The central nervous system (CNS) serves as the control hub of the human body, coordinating every thought, movement, and sensation. Medications that act on the CNS influence brain activity, producing effects that range from anesthesia and pain relief to mood alteration and sedation. This article explores the major categories of CNS-active drugs — focusing on general anesthetics, analgesics, and anti-inflammatory agents — with an emphasis on their pharmacological roles and representative examples.
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General Anesthetics: The Gateway to Pain-Free Surgery
General anesthetics are agents that induce a reversible loss of consciousness and sensation, allowing surgical procedures to be performed without pain or distress. They can be administered through inhalation or intravenous routes, each class offering unique advantages in onset, duration, and safety.
Inhalation Anesthetics
Inhalation anesthetics are delivered as gases or vapors and absorbed through the lungs. They produce anesthesia by depressing neuronal activity in the brain.
Halothane* – A potent agent known for its rapid induction and pleasant odor. However, its use has declined due to the risk of hepatotoxicity.
Methoxyflurane, Enflurane, Sevoflurane, Isoflurane, and Desflurane – These modern anesthetics are favored for their controllable depth of anesthesia and faster recovery times. Sevoflurane, in particular, is popular in pediatric anesthesia for its smooth induction and minimal irritation.
Ultra Short-Acting Barbiturates
Barbiturates are intravenous anesthetics that act quickly and wear off within minutes, making them ideal for induction of anesthesia before inhalation agents are introduced.
Methohexital sodium*, Thiamylal sodium, and Thiopental sodium are examples of ultra short-acting barbiturates.
They work by enhancing GABA-mediated inhibitory neurotransmission, leading to sedation and hypnosis. Their rapid onset and short duration make them valuable in emergency and short surgical procedures.
Dissociative Anesthetics
Dissociative anesthetics induce a unique type of anesthesia characterized by catalepsy, amnesia, and analgesia—without complete loss of consciousness.
Ketamine hydrochloride* is the prototypical drug in this category. It produces profound analgesia and amnesia while maintaining cardiovascular stability. However, its use is limited by hallucinations and delirium during recovery, especially in adults. Despite that, it remains indispensable in trauma and pediatric anesthesia.
Narcotic and Non-Narcotic Analgesics: Easing the Pain
Analgesics are substances that relieve pain without causing total loss of consciousness. They can be broadly divided into narcotic (opioid) and non-narcotic (non-opioid) types.
Morphine and Related Drugs
Morphine remains the gold standard for pain relief, acting primarily on μ-opioid receptors in the brain and spinal cord to block pain perception.
The structure–activity relationship (SAR) of morphine analogues has guided the development of numerous derivatives aimed at enhancing efficacy, reducing addiction potential, and modifying duration of action.
Notable examples include:
Morphine sulphate – the classical strong analgesic.
Codeine – a milder derivative, often used in cough preparations.
Meperidine hydrochloride, Anileridine hydrochloride, Diphenoxylate hydrochloride, and Loperamide hydrochloride – synthetic opioids with varied potency and gastrointestinal effects.
Fentanyl citrate* – an extremely potent, fast-acting analgesic used in surgical and cancer pain.
Methadone hydrochloride* – used both as an analgesic and in opioid dependence therapy due to its long duration of action.
Propoxyphene hydrochloride, Pentazocine, and Levorphanol tartarate – represent different structural classes that balance pain relief with fewer side effects.
Narcotic Antagonists
To counteract opioid overdose and respiratory depression, narcotic antagonists play a lifesaving role.
Nalorphine hydrochloride, Levallorphan tartarate, and Naloxone hydrochloride competitively block opioid receptors, reversing the effects of narcotics.
Naloxone is especially vital in emergency settings for rapid reversal of opioid toxicity.
Anti-Inflammatory Agents: Fighting Pain and Swelling
Inflammation is the body’s natural response to injury, but when excessive, it can cause pain and tissue damage. Anti-inflammatory agents reduce inflammation, alleviate pain, and lower fever. Most act by inhibiting prostaglandin synthesis through cyclooxygenase (COX) enzyme blockade.
Salicylates and Traditional NSAIDs
Sodium salicylate and Aspirin (acetylsalicylic acid) are classical salicylates that relieve pain, fever, and inflammation. Aspirin’s additional antiplatelet effect makes it invaluable in cardiovascular therapy.
Mefenamic acid* and Meclofenamate belong to the anthranilic acid group, known for their effectiveness in dysmenorrhea and musculoskeletal pain.
Indomethacin, Sulindac, Tolmetin, Zomepriac, Diclofenac, Ketorolac, Ibuprofen*, Naproxen, and Piroxicam are widely used nonsteroidal anti-inflammatory drugs (NSAIDs) offering relief in arthritis, migraine, and acute pain conditions.
Antipyretic and Analgesic Agents
For mild to moderate pain and fever, Phenacetin, Acetaminophen (Paracetamol), and Antipyrine are preferred due to their central action and minimal gastric irritation.
Phenylbutazone, though potent, is now less common because of its adverse effects on the blood and gastrointestinal tract.