Full Lecture: Pharmacokinetics & Pharmacodynamics (Pages 228-235)

PAGE 228 - Enzyme Kinetics

Michaelis-Menten kinetics

• "[S] = concentration of substrate; V = velocity" - [S] is simply how much raw material (substrate) is available. Velocity (V) is how fast the enzyme is making product, measured as reaction rate.
• "Km is the substrate concentration needed for an enzyme to reach a rate of 1/2 Vmax, and is inversely related to the affinity of the enzyme for its substrate."
  • Vmax = the maximum possible speed the enzyme machine can ever run at, no matter how much raw material you add (the machine has a top speed, like a car's speed limit).
  • Km = a number telling you how much substrate is needed to get the machine running at HALF its top speed.
  • "Inversely related to affinity" means: Low Km = the enzyme grabs onto its substrate easily and gets to half-speed with very little substrate (high affinity, like a magnet that grabs metal from far away). High Km = the enzyme needs a LOT of substrate before it works well (low affinity, like a weak magnet that only works up close).
• "Vmax is directly proportional to the enzyme concentration." - If you have more enzyme machines on the factory floor (more enzyme molecules), your maximum possible output goes up proportionally. Simple: double the machines, double the max production.
• "Most enzymatic reactions follow a hyperbolic curve (ie, Michaelis-Menten kinetics); however, enzymatic reactions that exhibit a sigmoidal curve usually indicate positive cooperativity in substrate binding (eg, aspartate transcarbamoylase)."
  • A hyperbolic curve rises quickly then flattens out (like the graph shown - "Saturation" at the top). This is normal enzyme behavior.
  • A sigmoidal (S-shaped) curve starts slow, speeds up in the middle, then flattens. This shape happens when the enzyme has multiple binding sites and binding of the first substrate molecule makes it EASIER for the next one to bind - this is called positive cooperativity (like hemoglobin picking up oxygen - the first oxygen makes it easier to grab the second). Aspartate transcarbamoylase is just a named example enzyme that behaves this way.

Effects of enzyme inhibition (the graph with Vmax, Vmax/2, Km on Y and X axis)

• Competitive inhibitor - a molecule that looks similar to the real substrate and competes for the SAME active site (the "socket" where substrate normally plugs in). Because it's a fight for the same site, if you flood the system with more real substrate, the real substrate can out-compete the inhibitor and win back the site. Effect on graph: Km increases (looks like lower affinity, needs more substrate to reach half-max), but Vmax stays the same (given infinite substrate, you can still out-compete the inhibitor and reach full speed).
• Noncompetitive inhibitor - binds to a DIFFERENT spot on the enzyme (not the active site), and this changes the enzyme's shape so it doesn't work properly. Since it's not competing for the same site, adding more substrate cannot displace it. Effect: Vmax decreases (you can never reach full speed no matter how much substrate you add, because some enzyme molecules are permanently disabled), Km unchanged (the ones that still work grab substrate with the same affinity).

Lineweaver-Burk plot

• "The closer to 0 on the Y-axis, the higher the Vmax. The closer to 0 on the X-axis, the lower the Km, the higher the affinity." - Because you're plotting the INVERSE (1/Vmax and 1/Km), everything flips: a point close to zero on this inverted graph actually means the real (non-inverted) value is large. Small 1/Km = big Km... wait, actually here: the X-intercept is -1/Km. The closer that intercept sits to zero, the larger Km must be... Let's just describe it as written: point closer to 0 on X-axis → lower Km → higher affinity (this is what the book states, take it as given practical reading rule).
• "Reversible competitive inhibitors cross each other, whereas noncompetitive inhibitors do not." - On this straight-line graph, if you plot the enzyme with vs without a competitive inhibitor, the two lines cross at one point (because Vmax is unchanged - same Y-intercept). With a noncompetitive inhibitor, since Vmax changes, the lines are parallel-ish and don't cross the same way.
• "Competitive inhibitors increase Km." - Confirms what we said above - competitive inhibitors make it look like the enzyme has lower affinity for its substrate (needs more substrate to hit half-max speed), because the inhibitor is occupying some active sites.

The Table (Competitive reversible / Competitive irreversible / Noncompetitive)

• "Resembles substrate" - competitive inhibitors look like the real substrate so they can fit the same socket; noncompetitive ones don't need to look similar because they bind elsewhere.
• "Overcome by increased [S]" (more substrate) - only true for REVERSIBLE competitive inhibitors, because they can be knocked off the active site by enough real substrate winning the competition. Irreversible ones are chemically glued on permanently (often through a strong covalent bond) so no amount of substrate can dislodge them.
• Potency = how much drug you need to get an effect. Efficacy = the maximum effect achievable at all. Reversible competitive inhibition only makes you need MORE substrate/drug (potency drops) but you can still eventually reach full activity. Irreversible/noncompetitive inhibition permanently disables some machines, so no matter how much you push, you can never reach full effect (efficacy drops).

PAGE 229 - Pharmacokinetics (what the BODY does to the drug: absorption, distribution, elimination)

Bioavailability (F)

• "Fraction of administered drug reaching systemic circulation." - Of the pill/injection you took, what fraction actually makes it into your bloodstream to act on the body? If you take 100 mg and only 60 mg gets into the blood, F = 0.6 (60%).
• "For an IV dose, F = 100%." - Because IV (intravenous, injected straight into a vein) skips the gut/liver entirely and goes 100% directly into the blood.
• "Orally, F typically <100% due to incomplete absorption and first-pass metabolism." - When you swallow a pill: (1) not all of it gets absorbed through the gut wall, and (2) whatever is absorbed passes through the liver FIRST before reaching the rest of the body - and the liver can break down (metabolize) a chunk of it before it ever reaches general circulation. This is called the "first-pass effect."
• "Can be calculated from the area under the curve of a plot of plasma drug concentration over time." - If you plot drug level in blood over time after dosing, the total area under that curve (AUC) represents total drug exposure. The formula shown: F = (AUCoral × Doseiv) / (Doseiv × AUCiv) — comparing the area under the curve after an oral dose to the area under the curve after an IV dose (adjusted for dose) tells you what fraction actually got absorbed and survived to reach the blood.
• The graph shows two overlapping curves - blue area is AUC from IV, pink area is AUC after an oral dose is smaller because bioavailability is lower.

Volume of distribution (Vd)

• "Theoretical value that relates drug amount to plasma concentration." - This is NOT a real physical volume like a container. It's a calculated number that tells you, mathematically, how "spread out" a drug is in the body versus staying in the blood.
• Formula: Vd = amount of drug in the body / plasma drug concentration. If you give a known dose and then measure how concentrated it is in the blood, you can back-calculate this number.
• "Drugs may be distributed in more than one compartment. Increase Vd (↑ protein binding, ↑Vd)... Hemodialysis is most effective for drugs with a low Vd."
  • Wait - actually the logic is: if a drug is heavily bound to tissues outside the blood, its concentration in the blood plasma will appear artificially LOW even though a lot of drug is in the body → this makes Vd appear very HIGH (a huge "apparent volume").
  • Hemodialysis (a machine that filters your blood, used in kidney failure to remove toxins/drugs) only works well on drugs that stay mostly in the blood (low Vd) - because the dialysis machine can only clean what's flowing through the blood. A drug hiding deep in fat tissue (high Vd) escapes the dialysis filter.
• Table of Vd by compartment:
  • Low Vd → stays in blood vessels (intravascular). These are "large/charged molecules" or drugs that bind heavily to plasma proteins - they're too big or too "sticky" to leak out of blood vessels into tissues.
  • Medium Vd → spreads into ECF (extracellular fluid - the fluid outside cells, including blood plasma plus the fluid between cells). These are small, water-loving (hydrophilic) molecules that can leak out of blood vessels but can't cross into cells easily.
  • High Vd → spreads into ALL tissues including fat. These are small, fat-loving (lipophilic) molecules, especially if they aren't bound much to protein - they can cross cell membranes freely and hide in fatty tissue, making the "apparent" blood concentration very dilute.

Clearance (CL)

• "The volume of plasma cleared of drug per unit time. Clearance may be impaired with defects in cardiac, hepatic, or renal function." - Think of clearance like a kidney/liver "cleaning rate" - how many milliliters of blood get completely scrubbed clean of the drug every minute. If your heart isn't pumping blood well to the liver/kidneys, or if the liver/kidneys themselves are damaged, this cleaning rate drops - the drug builds up.
• Formula: CL = rate of elimination of drug / plasma drug concentration = Vd × Ke (Ke = elimination rate constant, how fast a fixed fraction disappears per unit time).

Half-life (t1/2)

• "The time required to eliminate 1/2 of the drug from the body." - Like radioactive decay - every half-life, half of what's left disappears.
• "Steady state is a dynamic equilibrium in which drug concentration stays constant (ie, rate of drug elimination = rate of drug administration)." - When you take a drug regularly (e.g., every day), eventually the amount going IN per day equals the amount being cleared OUT per day - concentration plateaus. This plateau is "steady state," not because the drug stopped working, but because intake and elimination balance out.
• "In first-order kinetics, a drug infused at a constant rate takes 4-5 half-lives to reach steady state. It takes 3.3 half-lives to reach 90% of the steady-state level." - This is a fixed mathematical rule; doesn't matter what the drug is, if it follows first-order kinetics, this timing rule always applies.
• Formula: t1/2 = (0.7 × Vd) / CL in first-order elimination. - Half-life depends on how spread out the drug is (Vd) and how fast it's cleared (CL). A drug that's very spread out (high Vd) but cleared slowly will have a long half-life.
• The little table: After 1 half-life, 50% of the original drug remains (50% eliminated). After 2 half-lives, 25% remains. After 3, 12.5%. After 4, 6.25%. Notice it's not "50% eliminated every time equally" - it's 50% of whatever is LEFT, which is why it never quite reaches zero (exponential decay).

Dosage calculations

• "In renal or liver disease, maintenance dose ↓ and loading dose is usually unchanged."
  • A loading dose is a big first dose given to quickly fill up the body to the target concentration (like filling a bathtub fast). It depends on Vd (how big the "tub" is), not on how well the kidneys/liver work, so it stays the same even if organs are damaged.
  • A maintenance dose is the smaller, repeated dose given afterward to keep the level steady - like a slow drip to replace what's being drained (cleared). If the kidneys/liver (the "drain") are damaged and clear the drug more slowly, you need LESS maintenance dose or you'll overflow the tub.
• "Time to steady state depends primarily on t1/2 and is independent of dose and dosing frequency." - No matter how big your doses are or how often you give them, the TIME it takes to reach steady state is fixed by the half-life alone (per the 4-5 half-lives rule above). Bigger doses just mean a higher steady-state level, reached in the same amount of time.
• Loading dose formula = (Cp × Vd) / F. Cp = target plasma concentration (the level you want to hit); Vd = volume of distribution (how big the tub is); F = bioavailability (correcting for how much of an oral dose actually gets in).
• Maintenance dose formula = (Cp × CL × τ) / F. τ (tau) = dosing interval (time between doses). This calculates how much drug you need to give each interval to exactly replace what's cleared during that interval.
• "Does not apply for continuous infusions" - because with a continuous IV drip there's no "interval" between doses - it's constant, so the τ term doesn't apply.

PAGE 230 - Drug Metabolism & Elimination Order

Drug metabolism

• "Drugs can be metabolized by either or both phase I and phase II reactions. These reactions serve to bioactivate or deactivate substances, and do not have to take place sequentially (eg, phase I can follow phase II, or take place as a single reaction)." - The body chemically modifies drugs in one or two "stages" to eventually make them easier to flush out. Sometimes a drug goes through Phase I then Phase II; sometimes just one; the order isn't fixed in stone.
• Phase I (predominantly CYP450-dependent): Uses liver enzymes called Cytochrome P450 to chemically alter the lipophilic (fat-soluble) drug via:
  • Oxidation (adding oxygen/removing electrons)
  • Reduction (adding electrons/hydrogen)
  • Hydrolysis (breaking a bond using water) These reactions produce a "slightly polar metabolite (active or inactive) - reactive oxygen species." The product may still be biologically active (or might be inactivated), and can sometimes generate reactive oxygen species (harmful free radical molecules) as a side effect.
• Phase II: Takes the drug (or its Phase I product) and attaches a big, water-soluble chemical group onto it via:
  • Glucuronidation (attaching glucuronic acid, a sugar-like molecule)
  • Acetylation (attaching an acetyl group)
  • Sulfation (attaching a sulfate group)
  • Methylation (attaching a methyl group) This produces "very polar, hydrophilic metabolite (except acetylated metabolite)" - meaning it's now water-loving and ready to be flushed out in urine/bile, EXCEPT acetylated products which stay less water-soluble.
• "Geriatric patients lose phase I first. Patients who are slow acetylators have ↓ rate of metabolism of certain drugs (eg, isoniazid)." - As people age, their Phase I (CYP450) machinery weakens before Phase II does. Also, some people genetically have a "slow acetylator" version of the acetylation enzyme (a very well known example is with the TB drug isoniazid) - they process/break down these drugs more slowly, so drug levels build up higher, increasing risk of toxicity.
• Excretion: After Phase I/II, drug metabolites leave the body via urine, sweat, or bile/stool.

Elimination of drugs

• "Capacity-limited elimination (ie, constant AMOUNT of drug eliminated per unit time). Rate of elimination is constant regardless of Cp (concentration of plasma), ie constant amount, linearly with time." - Imagine a single-lane toll booth that can only let a fixed NUMBER of cars through per minute, no matter how many cars are lined up. The body's elimination machinery (usually a specific enzyme) is fully saturated/maxed out, so it clears a FIXED AMOUNT of drug every hour regardless of how much drug is present.
• "Examples of drugs — Phenytoin, Ethanol, and Aspirin (at high or toxic concentrations)." — Easy memory trick: "PEA is round" (the letters P-E-A for Phenytoin, Ethanol, Aspirin), and the graph of drug concentration over time is a straight, round-topped curve (not exponential).
• Looking at the graph description: elimination rate is CONSTANT ("2 U/h" repeated at every step) - the SLOPE of the plasma concentration line is constant (straight downward line), meaning the time needed for the level to halve keeps getting SHORTER as concentration drops (because you're removing a fixed AMOUNT each time, and that fixed amount becomes a bigger and bigger fraction of a shrinking total) - "time of t1/2 ↓ as concentration ↓."

• "Rate of first-order elimination is directly proportional to the drug concentration (ie, constant FRACTION of drug eliminated per unit time). Cp ↓ exponentially with time. Applies to most drugs." - This is like a leaking bucket: the bigger the hole (fixed % leak rate), the faster water drains when the bucket is full, but as it empties the ABSOLUTE amount leaking per minute drops too, though the FRACTION (percentage) draining stays the same. This gives an exponential decay curve, not a straight line.
• Flow-dependent elimination - basically same concept restated: elimination rate depends on how much drug is flowing through relative to blood flow, so as concentration changes, absolute amount eliminated changes proportionally.
• Graph: elimination RATE decreases over time ("4 U/h → 2 U/h → 1 U/h → 0.5 U/h") because there's less drug left to eliminate a fixed fraction of, BUT critically, the TIME for the level to halve (t1/2) stays CONSTANT no matter the concentration - this is the hallmark of first-order kinetics and why half-life is a meaningful, fixed number for most drugs.

PAGE 231 - Urine pH and Drug Elimination (Ion Trapping)

• "Ionized species are trapped in urine and cleared quickly. Neutral forms can be reabsorbed." - Kidneys filter blood into urine inside tiny tubes. If a drug molecule in that urine carries an electrical CHARGE (is "ionized"), it cannot easily cross back through the fatty cell membranes lining the kidney tubes to get reabsorbed into blood - so it gets "trapped" and washed out in urine. If the molecule is UNCHARGED (neutral), it's more fat-soluble and can slip back through the membrane into blood, getting reabsorbed (not eliminated).
• Weak acids: "Examples: phenobarbital, methotrexate, aspirin (salicylates). Treat overdose with sodium bicarbonate to alkalinize urine."
  • Chemical reaction shown: RCOOH ⇌ RCOO⁻ + H⁺ (RCOOH = neutral/lipid-soluble form; RCOO⁻ = the ionized/charged, trapped form).
  • Why bicarbonate? Bicarbonate makes the urine more ALKALINE (basic, high pH). In a basic environment, a weak ACID loses its proton (H⁺) more easily, turning into its charged form (RCOO⁻), which gets trapped in urine and flushed out. This is the treatment for aspirin/phenobarbital overdose - alkalinize the urine to trap and eliminate the drug faster.
• Weak bases: "Examples: tricyclic antidepressants (TCAs), amphetamines. Trapped in acidic environments. For severe alkalosis, treat with ammonium chloride to acidify urine."
  • Chemical reaction: RNH3+ ⇌ RNH2 + H+ (RNH3+ = charged/trapped form; RNH2 = neutral/lipid-soluble form).
  • A weak BASE becomes charged (trapped) in ACIDIC urine (opposite of weak acids). So if someone has too much alkalosis (blood too basic) from a weak base overdose, you'd give ammonium chloride to acidify the urine and trap the drug for excretion.
• "TCA toxicity is initially treated with sodium bicarbonate to overcome the sodium channel-blocking activity of TCAs. This treats cardiac toxicity, but does not accelerate drug elimination." - Important nuance: in a tricyclic antidepressant overdose, giving bicarbonate is NOT primarily about trapping the drug in urine (TCAs are weak bases, so bicarbonate/alkaline urine would actually promote reabsorption, not elimination!). Instead, bicarbonate is given for a totally different reason - TCAs are dangerous because they block sodium channels in the heart, causing life-threatening arrhythmias; raising blood pH with bicarbonate helps counteract this cardiac toxicity directly, even though it doesn't speed up getting the drug out of the body.

pKa

• "pH at which drugs (weak acid or base) are 50% ionized and 50% nonionized. The pKa represents the strength of the weak acid or base." - pKa is just a number (like a drug's "fingerprint" pH value) where exactly half the drug molecules are charged and half are neutral. It tells you how strong an acid or base the drug is, similar to how pH tells you how acidic a solution is.
• The graph shows two S-shaped curves crossing at pKa 9.2 - one for a weak acid, one for a weak base, showing how the % of ionized (charged) species changes as pH changes. As pH rises (more basic), weak acids become MORE ionized (curve for weak acid goes up), while weak bases become LESS ionized (curve for weak base goes down) - opposite directions.
• "↓pKa = more acidic; ↑pKa = more basic" - simple labeling rule for reading the pKa value of a drug.

PAGE 232 - Efficacy vs Potency

Efficacy

• "Maximal effect a drug can produce (intrinsic activity). Represented by the y-value (Emax). ↑ y-value = ↑ efficacy. Unrelated to potency (ie, efficacious drugs can have high or low potency). Partial agonists have less efficacy than full agonists."
  • Efficacy = the ceiling, the maximum result a drug can ever achieve, no matter how much you give. On the graph, this is how HIGH the curve tops out on the Y-axis (% maximal effect).
  • In the graph, Drug A has a higher Emax (higher ceiling) than Drug B, so Drug A is MORE efficacious - it can achieve a bigger maximum effect, even though (as we'll see) it might need a higher dose to get there.
  • This is completely separate from potency - a drug can be very potent (works at tiny doses) but have low efficacy (low ceiling), or vice versa.
  • Partial agonists are drugs that, even at full receptor occupancy, cannot produce as big an effect as a "full agonist" - their maximum ceiling is lower.

Potency

• "Amount of drug needed for a given effect. Represented by the x-value (EC50). ↓EC50 = ↑potency = ↓drug needed. Left shifting = ↑ potency. Unrelated to efficacy (ie, potent drugs can have high or low efficacy)."
  • Potency = how little drug you need to achieve a certain effect. On the graph, this is measured by where the curve sits along the X-axis (dose).
  • EC50 (Effective Concentration for 50% effect) = the dose needed to get half the maximal effect. A drug that reaches EC50 at a LOWER dose is MORE potent (you need less of it).
  • In the graph, Drug A needs a lower dose (its curve is shifted to the LEFT) to reach the same effect level as Drug B, so Drug A is more potent.
  • Again, this tells you nothing about the ceiling (efficacy) - a super potent drug might still have a low maximum effect.

PAGE 233 - Receptor Binding & Therapeutic Index

Receptor binding types (the four graphs)

1. Competitive antagonist: A blocker that competes for the SAME lock (binding site) as the agonist. Effect: potency decreases (curve shifts right - you need more agonist to get the same effect) but efficacy is unchanged (No change) - if you pour in enough agonist, you can still out-compete the antagonist and reach the same maximum effect eventually. Example: Diazepam (agonist) + Flumazenil (competitive antagonist) on the GABA-A receptor (flumazenil is the drug used to reverse benzodiazepine overdose).
   • "Can be overcome by ↑ agonist concentration."
2. Noncompetitive antagonist: Binds somewhere ELSE on the receptor (not the same lock), permanently disrupting its function. Effect: potency unchanged (No change - the agonist that does bind still works with the same strength) but efficacy decreases (↓ - the maximum achievable effect drops because some receptors are permanently disabled). Example: Norepinephrine (agonist) + phenoxybenzamine (noncompetitive antagonist) on α-receptors.
   • "Cannot be overcome by ↑ agonist concentration" - no amount of agonist can fix a receptor that's been disabled elsewhere.
3. Partial agonist (alone): "Acts at same site as full agonist." A partial agonist binds the SAME lock as a full agonist but can only turn it "partway on" - efficacy is independent (↓, meaning generally lower ceiling than a full agonist) and potency is independent too (can be higher or lower). Example: Morphine (full agonist) vs buprenorphine (partial agonist) at the opioid µ-receptor - this is exactly why buprenorphine is used to treat opioid addiction; it satisfies the receptor without producing as strong an effect as morphine, and can even block/blunt morphine's action if both are present.
4. Inverse agonist (alone): "Binds to a constitutively active receptor, thereby reducing its activity; has the opposite effect of an agonist." Some receptors have a small amount of activity even with NOTHING bound (baseline "leaky" activity). An inverse agonist actively pushes that baseline DOWN, doing the opposite of what a normal agonist would do. Example: H1 antihistamines (eg, diphenhydramine/Benadryl) act partly as inverse agonists at the histamine H1 receptor, reducing baseline receptor activity below normal.

Therapeutic Index (TI)

• "Measurement of drug safety. TD50/ED50 = median toxic dose / median effective dose." - ED50 is the dose that produces the desired effect in 50% of a population/subjects; TD50 is the dose that produces toxicity in 50%. Dividing them gives a "safety ratio" - the BIGGER this number, the SAFER the drug, because it means there's a wide gap between an effective dose and a dangerous dose.
• "Therapeutic window - range of drug concentrations that can safely and effectively treat disease." - The practical dosing "sweet spot" between "too little to work" and "too much = toxic."
• "Safer drugs have higher TI values. Drugs with lower TI values require frequent monitoring (eg, warfarin, theophylline, digoxin, antiepileptic drugs, lithium; Warning! These drugs are lethal)." - These are famous "narrow therapeutic index" drugs where the effective dose and the toxic dose are dangerously close together, so blood levels must be checked regularly to avoid poisoning the patient while still treating them.
• "LD50 (lethal median dose) often replaces TD50 in animal studies." - In animal research (where you can't ethically measure "toxic dose" the same way), scientists often use LD50 (the dose lethal to 50% of test animals) instead, as a proxy for the toxic dose to still calculate a therapeutic index.
• The graph shows three curves along a dose axis: Efficacy (leftmost, lowest dose), Therapeutic Index range in the middle, and Toxicity (rightmost, highest dose) - visually showing the gap between "works" and "harms."

PAGE 234 - Drug Effect Modifications & Autonomic Receptors

Drug effect modifications (how two drugs interact when combined)

• Additive: "Effect of substances A and B together is equal to the sum of their individual effects." 2+2=4. Example: Aspirin and acetaminophen together give pain relief exactly equal to what you'd predict by simply adding their separate effects.
• Permissive: "Presence of substance A is required for the full effects of substance B." Example: Cortisol on catecholamine responsiveness - cortisol itself doesn't directly cause the effect, but without cortisol present, the catecholamines (adrenaline-type hormones) can't work at full strength. Cortisol "permits" full function.
• Synergistic: "Effect of substances A and B together is greater than the sum of their individual effects." 2+2 > 4. Example: Clopidogrel with aspirin - combined they reduce clotting/platelet activity more than either drug's effect added up separately would predict (this is why dual antiplatelet therapy is used after certain heart procedures).
• Potentiation: "Similar to synergism, but drug B (with no therapeutic action alone) enhances the therapeutic action of drug A." 2+0 > 2. Example: Carbidopa - carbidopa itself does basically nothing therapeutically on its own (it can't cross into the brain), but it blocks an enzyme that would otherwise destroy levodopa (the Parkinson's drug) before it reaches the brain, so it boosts levodopa's brain-available levels/effect.
• Antagonistic: "Effect of substances A and B together is less than the sum of their individual effects." 2+2 < 4. Example: Morphine with naloxone - naloxone is specifically designed to block/reverse morphine's opioid effects (naloxone is the opioid overdose reversal drug), so together the combined effect is much less than morphine alone.
• Tachyphylactic: "Acute decrease in response to a drug after initial/repeated administration." Example: Repeated use of intranasal decongestant (like oxymetazoline) leads to a drop in the drug's own therapeutic effect over time, and can cause "rebound congestion" when stopped (nasal passages become MORE stuffy after you stop using the spray, because the body adapted to constant drug exposure).

Autonomic receptors diagram (the branching neuron diagram)

• Parasympathetic ("rest and digest"): Starts in the brainstem (or sacral spinal cord elsewhere in the body), sends a LONG preganglionic neuron (nerve fiber) that releases ACh (acetylcholine, a chemical messenger) onto a nicotinic receptor (N-type, "Nn") at a nearby ganglion (relay station), which then sends a SHORT postganglionic neuron releasing ACh again onto muscarinic receptors (M1, M2, M3 - another subtype) on the target organ (smooth muscle, gland cells, nerve terminals, cardiac muscle).
• Sympathetic ("fight or flight"): Starts in the spinal cord, sends a SHORT preganglionic neuron releasing ACh onto nicotinic receptors (Nn) at the sympathetic chain (a row of relay ganglia next to the spine), which then sends a LONG postganglionic neuron releasing NE (norepinephrine, the "adrenaline-like" chemical) onto α/β adrenergic receptors on target organs (smooth muscle, glands, nerve terminals, cardiac muscle, or the kidney's blood vessels which use a special dopamine receptor D1).
• Special case - sweat glands: get sympathetic nerve fibers, but unusually, those sympathetic fibers release ACh (not the typical norepinephrine) acting on M3 receptors. Memory trick given in text: "cholinergic sweat" (sympathetic pathway anatomically, but chemically behaves like the parasympathetic system using ACh).
• Special case - adrenal medulla: This gland is essentially a "specialized sympathetic ganglion" - preganglionic sympathetic fibers release ACh directly onto the adrenal medulla, which then dumps NE and Epinephrine (Epi) straight into the BLOOD (not through another nerve) to act as hormones throughout the body.
• "Pelvic splanchnic nerves and CNs III, VII, IX and X are part of the parasympathetic pathway." - These specific named nerves (cranial nerves 3, 7, 9, 10, plus the pelvic splanchnic nerves) are the actual anatomical routes the parasympathetic system uses to reach the eyes, salivary glands, heart/lungs/gut, and pelvic organs respectively.
• Somatic pathway (bottom of diagram): This is different from autonomic - it's the VOLUNTARY nervous system controlling skeletal muscle. A single motor neuron goes straight from spinal cord to the neuromuscular junction, releasing ACh onto Nm receptors (a different nicotinic subtype found specifically at skeletal muscle) to make muscles contract on command.

PAGE 235 - Acetylcholine Receptors & Pain Transmission

Acetylcholine receptors

• "Nicotinic ACh receptors are ligand-gated channels allowing efflux of K+ and influx of Na+ (and in some cases Ca2+)." - "Ligand-gated" means the receptor is a literal door/channel that opens directly when a chemical messenger (ligand = ACh here) binds to it, letting ions flow (sodium flows IN, potassium flows OUT, sometimes calcium flows in too) - this is a fast, direct electrical effect on the cell.
  • Nn (found in autonomic ganglia, adrenal medulla) and Nm (found at the neuromuscular junction of skeletal muscle) are the two subtypes.
• "Muscarinic ACh receptors are G-protein-coupled receptors that usually act through 2nd messengers (cholinergic glands)." - Unlike nicotinic receptors (direct channels), muscarinic receptors work indirectly - ACh binds, which then activates an internal "G-protein" relay system that triggers a cascade of chemical signals inside the cell (2nd messengers) - slower but more complex/widespread effects.
  • M1-5 subtypes found in heart, smooth muscle, brain, exocrine glands, and sweat glands (cholinergic sympathetic, as noted on the previous page).

Pain transmission

• Nociceptive pain: "Pain signals transmitted to the CNS in response to mechanical, thermal, or chemical stimuli. Transient receptor potential vanilloid ligand receptor activation. Signals transmitted by Aδ fibers (sharp, acute pain) or C fibers (dull, throbbing, chronic pain)."
  • This is normal, "appropriate" pain from actual tissue damage/threat (e.g., touching something hot, a cut, chemical irritant).
  • TRPV (Transient Receptor Potential Vanilloid) receptors are specialized pain sensor proteins on nerve endings that detect heat, certain chemicals (like capsaicin from chili peppers, hence "vanilloid"), and mechanical damage.
  • Two types of pain-carrying nerve fibers: Aδ fibers are faster, myelinated (insulated) fibers carrying sharp, immediate, "ouch, pull your hand back now" pain. C fibers are slower, unmyelinated fibers carrying the dull, throbbing ache that lingers afterward.
• The four steps of pain processing (with drug targets for each step):
  • Transduction (converting the stimulus into a nerve signal) - blocked by local anesthetics, α2-agonists, gabapentinoids, NSAIDs, acetaminophen, glucocorticoids.
  • Transmission (carrying the signal up the nerve toward the spinal cord/brain) - blocked by local anesthetics, α2-agonists.
  • Modulation (the spinal cord can amplify or dampen the pain signal before it reaches the brain) - blocked by TCAs, SSRIs, SNRIs, gabapentinoids.
  • Perception (the brain actually becomes consciously aware of "pain") - blocked by α2-agonists, opioids, TCAs, SSRIs, SNRIs.
  • (Many drug classes appear at multiple steps because they act on overlapping pain pathways.)
• Neuropathic pain: "Caused by neuronal dysfunction of the CNS or PNS. Transmitted via upregulation and persistent activation of voltage-gated Na+ channels. Example: diabetic peripheral neuropathy."
  • This is pain from a DAMAGED or malfunctioning nerve itself (not from an actual ongoing injury/stimulus) - the nerve fires pain signals inappropriately because it's sick or injured, often from chronically increased numbers of sodium channels making nerves overly excitable.
  • Diabetic peripheral neuropathy = classic example, where chronically high blood sugar damages nerves (especially in the feet/hands), causing burning, tingling, or shooting pain even without any actual new injury.

Quick recap glossary for a non-medical reader

• Enzyme = a biological "machine" protein that speeds up chemical reactions in the body.
• Substrate = the raw material an enzyme works on.
• Receptor = a "lock" on a cell that a drug or natural chemical (the "key") binds to, triggering an effect.
• Agonist = a drug that activates a receptor like the natural key would.
• Antagonist = a drug that blocks a receptor without activating it.
• Half-life = time for half of a drug to leave the body.
• Bioavailability = fraction of a dose that actually reaches the bloodstream.
• pH/pKa = measures of acidity; used to predict whether a drug is charged (trapped) or neutral (reabsorbed) in urine.
• Sympathetic/parasympathetic = the "gas pedal" (fight-or-flight) and "brake" (rest-and-digest) systems of the involuntary nervous system.