filtration vs reabsorption

“A coffee filter lets water drip through while catching grounds – like glomerular filtration letting plasma pass but retaining proteins.”

Filtration occurs at the glomerulus where hydrostatic pressure forces plasma (minus proteins) into Bowman's capsule; the net filtration pressure is PGC-PBS-πGC. Reabsorption happens mainly in the proximal tubule, where Na⁺, glucose and water are actively taken back, reducing the filtrate volume by ~99%. Trap: the huge 180 L/day filtered is not urine; only ~1 % survives to become urine.

Memory trick: Filtration = Force, Reabsorption = Return (F-R)

Memory trick: Primary urine → cheese, not milk

Resting and action potential

“A neuron’s resting voltage is like a negatively charged battery waiting for the switch to be flipped.”

Resting potential ≈ –70 mV is maintained by the Na⁺/K⁺-ATPase and K⁺ leak channels. When depolarisation reaches ≈ –55 mV, voltage-gated Na⁺ channels open, producing a rapid upstroke; the event is all-or-none. Repolarisation follows via K⁺ efflux, and the absolute refractory period prevents a second spike. Trap: forgetting that amplitude cannot vary with stimulus strength.

Memory trick: Threshold = flip switch → full blast.

Resting and action potential — common mistake

“Like a light switch, a neuron either flips on fully or stays off, regardless of how hard you press the switch.”

When membrane depolarization reaches threshold, voltage spikes to a fixed peak (~+30 mV) and repolarizes. Stimulus strength only changes firing frequency, not spike height. Exam trap: many write larger amplitude for stronger stimulus—remember amplitude is constant.

Memory trick: Threshold → full flip, no dimming.

Saltatory conduction

“Myelin sheaths are like insulated railway tracks, letting the signal hop from station to station.”

Myelin wraps increase membrane resistance and lower capacitance, so the depolarisation jumps between nodes of Ranvier where Na⁺ channels cluster. Conduction speed rises to 10–50 m/s versus 1–2 m/s in unmyelinated axons. Trap: assuming thicker myelin always means faster; disease can strip myelin and slow the signal.

Memory trick: Myelin = insulated hops.

Saltatory conduction — common mistake

“A garden hose with a smooth coating lets water flow faster than a rough one.”

Myelin does not slow; it insulates, reducing leak and allowing saltatory jumps, thus speeding conduction. The only exception is very thin unmyelinated fibers where diameter dominates speed. Trap: thinking myelin acts as a brake rather than a speed boost.

Memory trick: Myelin = speed boost, not brake.

Synaptic transmission

“Neurotransmitters ferry across the 20-nm synaptic gap like couriers on a bridge.”

Most synapses release chemicals that diffuse across the cleft to bind receptors; electrical synapses use gap-junction channels for direct ion flow and are confined to special regions (e.g., retina, brainstem). Exams assume chemical transmission, so overlooking electrical types is a common mistake.

Memory trick: C > E: Chemical dominates, Electrical rare

Synaptic transmission — common mistake

“Neuronal highways have traffic lights (chemical synapses) rather than all being welded together.”

Most synapses are chemical, using neurotransmitters to cross a small cleft; only a few specialized electrical synapses allow direct ion flow via gap junctions. Trap: confusing the minority electrical type with the rule; unless the question says ‘gap junction’, answer chemical.

Memory trick: Only a few 'electric bridges', most are chemical.

Reflex arc

“A knee-jerk reflex is a doorbell that lights the lamp directly, no hallway switch.”

The stretch reflex is monosynaptic: the sensory neuron synapses straight onto the motor neuron. Polysynaptic reflexes insert one or more interneurons (e.g., withdrawal). Assuming every reflex has an interneuron leads to wrong pathway counts.

Memory trick: Mono = direct, Poly = middle

Reflex arc — common mistake

“When you jerk your hand away, the signal loops locally in the spine before the brain even hears the alarm.”

Reflexes are mediated at the spinal cord level; the brain receives the information later for conscious awareness. This pre-brain pathway gives rapid protection. Trap: treating reflexes as conscious decisions; always check if the pathway skips the brain.

Memory trick: Reflex: brain is spectator, spine is player.

Endocrine signalling

“Hormones travel like postal letters in the bloodstream, reaching distant organs over hours, unlike instant text messages of nerves.”

Endocrine hormones are secreted into blood, travel systemically, and bind specific receptors on target cells; their effects appear seconds to hours later but last longer than neural signals. Trap: mixing up speed and duration—nerves are fast, brief; hormones are slower, prolonged.

Memory trick: Nerves: SMS, Hormones: snail mail.

Endocrine signalling — common mistake

“A hormone travels through blood like a courier parcel, arriving minutes later, while a nerve impulse is a flash of light reaching instantly.”

Nerve impulses travel along axons at up to 120 m/s, causing rapid, short-lasting actions. Hormones are released into the bloodstream and reach target cells in seconds to minutes, producing slower but prolonged effects. Exams often trap you by asking response time; remember only neural pathways give millisecond reactions.

Memory trick: Nerve = Nifty (fast), Hormone = Hushed (slow)

Feedback control

“Like a home thermostat, the pancreas raises insulin when blood sugar rises and cuts it off when it falls.”

Hormone secretion is chiefly governed by negative feedback: the hormone’s product suppresses its own release, keeping plasma levels stable. Notable exceptions (oxytocin surge in labor, LH surge before ovulation) require a positive loop. Ignoring these exceptions leads to wrong answers.

Memory trick: Negative feedback = “N” for “No more” hormone.

Feedback control — common mistake

“During labor, oxytocin spikes amplify contractions—a rare positive loop.”

Positive feedback occurs only in specific physiological events; the majority of hormonal pathways use negative feedback. Mistaking a positive loop for a negative one costs marks, especially for oxytocin and LH surges.

Memory trick: Positive = “Push” like labor.

Major glands

“Your pituitary is like a conductor, but the pancreas drums its own rhythm, responding to glucose, not the pituitary’s baton.”

The anterior pituitary releases tropic hormones (TSH, ACTH, LH, FSH) that regulate thyroid, adrenal cortex and gonads. It does NOT secrete any hormone that directly controls pancreatic β- or α-cells. Insulin and glucagon release are driven mainly by blood glucose levels and autonomic input. A common mistake is to link the pituitary directly to pancreatic secretion.

Memory trick: Pituitary → thyroid, adrenal, gonads; pancreas → glucose & nerves

Major glands — common mistake

“Confusing cortisol with aldosterone is like swapping salt and sugar in a recipe.”

Cortisol (zona fasciculata) controls carbohydrate metabolism; aldosterone (zona glomerulosa) regulates Na⁺/K⁺ balance and water retention. Both arise from adrenal cortex but different zones—mixing them loses points.

Memory trick: Cortisol = “Carb” metabolism, Aldo = “Salt” balance.

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