17. Parasympathomimetics and parasympathoytics Flashcards
(16 cards)
What is the parasympathetic nervous system?
The parasympathetic nervous system is part of the nervous system that helps control bodily functions when you are at rest. It is often called the “rest-and-digest” system, because it conserves energy and slows down the heart rate while increasing intestinal and gland activity
Signs that the parasympathetic nervous system has been activated:
- Constricted pupils
- Stimulated saliva
- Slow heartbeat
- Constricted airways
- Stimulated stomach activity
- Inhibited glucose release
- Stimulated intestinal activity
- Contracted bladder
- Promoted genital erection
Mechanism of action of the parasympathetic nervous system
Cholinergic drugs work by targeting muscarinic receptors, which are specific proteins on cells that acetylcholine can bind to.
How cholinergic drugs work:
1. Acetylcholine binds to the muscarinic receptors
2. The binding starts a chain reaction inside the cell involving something called a “G-protein”
3. Depending on the type of receptor and where it is, this will lead to different effects:
- M1, M3: They act as the “on-switches”
➤ Stomach and glands: Stimulating the stomach acid production and gland secretion
➤ Bronchial smooth muscle: causing airways in the lungs to constrict
➤ Eye ciliary muscle: They help focus the vision
➤ Vascular endothelium: They can cause blood vessels to relax, which lowers blood pressure
- M2: This is the “off-switch”
➤ Heart: slows down the heart rate
Antidote of the parasympathetic nervous system
Atropine
Side effects of the parasympathetic nervous system
- Bradycardia
- Bronchocontriction
- Miosis
- Salivation
- Vomiting
- Diarrhoea
- Urination
What are parasympathomimetics
= Cholinergic drugs
substances that enhance, inhibit, or mimic the action of acetylcholine, a key neurotransmitter in the parasympathetic nervous system
Parasympathomimetic, Active substances
- Neostigmine
- Physostigmine
- Organophosphate
- Pyridostigmine
Parasympathomimetic, mechanism of action
The drugs mimic or enhance the effect of acetylcholine. They either directly stimulate cholinergic receptors (muscarinic and/or nicotinic) or they inhibit the acetylcholinesterase, which is the enzyme that breaks down acetylcholine and therefore increases the acetylcholine levels.
Parasympathomimetic, pharmacological effect
- Eye: Affects the iris sphincter, lacrimal glands and intraocular pressure
- Cardiovascular system: Shows negative chronotropic effects, causes no influence of the level of contraction and no vasodilation
- GI tract: Leads to smooth muscle contraction, glandular hyperfunction, vomiting and diarrhoea
- Respiratory tract: Causes bronchoconstriction and increased bronchial secretion
- Urinary tract: Results in bladder contraction and sphincter relaxation
What are parasympatholytics
= Anticholinergic drugs, the antidotes of cholinergic drugs
Parasympatholytics, Active substances
- Atropine
- Glycopyrrolate
- Ipratropium
- Homatropine
- Tropicamide
- Benzetimide
- Butil – scopolamine
Parasympatholytics, mechanism of action
The drugs block or inhibit the action of acetylcholine at muscarinic receptors, and prevent parasympathetic nerve impulses by preventing the acetylcholine to bind to muscarinic receptors
Parasympatholytics, pharmacological effect
Eye
- Causes blurry vision by weakening the muscles that help to focus (cycloplegia)
- Can also help with certain eye inflammation = uveitis
- Mydriasis (makes the pupil bigger)
- Used to examine the eyes
- Prevents the iris from sticking to the lens in certain eye conditions
Heart
- Tachycardia ⭢ increased heart rate by blocking the signals which normally decreases the heart rate
- Before, it was used before surgery, but today there are better options.
Stomach and gut
- dries up the saliva
- decreased digestion, which can cause constipation
- reduces stomach acid – but only in higher doses
Lungs
- Opens up the airways (bronchodilation) and reduces the mucus, used before surgery
Parasympatholytics, indications
- Premedication before surgery
- Eye treatment
- Bronchodilation
- Used as an antidote for atropine
- Antidiarrheals
Parasympatholytics, toxicity
They have a smaller therapeutic index = more toxic. The effective dose is not much lower than the toxic dose.
Herbivores and carnivores have different sensitivities
Rabbits are less affected by atropine because they can break it down more easily and by the liver
Parasympatholytics, symptoms of toxicosis
- Dry mouth
- Tachycardia
- Mydriasis
- Constipation
- Convulsions
- Coma
- Death
