Resting potential ~ -70 mV

“A neuron’s resting potential is the idle voltage of a battery before you switch it on.”

At rest the membrane sits around –70 mV due to high K⁺ permeability and the Na⁺/K⁺ ATPase maintaining ion gradients. Small variations exist among cell types. Trap: assuming every neuron has exactly –70 mV or that the value is fixed regardless of ion changes.

Resting potential ~ -70 mV

Memory trick: -70 mV = quiet night (negative quiet)

Action potential is all-or-none

“An action potential fires like a stadium’s fire alarm—once the threshold is hit, the whole system blares.”

All-or-none means that once membrane voltage reaches threshold (~ –55 mV), a full-amplitude spike occurs, independent of stimulus strength. Only the firing frequency changes with stimulus intensity; mistaking graded potentials for spikes is a common error.

Action potential is all-or-none

Memory trick: Threshold = “Trigger” → full blast.

Insulin lowers, glucagon raises blood glucose

“After a sugary drink, insulin works like a drain pulling glucose into cells, while glucagon opens the tap releasing stored sugar.”

Insulin and glucagon are antagonistic pancreatic hormones. Insulin drives glucose into muscle and fat cells and promotes glycogen synthesis, while glucagon triggers glycogenolysis and gluconeogenesis in liver. The trap: they are not mutually exclusive—both can be released, but the net blood glucose change depends on which hormone dominates.

Insulin lowers, glucagon raises blood glucose

Memory trick: Insulin = IN (into cells), Glucagon = GLU (goes out)

Pituitary is the master gland

“Pituitary is a train station: hypothalamus hands tickets (releasing hormones) and pituitary sends trains (tropic hormones) to other glands.”

The anterior pituitary releases true tropic hormones—TSH, ACTH, LH, FSH—that act on other endocrine glands. Their secretion is governed by hypothalamic releasing or inhibiting hormones, so the pituitary is not autonomous. GH, prolactin etc. act directly on target tissues; assuming all pituitary hormones are tropic is a common trap.

Pituitary is the master gland

Memory trick: TROPIC = TSH, ACTH, LH, FSH; GH is NOT

nervous control vs hormonal control

“When you touch a hot stove, nerves fire like instant emails, while hormonal responses are like postal letters arriving later.”

Nervous control transmits electrical impulses via neurons, producing responses in milliseconds; hormonal control releases chemicals into blood, taking seconds to minutes for effects. Both systems can interact—e.g., stress triggers neural release of adrenaline. Trap: assuming hormones are always slower; catecholamines act fast but still follow hormonal pathways.

Memory trick: Nerve = N for Now, Hormone = H for Hours

insulin vs glucagon

“After dinner, muscle cells quickly pull glucose from blood, while the liver stores the surplus as glycogen under insulin’s cue.”

Insulin moves GLUT4 to muscle and adipose membranes, boosting uptake; liver glucose entry uses GLUT2 (insulin-independent) but insulin drives hepatic glycogen synthesis, glycolysis and lipogenesis. Glucagon chiefly triggers liver glycogenolysis and gluconeogenesis; its receptors are minimal in adipose. Trap: don’t claim insulin ‘helps liver take up glucose’—the liver’s uptake isn’t insulin-dependent.

Memory trick: Insulin = muscle-GLUT4, liver-store; Glucagon = liver-release.

myelinated vs unmyelinated axon

“A myelinated axon is like a train on a high-speed track, jumping stations, while an unmyelinated one crawls along the track without jumps.”

Myelinated axons have insulating myelin and nodes of Ranvier, allowing the impulse to jump (saltatory conduction) and travel 10–100 m s⁻¹. Unmyelinated axons lack this sheath, so the impulse moves continuously at 0.5–2 m s⁻¹. Trap: larger diameter also speeds conduction, so a thick unmyelinated fibre can outpace a thin myelinated one.

Memory trick: Myelin = M for Mile-per-second, Unmyelin = U for U-slow

Memory trick: AP = All-or-None, not Amplitude-Proportional

Gametogenesis

“Spermatogenesis is like a nonstop car factory, while oogenesis is a seasonal bakery that bakes one loaf and discards the crumbs.”

Each primary spermatocyte completes meiosis I and II, producing 4 motile sperm after 64 days; meiosis proceeds without arrest. In oogenesis, the primary oocyte arrests in prophase I at birth, resumes meiosis I at puberty, and arrests again at metaphase II, yielding one ovum and up to 3 polar bodies. The trap: assuming equal gamete numbers from both sexes.

Memory trick: 4 sperm vs 1 egg + polar bodies = 4:1 ratio.

Gametogenesis — common mistake

“Imagine a baker making one big loaf and three crumbs; the crumbs (polar bodies) never become bread.”

A common error is to think oogenesis produces four equal eggs. In reality, meiosis yields one ovum and up to three polar bodies that receive minimal cytoplasm and later degenerate. Remember the asymmetry of cytoplasmic allocation. Trap: writing ‘4 eggs’ in answers.

Memory trick: 1 egg + polar bodies, not 4 eggs.

Menstrual cycle

“Think of the uterus as a hotel: days 1-14 prep rooms (follicular), day 14 checks in a guest (ovulation), days 15-28 serve meals (luteal).”

The menstrual cycle (~28 days) has three phases. Follicular phase: FSH-driven follicle growth, estrogen rises. Ovulatory phase: a rapid LH surge triggers oocyte release around day 14. Luteal phase: corpus luteum secretes progesterone (and estrogen) to maintain the endometrium. Trap: confusing estrogen-dominant follicular phase with progesterone-dominant luteal phase.

Memory trick: F-E, O-LH, L-P.

Menstrual cycle — common mistake

“A calendar shows the period as night-time; the sunrise (ovulation) always comes around day 14, not during the dark days.”

Many think ovulation occurs during menstruation. In fact, bleeding marks shedding of the previous endometrial layer, while ovulation happens mid-cycle (≈ day 14) after the estrogen-driven LH surge. Trap: linking the two events; keep them separate.

Memory trick: Bleeding ≠ ovulation; ovulation ≈ day 14.

Hormonal control

“Pituitary hormones act like a conductor’s baton: FSH lifts the follicle’s curtain, LH cues the solo (ovulation), estrogen & progesterone set the stage for implantation.”

FSH from anterior pituitary stimulates follicular growth; rising estrogen exerts positive feedback, causing the LH surge that triggers ovulation. The ruptured follicle becomes corpus luteum, secreting progesterone (and some estrogen) to prepare the uterus for possible implantation. Trap: forgetting estrogen’s positive feedback on LH; progesterone does not cause the surge.

Memory trick: FSH → follicle, estrogen → LH surge, LH → ovulation, progesterone → luteal.

Hormonal control — common mistake

“A firecracker’s sudden burst mirrors the LH surge that makes the egg burst from its follicle.”

Rising estradiol gives positive feedback on the pituitary, causing a brief LH surge that triggers ovulation about 36 h later. FSH drives follicular growth, not ovulation. The exam trap is confusing LH with FSH; only LH initiates the egg’s release.

Memory trick: LH = Let’s Hatch

Fertilisation

“A single key fits the lock; once turned, the lock’s pins drop, stopping any other keys.”

Only one sperm penetrates the zona pellucida and fuses with the oocyte membrane. This triggers cortical granule release, modifying the zona to block any additional sperm (polyspermy block). Students often forget the block occurs after the first fusion.

Memory trick: One sperm, one lock – cortical block

Fertilisation — common mistake

“The first runner crosses the finish line, then the gate shuts, barring all others from entering.”

Polyspermy is prevented by the cortical reaction; only the first sperm succeeds. A common mistake is thinking several sperm can fertilise the same egg. Remember the block is immediate after the first sperm entry.

Memory trick: First sperm wins; gate closes

Implantation and placenta

“A hotel concierge hands luggage between guests without ever letting the guests meet.”

The blastocyst adheres to the uterine lining and the chorionic villi develop into the placenta. Maternal and fetal blood flow in separate vessels, separated by a thin syncytial membrane where gases and nutrients diffuse. Exams may ask if blood mixes— it does not.

Memory trick: Placenta = private exchange, no blood blend

Implantation and placenta — common mistake

“Two rivers flow side-by-side, their waters never mix but exchange nutrients at the banks.”

In the placenta, maternal blood bathes the villi while fetal blood circulates within capillaries; a thin barrier allows exchange but keeps the two circulations distinct. A frequent trap is assuming direct blood mixing; the barrier prevents it.

Memory trick: Maternal ↔ fetal, but no blend

Pregnancy hormones

“A starter battery powers a car until the alternator takes over, just like hCG and the placenta.”

hCG, secreted by the syncytiotrophoblast, mimics LH and maintains the corpus luteum for progesterone production in the first trimester. After ~10 weeks the placenta takes over hormone synthesis. Students sometimes think hCG stays high throughout pregnancy.

Memory trick: hCG = starter, placenta = alternator

Pregnancy hormones — common mistake

“A home pregnancy test lights up because it detects hCG secreted by the embryo’s syncytiotrophoblast.”

hCG is produced by the syncytiotrophoblast soon after implantation and rises ~doubling every 48 h. It shares the α-subunit with LH, so students often mix them up; remember hCG, not LH, triggers the test.

Memory trick: hCG = Home test’s Catchy Gene

Lactation

“When a newborn sucks, the mother’s pituitary releases prolactin for milk and oxytocin for the let-down reflex.”

Prolactin (anterior pituitary) stimulates milk synthesis; oxytocin (posterior pituitary) causes myoepithelial contraction for ejection. A trap is assuming both are anterior hormones; oxytocin’s release is neuro-hormonal, not endocrine.

Memory trick: Pro = Produce, Oxy = Oust milk

Lactation — common mistake

“Prolactin is the milk-maker, oxytocin the milk-sprayer—like a coffee machine that brews then a pump that dispenses.”

Prolactin from anterior pituitary initiates milk synthesis in alveolar cells; oxytocin from posterior pituitary triggers myoepithelial contraction to eject milk. During pregnancy high estrogen/progesterone block prolactin’s effect despite its high level. Trap: thinking oxytocin also stimulates milk production.

Memory trick: Pro-makes, Ox-pushes.

Spermatogenesis: 1 cell -> 4 sperm

“A spermatogonium is like a four-way traffic splitter, ending in four haploid sperm.”

One spermatogonium undergoes mitosis, then meiosis I and II, producing 4 spermatids each with 23 chromosomes. Toppers note that each primary spermatocyte yields exactly four sperm, but many are lost before ejaculation.

Spermatogenesis: 1 cell -> 4 sperm

Memory trick: 1→4 sperm, like a forked road

Oogenesis: 1 cell -> 1 ovum + polar bodies

“A girl’s egg is like a paused marathon: it starts in the womb, stops at the first checkpoint, and only finishes after fertilization.”

Primary oocytes enter meiosis I during fetal life and arrest in prophase I. After puberty, each month one resumes, completes meiosis I to yield a secondary oocyte and a first polar body, then arrests at metaphase II; meiosis II finishes only after fertilization, giving the ovum and a second polar body. Traps: assuming meiosis I ends before birth or that polar bodies appear post-fertilization.

Oogenesis: 1 cell -> 1 ovum + polar bodies

Memory trick: Fetal-Prophase-I, Puberty-Resume, Ovulation-Metaphase-II, Fertilization-Finish (F-P-O-F).