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Muscarinic antagonists

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Muscarinic antagonists

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Muscarinic antagonists

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is an anticholinergic that is administered with the antidiarrheal diphenoxylate to discourage deliberate opioid overdose via the undersirable anticholingeric effects.

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A 70-year-old male presents to the office with his wife who states the patient has been confused for the past 2 days. History also reveals lethargy. Past medical history is significant for Parkinson's disease. Vital signs reveal a temperature of 37.3ºC (99.1ºF), a blood pressure of 130/80 mmHg, and a heart rate of 80/min. Physical examination reveals dry oral mucus membranes and dilated pupils. Which of the following is most likely causing his symptoms? 

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Muscarinic antagonists, or antimuscarinic medications, are a class of medications that prevent muscarinic receptors of the parasympathetic nervous system from getting stimulated by acetylcholine.

Okay, first things first, the nervous system is divided into the central nervous system, so the brain and spinal cord; and the peripheral nervous system.

The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles; and the autonomic nervous system, which is further divided into the sympathetic and parasympathetic nervous systems, and controls the involuntary movement of the smooth muscles, and the glands of our organs.

Now, the autonomic nervous system is made up of a relay that includes two neurons.

We’ll focus on just the parasympathetic nervous system.

Signals for the parasympathetic nervous system start in the hypothalamus.

These hypothalamic neurons synapse with nuclei in the brainstem or spinal cord, which send out signals to preganglionic neurons that travel to the rest of the body.

Their targets are the parasympathetic ganglion, which consist of many postganglionic neuron cell bodies and are located nearby or directly in the target organs.

The postganglionic neurons extend the rest of the way to the target cell, where they release the neurotransmitter acetylcholine, which is why they are also called cholinergic neurons.

Acetylcholine binds to a type of receptor, known as muscarinic receptors, on the cells of target organs, which allow the parasympathetic nervous system to trigger a ‘rest and digest’ response, meaning that it keeps body-energy use as low as possible to stimulate activities like digestion.

It acts in the heart, slowing the heart rate and reducing the cardiac output.

In the gastrointestinal tract, it increases its motility to stimulate digestion and defecation.

In the bladder, it causes constriction of the bladder muscle, called the detrusor muscle, which stimulates urination.

In salivary, sweat, and lacrimal glands, it increases their secretions.

In the liver, it triggers glucose storage to reduce blood glucose levels.

In the lungs, it causes bronchoconstriction, since in a relaxed state our cells do not consume as much oxygen.

In the eyes, it triggers miosis, or constriction of the pupils, to improve close vision and stimulates the contraction of the ciliary muscles, increasing the outflow of aqueous humor, which is the fluid in the anterior chamber of the eye, and this decreases the intraocular pressure.

Finally, its effects on the brain are extremely complicated, but generally, they cause overall stimulation, and they participate in many wanted and unwanted functions, such as movement control and vomiting, respectively.

Alright, so medications that block the effects of acetylcholine on muscarinic receptors are called muscarinic antagonists or antimuscarinic medications.

Now, the most famous muscarinic antagonist is atropine.

Atropine blocks the “rest and digest” effect from the parasympathetic system, so clinically, it can be used to treat bradycardia, or slowed heart rate.

It decreases bladder smooth muscle contraction so it’s useful for preventing nocturnal enuresis, or bedwetting, in children.

Ophthalmologists can also use atropine to dilate the pupils.

Finally, it’s an antidote for poisoning by anticholinesterases like organophosphates found in pesticides. These agents prolong the effect of acetylcholine by inhibiting their break down, so atropine can help by simply blocking the muscarinic receptors.

Now, for side effects, atropine can produce: tachycardia, or increased heart rate; constipation and urinary retention; dry mouth, skin and eyes; and blurry vision.

The more severe side effects include hyperthermia, dizziness, confusion, and delirium.

It’s also contraindicated in individuals suffering from narrow angle glaucoma since it can worsen the obstruction of aqueous humor drainage.

Sources
  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
  4. "Muscarinic receptor subtypes in airways" Life Sci (1993)
  5. "Pharmacologic therapy of obstructive airway disease" Clin Chest Med (1990)
  6. "[Atropine. Principles and rules of utilization]" Rev Prat (2001)
  7. "Antimuscarinic drugs" Prof Nurse (2004)