Parasympatholytic
Encyclopedia
A parasympatholytic agent is a substance or activity that reduces the activity of the parasympathetic nervous system
. (The parasympathetic nervous system is often colloquially described as the "Feed and Breed" or "Rest and Digest" portion of the autonomic nervous system
. The parasympathetic nervous system becomes strongly engaged during or after a meal and during times when the body is at rest.)
The term parasympatholytic typically refers to the effect of a drug
, although some poison
s act to block the parasympathetic nervous system as well. Most drugs with parasympatholytic properties are anticholinergic
s.
"Parasympatholytic" and sympathomimetic agents have similar effects, but not identical. For example, both cause mydriasis
, but parasympatholytics reduce accommodation
(cycloplegia
) while sympathomimetics do not.
s or bradydysrhythmias) caused by myocardial infarction
s or other pathologies, as well as to treat conditions which cause bronchiole
s in the lung to constrict, such as asthma
. By blocking the parasympathetic nervous system, parasympatholytic drugs can increase heart rate in patients with bradycardic heart rhythms, and open up airways and reduce mucous production in patients suffering from asthma.
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...
. (The parasympathetic nervous system is often colloquially described as the "Feed and Breed" or "Rest and Digest" portion of the autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
. The parasympathetic nervous system becomes strongly engaged during or after a meal and during times when the body is at rest.)
The term parasympatholytic typically refers to the effect of a drug
Drug
A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...
, although some poison
Poison
In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....
s act to block the parasympathetic nervous system as well. Most drugs with parasympatholytic properties are anticholinergic
Anticholinergic
An anticholinergic agent is a substance that blocks the neurotransmitter acetylcholine in the central and the peripheral nervous system. An example of an anticholinergic is dicycloverine, and the classic example is atropine....
s.
"Parasympatholytic" and sympathomimetic agents have similar effects, but not identical. For example, both cause mydriasis
Mydriasis
Mydriasis is a dilation of the pupil due to disease, trauma or the use of drugs. Normally, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day...
, but parasympatholytics reduce accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....
(cycloplegia
Cycloplegia
Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation.-Anatomy:The iris is the heavily pigmented colored part of the eye. It has a contractile diaphragm in front of the lens with a central opening called the pupil...
) while sympathomimetics do not.
Clinical significance
Parasympatholytic drugs are sometimes used to treat slow heart rhythms (bradycardiaBradycardia
Bradycardia , in the context of adult medicine, is the resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. It may cause cardiac arrest in some patients, because those with bradycardia may not be pumping enough oxygen to their heart...
s or bradydysrhythmias) caused by myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
s or other pathologies, as well as to treat conditions which cause bronchiole
Bronchiole
The bronchioles or bronchioli are the first airway branches that no longer contain cartilage or glands in their submucosa. They are branches of the bronchi.The bronchioles terminate by entering the circular sacs called alveoli.- Structure :...
s in the lung to constrict, such as asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
. By blocking the parasympathetic nervous system, parasympatholytic drugs can increase heart rate in patients with bradycardic heart rhythms, and open up airways and reduce mucous production in patients suffering from asthma.