Unit-3 Pharmacology of drugs acting on peripheral nervous system

Peripheral Nervous System Pharmacology Unit 3 Cover

Fig 1: Peripheral Nervous System (PNS) Overview

Welcome to DrX Whiz Niraj. In this post, we provide comprehensive notes for B.Pharm 4th Semester, Pharmacology I (Unit 3). This unit focuses on the Peripheral Nervous System, Autonomic Drugs, Local Anesthetics, and neuromuscular agents.

Introduction to PNS & ANS

The Peripheral Nervous System (PNS) controls communication between the CNS and body organs. It is divided into the Somatic Nervous System (voluntary) and the Autonomic Nervous System (ANS) (involuntary).

The ANS regulates involuntary functions like heart rate, digestion, respiration, blood pressure, and gland secretion. It is further divided into two opposing systems:

1. Sympathetic Nervous System

Known as the "Fight or Flight System".
Functions: Increases HR, dilates pupils & bronchi, inhibits digestion.
Main Neurotransmitter: Noradrenaline.

2. Parasympathetic Nervous System

Known as the "Rest and Digest System".
Functions: Decreases HR, constricts pupils, stimulates digestion & secretions.
Main Neurotransmitter: Acetylcholine.

Fig 2: Sympathetic vs Parasympathetic Nervous System

Comparison Table: Sympathetic vs Parasympathetic

Organ / System	Sympathetic Action	Parasympathetic Action
Heart	Increases rate (Tachycardia)	Decreases rate (Bradycardia)
Pupil of Eye	Dilates (Mydriasis)	Constricts (Miosis)
Bronchi	Dilates (Relaxation)	Constricts
Gastrointestinal Tract	Decreases motility & secretion	Increases motility & secretion
Urinary Bladder	Relaxes detrusor muscle	Contracts detrusor muscle

Neurohumoral Transmission

Definition: Neurohumoral transmission is the process by which nerve impulses are transmitted across a synapse or neuroeffector junction through chemical mediators called neurotransmitters.

Steps of Neurohumoral Transmission:

1. Synthesis: Neurotransmitter is formed inside the nerve ending.
2. Storage: Stored in synaptic vesicles.
3. Release: Nerve impulse (Action Potential) causes release into the synaptic cleft.
4. Receptor Binding: Neurotransmitter binds to specific receptors on the post-synaptic cell.
5. Response: Biological effect occurs.
6. Termination: Neurotransmitter is destroyed by enzymes or re-uptaken into the neuron.

Neurohumoral Transmission Synapse Diagram

Fig 3: Mechanism of Neurohumoral Transmission at Synapse

Classification of Neurotransmitters

Cholinergic & Adrenergic

Cholinergic: Acetylcholine.
Adrenergic: Noradrenaline, Adrenaline, Dopamine.

Amino Acids

GABA (Inhibitory), Glycine, Glutamate (Excitatory).

Peptides & Purines

Peptides: Substance P, Endorphins.
Purines: ATP, Adenosine.

Co-Transmission: The release of multiple neurotransmitters from the same nerve ending (e.g., ATP with Noradrenaline).

Drugs Acting on ANS

Parasympathomimetics (Cholinergic Agonists)

Action: Stimulate muscarinic receptors (mimic Ach).
Effects: Constricts pupil, increases saliva & GI motility, decreases HR.
Drugs: Pilocarpine, Bethanechol.
Uses: Glaucoma, Urinary retention.

Parasympatholytics (Anticholinergics)

Action: Block muscarinic receptors.
Effects: Dilates pupil, dry mouth, increases HR.
Drugs: Atropine, Scopolamine.
Uses: Asthma, Bradycardia, Organophosphate poisoning.

Sympathomimetics (Adrenergic Agonists)

Action: Stimulate adrenergic receptors.
Effects: Increases HR, Bronchodilation, Vasoconstriction.
Drugs: Phenylephrine (\(\alpha\)), Salbutamol (\(\beta\)).
Uses: Asthma, Shock, Cardiac arrest.

Sympatholytics (Adrenergic Antagonists)

Action: Block adrenergic receptors.
Effects: Reduces HR, Vasodilation, lowers BP.
Drugs: Prazosin (\(\alpha\)-blocker), Propranolol (\(\beta\)-blocker).
Uses: Hypertension, Angina.

Neuromuscular & Skeletal Muscle Agents

1. Neuromuscular Blocking Agents

Produce skeletal muscle relaxation by acting at the neuromuscular junction.

Non-Depolarizing Blockers: Competitive blockade of nicotinic receptors. (Ex: Tubocurarine, Pancuronium)
Depolarizing Blockers: Persistent depolarization causing paralysis. (Ex: Succinylcholine)
Uses: Surgical anesthesia, Intubation.

2. Skeletal Muscle Relaxants

Reduce muscle tone and spasm.

Centrally Acting: Baclofen, Diazepam.
Peripherally Acting: Dantrolene.
Uses: Muscle spasms, Cerebral palsy.

Local Anesthetics (LA)

Definition: Local anesthetics are drugs that reversibly block nerve conduction locally when applied to a specific area.

Sodium Channel Blockade by Local Anesthetics

Fig 4: Mechanism of Action of Local Anesthetics (Sodium Channel Blockade)

Mechanism: LA Drug \(\rightarrow\) Blocks Na⁺ Channels \(\rightarrow\) Prevents Action Potential Generation

Ester Type: Procaine, Benzocaine.
Amide Type: Lidocaine, Bupivacaine.
Uses: Minor surgery, Dental procedures, Spinal anesthesia.

Myasthenia Gravis & Glaucoma

Myasthenia Gravis

An autoimmune disorder causing muscle weakness due to the destruction of nicotinic (Nm) receptors. Symptoms include ptosis, difficulty swallowing, and fatigue.

Drugs Used (Anticholinesterases): Neostigmine, Pyridostigmine.
Mechanism: They inhibit acetylcholinesterase, increasing acetylcholine levels at the NMJ to restore muscle strength.

Glaucoma

A disease characterized by optic nerve damage due to increased intraocular pressure (IOP). Types: Open-angle and Closed-angle.

Cholinergic Drugs: Pilocarpine.
Beta Blockers: Timolol.
Carbonic Anhydrase Inhibitors: Acetazolamide.
Prostaglandin Analogues: Latanoprost.
Mechanism: Either reduce aqueous humor production or increase its drainage.

VVI Exam Questions

2 Marks Questions (Short Answers)

Define Neurohumoral Transmission.
What is Co-transmission? Give an example.
What is the difference between Sympathomimetic and Sympatholytic drugs?
Name two Local Anesthetics (one Ester type and one Amide type).
What is Glaucoma? Name two drugs used to treat it.

5 Marks Questions (Short Essays)

Differentiate between Sympathetic and Parasympathetic Nervous System.
Explain the steps involved in Neurohumoral Transmission.
Classify Neuromuscular blocking agents with suitable examples.
Write a short note on the pharmacology of drugs used in Myasthenia Gravis.

10 Marks Questions (Long Essays)

Classify Sympathomimetics and Sympatholytics with examples. Discuss their therapeutic uses and adverse effects in detail.
Define Local Anesthetics. Classify them and explain their mechanism of action, therapeutic uses, and adverse effects.
Describe the pharmacology of various drugs used in the treatment of Glaucoma.

Unit-3 Pharmacology of drugs acting on peripheral nervous system