DRUGS ACTING ON THE CENTRAL NERVOUS SYSTEM: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacologists and healthcare professionals!
The Central Nervous System (CNS) is the most complex and vital system in the human body, controlling everything from consciousness and memory to movement and emotion. Drugs acting on the CNS are among the most powerful and widely used medications in clinical practice—from general anaesthetics that make surgery possible to antidepressants that restore mental health. As a pharmacology educator with years of experience teaching CNS pharmacology, I have observed that students often find this topic challenging due to the sheer number of drugs and their mechanisms. Let me tell you: Understanding drugs acting on the CNS is essential for every healthcare professional.
In this comprehensive guide, I will walk you through the major classes of CNS drugs—general anaesthetics, sedatives and hypnotics, anticonvulsants, anti-anxiety drugs, antidepressants, antipsychotics, nootropic agents, centrally acting muscle relaxants, and opioid analgesics. By the end of this article, you will have a thorough understanding of these essential pharmacological agents. Let us begin!
Dpharmguru’s exam insights:
CNS drugs are frequently tested in pharmacology exams. Remember: General anaesthetics produce reversible loss of consciousness. Sedatives reduce excitement; hypnotics induce sleep. Anticonvulsants control seizures. Antidepressants treat depression. Antipsychotics treat schizophrenia. Opioid analgesics relieve severe pain. Pay special attention to the mechanisms of action and classifications—these are almost always asked in exams!
GENERAL ANAESTHETICS
General Anaesthesia is referred to as a state of controlled unconsciousness. General Anaesthetics are the drugs that produce reversible loss of sensation and consciousness.
Basic components of general anaesthesia include:
- Analgesia (pain relief)
- Amnesia (loss of memory)
- Immobilization
- Diminution of motor response
- Reversibility
Stages of General Anaesthesia
- Stage I – Analgesia: Loss of consciousness; patient experiences drowsiness/dizziness, auditory and visual hallucinations.
- Stage II – Excitement (Delirium): Patient is excited, struggling, showing delirious behaviour; amnesic; irregular breathing may occur.
- Stage III – Surgical Anaesthesia:
- Plane 1: Respiration regular, pupil constricts, muscle tone decreases
- Plane 2: Respiration even, thoracic respiration decreases, pupil becomes large
- Plane 3: Thoracic respiration inhibited, pupil dilates, muscle tone inhibited
- Plane 4: Respiration almost stops, pupil dilates, atonia of skeletal muscles
- Stage IV – Medullary Depression: Respiration stops; vasomotor and respiratory centres depressed; leads to coma and death if no action is taken.
Classification of General Anaesthetics
- Inhalational Anaesthetics: Enflurane, Desflurane, Halothane, Ether
- Intravenous Anaesthetics: Thiopentone sodium, Etomidate, Ketamine, Fentanyl, Diazepam
- Gaseous Anaesthetics: Nitrous oxide
Nitrous Oxide
Pharmacological Action: Causes light anaesthesia without depressing respiratory or vasomotor centre.
Indications: Maintains surgical anaesthesia in combination with other agents; Entonox provides analgesia for obstetric practice, emergency management of injuries.
Contraindications: Collections of air in pleural, pericardial, or peritoneal spaces; intestinal obstruction; arterial air embolism; decompression sickness; severe COPD; emphysema.
Adverse Effects: Nausea and vomiting; interferes with methionine, deoxythymidine, and DNA synthesis; megaloblastic changes in bone marrow if given for >4 hours.
Thiopentone Sodium (Thiopental)
Pharmacological Action: Binds to GABA-A receptor, increasing chloride ion channel opening time; exerts inhibitory effects on CNS, mainly at cerebral cortex and reticular formation. Rapid onset due to high lipid solubility; short duration (5 minutes) due to redistribution.
Indications: Inducing general anaesthesia; surgeries of short duration; anaesthetic in patients with malignant hyperthermia; anticonvulsant in intractable convulsions.
Contraindications: Liver disease, Addison’s disease, myxoedema, severe heart disease, severe hypotension, severe breathing disorder, family history of porphyria.
Ketamine
Pharmacological Action: Blocks excitatory NMDA receptors; allows sodium and calcium ion influx, increasing neural excitation probability.