Drugs Acting on Central Nervous System
The central nervous system controls thinking, movement, pain sensation, consciousness, emotions, and memory. Drugs acting on the central nervous system (CNS) either stimulate or depress brain activity. These medicines are used for major conditions such as mental illness, seizures, pain management, anesthesia, anxiety, and sleep disorders.
Anaesthetics
Anaesthetics cause loss of sensation by suppressing the activity of brain or nerves. They are given during medical procedures to prevent pain. Based on their depth of action, these drugs are divided into:
- Local Anaesthetics: numb a specific area (used in dental procedures and minor surgeries).
- General Anaesthetics: induce complete loss of consciousness and are used in major surgeries, childbirth, and emergency procedures.
Stages of General Anaesthesia
General anaesthetics depress brain functions in four stages:
- Stage I (Analgesia): sensation reduces with mild consciousness loss.
- Stage II (Delirium): uncontrolled activity and excitability appear.
- Stage III (Surgical Anaesthesia): complete unconsciousness with relaxed muscles; used for surgery.
- Stage IV (Medullary Paralysis): avoided stage; it may lead to respiratory and cardiac failure.
Classification of General Anaesthetics
- Inhalational Agents: Halothane, Enflurane, Ether.
- Intravenous Agents: Thiopentone Sodium, Ketamine, Propofol, Etomidate.
- Gaseous Anaesthetics: Nitrous Oxide.
Example: Thiopental Sodium
Thiopental sodium is a short-acting intravenous anaesthetic. It enhances GABA activity in the brain, causing a deep sedative effect. It is used for rapid induction of anaesthesia, short procedures, controlling convulsions, and surgery in brain injury patients.
Example: Ketamine Hydrochloride
Ketamine produces strong anaesthesia by blocking NMDA receptors. Unlike many agents, it does not act through GABA receptors. It is useful in painful procedures, emergency anesthesia, and is often chosen for children. It also helps in cases of depression in special medical settings.
Example: Propofol
Propofol activates GABA receptors, causing hypnosis and sedation. It is commonly used to maintain sleep during surgeries and to sedate ventilated patients in intensive care units.
Sedatives and Hypnotics
These drugs depress the CNS. At low doses they calm nerves (sedation), while at higher doses they induce sleep (hypnosis). They do not relieve pain, but they reduce anxiety and promote natural sleep. Common groups include barbiturates, benzodiazepines, and modern hypnotics like zolpidem.
Example: Diazepam
Diazepam enhances the effect of GABA, causing relaxation, decreased anxiety, and seizure control. It is used for severe anxiety, alcohol withdrawal, muscle spasms, and sleep disorders.
Example: Alprazolam
Alprazolam acts on benzodiazepine receptors linked with GABA. It is a popular medicine for panic attacks and anxiety disorders, but should be used cautiously due to dependence risk.
Example: Nitrazepam
Nitrazepam is a long-acting benzodiazepine used for insomnia, panic disorders, and seizure control. It enhances GABA binding to reduce brain excitability.
Example: Phenobarbital
Phenobarbital is a barbiturate with strong CNS depressant action. It increases seizure threshold and is used for epilepsy and sedation in tension states.
Antipsychotics
Antipsychotic drugs manage disorders like schizophrenia, bipolar disease, hallucinations, and severe behavioural problems. They work mainly by affecting dopamine and serotonin activity in the brain.
Example: Chlorpromazine
Chlorpromazine blocks dopamine receptors to treat schizophrenia and control nausea, vomiting, restlessness, manic behaviour, and severe childhood hyperactivity.
Example: Haloperidol
Haloperidol suppresses dopamine activity and is used in drug-induced psychosis, violent behaviour, agitation, and severe nausea.
Example: Risperidone
Risperidone blocks both dopamine and serotonin receptors. It is widely prescribed for schizophrenia, bipolar mood disorders, and depression with psychosis.
Anticonvulsants
Anticonvulsants prevent abnormal and excessive neuronal activity in the brain. They are used for epilepsy, seizures, neuropathic pain, and psychiatric disorders.
Example: Phenytoin
Phenytoin stabilizes sodium channels and prevents spread of seizure activity. It is used in status epilepticus, trigeminal neuralgia, and digitalis-induced arrhythmias.
Example: Carbamazepine
Carbamazepine inhibits repetitive neuron firing. Apart from epilepsy, it treats trigeminal neuralgia and bipolar disorders.
Example: Valproic Acid
Valproic acid increases GABA levels and controls absence seizures, tonic-clonic seizures, bipolar mania, and migraine prophylaxis.
Antidepressants
Antidepressants elevate mood and treat emotional disorders. They work by increasing serotonin, norepinephrine, or both. Major classes include SSRIs, SNRIs, MAO inhibitors, and tricyclic antidepressants.
Example: Amitriptyline
Amitriptyline enhances serotonin and norepinephrine levels. It is used for depression, neuropathic pain, anxiety, and mood disorders.
Example: Fluoxetine
Fluoxetine is an SSRI that selectively increases serotonin concentration. It is one of the most prescribed drugs for depression, OCD, and obesity-related mood disorders.
Example: Sertraline
Sertraline increases serotonin activity and is highly effective in depression, panic disorder, PTSD, anxiety, and obsessive-compulsive disorder.
