Parasympathetic system and pharmacology

 **Pharmacology of the Parasympathetic Nervous System**

The parasympathetic nervous system (PSNS) is one of the divisions of the autonomic nervous system, often referred to as the "rest and digest" system. It plays a crucial role in maintaining homeostasis, promoting relaxation, and conserving energy. The PSNS uses acetylcholine (ACh) as its primary neurotransmitter to transmit signals and regulate various physiological functions. Pharmacological agents targeting the parasympathetic system can either enhance or inhibit its actions for therapeutic purposes. Let's explore the pharmacology of the parasympathetic nervous system:

**1. Parasympathetic Neurotransmitter:**

   - **Acetylcholine (ACh):** It is the primary neurotransmitter of the parasympathetic nervous system. ACh is released from postganglionic nerve endings to activate cholinergic receptors.

**2. Cholinergic Receptors:**

   - **Nicotinic Receptors:** These receptors are found in the ganglia of both sympathetic and parasympathetic nervous systems, as well as at the neuromuscular junction of skeletal muscles.

   - **Muscarinic Receptors:** These receptors are located on target organs innervated by the parasympathetic system, such as the heart, smooth muscles, and glands.

**3. Parasympathomimetic Drugs (Cholinergic Agonists):**

   - **Bethanechol:** It is used to increase bladder tone and contractility in conditions like urinary retention.

   - **Pilocarpine:** Used to treat glaucoma and xerostomia (dry mouth) by increasing the secretion of saliva and tears.

   - **Carbachol:** Used to lower intraocular pressure in glaucoma and to induce miosis during eye surgery.

**4. Parasympatholytic Drugs (Anticholinergic Agents):**

   - **Atropine:** Blocks muscarinic receptors and is used to increase heart rate, dilate pupils, and reduce secretions during surgery.

   - **Scopolamine:** Used as a motion sickness treatment due to its anticholinergic effects on the vestibular system.

   - **Ipratropium:** Used as a bronchodilator in asthma and chronic obstructive pulmonary disease (COPD).

**5. Clinical Applications:**

   - **Urinary Disorders:** Parasympathomimetic drugs can increase bladder tone and contractility, while parasympatholytic drugs can relax the bladder and reduce urgency.

   - **Ophthalmology:** Parasympathomimetics are used to treat conditions like glaucoma, while anticholinergics can dilate pupils for eye examinations.

   - **Gastrointestinal Disorders:** Parasympathomimetic drugs can increase gut motility and secretion, while anticholinergic drugs can be used to reduce hypermotility in conditions like irritable bowel syndrome.

**6. Side Effects and Precautions:**

   - Parasympathomimetic drugs may cause bradycardia, increased salivation, and gastrointestinal disturbances.

   - Parasympatholytic drugs may cause dry mouth, blurred vision, constipation, and urinary retention.

In conclusion, understanding the pharmacology of the parasympathetic nervous system and drugs that target its receptors is essential for appropriate therapeutic use and managing various medical conditions.

MCQs on Pharmacology of Parasympathetic Nervous System

Q1. What is the primary neurotransmitter of the parasympathetic nervous system?

Q2. Which type of cholinergic receptor is located on target organs innervated by the parasympathetic nervous system?

Q3. Which drug is used to increase bladder tone and contractility in conditions like urinary retention?

Q4. Which drug is used to treat glaucoma and xerostomia (dry mouth) by increasing the secretion of saliva and tears?

Q5. Which drug is commonly used to increase heart rate, dilate pupils, and reduce secretions during surgery?

Q6. Which drug is commonly used as a bronchodilator in asthma and chronic obstructive pulmonary disease (COPD)?

Q7. Which drug is used to lower intraocular pressure in glaucoma and to induce miosis during eye surgery?

Q8. Which drug blocks muscarinic receptors and is used to increase heart rate, dilate pupils, and reduce secretions?

Q9. Which drug is used as a motion sickness treatment due to its anticholinergic effects?

Q10. Which drug is used to relax the bladder and reduce urgency in urinary disorders?