Introduction
The autonomic nervous system (ANS) controls involuntary functions of the body such as heart rate, digestion, breathing, glandular secretion and smooth muscle activity. Drugs acting on the ANS are widely used in clinical practice for conditions like asthma, hypertension, shock, glaucoma and digestive disorders. Understanding the basic organisation of ANS helps in learning how these drugs work.
Divisions of the ANS
The ANS has two major divisions:
- Sympathetic nervous system (SNS)
- Parasympathetic nervous system (PNS)
Both systems work together to maintain balance (homeostasis). The sympathetic system prepares the body for emergencies (“fight or flight”), while the parasympathetic system maintains normal daily functions (“rest and digest”).
Sympathetic Nervous System
Sympathetic nerves originate from the thoracic and lumbar regions of the spinal cord. They supply the heart, lungs, blood vessels, glands and smooth muscles.
Key Features
- Pre-ganglionic nerve fibres are short.
- Post-ganglionic fibres are long.
- Main neurotransmitters: acetylcholine (ACh) at the ganglia and noradrenaline (NA) at the target organs.
Important Receptors
- Alpha receptors: involved in blood vessel contraction and pupil dilatation.
- Beta receptors: control heart rate, bronchodilatation and metabolic effects.
Physiological Actions
- Heart rate and blood pressure increase.
- Bronchi dilate.
- Pupils dilate.
- Blood diverted to muscles.
- Gastrointestinal activity decreases.
Parasympathetic Nervous System
Parasympathetic nerves arise mainly from the cranial nerves (especially vagus nerve) and sacral spinal cord.
Key Features
- Pre-ganglionic fibres are long and reach close to organs.
- Post-ganglionic fibres are short.
- Main neurotransmitter: acetylcholine at both ganglia and effector organs.
Important Receptors
- Muscarinic receptors (M1–M5): present on smooth muscles, glands and heart.
- Nicotinic receptors: present at ganglia and neuromuscular junction.
Physiological Actions
- Heart rate decreases.
- Bronchi constrict.
- Pupils constrict.
- Digestion and glandular secretion increase.
Neurotransmission in the ANS
Communication in the ANS occurs through chemical messengers called neurotransmitters.
Cholinergic Transmission (ACh)
- Released from pre-ganglionic fibres of both SNS and PNS.
- Acts at muscarinic and nicotinic receptors.
Adrenergic Transmission (NA/Adrenaline)
- Released by sympathetic post-ganglionic fibres.
- Acts on alpha and beta receptors.
How Drugs Act on the ANS
Drugs acting on ANS target neurotransmitters or their receptors. They either mimic or block normal autonomic activity.
1. Cholinergic Drugs (Parasympathomimetics)
These drugs increase the action of the parasympathetic system.
Effects
- Decrease heart rate
- Increase salivation and digestion
- Constrict pupils
Examples
- Pilocarpine – used in glaucoma.
- Neostigmine – increases muscle strength in myasthenia gravis.
2. Anticholinergic Drugs (Parasympatholytics)
These drugs block the action of acetylcholine.
Effects
- Increase heart rate
- Reduce secretions
- Cause pupil dilation
- Relax smooth muscles
Examples
- Atropine – used for bradycardia.
- Ipratropium – used in asthma.
3. Adrenergic Drugs (Sympathomimetics)
These drugs mimic the sympathetic system.
Effects
- Increase heart rate and blood pressure
- Dilate bronchi
- Increase blood glucose
Examples
- Adrenaline – used in anaphylaxis.
- Salbutamol – used in asthma.
4. Adrenergic Blockers (Sympatholytics)
These drugs block sympathetic activity.
Types
- Alpha-blockers: prazosin (used in hypertension).
- Beta-blockers: propranolol, atenolol (used in hypertension, angina).
Clinical Importance of ANS Drugs
Drugs acting on ANS are important in many clinical conditions:
- Asthma – bronchodilators like salbutamol.
- Hypertension – beta-blockers and alpha-blockers.
- Glaucoma – pilocarpine, beta-blockers.
- Shock and cardiac arrest – adrenaline.
- Pre-anaesthetic medication – atropine.
Detailed Notes:
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