Memory tricks for neurons, signals, and nervous system divisions
Neuron anatomy, action potentials, CNS and PNS divisions, synaptic transmission, the autonomic nervous system, and reflexes — these memory tricks connect the structure of the nervous system with exactly how it functions.
Proven Mnemonics & Acronyms — fast to learn, hard to forget.
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Nervous System Divisions
CNS = Brain + Spinal cord · PNS = Everything else
Central (protected by bone) · Peripheral (all nerves outside CNS)
Two divisions of the nervous system — and their subdivisions
CNS (Central Nervous System): brain + spinal cord — housed within bone (skull and vertebral column). Integration and command center. PNS (Peripheral Nervous System): all nervous tissue outside the CNS — cranial nerves (12 pairs), spinal nerves (31 pairs), ganglia, sensory receptors. PNS has two functional divisions: Sensory (afferent) division carries signals TO the CNS. Motor (efferent) division carries signals FROM the CNS. Motor division splits further: Somatic nervous system (voluntary — skeletal muscle) and Autonomic nervous system (involuntary — smooth muscle, cardiac muscle, glands).
Afferent
Sensory — carries signals TO CNS. Think: Afferent = Arriving at CNS.
Efferent
Motor — carries signals FROM CNS. Think: Efferent = Exiting CNS.
Somatic
Voluntary. Single motor neuron from CNS to skeletal muscle. Always excitatory.
Autonomic
Involuntary. Two-neuron chain (preganglionic + postganglionic). Sympathetic or parasympathetic.
Neuron Anatomy
Dendrites receive · Cell body integrates · Axon transmits
Three functional regions — input, processing, and output
Neuron structure — three functional regions and key features of each
Dendrites: branching extensions — receive signals from other neurons or sensory receptors. Increase in number with complexity. Cell body (soma): contains nucleus and organelles — integrates incoming signals. If threshold reached at axon hillock → fires action potential. Axon: single long process — conducts action potentials away from cell body. One per neuron. Axon hillock: where axon joins cell body — decision point, lowest threshold. Myelin sheath: lipid insulation around axon from Schwann cells (PNS) or oligodendrocytes (CNS). Speeds conduction by forcing AP to jump between nodes of Ranvier (saltatory conduction). Axon terminals (synaptic knobs): release neurotransmitters into synaptic cleft.
Dendrites
Input — receive signals. Many per neuron. Short, branching, no myelin.
Axon hillock
Trigger zone — lowest threshold. Where AP is initiated if summation reaches -55 mV.
Myelin
Fatty insulation. Speeds conduction. Absent at nodes of Ranvier (signal regenerates here).
Axon terminals
Output — synaptic knobs release neurotransmitters by exocytosis when AP arrives.
Resting -70 mV → threshold -55 mV → peak +30 mV → back to -70 mV
The action potential — what happens at each phase and which ions move
Resting membrane potential: -70 mV (K+ leaks out, Na+ pumped out by Na+/K+ ATPase). Depolarization: stimulus → Na+ channels open → Na+ rushes IN → membrane goes from -70 to +30 mV. Repolarization: Na+ channels inactivate → K+ channels open → K+ rushes OUT → returns toward -70 mV. After-hyperpolarization: K+ channels slow to close → briefly below -70 mV (~-80 mV). Refractory period: absolute (Na+ channels inactivated — no new AP possible) → relative (need larger-than-normal stimulus). All-or-none: once threshold is reached, the AP is always the same size — frequency encodes stimulus strength, not amplitude.
Threshold
-55 mV — point of no return. Reach it → AP fires automatically, all-or-none.
Depolarization
Na+ in → inside becomes positive. Fast. +30 mV at peak.
AP always same size. Frequency (rate of firing) encodes stimulus intensity.
Synaptic Transmission
AP → Ca2+ IN → Vesicle fusion → NT released → Receptor binds → Response
Six steps from action potential to postsynaptic response
How one neuron communicates with the next — chemical synapse step by step
Action potential reaches axon terminal → voltage-gated Ca2+ channels open → Ca2+ flows in → Ca2+ triggers synaptic vesicle fusion with presynaptic membrane → neurotransmitter released into synaptic cleft by exocytosis → NT diffuses across cleft → binds postsynaptic receptors → ion channels open or close → EPSP (excitatory, depolarizing) or IPSP (inhibitory, hyperpolarizing) generated in postsynaptic cell. NT then removed: reuptake into presynaptic terminal (most NTs), enzymatic degradation (ACh by acetylcholinesterase), or diffusion. Summation of EPSPs and IPSPs at axon hillock determines if AP fires.
Ca2+ role
Trigger for vesicle fusion. No Ca2+ influx = no NT release. Blocked by Mg2+.
EPSP
Excitatory — Na+ or Ca2+ in → depolarizes. Brings membrane closer to threshold.
IPSP
Inhibitory — K+ out or Cl- in → hyperpolarizes. Moves membrane away from threshold.
AChE
Acetylcholinesterase — degrades ACh in synaptic cleft. Nerve agents inhibit this → overstimulation.
Sensory relay — all senses except smell route through here to cortex.
Hypothalamus
Master homeostasis. Controls pituitary (HPG, HPA, HPT axes). Temperature set point.
Cerebellum
Coordination and balance. Receives efference copy of motor commands. Ataxia if damaged.
Spinal Cord Organization
Dorsal = Sensory IN · Ventral = Motor OUT — DSMO
Dorsal horn = sensory · Ventral horn = motor · Bell-Magendie Law
Spinal cord anatomy — how sensory and motor information are separated
The spinal cord has a butterfly-shaped gray matter (cell bodies) surrounded by white matter (axon tracts). Dorsal (posterior) horn: receives sensory input — pain, temperature, touch enter here. Dorsal root carries sensory fibers (afferent). Dorsal root ganglion: contains cell bodies of sensory neurons. Ventral (anterior) horn: motor output — alpha motor neurons here control skeletal muscle. Ventral root carries motor fibers (efferent). Bell-Magendie Law: dorsal roots = sensory, ventral roots = motor. Spinal nerves form when dorsal and ventral roots merge — mixed (both sensory and motor). 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
Dorsal root
Sensory (afferent) — carries signals TO spinal cord. Has dorsal root ganglion.
Ventral root
Motor (efferent) — carries signals FROM spinal cord to muscles/glands.
Spinal nerve
Mixed — formed by merging of dorsal and ventral roots. 31 pairs.
Dermatome
Area of skin innervated by single spinal nerve. Used to locate spinal cord lesions clinically.
Autonomic Nervous System
Sympathetic = Fight or Flight · Parasympathetic = Rest and Digest
Two divisions with opposite effects on most target organs
Sympathetic vs parasympathetic — effects on every major organ
Sympathetic (fight or flight): thoracolumbar origin (T1-L2). Short preganglionic, long postganglionic. NE at target (except sweat glands = ACh). Effects: ↑HR, ↑BP, bronchodilation, ↓digestion, ↑blood glucose, pupil dilation, piloerection. Parasympathetic (rest and digest): craniosacral origin (CN III, VII, IX, X + S2-S4). Long preganglionic, short postganglionic. ACh throughout. Effects: ↓HR, ↑digestion, bronchoconstriction, pupil constriction, bladder contraction. Vagus nerve (CN X): carries 75% of parasympathetic fibers — heart, lungs, GI tract. Both divisions active simultaneously — the balance determines organ response. ANS control is largely subconscious — hypothalamus is the primary control center.
Sympathetic NTs
ACh (preganglionic) → NE (postganglionic). Exception: sweat glands use ACh postganglionic.
Parasympathetic NTs
ACh throughout — nicotinic receptors at ganglia, muscarinic at target organs.
Vagus nerve
CN X — 75% of parasympathetic outflow. Heart, lungs, GI. Vagal syncope = vasovagal response.
Dual innervation
Most organs receive both sympathetic and parasympathetic — balance determines activity.
Five components of a reflex arc — rapid involuntary response
The reflex arc — how reflexes bypass the brain for speed
Reflexes are rapid, involuntary, predictable responses to stimuli — processed at the spinal cord level without waiting for brain input. Five components: Receptor: detects stimulus. Afferent (sensory) neuron: carries signal to spinal cord. Integration center (spinal cord): processes signal. Efferent (motor) neuron: carries response signal. Effector: muscle or gland that responds. Stretch reflex (patellar/knee-jerk): monosynaptic — one synapse between afferent and efferent. Only monosynaptic reflex. Withdrawal reflex: polysynaptic — pulls limb away from pain. Crossed extensor reflex: simultaneous withdrawal of one limb and extension of other (weight shift). Reflexes test spinal cord integrity — hyperreflexia (UMN lesion), hyporeflexia (LMN lesion).
Stretch reflex
Monosynaptic — Ia afferent directly onto alpha motor neuron. Knee jerk test.
Withdrawal reflex
Polysynaptic — pulls away from painful stimulus. Interneurons involved.
Diminished/absent reflex — LMN lesion damages the reflex arc itself.
🎓 Common Exam Questions
Q: Describe the action potential — phases and the ions responsible.
A: Resting membrane potential: -70 mV (inside negative). Na+/K+ ATPase: 3 Na+ out, 2 K+ in — maintains gradient. K+ leaks out slightly — makes inside more negative. Phases: (1) Depolarization: stimulus reaches threshold (-55 mV) → voltage-gated Na+ channels open → Na+ rushes in → membrane potential rises to +30 mV. (2) Repolarization: Na+ channels inactivate → voltage-gated K+ channels open → K+ rushes out → membrane potential falls. (3) Hyperpolarization (afterpotential): K+ channels slow to close → overshoots resting potential → brief period more negative than -70 mV. (4) Resting potential restored: K+ channels close, Na+/K+ pump restores gradients. All-or-nothing principle: if threshold reached, full AP occurs. Refractory period: absolute (no AP possible — Na+ channels inactivated), relative (stronger stimulus can trigger AP).
Q: How does synaptic transmission work and what are neurotransmitters?
A: Steps: AP arrives at axon terminal → voltage-gated Ca2+ channels open → Ca2+ influx → synaptic vesicles fuse with presynaptic membrane → neurotransmitter released by exocytosis → diffuses across synaptic cleft (20-40 nm) → binds postsynaptic receptors → ion channels open or second messengers activated → EPSP (excitatory, depolarizes) or IPSP (inhibitory, hyperpolarizes) → NT inactivated (reuptake by presynaptic terminal, enzymatic breakdown, diffusion). Key neurotransmitters: ACh — neuromuscular junction, parasympathetic, CNS (memory). Dopamine — reward, movement (deficient in Parkinson). Serotonin — mood, sleep (target of SSRIs). Norepinephrine — sympathetic, alertness. GABA — major CNS inhibitory (target of benzodiazepines, alcohol). Glutamate — major CNS excitatory. Summation: spatial (multiple synapses at once) or temporal (rapid successive stimuli) determine if threshold is reached.
Q: What are the divisions of the brain and their functions?
A: Brainstem: Medulla oblongata (vital centers: cardiac, respiratory, vomiting, swallowing), Pons (connects cerebellum to rest of brain, sleep regulation, CN V, VI, VII, VIII), Midbrain (visual and auditory reflexes, CN III, IV, dopamine nuclei — substantia nigra). Cerebellum: coordination, balance, fine motor learning — compares intended vs actual movement. Diencephalon: Thalamus (sensory relay station — all sensory except olfaction goes through thalamus), Hypothalamus (homeostasis — temperature, hunger, thirst, circadian rhythm, controls anterior pituitary via releasing hormones, posterior pituitary produces ADH and oxytocin). Cerebrum: Frontal lobe (motor cortex, executive function, Broca area [speech production]), Parietal lobe (somatosensory cortex, spatial awareness), Temporal lobe (auditory cortex, Wernicke area [speech comprehension], memory with hippocampus), Occipital lobe (primary visual cortex). Limbic system: amygdala (emotion, fear), hippocampus (memory consolidation).
Q: Compare the sympathetic and parasympathetic nervous systems.
A: Sympathetic ('fight or flight'): origin = thoracolumbar (T1-L2 lateral horn). Short preganglionic → sympathetic chain (paravertebral) or collateral ganglia (prevertebral). Long postganglionic. NT: preganglionic = ACh (nicotinic receptor); postganglionic = norepinephrine (adrenergic receptors, mostly). Effects: increased HR and BP, bronchodilation, pupil dilation (mydriasis), glycogenolysis, decreased GI motility, vasoconstriction (skin, GI), vasodilation (skeletal muscle), piloerection, sweating (ACh exception). Parasympathetic ('rest and digest'): origin = craniosacral (CN III, VII, IX, X + S2-S4). Long preganglionic → ganglia near/in target organ. Short postganglionic. NT: both pre and postganglionic = ACh. Effects: decreased HR, bronchoconstriction, pupil constriction (miosis), increased GI motility and secretions, urination, erection. Memory: SLUD = Salivation, Lacrimation, Urination, Defecation = parasympathetic.
Q: What is a reflex arc and what are the five components?
A: Reflex arc: a rapid, involuntary response to a stimulus that does not require conscious brain involvement. Five components: (1) Receptor — detects stimulus (muscle spindle, skin receptor). (2) Afferent (sensory) neuron — carries signal to CNS via dorsal horn of spinal cord. (3) Integration center — in spinal cord gray matter (interneuron for most reflexes). (4) Efferent (motor) neuron — carries signal from ventral horn to effector. (5) Effector — muscle or gland that responds. Stretch reflex (monosynaptic): muscle stretched → muscle spindle activated → afferent neuron → directly synapses on motor neuron → muscle contracts to resist stretch. Example: patellar reflex (knee jerk). Withdrawal reflex (polysynaptic): pain → interneurons → flexor muscles contract (withdraw from pain) + extensor muscles relax on same side (reciprocal inhibition) + crossed extensor reflex (opposite leg extends to bear weight). Clinical use: reflexes test nerve and spinal cord integrity.