LOCAL ANESTHETICS - Study Notes

1. Definition

• Drugs that reversibly block impulse conduction along nerve axons and other excitable membranes
• Block pain impulses or undesirable autonomic responses (e.g., sympathetic vasoconstriction)
• Produce temporary but complete analgesia of well-defined body regions

2. Basic Chemistry

• Optimal activity requires a balance between the lipophilic and hydrophilic ends
• Available as hydrochloride salts for solubility and stability
• Weak bases; exist as protonated or non-protonated species in tissues (ratio determined by pKa and tissue pH)
• Most drugs have pKa 8-9 → predominantly protonated at physiologic pH

Protonated vs. Non-protonated forms:

3. Classification

Amides

• Lignocaine (lidocaine)
• Mepivacaine
• Bupivacaine
• Prilocaine
• Ropivacaine
• Etidocaine

Esters

• Benzocaine - topical only (low solubility; risk of methemoglobinemia)
• Procaine
• Tetracaine - good for nerve blocks (long intracellular t½)
• Cocaine - topical only (significant systemic side effects)

4. Mechanism of Action

• Bind to intracellular sites on voltage-gated Na⁺ channels → blockade
• Binding only possible in active (open) and inactivated (closed) states - NOT in the resting state
• More nerve stimulation = more channels blocked (use-dependence)
• Binding/unbinding is noncovalent

Progressive effects of Na⁺ channel blockade:

1. Stimulation threshold increases
2. Action potential amplitude decreases
3. Impulse conduction slows
4. Action potential generation is ultimately blocked

Additional channel dynamics:

• Recovery from LA-induced block is up to 1000x slower than normal inactivation → refractory period greatly increased
• Blockade is voltage- and time-dependent
  • More negative membrane potential → channels mostly closed → low LA affinity
  • More positive membrane potential → channels open or inactivated → high LA affinity
• Effect more marked on rapidly firing axons than resting nerve fibers
• Resting membrane potential is not significantly altered (K⁺ efflux counteracts Na⁺ influx)

Modifiers of blockade:

• Elevated extracellular Ca²⁺ → partially antagonizes LA (favors resting/closed channel state)
• Elevated extracellular K⁺ → potentiates LA (favors depolarized/inactivated channel state)

5. Effects on Different Nerve Types

• Affects all nerve types: sensory, somatic motor, autonomic
• Myelinated > non-myelinated nerves: easier to block
• Smaller diameter = faster blockade: order of susceptibility:

  Pain > Autonomic > Motor

Spinal anesthesia-specific risks:

• Motor impairment → paralysis of respiratory muscles
• Autonomic impairment → hypotension

Fiber position in nerve bundle:

• Large nerve trunks: Motor fibers are outermost → motor block may precede sensory block
• Extremities: Proximal sensory fibers outermost; distal fibers innermost → anesthesia develops proximally first, spreads distally

6. Local Anesthetics in Infected Tissue

• Problem: Infected tissue is acidic (low pH) → protonated form predominates → poor cell penetration → reduced efficacy of infiltration anesthesia
• Solution: Use a regional nerve block instead (drug deposited away from the acidic area)

7. Routes of Administration

8. Drug Selection by Duration

• Topical: benzocaine (powder or gel formulations)
• Short/intermediate-acting drugs can be extended with vasoconstrictors
• Short-acting drugs (procaine, chlorprocaine) preferred for regional (IV) blocks to limit systemic effects

9. Pharmacokinetics

Absorption

• Systemic absorption is undesirable (risk of CVS and CNS toxicity)
• Factors affecting systemic absorption:
  1. Dose administered
  2. Vascularity of injection site
  3. Presence/absence of vasoconstrictor
  4. Drug-tissue binding
  5. Physicochemical properties of drug
• Greater drug-tissue binding → longer t½, slower onset
• Smaller + more lipophilic molecules → faster interaction with Na⁺ channels

Metabolism & Excretion

• Metabolized in liver and plasma → water-soluble metabolites → excreted in urine
• Esters: Hydrolyzed rapidly in plasma/tissues by pseudocholinesterase
• Amides: Metabolized by hepatic CYP450 enzymes
• Liver disease increases systemic toxicity risk for amides (lignocaine t½: 2 hrs → 8 hrs in liver disease)
• Urinary acidification promotes excretion of non-protonated drug

10. Vasoconstrictors

How they help:

• Prolong LA effect by up to 50% by limiting systemic escape of drug
• Increase local tissue drug concentration → enhanced nerve cell diffusion
• Reduce systemic toxicity
• Allow use of lower total drug dose (improved safety margin)
• Adrenaline in spinal LA also stimulates α₂ adrenoceptors → inhibits release of substance P (neurokinin-1) → reduces sensory neuronal firing
• Other α₂ agonists (e.g., clonidine) can augment LA by the same mechanism

Limitations:

• Little effect on reducing systemic effects of long-acting drugs (bupivacaine, prilocaine)

11. Adverse Effects

CNS (dose-dependent, in order of increasing severity):

• Sleepiness
• Light-headedness
• Visual disturbances
• Auditory disturbances
• Nystagmus
• Restlessness, anxiety
• Convulsions (tonic-clonic) ← higher drug levels
• CNS depression, coma, death ← highest drug levels

Cardiovascular:

• Cardiac arrhythmias, bradycardia
• Vasodilation and hypotension (autonomic nerve effects)
• Methemoglobinemia - with benzocaine
• Vasoconstriction, tachycardia, euphoria - cocaine-specific
• Pregnancy increases susceptibility → use decreased dose + vasoconstrictor
• Mepivacaine - particularly toxic to newborns → contraindicated in pregnancy

Key rule: Always use the lowest effective dose to minimize systemic toxicity

LOCAL ANESTHETICS — MCQs

SECTION A: Chemical Structure & Classification

• A) Lipophilic aromatic ring
• B) Tertiary amine hydrophilic group
• C) Intermediate chain
• D) Hydrochloride salt formulation

• A) Procaine
• B) Tetracaine
• C) Benzocaine
• D) Lignocaine

• A) Amides
• B) Esters
• C) Both equally
• D) Neither — pseudocholinesterase is irrelevant to LAs

SECTION B: Mechanism of Action

• A) Resting state only
• B) Active (open) and inactivated (closed) states only
• C) All three states equally
• D) Inactivated state only

• A) Less blockade due to receptor saturation
• B) More blockade due to increased channel cycling into open/inactivated states
• C) No difference — LA blockade is independent of firing rate
• D) More blockade due to increased K⁺ efflux

• A) LAs also block K⁺ channels simultaneously
• B) K⁺ efflux counteracts the Na⁺ influx blocked by LAs
• C) LAs activate Cl⁻ channels which stabilize the membrane
• D) The resting Na⁺/K⁺ ATPase compensates immediately

• A) Reduces LA efficacy — channels remain in resting state
• B) Has no effect on LA action
• C) Potentiates LA action — channels favor the depolarized/inactivated state
• D) Antagonizes LA action by stabilizing the membrane

SECTION C: Pharmacokinetics & Ionization

• A) Non-protonated form; responsible for cell membrane penetration
• B) Protonated form; responsible for diffusion from injection site and retention inside nerve cell
• C) Non-protonated form; responsible for receptor binding at the sodium channel
• D) Equal amounts of both forms; no clinical significance

• A) Lidocaine is ineffective for dental procedures
• B) The patient has pseudocholinesterase deficiency
• C) The acidic environment of the abscess increases the protonated fraction, impairing cell membrane penetration
• D) Lidocaine requires a vasoconstrictor to work in the oral cavity

• A) t½ decreases from 2 hours to 30 minutes
• B) t½ increases from 2 hours to 8 hours
• C) No change — lignocaine is metabolized by plasma esterases
• D) Increased renal clearance compensates for reduced hepatic metabolism

SECTION D: Routes of Administration & Drug Selection

• A) Bupivacaine
• B) Ropivacaine
• C) Tetracaine
• D) Procaine

• A) Allergy to the local anesthetic
• B) Autonomic nerve blockade causing hypotension; motor blockade of respiratory muscles
• C) Systemic absorption causing CNS depression
• D) Vasoconstrictor effect of the drug

• A) Motor fibers have smaller diameters than sensory fibers
• B) Motor fibers are located circumferentially at the outermost part of the nerve trunk, exposed to LA first
• C) LA has higher affinity for motor than sensory receptors
• D) Motor fibers are non-myelinated and easier to block

SECTION E: Vasoconstrictors

• A) Direct blockade of Na⁺ channels
• B) Stimulation of α₂ adrenoceptors → inhibition of substance P release
• C) β₂ receptor stimulation → muscle relaxation
• D) Activation of opioid receptors in the dorsal horn

• A) Adrenaline is metabolized too quickly at epidural sites
• B) Bupivacaine has such high protein binding it is not absorbed systemically
• C) Vasoconstrictors have little effect on reducing systemic absorption of long-acting drugs like bupivacaine
• D) Adrenaline is contraindicated with amide-type LAs

SECTION F: Adverse Effects & Special Populations

• A) Seizures → light-headedness → coma → death
• B) Light-headedness → visual/auditory disturbances → restlessness → seizures → CNS depression → coma → death
• C) Coma → seizures → light-headedness → death
• D) Restlessness → visual disturbances → bradycardia → seizures

• A) Bronchospasm; give salbutamol
• B) Methemoglobinemia; give methylene blue
• C) LA systemic toxicity; give lipid emulsion
• D) Anaphylaxis; give adrenaline

• A) Lignocaine
• B) Bupivacaine with adrenaline
• C) Mepivacaine
• D) Prilocaine

• A) Sodium bicarbonate infusion
• B) Intravenous lipid emulsion (Intralipid)
• C) Activated charcoal
• D) Dialysis

• A) Cocaine blocks Na⁺ channels causing direct cardiac stimulation
• B) Cocaine blocks reuptake of catecholamines at sympathetic nerve terminals → sympathomimetic effects
• C) Cocaine stimulates opioid receptors → increased sympathetic tone
• D) Cocaine activates muscarinic receptors → reflex sympathetic activation

HIGH-YIELD PEARL SUMMARY

1. Local Vasoconstriction (Primary Mechanism)

• Blood flow to the area is reduced
• Lidocaine is "trapped" locally - it cannot be carried away by the bloodstream as quickly
• Higher concentration of lidocaine remains at the nerve for longer
• Duration of action is extended by up to 50%
• Onset may be slightly faster because more drug is available at the target nerve

2. Reduced Systemic Absorption → Lower Toxicity Risk

• Peak plasma lidocaine levels are significantly lower
• Risk of CNS toxicity (seizures) and cardiovascular toxicity (arrhythmias) is reduced
• This also means you can use a lower total dose of lidocaine while achieving the same effect - a major safety advantage

3. Allows Use of a Smaller Dose

4. α₂ Adrenoceptor - Mediated Analgesia (Spinal Use)

• It stimulates α₂ adrenoceptors on dorsal horn neurons
• This inhibits the release of substance P (neurokinin-1) - a key neurotransmitter that drives sensory neuronal firing and pain transmission
• This adds a direct analgesic effect on top of the sodium channel blockade

5. Why It Works Best With Lidocaine Specifically

Summary Table

Short Answer

Lidocaine in Pregnancy

• Lidocaine crosses the placenta (it is lipophilic and has low molecular weight)
• However, it is widely used and accepted in pregnancy — dental procedures, epidurals, and regional blocks routinely use lidocaine in pregnant women
• The key principle from your slides: pregnancy increases susceptibility to LA nerve block and systemic toxicity → use a reduced dose
• Lidocaine is FDA Category B historically (no proven fetal harm at clinical doses)

Epinephrine in Pregnancy — Where It Gets Complicated

⚠️ Concern 1: Uterine Vasoconstriction

• Epinephrine stimulates α₁ receptors on uterine blood vessels
• This can reduce uteroplacental blood flow → potential fetal hypoxia
• Risk is highest with intravascular injection or high systemic doses
• At the low concentrations used in dental/local infiltration (1:100,000 or 1:200,000), the systemic effect is minimal and generally considered safe

✅ Concern 2: Actually Protective in the Right Context

• By limiting systemic absorption of lidocaine, epinephrine prevents maternal systemic toxicity
• A seizing or cardiovascularly compromised mother is far more dangerous to the fetus than the low-dose epinephrine itself
• Your slides explicitly state: for pregnant patients, use a decreased dose, possibly with a vasoconstrictor — meaning epinephrine is not contraindicated

The Real Contraindication: Mepivacaine, Not Lidocaine

• Mepivacaine is contraindicated in pregnancy (toxic to newborns)
• Lidocaine is NOT contraindicated — it is the preferred amide LA in pregnancy when a local anesthetic is needed

Practical Clinical Rules

Key Physiological Reason Epi Is Often Kept

Lidocaine + Epinephrine During Labour

1. Labour Epidural Analgesia (Pain Relief During Labour)

• Labour epidurals typically use very low-concentration local anaesthetics (e.g., bupivacaine 0.0625–0.1% or ropivacaine) combined with an opioid (fentanyl)
• If epinephrine is added at all, it is used at very low doses (2 μg/mL) purely as an α₂ agonist adjuvant for analgesia enhancement - NOT for vasoconstriction
• A 2021 RCT (Haidl et al., PMID 33964196) found that adding adrenaline 2 μg/mL to a labour epidural solution had no significant effect on maternal haemodynamics (blood pressure, cardiac output, heart rate) - reassuring, but also illustrating that the dose is so low it barely affects vasomotor tone

Why high-concentration epinephrine is avoided in active labour:

2. Emergency Conversion to Caesarean Section (Surgical Anaesthesia)

2% lidocaine with epinephrine 5 μg/mL (1:200,000) - used for rapid conversion of existing labour epidural to surgical anaesthesia for urgent Caesarean delivery

• "Lidocaine should be administered with epinephrine, as lidocaine without epinephrine does not consistently provide satisfactory surgical anaesthesia"
• Sodium bicarbonate (1 mEq/10 mL) and fentanyl are often added to speed onset further
• This is time-critical - the combination provides faster, denser block than alternatives

3. The Key Distinction

4. Why Bupivacaine is No Longer Preferred for C-section

The Key Concept: The Uterus Has Two Systems Driving Contractions

1. Uterine Contractions Are Primarily Myogenic

• Uterine smooth muscle (myometrium) has intrinsic pacemaker activity — it generates its own rhythmic electrical activity and contractions without any nerve input
• This is similar to how the heart beats without neural input (cardiac automaticity)
• The signal for labour contractions originates within the myometrium itself, driven by:
  • Rising oestrogen levels (increase excitability and oxytocin receptor expression)
  • Oxytocin from the posterior pituitary (hormonal, travels via blood — not nerves)
  • Prostaglandins (local tissue hormones, not nerve-mediated)
  • Gap junctions between myometrial cells that spread electrical activity like a wave

2. What the Epidural/Nerve Block Actually Blocks

3. Clinical Evidence: Women Contract Perfectly Well Under Epidural

• Women with complete spinal cord injuries (no nerve connection to uterus whatsoever) still go into labour spontaneously and deliver vaginally
• Epidural anaesthesia does not stop labour - in fact, it is routinely used precisely during active labour
• Contractions on the CTG (cardiotocograph) continue normally after epidural placement
• Uterine activity is monitored with a tocometer and remains unchanged

4. The Nuance: What CAN Affect Contractions

5. What May Actually Change With Epidural Analgesia

• Sensory block reduces the Ferguson reflex (the urge to push triggered by fetal head pressure on the cervix/pelvic floor)
• Motor block of abdominal and pelvic floor muscles weakens voluntary expulsive effort
• But contractions themselves still occur — the woman just cannot feel or coordinate pushing as effectively

Summary

First: What Is the Positive Feedback Loop in Labour?

Fetal head descends
        ↓
Presses on cervix & lower uterine segment
        ↓
Stretch receptors in cervix activated
        ↓
Nerve signal travels via pelvic nerves → spinal cord → hypothalamus
        ↓
Posterior pituitary releases MORE oxytocin into blood
        ↓
Oxytocin binds myometrial receptors → stronger contractions
        ↓
Fetal head pushed down harder → more cervical stretch
        ↓
(cycle amplifies until delivery)

What Happens to This Loop Under Epidural/Nerve Block?

Arm 1: The Afferent Limb (Cervix → Brain) — PARTIALLY DISRUPTED

• The stretch receptors in the cervix send signals via pelvic splanchnic nerves (S2-S4) and hypogastric nerves (T10-L1)
• An epidural at lumbar level can block these afferent signals from reaching the spinal cord
• This means the hypothalamus receives a reduced or absent stretch signal
• Result: less reflex oxytocin release from the posterior pituitary

Arm 2: The Efferent Limb (Oxytocin → Uterus) — INTACT

• Oxytocin travels via the bloodstream, not via nerves
• Once released (from whatever signal reaches the hypothalamus), it binds oxytocin receptors on the myometrium directly
• The nerve block has zero effect on this arm
• The uterus still responds fully to whatever oxytocin is circulating

Arm 3: The Myometrial Response — INTACT

• As established, myometrial contractions are myogenic
• The muscle still contracts in response to oxytocin, prostaglandins, and intrinsic pacemakers
• The nerve block does not impair muscle contractility itself

Net Clinical Result: Loop Is Weakened But Not Broken

How Clinicians Compensate

• Exogenous oxytocin (Syntocinon) infusion — bypasses the entire neural reflex arc by directly supplying what the posterior pituitary is not releasing enough of
• This is why oxytocin augmentation is more commonly needed in women with epidurals
• It essentially replaces the missing positive feedback signal with a pharmacological substitute

The Ferguson Reflex Also Has a Second Job — The Pushing Urge

• The woman may not feel the urge to push even when fully dilated
• Pushing becomes a voluntary, coached effort rather than an overwhelming involuntary reflex
• This is a major reason for prolonged second stage with epidurals — not lack of contractions, but loss of the reflexive pushing drive

Summary

Why pKa Controls Onset

pH = pKa + log [non-protonated] / [protonated]

At tissue pH 7.4:

log [B] / [BH⁺] = 7.4 − pKa

Practical Comparison

The Other Factors (Secondary)

The Clinical Trick: Bicarbonate Alkalinisation

One-Line Answer

NEUROTOXINS & NEUROTRANSMITTER ANTAGONISTS

Quick-Reference Study Tables + Mnemonics

TABLE 1: Master Classification Table

TABLE 2: Toxin by Source/Origin

TABLE 3: Clinically Used Agents (High-Yield for Quiz)

MNEMONICS

🧠 "SARAH Blocks Release" — Agents that block AP propagation

Saxitoxin, A (local) Anesthetics, Remember Hiding in puffer fish (Tetrodotoxin)

• Puffer fish = TTX
• Sea microbe = Saxitoxin
• Local anesthetics = clinical version

🧠 "HAVE BRAT" — Steps of neurotransmission that can be blocked (in order)

Hemicholinium (synthesis) → Action potential → Vesamicol/VMAT (storage) → Exocytosis (botulinum) → Blockade of receptor (atropine/curare) → Reuptake (cocaine/TCAs) → Agonism/excess (latrotoxin/dendrotoxin) → Termination

🧠 "BIG VMAT" — Reserpine's target

• Blocks
• Into
• Granules: Vesicular Monoamine Amine Transporter
• = Reserpine/Tetrabenazine → depletes NE/DA → antihypertensive

🧠 "VAT traps ACh" — Vesamicol

• Vesamicol blocks VAT → ACh can't get into Vesicles
• Research only (no clinical use)

🧠 "BOTS can't fuse" — Botulinum toxin

• Botulinum Only Targets SNARE/VAMP fusion proteins
• Cleaves 2 amino acids → vesicle can't fuse → no ACh release
• Recovery takes months (must make new proteins)

🧠 "Black widow EXPLODES both" — α-Latrotoxin

• Black widow → BOTH cholinergic AND adrenergic explosive release
• (contrast with botulinum: only cholinergic, blocks release)

🧠 "MAMBA delays K⁺" — Dendrotoxin

• Mamba = Dendrotoxin
• Blocks K⁺ channels → delayed repolarisation → prolonged AP → more ACh
• Results: peripheral convulsions + diarrhea; central hyperexcitability + death

🧠 "CATS block nicotinic" — Curare agents

Curare (tubocurarine) / Atracurium / Tubocurarine analogues / Surgical relaxants (pancuronium, vecuronium)

• All: competitive antagonists at NM junction nicotinic receptors
• Quaternary ammonium → poor oral absorption (arrow poisons work parenterally)

🧠 "ABCD of Atropine"

Anti-muscarinic | Bella donna source | Children get excited + dry + hot + blind | Drug: physostigmine is antidote

🧠 "Strychnine STOPS Glycine"

• Strychnine blocks glycine receptors in spinal cord
• Glycine = main inhibitory NT on motor neurons
• Block it → unopposed excitation → extensor spasms (triggered by ANY stimulus)
• Tetanus toxin = same outcome but different mechanism (blocks glycine release)

"Strychnine STOPS the receptor; Tetanus STOPS the release"

🧠 "ABRA — 4 clinical drugs to know" (from class objectives)

Alpha-methyltyrosine | Botulinum toxin | Reserpine | Atropine

NEUROTOXINS & NEUROTRANSMITTER ANTAGONISTS — Clinical Case MCQs

SECTION A: Toxins That Block Action Potential Propagation

• A) Administer activated charcoal; toxin blocks muscarinic receptors
• B) Supportive care and mechanical ventilation; toxin blocks voltage-gated Na⁺ channels abolishing nerve conduction
• C) Give calcium gluconate; toxin blocks Ca²⁺ channels at the NM junction
• D) Administer physostigmine; toxin inhibits acetylcholinesterase

• A) Vibrio cholerae; activates adenylyl cyclase causing secretory diarrhea
• B) Gonyaulax species; produces saxitoxin which blocks voltage-gated Na⁺ channels
• C) Clostridium botulinum; blocks ACh exocytosis at cholinergic terminals
• D) Bungarus multicinctus; produces α-bungarotoxin blocking nicotinic receptors

SECTION B: Toxins That Block Transmitter Synthesis

• A) Reserpine
• B) Phentolamine
• C) α-methyltyrosine (metyrosine)
• D) Atropine

SECTION C: Toxins That Block Transmitter Storage

• A) Cocaine; blocks norepinephrine reuptake
• B) Reserpine; blocks VMAT preventing NE/DA entry into vesicles
• C) α-methyltyrosine; inhibits tyrosine hydroxylase
• D) Phentolamine; blocks α₁ adrenoceptors

• A) Hemicholinium; blocks choline reuptake transporter
• B) Vesamicol; blocks VAT (vesicle-associated transporter) preventing ACh entry into vesicles
• C) Botulinum toxin; cleaves VAMP fusion proteins
• D) α-bungarotoxin; blocks nicotinic receptors

SECTION D: Botulinum Toxin

• A) Guillain-Barré syndrome; autoimmune demyelination of peripheral nerves
• B) Infant botulism; botulinum toxin cleaves VAMP proteins blocking ACh exocytosis
• C) Myasthenia gravis; autoantibodies against nicotinic receptors
• D) Tetanus; blockade of glycine release causing spastic paralysis

Key distinction: Botulinum = flaccid (no release) | Tetanus = spastic (no inhibition)

• A) Synthesis of new acetylcholinesterase enzyme over 2–4 weeks
• B) Regeneration of new nerve terminals and synthesis of new VAMP fusion proteins over several months
• C) Clearance of the toxin by the liver within 1–2 weeks
• D) Upregulation of nicotinic receptors at the NM junction over 4 weeks

SECTION E: Toxins Causing Excessive Neurotransmission

• A) Blocks voltage-gated Na⁺ channels → flaccid paralysis
• B) Binds vesicles in both cholinergic and adrenergic terminals → explosive transmitter release
• C) Blocks glycine receptors → extensor spasms triggered by stimuli
• D) Cleaves VAMP proteins → prevents ACh exocytosis

• A) α-Bungarotoxin; blocks nicotinic receptors at NM junction
• B) Tetrodotoxin; blocks Na⁺ channels
• C) Dendrotoxin; blocks K⁺ channels → delayed repolarisation → prolonged AP → ↑ ACh release
• D) α-Latrotoxin; causes explosive ACh release from vesicles

SECTION F: Agents Blocking Post-Synaptic Receptors

• A) Succinylcholine; depolarising blocker; highly lipophilic
• B) Atracurium; non-depolarising competitive nicotinic antagonist; quaternary ammonium compound (charged → poor oral absorption)
• C) Atropine; muscarinic antagonist; crosses blood-brain barrier easily
• D) Botulinum toxin; blocks VAMP; protein structure prevents oral absorption

• A) Treatment of myasthenia gravis by blocking autoantibody binding sites
• B) Assaying and quantifying nicotinic receptor content in tissue preparations
• C) Investigating voltage-gated Na⁺ channel distribution in neurons
• D) Blocking muscarinic receptors to study parasympathetic function

SECTION G: Atropine

• A) Organophosphate poisoning; atropine is the antidote
• B) Atropine/belladonna poisoning; physostigmine is the antidote
• C) Strychnine poisoning; diazepam is the antidote
• D) Botulinum toxicity; supportive care only

• A) Physostigmine; inhibits AChE at nicotinic receptors
• B) Pralidoxime; reactivates acetylcholinesterase
• C) Atropine; competitive muscarinic receptor antagonist — blocks parasympathetic overstimulation
• D) Reserpine; depletes ACh stores

SECTION H: Strychnine & Tetanus Toxin

• A) Blocks VAMP proteins → no ACh release → flaccid paralysis
• B) Binds glycine receptors in the spinal cord → removes inhibitory control of motor neurons → violent extensor spasms
• C) Blocks K⁺ channels → prolongs action potential → excessive motor firing
• D) Explosive release of all neurotransmitters → sympathetic storm

• A) Tetanus toxin blocks VAMP → no ACh release at NM junction (same as botulinum)
• B) Tetanus toxin blocks glycine release from inhibitory interneurons in the spinal cord (presynaptic); strychnine blocks glycine receptors (postsynaptic) — same clinical outcome via different sites
• C) Tetanus toxin blocks nicotinic receptors competitively like curare
• D) Tetanus toxin and strychnine are identical in mechanism

• Tetanus toxin: blocks release of glycine from inhibitory interneurons (presynaptic)
• Strychnine: blocks glycine receptors on motor neurons (postsynaptic)

SECTION I: Capsaicin

• A) Blocks voltage-gated Na⁺ channels in sensory nerves → immediate anesthesia
• B) Binds nociceptor neurons → releases then depletes substance P; high doses destroy the neurons causing prolonged analgesia
• C) Blocks TRPV1 receptors → prevents pain signal generation
• D) Inhibits prostaglandin synthesis → anti-inflammatory analgesia

SECTION J: Reuptake Blockers

• A) Cocaine blocks muscarinic receptors causing sympathetic dominance
• B) Cocaine blocks NET (norepinephrine transporter) → NE accumulates at sympathetic synapses → intense α₁ and β₁ stimulation → vasoconstriction, tachycardia, hypertension, coronary spasm
• C) Cocaine blocks VMAT → NE floods out of vesicles
• D) Cocaine stimulates β₂ receptors on the heart causing tachycardia

• A) Anticholinergic syndrome from muscarinic receptor blockade
• B) Serotonin syndrome from excessive serotonergic activity due to dual reuptake inhibition combined with tramadol's serotonin release
• C) Norepinephrine storm from NET blockade + α₂ receptor overstimulation
• D) Malignant hyperthermia from skeletal muscle Na⁺ channel activation

HIGH-YIELD COMPARISON TABLE