DRUG ACTING ON AUTONOMIC NERVOUS SYSTEM: The autonomic nervous system (ANS) is a vital component of the peripheral nervous system, responsible for regulating involuntary bodily functions. It maintains equilibrium between its two divisions, the sympathetic and parasympathetic, which work antagonistically to control various physiological processes such as heart rate, digestion, respiratory rate, and glandular secretions. In order to influence the ANS, medications can target specific receptors and neurotransmitters, thus altering their functions and providing therapeutic benefits.

Neurotransmitter

In the autonomic nervous system, two primary neurotransmitters play significant roles:

1. Adrenergic receptors: These receptors get activated by the neurotransmitters adrenaline (epinephrine) and noradrenaline (norepinephrine). They are crucial for the “fight or flight” response in the sympathetic nervous system.
2. Cholinergic receptors: These receptors get activated by the neurotransmitter acetylcholine and are integral to the parasympathetic nervous system’s “rest and digest” functions.

Nicotinic Receptors

Nicotinic receptors, a type of cholinergic receptor, are present in the autonomic ganglia and skeletal muscles.

Parasympathomimetic or Cholinergic Drugs

Parasympathomimetic drugs, also known as cholinergic drugs, mimic the effects of acetylcholine and stimulate cholinergic receptors.

Classification of Parasympathomimetic Drugs

1. Direct-Acting Cholinergic Agonists: These drugs directly bind to cholinergic receptors, activating them. Examples include Bethanechol and Pilocarpine.
2. Indirect-Acting Cholinergic Agonists or Cholinesterase Inhibitors: These drugs inhibit the enzyme acetylcholinesterase, responsible for breaking down acetylcholine, resulting in increased acetylcholine levels at the synapses. Examples include Donepezil and Physostigmine.

Pharmacological Actions

Parasympathomimetic drugs primarily stimulate the parasympathetic nervous system, leading to various physiological responses, including:

• Increased gastrointestinal motility and secretion
• Pupillary constriction (miosis)
• Bronchoconstriction
• Decreased heart rate
• Increased urinary bladder contraction
• Increased glandular secretions

Therapeutic Uses

These drugs find applications in various medical conditions, such as:

• Treating gastrointestinal disorders like gastroparesis and urinary retention using Bethanechol.
• Reducing intraocular pressure in glaucoma with Pilocarpine.
• Improving cognitive functions in Alzheimer’s patients using Donepezil.
• Alleviating muscle weakness in patients with myasthenia gravis with Pyridostigmine.

Anticholinesterase Drugs

Anticholinesterase drugs are agents that inhibit the activity of acetylcholinesterase, an enzyme responsible for breaking down acetylcholine.

Classification of Anticholinesterases

1. Reversible Anticholinesterases: These drugs temporarily bind to acetylcholinesterase, resulting in transient inhibition of the enzyme. Examples include Neostigmine and Edrophonium.
2. Irreversible Anticholinesterases: These drugs form stable complexes with acetylcholinesterase, leading to long-lasting inhibition. Examples include Organophosphorus compounds like Parathion and Malathion.

Pharmacological Actions

Anticholinesterases prolong the action of acetylcholine, leading to enhanced cholinergic effects.

Therapeutic Uses

Reversible anticholinesterases like Pyridostigmine are used to improve muscle strength in patients with myasthenia gravis.

Additionally, Neostigmine is used to reverse the effects of non-depolarizing neuromuscular blocking agents after surgery.

Organophosphorus compound poisoning can lead to severe cholinergic overstimulation.

Parasympatholytic Drugs or Cholinergic-Blocking Agents

Parasympatholytic drugs, also known as cholinergic-blocking agents, block the action of acetylcholine at cholinergic receptors.

Classification of Cholinergic-Blocking Agents

1. Tertiary Amines: These drugs have a higher affinity for muscarinic receptors and may have central effects due to their ability to cross the blood-brain barrier. Examples include Atropine and Scopolamine.
2. Quaternary Ammonium Compounds: These drugs have limited ability to cross the blood-brain barrier, resulting in predominantly peripheral effects. Examples include Ipratropium and Tiotropium.

Pharmacological Actions

Cholinergic-blocking agents inhibit the actions of the parasympathetic nervous system, leading to various physiological responses, including:

• Decreased gastrointestinal motility and secretion
• Pupillary dilation (mydriasis)
• Bronchodilation
• Increased heart rate
• Decreased urinary bladder contraction
• Decreased glandular secretions

Therapeutic Uses

Cholinergic-blocking agents are used in several medical conditions, such as:

• Reducing salivary and respiratory secretions before surgery using Atropine.
• Dilating the pupil during eye examinations using Tropicamide.
• Serving as bronchodilators in conditions like asthma and chronic obstructive pulmonary disease (COPD) using Ipratropium and Tiotropium.

Sympathomimetics Drugs

Sympathomimetic drugs, also known as adrenergic agonists, mimic the effects of adrenaline and noradrenaline.

Classification of Sympathomimetic Drugs

1. Direct-Acting Adrenergic Agonists: These drugs directly bind to adrenergic receptors and activate them. Examples include Epinephrine and Norepinephrine.
2. Indirect-Acting Adrenergic Agonists: These drugs stimulate the release of adrenaline and noradrenaline from nerve terminals. Examples include Amphetamine and Ephedrine.

Pharmacological Actions

Sympathomimetic drugs primarily stimulate the sympathetic nervous system, leading to various physiological responses, including:

• Increased heart rate and contractility
• Bronchodilation
• Pupillary dilation (mydriasis)
• Increased blood pressure
• Increased blood flow to skeletal muscles
• Decreased gastrointestinal motility

Therapeutic Uses

Sympathomimetic drugs find applications in various medical conditions, such as:

• Serving as an emergency treatment for severe allergic reactions using Epinephrine.
• Stimulating the heart during cardiopulmonary resuscitation (CPR) using Epinephrine.
• Acting as bronchodilators in acute asthma attacks using short-acting beta-agonists like Albuterol.

Sympathetic Blocking Drugs

Sympathetic blocking drugs, also known as adrenergic antagonists, block the actions of adrenaline and noradrenaline at adrenergic receptors.

Classification of Sympathetic Blocking Drugs

1. Alpha-Blockers: These drugs block alpha-adrenergic receptors, resulting in vasodilation and reduced blood pressure. Examples include Prazosin and Doxazosin.
2. Beta-Blockers: These drugs block beta-adrenergic receptors, leading to decreased heart rate and contractility. Examples include Atenolol and Metoprolol.

Pharmacological Actions

Sympathetic blocking drugs inhibit the actions of the sympathetic nervous system, resulting in various physiological responses:

• Vasodilation and reduced blood pressure (alpha-blockers)
• Decreased heart rate and contractility (beta-blockers)

Therapeutic Uses

Sympathetic blocking drugs are used in several medical conditions, such as:

• Treating hypertension and managing blood pressure levels using alpha-blockers and beta-blockers.
• Managing irregular heart rhythms using beta-blockers.
• Migraine prophylaxis using beta-blockers.

Conclusion

The autonomic nervous system’s role in regulating involuntary bodily functions is pivotal. Different drugs act on this system, targeting specific receptors and neurotransmitters, to achieve therapeutic effects. Parasympathomimetic drugs stimulate the parasympathetic nervous system, while anticholinesterases inhibit acetylcholine breakdown. Parasympatholytic drugs block acetylcholine actions, and sympathomimetic drugs imitate adrenaline and noradrenaline effects. Sympathetic blocking drugs, in turn, inhibit the actions of these neurotransmitters. Understanding the pharmacological actions of these drugs is vital for their safe and effective use in various medical conditions.

FAQs

1. What are sympathomimetic drugs used for? Sympathomimetic drugs are utilized to stimulate the sympathetic nervous system and are applied in conditions like anaphylaxis, cardiac arrest, and asthma.
2. What is the primary function of parasympatholytic drugs? Parasympatholytic drugs block acetylcholine actions and inhibit the parasympathetic nervous system, leading to various physiological responses.
3. What is the role of anticholinesterase drugs in myasthenia gravis? Anticholinesterases like Pyridostigmine are employed to enhance muscle strength in patients with myasthenia gravis.
4. What are the pharmacological actions of alpha-blockers? Alpha-blockers cause vasodilation and reduce blood pressure by blocking alpha-adrenergic receptors.
5. Are sympathetic-blocking drugs used in the treatment of hypertension? Yes, both alpha-blockers and beta-blockers are used to treat hypertension and manage blood pressure levels.

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