← MCAT®

Bio/Biochem 3A: Structure and Function of the Nervous and Endocrine Systems

Two control systems for one body: one that signals in milliseconds down wires, one that signals in hours through the bloodstream.

23 topics · 99 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

3A. Organization of the Vertebrate Nervous System

  1. How it worksWalk me through the split from central to peripheral, then peripheral into somatic and autonomic.
  2. How it worksHow does the nervous system pull signals from every organ into one coordinated response?
  3. Going furtherWhat happens to the whole hierarchy when the spinal cord is severed partway down?
Your turneasy1 of 3
A sensory signal travels 91 cm from a toe receptor to the spinal cord, and the motor command returns over the same 91 cm, both at 66 m/s. Find the total conduction time for the round trip.

3A. Sensor and Effector Neurons

  1. How it worksHow do sensory, inter-, and motor neurons form a three-cell chain from skin to muscle?
  2. How it worksWalk me through afferent and efferent fibers entering and leaving the cord on different roots.
  3. Common mistakeWhy isn't every sensory signal carried all the way up to the brain before anything happens?
  4. Going furtherWhat happens to sensation and to movement when only the dorsal root is damaged?
Your turneasy1 of 3
A mixed spinal nerve carries 700 axons, and 68 percent of them are afferent fibres entering the cord through the dorsal root. How many efferent fibres leave through the ventral root?

3A. Reflexes, Feedback Loops, and the Reflex Arc

  1. How it worksWalk me through the knee-jerk arc from stretch receptor to quadriceps with a single synapse.
  2. How it worksHow does a withdrawal reflex add an interneuron and relax the opposing muscle at the same time?
  3. Worked exampleTrace the path and count the synapses in a withdrawal reflex running from fingertip to biceps.
  4. Common mistakeWhy isn't the brain doing the work in a spinal reflex, even though you feel it happen?
  5. Going furtherWhat happens to reflexes when descending control from the brain is cut away?
Your turnmedium1 of 3
In a withdrawal reflex the afferent and efferent axons together span 108 cm and conduct at 76 m/s, and the arc contains 4 chemical synapses, each adding 0.8 ms of delay. Find the total latency of the reflex.

3A. Sympathetic and Parasympathetic Antagonistic Control

  1. How it worksHow do the two autonomic branches differ in ganglion position, transmitter, and effect on a target?
  2. How it worksWalk me through fight-or-flight arriving at heart, lungs, gut, and pupils all at once.
  3. Worked exampleTrace heart rate, airway width, and digestion when a drug blocks muscarinic receptors.
  4. Common mistakeWhy isn't the parasympathetic system simply the sympathetic system switched off?
Your turnmedium1 of 3
A denervated heart beats at its intrinsic sinoatrial rate of 105 beats per minute. With both autonomic supplies restored, vagal tone slows it by 35 percent of that intrinsic rate while sympathetic tone adds back 11 beats per minute. Find the resting heart rate.

3A. Nerve Cell: Cell Body, Dendrites, and Axon

  1. How it worksWalk me through a signal entering the dendrites, summing at the hillock, and racing down the axon.
  2. How it worksHow does the axon hillock decide whether the whole cell fires at all?
  3. Going furtherWhat happens to a neuron's input capacity when its dendritic tree doubles its branching?
  4. Common mistakeWhy isn't the cell body where the action potential actually begins?
Your turneasy1 of 3
A neuron's dendritic tree branches 7 times, and every branch point doubles the number of tips. If each tip carries 31 synapses, how many synaptic inputs does the cell receive?

3A. Glial Cells and the Myelin Sheath

  1. How it worksHow do oligodendrocytes and Schwann cells each wrap axons, and where does each one work?
  2. How it worksWalk me through astrocytes, microglia, and ependymal cells doing three unrelated jobs.
  3. Worked exampleTrace what happens to conduction speed and signal reliability as myelin is stripped off an axon.
  4. Common mistakeWhy aren't glial cells just packing material filling the gaps between neurons?
Your turneasy1 of 3
One oligodendrocyte myelinates 41 separate axons, wrapping a 654 micrometre internode on each. In a tract containing 77 such oligodendrocytes, what total length of axon, in millimetres, is sheathed?

3A. The Resting Potential and Its Electrochemical Gradient

  1. How it worksHow do potassium leak channels plus the sodium-potassium pump build and hold that voltage?
  2. How it worksWalk me through why the resting voltage sits near potassium's equilibrium and nowhere near sodium's.
  3. Worked exampleFind which way the resting potential moves when extracellular potassium rises from 5 mM to 10 mM.
  4. Going furtherWhat happens to the resting voltage when the membrane's potassium permeability suddenly drops?
Your turneasy1 of 3
Extracellular potassium is 8 mM and intracellular potassium is 139 mM. Using E = 61.5 times log10 of the ratio of outside to inside, find the equilibrium potential for potassium in millivolts.

3A. The Action Potential

  1. How it worksWalk me through depolarization, repolarization, and hyperpolarization as gates open and shut in order.
  2. How it worksHow do a sodium channel's fast activation gate and slow inactivation gate shape the whole spike?
  3. Worked exampleTrace the voltage point by point from minus 70 up to plus 30 and back down past minus 80.
  4. Common mistakeWhy isn't an action potential a wave of ions flowing lengthwise down the inside of the axon?
  5. Going furtherWhat happens to the spike when a toxin plugs every voltage-gated sodium channel?
Your turneasy1 of 3
A neuron rests at minus 72 mV, fires at a threshold of minus 55 mV, and its spike peaks at plus 39 mV. How many millivolts of the climb from rest to peak happen after threshold has been crossed?

3A. Threshold, All-or-None Firing, and the Refractory Periods

  1. How it worksHow does a threshold turn a smoothly graded input into an all-or-none output?
  2. How it worksWalk me through the absolute and relative refractory periods and what makes each one different.
  3. Common mistakeWhy isn't a bigger action potential possible when the stimulus is enormous?
  4. Going furtherWhat happens to the maximum firing rate when the refractory period gets shorter?
  5. Ties togetherHow does the refractory period force a signal to travel one way down the axon and never back?
Your turneasy1 of 3
Each spike in an axon lasts 2 ms and is followed by an absolute refractory period of 1.97 ms. What is the highest frequency, in spikes per second, this axon can sustain?

3A. Saltatory Conduction and the Nodes of Ranvier

  1. How it worksWalk me through current leaping node to node while the myelinated stretches carry it passively.
  2. How it worksHow does crowding sodium channels into the bare nodes make those jumps possible at all?
  3. Worked exampleCompare a myelinated axon conducting at 100 meters per second with a bare one at 1.
  4. Going furtherWhat happens to conduction when demyelination leaves the nodes intact but the sheath patchy?
  5. Common mistakeWhy isn't the action potential regenerated underneath the myelin as well as at the nodes?
Your turneasy1 of 3
Nodes of Ranvier sit every 1287 micrometres along a myelinated axon. Over a 46 mm stretch of that axon, how many nodes must the spike be regenerated at?

3A. The Synapse and Transmitter Molecules

  1. How it worksWalk me through calcium entering the terminal and vesicles fusing to dump transmitter into the cleft.
  2. How it worksHow does transmitter get cleared from the cleft fast enough for the next signal to read clean?
  3. Common mistakeWhy isn't a neurotransmitter excitatory or inhibitory by its own nature?
  4. Worked exampleTrace what happens at a cholinergic synapse when acetylcholinesterase is inhibited.
  5. Going furtherWhat changes at an electrical synapse where the two cells are joined by gap junctions?
Your turneasy1 of 3
Each synaptic vesicle holds 10 thousand transmitter molecules, and one action potential triggers the fusion of 158 vesicles. How many million transmitter molecules are dumped into the cleft?

3A. Excitatory and Inhibitory Inputs and Summation

  1. How it worksHow do temporal and spatial summation add postsynaptic potentials across time and across the cell?
  2. How it worksWalk me through EPSPs and IPSPs arriving together and cancelling out at the axon hillock.
  3. Worked exampleFind whether a cell at minus 70 fires when three 4 millivolt EPSPs and one 6 millivolt IPSP arrive together.
  4. Common mistakeWhy aren't postsynaptic potentials all-or-none the way action potentials are?
Your turnmedium1 of 3
A neuron rests at minus 72 mV with a threshold of minus 56 mV. 6 EPSPs of 4 mV each and one IPSP of 4 mV arrive together at the hillock. What membrane potential results, in millivolts?

3A. Electrochemistry: Concentration Cells and the Nernst Equation

  1. How it worksHow does the Nernst equation turn a ten-to-one concentration ratio into about 60 millivolts?
  2. Worked exampleFind the equilibrium potential for sodium at 145 mM outside and 12 mM inside at body temperature.
  3. Common mistakeWhy isn't the measured resting voltage equal to any single ion's Nernst potential?
  4. Going furtherWhat happens to a calculated equilibrium potential when temperature rises from 20 to 37 degrees C?
Your turneasy1 of 3
Sodium sits at 145 mM outside a neuron and 14 mM inside. Using E = 61.5 times log10 of outside over inside for a singly charged cation at body temperature, find the sodium equilibrium potential in millivolts.

3A. Voltage-Gated and Ligand-Gated Ion Channels

  1. How it worksWalk me through a charged voltage sensor sliding in the membrane field and dragging a gate open.
  2. How it worksHow does a ligand-gated channel open the instant its transmitter binds and shut when it leaves?
  3. Going furtherWhat happens at a synapse whose ligand-gated channel passes chloride instead of sodium?
  4. Common mistakeWhy isn't a leak channel just a gated channel that happens to be stuck open?
Your turneasy1 of 3
A membrane 7 nm thick holds a potential difference of 81 mV across it. What electric field, in millions of volts per metre, does the voltage sensor of a channel sit in?

3A. G Protein-Coupled Receptors and Second Messengers

  1. How it worksWalk me through a receptor activating its G protein, which activates adenylate cyclase, which makes cAMP.
  2. How it worksHow does every step of a cascade multiply the number of active molecules below it?
  3. Worked exampleTrace the amplification from one bound epinephrine to the glucose released by a liver cell.
  4. Common mistakeWhy isn't the G protein left permanently switched on once the ligand has bound?
  5. Going furtherWhat happens to the cascade when caffeine blocks the enzyme that destroys cAMP?
Your turneasy1 of 3
One bound receptor activates 38 G proteins, and each of those switches on one adenylate cyclase that makes 360 molecules of cAMP before it shuts off. How many cAMP molecules follow from one bound hormone?

3A. Receptor Enzymes and Signal Amplification

  1. How it worksWalk me through two receptor halves pairing up and phosphorylating each other's tails.
  2. How it worksHow does a phosphorylated tail become a docking site that launches a whole signalling branch?
  3. Common mistakeWhy isn't a receptor kinase's signal switched off the moment the ligand leaves?
  4. Going furtherWhat happens downstream when a phosphatase strips off the phosphates the receptor added?
Your turnmedium1 of 3
Each activated insulin receptor dimer phosphorylates 6 tyrosine residues, and every phosphotyrosine docks an adaptor that activates 46 kinase molecules. If 19 thousand receptors are bound, how many million kinase molecules are switched on?

3A. Lipids as Signals: Steroids, Terpenes, and Terpenoids

  1. How it worksHow do four fused rings make a steroid greasy enough to cross and specific enough to bind?
  2. How it worksWalk me through cholesterol being rebuilt into cortisol, testosterone, and aldosterone.
  3. Going furtherWhat changes about response time when the receptor waits inside the cell rather than on it?
  4. Ties togetherHow do terpenes assembled from isoprene units relate to the steroids built from the same start?
Your turneasy1 of 3
Squalene, the thirty-carbon precursor of every steroid, is assembled from five-carbon isoprene units. A tissue builds 687 thousand squalene molecules per second. How many million isoprene units per second does that consume?

3A. The Endocrine System: Glands, Hormones, and Their Sources

  1. How it worksWalk me through the major glands from pineal down to gonads and what each one secretes.
  2. How it worksHow does a hormone poured into the blood reach only the tissues meant to hear it?
  3. Common mistakeWhy isn't an exocrine gland just an endocrine gland that happens to have a duct?
  4. Going furtherWhat happens to a message's speed and duration when it travels by blood instead of by axon?
Your turneasy1 of 3
A gland secretes 69 micrograms of hormone into a plasma volume of 3.47 litres. What plasma concentration results, in nanograms per millilitre?

3A. The Anterior and Posterior Pituitary

  1. How it worksWalk me through the anterior pituitary's six hormones and the target each one drives.
  2. How it worksHow does the posterior pituitary release hormones it never manufactured itself?
  3. Worked exampleTrace ADH from a cell body in the hypothalamus to the collecting duct where it acts.
  4. Common mistakeWhy isn't the posterior pituitary a gland in the same sense the anterior one is?
Your turneasy1 of 3
ADH is made in a hypothalamic cell body and carried 17 mm down an axon to the posterior pituitary at 5 mm per hour. How many hours does one molecule take to arrive?

3A. Thyroid, Parathyroid, and Adrenal Hormones

  1. How it worksHow do T3 and T4 raise basal metabolic rate down at the level of individual mitochondria?
  2. How it worksWalk me through calcitonin and parathyroid hormone dragging blood calcium in opposite directions.
  3. How it worksShow me the adrenal cortex's three zones making aldosterone, cortisol, and androgens.
  4. Common mistakeWhy isn't the adrenal medulla an endocrine gland in the ordinary sense?
Your turneasy1 of 3
A hypothyroid patient's basal metabolic rate is 1618 kcal per day. Thyroid hormone replacement raises it by 34 percent. What is the new basal metabolic rate?

3A. Pancreatic and Gonadal Hormones

  1. How it worksWalk me through insulin and glucagon pushing blood glucose back toward 90 milligrams per deciliter.
  2. How it worksHow do the gonads turn out both gametes and hormones from the same organ?
  3. Worked exampleTrace blood glucose, insulin, and glucagon over the four hours after a large meal.
  4. Going furtherWhat happens to glucose control once the pancreatic beta cells are destroyed?
Your turneasy1 of 3
Blood glucose peaks at 196 mg/dL after a meal, and insulin drives it back toward a set point of 90 mg/dL at 46 mg/dL per hour. How many hours does the return take?

3A. Cellular Mechanisms of Peptide and Steroid Hormones

  1. How it worksHow does a peptide hormone work entirely through a surface receptor and a second messenger?
  2. How it worksWalk me through a steroid crossing the membrane and turning its receptor into a transcription factor.
  3. Worked exampleCompare how fast epinephrine and cortisol take hold, and how long each effect lasts.
  4. Common mistakeWhy isn't a steroid's slow action a sign that it is the weaker signal?
  5. Going furtherWhat happens to a peptide hormone's effect when it is swallowed instead of injected?
Your turnhard1 of 3
Epinephrine's effect begins 11 seconds after it reaches a cell and lasts 7 minutes; cortisol's begins 115 minutes after it arrives and lasts 12 hours. How many times longer than epinephrine's is cortisol's effect?

3A. Negative Feedback, Hormone Transport, and Target Specificity

  1. How it worksWalk me through the hypothalamus, pituitary, and thyroid loop shutting its own signal down.
  2. How it worksHow does a carrier protein in the blood keep a steroid available but inactive until needed?
  3. Worked exampleTrace TRH, TSH, and T4 over several weeks in someone taking thyroid hormone from a bottle.
  4. Common mistakeWhy isn't target specificity about which tissues the hormone physically manages to reach?
  5. Ties togetherHow does endocrine negative feedback mirror the feedback loop built into a spinal reflex?
Your turneasy1 of 3
Total plasma cortisol is 20 micrograms per decilitre, and 93 percent of it is bound to corticosteroid-binding globulin. How many micrograms per decilitre are free and biologically active?

AP®, SAT® and PSAT/NMSQT® are trademarks registered by the College Board (PSAT/NMSQT® jointly with the National Merit Scholarship Corporation). ACT® is a registered trademark of ACT Education Corp. GRE® is a registered trademark of ETS. LSAT® is a registered trademark of the Law School Admission Council. MCAT® is a registered trademark of the Association of American Medical Colleges. ISEE® and SSAT® are registered trademarks of their owners. None of these organizations is affiliated with, or endorses, Jentoo.