Table 42.1 — Ion distribution and equilibrium potentials
4 rows| Ion | Extracellular fluid | Cytoplasm | Ratio (out:in) | Equilibrium potential |
|---|---|---|---|---|
| Na⁺ | 150 mM | 15 mM | 10 : 1 | +60 mV |
| K⁺ | 5 mM | 150 mM | 1 : 30 | -90 mV |
| Cl⁻ | 110 mM | 7 mM | 15 : 1 | -70 mV |
| Measured resting potential | — | — | — | -70 mV (K⁺ value pulled up by Na⁺ leak) |
The action potential, phase by phase
7 rows| Phase | Channels | Ion movement | Membrane potential |
|---|---|---|---|
| Resting | Only leakage channels open | K⁺ leaks out, balanced by inward electrical pull | -70 mV, steady |
| Stimulus to threshold | Ligand-gated Na⁺ channels open | A little Na⁺ in | -70 → -55 mV |
| Rising phase (depolarization) | Voltage-gated Na⁺ activation gates open | Na⁺ rushes IN | -55 → climbing fast |
| Peak | Na⁺ inactivation gates close; K⁺ gates begin opening | Na⁺ entry stops | +35 to +40 mV |
| Falling phase (repolarization) | Voltage-gated K⁺ channels open | K⁺ flows OUT | Falls back toward -70 mV |
| Undershoot (hyperpolarization) | K⁺ channels close slowly | A little too much K⁺ leaves | Dips below -70 mV |
| Return to rest | All gated channels closed; Na⁺ inactivation gates reset | Na⁺/K⁺ pump restores gradients | Back to -70 mV |
Table 42.2 — Conduction velocity: diameter vs myelin
5 rows| Axon | Diameter | Myelin | Velocity |
|---|---|---|---|
| Squid giant axon | 500 µm | None | 25 m/s |
| Large motor axon to human leg muscle | 20 µm | Yes | 120 m/s |
| Axon from human skin pressure receptor | 10 µm | Yes | 50 m/s |
| Axon from human skin temperature receptor | 5 µm | Yes | 20 m/s |
| Motor axon to human internal organ | 1 µm | None | 2 m/s |
Table 42.4 — CNS subdivisions and functions
11 rows| Division | Structure | Function |
|---|---|---|
| Spinal cord | — | Spinal reflexes; relays sensory and motor information |
| Hindbrain | Medulla oblongata | Sensory nuclei, reticular activating system, autonomic functions |
| Hindbrain | Pons | Reticular activating system, autonomic functions |
| Hindbrain | Cerebellum | Coordination of movement and balance |
| Midbrain | Optic tectum | Reflexes involving the eyes and ears; vision processing in fish |
| Forebrain — diencephalon | Thalamus | Relay station for ascending sensory and descending motor tracts |
| Forebrain — diencephalon | Hypothalamus | Autonomic functions, neuroendocrine control of the pituitary |
| Forebrain — telencephalon | Basal nuclei | Motor control |
| Forebrain — telencephalon | Corpus callosum | Relays information between the hemispheres |
| Forebrain — telencephalon | Hippocampus / limbic system | Memory and emotion |
| Forebrain — telencephalon | Cerebral cortex | Higher cognition; integrates sensory input, organizes motor output |
Table 42.5 — Somatic vs autonomic nervous system
5 rows| Feature | Somatic | Autonomic |
|---|---|---|
| Effectors | Skeletal muscle | Cardiac muscle, smooth muscle, exocrine glands |
| Effect on motor nerves | Excitation only | Excitation or inhibition |
| Innervation of effector cells | Always single | Typically dual |
| Neurons in series to the effector | One | Two (preganglionic + postganglionic) |
| Neurotransmitter | Acetylcholine | Acetylcholine and norepinephrine |
Table 42.6 — Sympathetic vs parasympathetic effects
13 rows| Target tissue | Sympathetic ("fight or flight") | Parasympathetic ("rest and repose") |
|---|---|---|
| Pupil of eye | Dilates | Constricts |
| Salivary glands | Vasoconstriction, slight secretion | Vasodilation, copious secretion |
| Gastric glands | Inhibits secretion | Stimulates gastric activity |
| Liver | Stimulates glucose release | Inhibits glucose release |
| Sweat glands | Sweating | No effect |
| GI sphincters / GI wall | Increased sphincter tone, decreased wall tone | Decreased sphincter tone, increased motility |
| Gallbladder | Relaxation | Contraction |
| Bladder muscle / sphincter | Muscle relaxes, sphincter contracts | Muscle contracts, sphincter relaxes |
| Heart muscle | Increased rate and strength | Decreased rate |
| Lungs | Dilates bronchioles | Constricts bronchioles |
| Blood vessels (muscle / skin / viscera) | Muscle dilate, skin constrict, viscera constrict | No effect / no effect / viscera dilate |
| Origin of preganglionic neurons | Thoracic and lumbar cord | Brain and sacral cord |
| Postganglionic transmitter | Norepinephrine | Acetylcholine |
Table 42.7 — The 12 cranial nerves
12 rows| # | Nerve | Function |
|---|---|---|
| I | Olfactory | Smell |
| II | Optic | Vision |
| III | Oculomotor | Motor control of some eye muscles and the eyelid |
| IV | Trochlear | Motor control of some eye muscles |
| V | Trigeminal | Chewing muscles and some facial sensation |
| VI | Abducens | Motor control of some eye muscles |
| VII | Facial | Facial muscles, salivation, taste, cutaneous sensation |
| VIII | Acoustic (vestibulocochlear) | Hearing, equilibrium, static sense |
| IX | Glossopharyngeal | Salivation; sensation from skin, taste, viscera |
| X | Vagus | Motor control of heart and viscera; sensation from thorax, pharynx, abdomen |
| XI | Accessory | Motor impulses to pharynx and shoulder |
| XII | Hypoglossal | Motor control of the tongue; some skeletal muscle and visceral fibers |
Exam traps
12 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
Afferent (sensory)vsEfferent (motor)
Afferent Arrives at the CNS from a receptor. Efferent Exits the CNS to an effector. If the question mentions skin, eye, or ear, it is afferent; if it mentions muscle or gland, it is efferent.
SympatheticvsParasympathetic
Sympathetic = fight or flight, preganglionic from thoracic/lumbar cord, long chain of ganglia beside the cord, postganglionic releases norepinephrine. Parasympathetic = rest and repose, preganglionic from brain/sacral cord, ganglia sit at the organ, postganglionic releases ACh. The postganglionic transmitter is the cleanest tell.
EPSPvsIPSP
EPSP = depolarization (e.g. -70 → -65), moves the cell toward -55 mV threshold, typically Na⁺ entering. IPSP = hyperpolarization (e.g. -70 → -85), moves it away, typically Cl⁻ entering via GABA or glycine.
Temporal summationvsSpatial summation
Temporal = one synapse firing again and again within ~15 ms (time). Spatial = many different synapses firing at once (space). One shovel working fast vs many shovels working together.
Gray mattervsWhite matter
Gray = cell bodies and dendrites, unmyelinated. White = myelinated axons. Then flip the geography: in the cerebrum gray is on the outside (cortex); in the spinal cord gray is on the inside.
Absolute refractory periodvsRelative refractory period
Absolute = Na⁺ inactivation gates are shut, no stimulus of any size can fire the cell. Relative = the cell can fire, but only with a stronger stimulus and only a smaller-amplitude spike.
Graded potentialvsAction potential
Graded: small, size varies with stimulus, fades with distance, sums, made by ligand-gated channels, happens in dendrites and cell body. Action: fixed size, all-or-none, does not fade or sum, made by voltage-gated channels, starts where the axon leaves the cell body.
Schwann cellvsOligodendrocyte
Both make myelin. Schwann = PNS, one cell per axon segment. Oligodendrocyte = CNS, one cell wraps segments of several axons. Mnemonic: oligoDENDROcyte lives with the CNS dendrites of the brain.
Depolarization vs repolarizationvsHyperpolarization
Depolarization = less negative (Na⁺ in). Repolarization = returning back down to resting (K⁺ out). Hyperpolarization = past resting, more negative than -70 mV — either the undershoot at the end of a spike or an IPSP.
Ligand-gated channelvsVoltage-gated channel
Ligand-gated opens when a chemical binds → graded potentials, at the synapse. Voltage-gated opens when the membrane potential changes → the action potential, along the axon. Ca²⁺ channels at the axon terminal are voltage-gated.
Nicotinic ACh receptorvsMuscarinic ACh receptor
Same transmitter, opposite outcomes. Nicotinic (skeletal muscle) is itself a Na⁺ channel → fast depolarization, excitation. Muscarinic (heart) works through a G protein to open K⁺ channels → hyperpolarization, slowing. Proof that the receptor, not the transmitter, decides.
Monosynaptic reflex (knee-jerk)vsCutaneous reflex
Knee-jerk = no interneuron; sensory neuron synapses directly onto the motor neuron. Cutaneous = at least one interneuron, plus extra interneurons that inhibit the antagonistic muscle. If the question says "interneuron," it is not the knee-jerk.