Preparatory Mindset
CNS drugs are organized by the neurotransmitter system they affect. For each drug class, know: mechanism, clinical uses, adverse effects, and contraindications. Opioids are the high-yield topic here — understand the receptor types (μ, κ, δ), their effects, and the triad of opioid overdose (respiratory depression, miosis, coma).
Core Concepts
Ch03: Central Nervous System
Neurotransmitters at a Glance
| Neurotransmitter | Key Function | Related Disorder |
|-----------------|-------------|-----------------|
| GABA | Primary inhibitory NT; sleep regulation | Anxiety, epilepsy |
| Dopamine | Motor control, reward, emotion | Parkinson's (↓), Schizophrenia (↑) |
| Serotonin (5-HT) | Mood regulation, sleep | Depression (↓) |
| Acetylcholine (Central) | Learning, memory, motor regulation | Alzheimer's (↓) |
| Glutamate | Primary excitatory NT | Excitotoxicity in stroke |
Key Drug Tables
Opioid Analgesics
| Drug | Receptor | Effect | Use | Key Side Effect |
|------|---------|--------|-----|----------------|
| Morphine | μ (main) + κ + δ | Analgesia, sedation, euphoria, cough suppression, miosis, ↓ GI motility | Severe pain, cardiac asthma, diarrhea, cough | Respiratory depression (DEADLIEST), constipation, addiction, miosis |
| Codeine | μ (weak) | Mild analgesia, cough suppression | Mild-moderate pain, cough | Less potent, better oral bioavailability |
| Naloxone/Naltrexone | μ antagonist | Reverses opioid effects | Opioid overdose, alcohol dependence | Acute withdrawal in dependent patients |
Benzodiazepines
| Drug | Receptor | Effect | Clinical Use |
|------|---------|--------|-------------|
| Diazepam | GABA-A (α/γ subunit) | Anxiolytic, sedative, anticonvulsant, muscle relaxant | Status epilepticus (IV), anxiety, muscle spasm |
| Antidote: Flumazenil | Competitive BZ antagonist | Reverses BZ overdose | BZ poisoning |
Antiepileptics
| Seizure Type | 1st Choice | Others |
|-------------|-----------|--------|
| Tonic-clonic (Grand mal) | Phenytoin | Carbamazepine, Valproate |
| Absence (Petit mal) | Ethosuximide | Valproate |
| Status epilepticus | Diazepam (IV) | Phenytoin, Phenobarbital |
NSAIDs (Aspirin)
| Dose | Effect | Mechanism |
|------|--------|-----------|
| Low (50-100 mg/d) | Antiplatelet | Irreversibly inhibits COX-1 → ↓ TXA2 |
| Middle | Antipyretic + Analgesic | Inhibits CNS COX-2 + peripheral PGs |
| High (3-5 g/d) | Anti-inflammatory | Inhibits COX-2 in inflamed tissues |
Antipsychotic (Chlorpromazine)
| Use | Mechanism | Side Effects |
|-----|-----------|-------------|
| Schizophrenia, hypothermic anesthesia | Blocks D2 receptors (mesolimbic) | Extrapyramidal symptoms, sedation, anticholinergic effects |
Exam-Frequency Core Points (from exam notes)
I. Sedative-Hypnotics (Exam frequency: ~15%)
| Exam Category | Specific Exam Content | Frequency | 【Must-Know Details】 |
| --- | --- | --- | --- |
| Benzodiazepines (BZ) – Core points | Mechanism of action | ★★★ | Binds to the allosteric site of the GABAA receptor, increases the frequency of Cl⁻ channel opening, enhances the central inhibitory effect of GABA. No direct activation of the GABAA receptor, and even high doses do not cause anaesthesia. |
| Benzodiazepines (BZ) – Core points | Drug classification | ★★ | Long-acting: diazepam, flurazepam; Intermediate-acting: estazolam; Short-acting: triazolam. |
| Benzodiazepines (BZ) – Core points | Core points on diazepam | ★★★ | Pharmacological effects: anxiolytic, sedative-hypnotic, anticonvulsant/antiepileptic, central muscle relaxant. Clinical uses: anxiety, insomnia, first choice for status epilepticus, premedication before surgery, relief of muscle spasm. Adverse effects: residual effects (hangover), tolerance, dependence, ataxia at high doses. |
| Benzodiazepines (BZ) – Core points | Specific antidote | ★★★ | Flumazenil is a competitive antagonist at the benzodiazepine receptor and is the specific antidote for acute benzodiazepine poisoning. |
| Barbiturates – Core points | Mechanism of action & safety risks | ★★★ | Binds to the allosteric site of the GABAA receptor, increases the duration of Cl⁻ channel opening. At high doses, it can directly activate the GABAA receptor. With increasing doses, sedation, hypnosis, anticonvulsant effects, anaesthesia, and central paralysis leading to death occur sequentially. Narrow therapeutic window, no specific antidote; no longer used as routine sedative-hypnotics, now only for anticonvulsant effects and anaesthetic induction. |
II. Antiepileptics and Anticonvulsants (Exam frequency: ~30%, core focus of the CNS module)
| Exam Content | Frequency | 【Must-Know Details】 |
| --- | --- | --- |
| First-choice drugs by seizure type | ★★★ | ① Status epilepticus: diazepam IV is first choice; ② Grand mal (tonic-clonic seizures): phenytoin sodium is first choice; ③ Absence seizures: ethosuximide is first choice; ④ Psychomotor seizures: carbamazepine is first choice; ⑤ Broad-spectrum antiepileptic (effective for all seizure types): sodium valproate. |
| Core points on phenytoin sodium | ★★★ | No sedative-hypnotic effect, does not affect normal cognition. Adverse effects: gingival hyperplasia, megaloblastic anaemia, allergic reactions, teratogenicity (contraindicated in pregnancy). Ineffective for absence seizures. |
| Core points on carbamazepine | ★★★ | Superior to phenytoin sodium for trigeminal neuralgia and glossopharyngeal neuralgia; first-choice drug for neuropathic pain. |
| Core points on sodium valproate | ★★ | Most severe adverse effect is fatal hepatotoxicity; liver function monitoring required in children. |
| Core points on magnesium sulfate | ★★★ | Different routes of administration produce completely different effects: ① Oral: catharsis and cholagogic action; ② Intravenous: anticonvulsant and antihypertensive effects, used for eclampsia, tetanus convulsions, and hypertensive crisis; ③ Topical: reduces swelling and relieves pain. Rapid IV injection can cause respiratory depression and sudden hypotension; calcium gluconate is the antidote. |
III. Antipsychotic Drugs (Exam frequency: ~20%)
| Exam Category | Specific Exam Content | Frequency | 【Must-Know Details】 |
| --- | --- | --- | --- |
| Antipsychotics (core points on chlorpromazine) | Mechanism of action | ★★★ | Blocks central dopamine (DA) receptors, α‑receptors, M‑receptors, and 5‑HT receptors. |
| Antipsychotics (core points on chlorpromazine) | Clinical applications | ★★★ | ① Schizophrenia (mainly positive symptoms); ② Vomiting and intractable hiccups (ineffective for motion sickness‑induced vomiting); ③ Hypothermic anaesthesia and artificial hibernation (combined with pethidine and promethazine as the “hibernation cocktail”). |
| Antipsychotics (core points on chlorpromazine) | Adverse effects | ★★★ | ① Extrapyramidal symptoms: Parkinsonism, akathisia, acute dystonia, tardive dyskinesia (the first three can be relieved by trihexyphenidyl; no specific antidote for tardive dyskinesia); ② Orthostatic hypotension (due to α‑receptor blockade; adrenaline is contraindicated for reversal; noradrenaline should be used instead); ③ Endocrine disturbances: elevated prolactin leading to galactorrhoea and amenorrhoea, decreased growth hormone secretion; ④ Dry mouth, constipation, blurred vision (due to M‑receptor blockade). |
| Core points on antidepressants | First‑line drug classes | ★★ | Selective serotonin reuptake inhibitors (SSRIs): fluoxetine, paroxetine, sertraline – first‑choice antidepressants with fewer adverse effects. |
IV. Analgesics (Opioid and Non‑opioid) – Exam frequency: ~20%
| Exam Category | Specific Exam Content | Frequency | 【Must-Know Details】 |
| --- | --- | --- | --- |
| Opioid analgesics (core points on morphine) | Pharmacological effects | ★★★ | ① Central effects: analgesia, sedation, respiratory depression, antitussive, miosis (pinpoint pupils are characteristic of morphine poisoning), emetic; ② Peripheral effects: stimulation of gastrointestinal smooth muscle causing constipation, vasodilation causing orthostatic hypotension. |
| Opioid analgesics (core points on morphine) | Clinical applications | ★★★ | ① Severe acute pain (severe trauma, burns, surgical pain, cancer pain – contraindicated for labour pain); ② Cardiac asthma (contraindicated in bronchial asthma); ③ Diarrhoea (non‑infectious debilitating diarrhoea). |
| Opioid analgesics (core points on morphine) | Contraindications | ★★★ | Contraindicated in bronchial asthma, cor pulmonale, head injury with raised intracranial pressure, labour pain, nursing mothers, and severe hepatic insufficiency. |
| Opioid analgesics (core points on morphine) | Antidote for poisoning | ★★★ | Acute morphine poisoning presents with the triad of coma, pinpoint pupils, and respiratory depression. The specific antidote is naloxone (a competitive opioid receptor antagonist). |
| Synthetic analgesics | Core points on pethidine (meperidine) | ★★ | Weaker dependence potential than morphine; clinically can replace morphine for analgesia, cardiac asthma, preanaesthetic medication, and artificial hibernation. Has no antidiarrhoeal or antitussive effects; not used for labour pain (may cause neonatal respiratory depression). |
V. Antipyretic‑Analgesic and Anti‑Inflammatory Drugs (Non‑Steroidal Anti‑Inflammatory Drugs / NSAIDs) – Exam frequency: ~15%
| Exam Content | Frequency | 【Must-Know Details】 |
| --- | --- | --- |
| Common mechanism of action | ★★★ | Inhibition of cyclo‑oxygenase (COX) activity, reducing synthesis of prostaglandins (PGs). |
| Core points on aspirin (acetylsalicylic acid) | ★★★ | Pharmacological effects: low‑dose – antiplatelet aggregation (prevents cardiovascular and cerebrovascular thrombosis); medium‑dose – antipyretic and analgesic; high‑dose – anti‑inflammatory and anti‑rheumatic. Adverse effects: gastrointestinal reactions (most common), coagulation disorders, salicylism, allergic reactions (aspirin‑induced asthma), Reye’s syndrome (aspirin is contraindicated in children with febrile viral illness; paracetamol should be used instead). |
| Core points on paracetamol (acetaminophen) | ★★★ | Potent antipyretic and analgesic effects; almost no anti‑inflammatory or anti‑rheumatic effects; no antiplatelet action; few adverse effects. It is the first‑choice antipyretic for children. Overdose can cause acute hepatic necrosis; the antidote is acetylcysteine. |
| Core points on selective COX‑2 inhibitors | ★★ | Representative drugs: celecoxib, etoricoxib. Gastrointestinal adverse effects are significantly lower than those of non‑selective COX inhibitors, but they may increase the risk of adverse cardiovascular events. |
High-Yield Points
- Opioid triad: respiratory depression, miosis, coma — Rx: naloxone
- Morphine: μ agonist — analgesia, euphoria, constipation, respiratory depression
- Benzodiazepines: potentiate GABA-A — anxiolytic, sedative, anticonvulsant
- Flumazenil: BDZ antagonist — reverses overdose
- Antiepileptics: phenytoin (Na channel), valproate (multiple), carbamazepine (Na channel)
- Antipsychotics: D2 blockade — extrapyramidal side effects
- SSRIs: first-line for depression — fewer side effects than TCAs
Topic Summary
CNS drugs include opioids (μ agonists = analgesia, respiratory depression), benzodiazepines (GABA-A potentiation), antiepileptics (various mechanisms), antipsychotics (D2 blockade), and antidepressants (SSRIs, SNRIs). Opioid overdose triad: respiratory depression, miosis, coma — treat with naloxone.