Drugs Acting on the Central Nervous System

Alumkal

Introduction

  • The central nervous system (CNS) consists of the brain and spinal cord, which control and coordinate all body functions.

  • It regulates consciousness, movement, sensation, emotions, memory, and reflex activities.

  • Disorders of the CNS include pain, anxiety, epilepsy, insomnia, psychiatric disorders, and neurological diseases.

  • Various drugs act on the CNS to stimulate, depress, or modify neural activity to produce therapeutic effects.

  • These drugs influence the transmission of nerve impulses through neurotransmitters such as acetylcholine, dopamine, serotonin, and gamma-aminobutyric acid (GABA).

  • Important classes of CNS drugs include sedatives, hypnotics, analgesics, antiepileptics, antidepressants, antipsychotics, local anesthetics, and general anesthetics.

  • Among these, local and general anesthetics are widely used during medical and surgical procedures to relieve pain.

  • Understanding the classification, mechanism of action, therapeutic uses, side effects, and adverse effects of CNS drugs is essential for safe clinical practice.

 

Drugs Acting on the CNS

 

Drug Group Examples Main Function
Sedatives and hypnotics Diazepam, Phenobarbital Reduce anxiety and induce sleep
Analgesics Morphine, Codeine Relieve pain
Antiepileptic drugs Phenytoin, Valproate Control seizures
Antidepressants Fluoxetine, Amitriptyline Treat depression
Antipsychotics Haloperidol, Chlorpromazine Manage psychotic disorders
Local anesthetics Lidocaine, Procaine Produce loss of sensation locally
General anesthetics Propofol, Isoflurane Produce reversible loss of consciousness

 

Local Anesthetics

Local anesthetics are drugs that produce reversible loss of sensation in a specific part of the body without loss of consciousness.

Mechanism of Action

  • Local anesthetics block voltage-gated sodium channels in nerve membranes.

  • This prevents generation and conduction of nerve impulses, resulting in loss of pain sensation.

Classification of Local Anesthetics

Type Examples Characteristics
Ester type Procaine, Benzocaine Short acting
Amide type Lidocaine, Bupivacaine Longer duration

Methods of Administration

  • Topical anesthesia – applied on skin or mucous membranes.

  • Infiltration anesthesia – injected into tissue to block nerve endings.

  • Nerve block anesthesia – injected near a nerve trunk.

  • Spinal anesthesia – injected into subarachnoid space.

  • Epidural anesthesia – injected into epidural space.

Therapeutic Uses

  • Minor surgical procedures.

  • Dental procedures.

  • Obstetric procedures such as epidural anesthesia during labor.

  • Pain relief in minor injuries and diagnostic procedures.

  • Used in regional anesthesia during surgery.

Side Effects

  • Local irritation

  • Mild dizziness

  • Temporary numbness beyond the injection site

  • Allergic reactions

Adverse Effects

  • Severe hypotension

  • Convulsions due to CNS toxicity

  • Cardiac arrhythmias

  • Respiratory depression in severe cases

 

General Anesthetics

General anesthetics are drugs that produce reversible loss of consciousness, analgesia, muscle relaxation, and absence of reflexes during surgical procedures.

Characteristics of an Ideal General Anesthetic

  • Rapid induction and recovery

  • Adequate analgesia

  • Muscle relaxation

  • Minimal toxicity

  • Easy control of depth of anesthesia

 

Classification of General Anesthetics

1. Inhalational Anesthetics

Drug Features
Nitrous oxide Rapid onset, weak anesthetic
Halothane Potent anesthetic
Isoflurane Widely used in surgery
Sevoflurane Rapid induction and recovery

2. Intravenous Anesthetics

Drug Characteristics
Propofol Rapid induction
Thiopental sodium Ultra-short acting
Ketamine Produces dissociative anesthesia
Etomidate Used in induction of anesthesia

 

Stages of General Anesthesia

  1. Stage of analgesia – patient experiences reduced pain sensation.

  2. Stage of excitement – irregular breathing and involuntary movements may occur.

  3. Stage of surgical anesthesia – suitable for surgery with muscle relaxation.

  4. Stage of medullary paralysis – severe depression of vital centers (dangerous stage).

Therapeutic Uses of General Anesthetics

  • Major surgical procedures.

  • Pain control during complex medical interventions.

  • Used in diagnostic procedures requiring unconsciousness.

  • Facilitate muscle relaxation during surgery.

  • Used in emergency surgical operations.

Side Effects of General Anesthetics

  • Nausea and vomiting

  • Dizziness

  • Headache

  • Temporary confusion

  • Shivering after surgery

Adverse Effects of General Anesthetics

  • Severe respiratory depression

  • Hypotension and cardiovascular depression

  • Malignant hyperthermia (rare but serious reaction)

  • Liver or kidney toxicity (with some agents)

  • Allergic reactions

 

Comparison Between Local and General Anesthesia

Feature Local Anesthesia General Anesthesia
Consciousness Patient remains conscious Patient becomes unconscious
Area affected Specific localized area Entire body
Use Minor procedures Major surgery
Risk level Lower risk Higher risk
Recovery Faster recovery Longer recovery period

 

Precautions in CNS Drug Therapy

  • Use anesthetic drugs under medical supervision only.

  • Monitor vital signs such as blood pressure, heart rate, and respiration.

  • Check for drug allergies before administration.

  • Adjust dosage based on age, weight, and health status of the patient.

  • Maintain emergency equipment and resuscitation facilities during anesthesia.