01Core principlesThe concepts and mechanisms needed to understand the subject.
Neurons maintain unequal sodium, potassium, chloride and calcium concentrations using pumps and transporters. At rest, permeability is weighted toward potassium, placing voltage near but not exactly at its equilibrium potential. Depolarising current that reaches threshold recruits voltage-gated sodium channels faster than opposing currents, creating the upstroke. Sodium-channel inactivation and potassium-channel opening repolarise the membrane and produce refractory periods. Action-potential amplitude does not grade with stimulus strength once threshold is reached; intensity is encoded by firing frequency and population recruitment.
Cable properties determine spread. Larger axon diameter lowers internal resistance. Myelin reduces current leak and the charge needed to change membrane voltage, so depolarisation travels farther and faster to the next node. Saltatory conduction is still continuous current flow between regenerative nodal events, not a literal jump through space. Demyelination exposes membrane, increases capacitance and leak and can cause temporal dispersion or block. Peripheral nerve injury may also produce ectopic firing, altered channel expression and sensitisation, linking structural damage to positive symptoms as well as loss.
At a chemical synapse, arriving depolarisation opens voltage-gated calcium channels. Calcium triggers vesicle fusion and transmitter release; transmitter then acts on ionotropic receptors for rapid conductance change or metabotropic receptors for slower signalling. Postsynaptic potentials sum across space and time at the trigger zone. Inhibition can hyperpolarise the cell or shunt excitatory current by increasing conductance. Neuromuscular transmission has a high safety factor under ordinary conditions, but presynaptic release, receptor number or muscle excitability can each become limiting.
Nociceptors transduce mechanical, thermal or chemical threats. Fast thinly myelinated fibres and slower unmyelinated fibres convey different temporal qualities. Primary afferents synapse in dorsal horn, where projection neurons cross and ascend in anterolateral pathways to thalamic and distributed cortical regions. Local interneurons and descending pathways can amplify or suppress transmission. Peripheral sensitisation lowers nociceptor threshold after inflammation; central sensitisation increases spinal and supraspinal responsiveness, permitting allodynia, hyperalgesia and persistent pain after the original input falls. Neuropathic pain arises from lesion or disease of the somatosensory system and may coexist with nociceptive mechanisms.
Key points
- Resting potential reflects selective permeability and maintained ion gradients; threshold opens regenerative voltage-gated channels, producing an all-or-none action potential.
- Myelin raises membrane resistance and lowers capacitance, allowing saltatory conduction between nodes; demyelination slows or blocks propagation by reducing current safety margin.
- Chemical synapses convert presynaptic depolarisation into calcium-dependent transmitter release, receptor activation and excitatory or inhibitory postsynaptic potentials.
- Sensory systems encode modality through receptor and pathway identity, intensity through firing and recruitment, and location through receptive fields and central maps.
- Nociception is neural encoding of potentially damaging stimuli, while pain is a sensory and emotional experience shaped by context; the terms are related but not interchangeable.
- Ascending nociceptive pathways, spinal gating and descending modulation explain why tissue input, sensitisation and perceived pain can diverge.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Subthreshold voltage changes grade with input, but once regenerative channel recruitment exceeds opposing current, the propagated action potential has stereotyped amplitude.
Inactivated sodium channels cannot reopen immediately, preventing another action potential and enforcing one-way propagation along recently activated membrane.
Myelin lets local current spread efficiently between nodes, where dense voltage-gated channels regenerate the signal.
Multiple small postsynaptic potentials combine over time and membrane location, determining whether the axon initial segment reaches threshold.
Inflammatory mediators lower nociceptor threshold and increase response near injured tissue, producing tenderness and primary hyperalgesia.
Sustained neural input can enhance central responsiveness, expand receptive effects and make normally innocuous input painful.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Equilibrium-potential reasoning - Why
- Predict the voltage toward which a selective conductance drives membrane.
- Interpretation and limitations
- Use ion gradient and charge direction together; opening a channel shifts voltage toward, not necessarily all the way to, that ion's equilibrium potential.
- 02
Conduction comparison - Why
- Explain speed differences across diameter and myelination.
- Interpretation and limitations
- Larger diameter reduces axial resistance and myelin improves cable spread; temperature, channel function and internodal geometry also affect propagation.
- 03
Synaptic sequence map - Why
- Locate failure in presynaptic release, cleft handling, receptor activation or postsynaptic excitability.
- Interpretation and limitations
- Predict how quantal events, stimulation frequency and receptor response differ, rather than calling every transmission problem a receptor disorder.
- 04
Sensory pathway localisation - Why
- Use modality, distribution and crossing to localise a lesion.
- Interpretation and limitations
- Map first-order entry, spinal ascent or synapse, decussation and cortical destination; partial lesions and anatomical variation limit perfect textbook patterns.
- 05
Pain-mechanism classification - Why
- Distinguish nociceptive, neuropathic and nociplastic contributions.
- Interpretation and limitations
- Mechanisms can coexist and cannot be assigned from one adjective; distribution, sensory findings, tissue context and course all contribute.
- 06
Modulation analysis - Why
- Explain why pain experience differs from peripheral stimulus magnitude.
- Interpretation and limitations
- Account for spinal inhibition, descending control, attention, expectation, sleep and previous sensitisation without implying that pain is unreal.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked examplePredict the effect of focal demyelinationA model axon has normal ion gradients and nodal sodium channels, but one internodal segment loses myelin, increasing membrane capacitance and current leak.+
- 1Recognise that local current leaving the upstream node must charge the downstream membrane to threshold.
- 2Loss of myelin increases the charge required and allows more current to escape across the exposed internode.
- 3The downstream node reaches threshold later or not at all, producing slowed conduction, temporal dispersion or block depending on safety margin.
- 4The final predicted physiological outcome is unreliable propagation despite intact upstream action-potential generation.
- 5Verify by distinguishing axonal loss: demyelination primarily disrupts conduction properties, whereas loss of axons reduces the number of conducting fibres.
02Synaptic reasoningReduce presynaptic calcium entryAn arriving action potential is normal but fewer presynaptic voltage-gated calcium channels open.+
- 1Keep axonal conduction separate because the terminal still depolarises normally.
- 2Predict less calcium-triggered vesicle fusion and reduced transmitter release probability.
- 3Expect smaller or less frequent postsynaptic responses despite intact postsynaptic receptors.
- 4Check that a direct receptor agonist could still produce a postsynaptic response in this simplified model.
03Pain-pathway reasoningLocalise a hemicord patternBelow a unilateral spinal lesion, a model shows ipsilateral loss of vibration and proprioception with contralateral loss of pain and temperature beginning a short distance lower.+
- 1Trace dorsal-column fibres as ascending ipsilaterally within the spinal cord before brainstem crossing.
- 2Trace spinothalamic fibres as crossing near spinal entry after a short segmental ascent or descent.
- 3Localise the combination to one side of the spinal cord rather than a single peripheral nerve.
- 4Verify the side from the dorsal-column loss and explain the offset in pain-temperature boundary from segmental crossing anatomy.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Check excitability answers by separating graded local potentials, threshold and propagated action potentials.
- Draw presynaptic calcium entry before transmitter release to avoid reversing cause and effect.
- Verify lesion localisation by marking where each sensory pathway crosses.
- Use pain terms precisely: tissue nociception, neuropathic lesion and lived pain experience describe different levels.
- Review whether a mechanism explains positive symptoms, negative loss, temporal course and distribution rather than only one feature.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Inhibition may be shunting
Opening channels near resting potential can suppress excitation by lowering membrane resistance even without producing a large hyperpolarisation.
Myelin changes capacitance
Reduced capacitance means less charge is required to alter membrane voltage, complementing myelin's reduction of current leak.
Dermatomes overlap
Segmental maps guide localisation but adjacent roots and individual variation mean a single root lesion rarely creates a perfectly sharp sensory border.
Pain is not a voltmeter
Perceived pain integrates sensory, emotional, cognitive and contextual processing, so it cannot be inferred directly from tissue damage magnitude.
Plasticity can persist
Channel expression, synaptic strength and inhibitory control can remain altered after injury, supporting symptoms after the initiating event changes.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling larger action potentials the mechanism for stronger stimuli instead of frequency and recruitment.
- 02
Describing saltatory conduction without explaining reduced leak, capacitance and nodal regeneration.
- 03
Assuming an inhibitory postsynaptic potential must always make voltage more negative.
- 04
Localising every dermatomal symptom to a root without checking motor, reflex and peripheral nerve alternatives.
- 05
Using nociception and pain as synonyms or interpreting central modulation as evidence that symptoms are fabricated.