contains both cranial and spinal segments. They end near the tailbone or the spectrum. The PNS nerves come from the second and third segments of the spine. Most of the PNS segments contain sensory components. The sensory cells monitor so many vital functions like blood pressure, blood sugar, stomach and bowel contents and even the motor neurons. The sensory nerves are grouped into ganglia and are allowed to target the main parts of the body. They modulate the response from several parts of the body to the brain and send messages back and forth.
There are hundreds of chemical messengers involved in carrying these messages back and forth. For example acetylcholine is the primary chemical messenger that acts on the parasympathetic nervous system. These chemical messengers also activate other chemicals and receptors in the brain and the body. Some viral infections, hereditary complications and trauma can lead to several causative factors. They can even cause heart attacks, anxiety, reduced blood pressure, increase in heart rate and so much more. All these are caused by a condition called dysautonomia which causes the dysfunction of the autonomic nervous system. However, the causes for this condition are not exactly known.
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WiseGeek: What Is The Parasympathetic Nervous System?