--- module: 027-02 language: en chapter: 27 title: "Neural Coding, Synapses, Sensory Systems, and Pain" module_title: "Population coding, predictive perception, and persistent pain networks" source_sha256: e47e4e41a3eac035188a5f425cdbc91768b1b34c9e0c706f5bd44690efe03e42 --- # Population coding, prediction, and persistent pain ## Distributed neural information ### No single spike carries stimulus identity ### Which neurons, rate, timing, synchrony, network state ### Population codes represent continuous ranges ### Rate coding limited by refractoriness and cost ### Temporal coding uses precise spike timing ### Recruitment extends dynamic range ### Adaptation devotes activity to change ### Perception combines evidence with prior expectation #### Overly strong priors cause illusion and threat bias ## Receptive fields and contrast ### Central field differs from transduction zone #### Convergence enlarges, inhibition sharpens ### Small fields and dense innervation give acuity ### Lateral inhibition raises edge contrast #### Retinal centre-surround highlights change #### Improves two-point discrimination ### Convergence raises sensitivity, lowers localisation #### Visceral-somatic convergence explains referred pain ### Divergence spreads noxious input to many systems ## Synaptic gain and network state ### Release, receptors, driving force, location ### Inhibition near axon initial segment is strong ### Shunting inhibition without large hyperpolarisation ### Neuromodulators change gain, not on or off #### Noradrenaline: arousal and signal-to-noise #### Acetylcholine: attention and plasticity #### Dopamine: salience, learning, action value ### Astrocytes clear glutamate and potassium #### Failed uptake promotes seizures, excitotoxicity ### Microglial signalling changes synapses in pain ## Adaptation and sensitisation ### Rapid adaptation detects vibration and movement ### Slow adaptation encodes pressure and position ### Inflammatory mediators phosphorylate channels ### Repeated input removes magnesium block #### Calcium, transcription, stronger dorsal horn ### Sensory loss can coexist with pain #### Ectopic impulses, cross-excitation, central gain ## Visual computation ### Photoreceptors reduce release when hyperpolarised ### Bipolar pathways create on and off channels ### Colour compares cone classes #### Constancy infers reflectance from context ### Depth from many cues, one loss is tolerated ### Streams for identity, space, and action ### Ordered geometry localises field defects #### Optic nerve: monocular loss #### Chiasm: bitemporal fields #### Post-chiasm: contralateral homonymous defect ## Auditory and vestibular inference ### Travelling wave converts frequency to place ### Outer hair cell damage raises thresholds ### Conductive loss spares cochlear machinery ### Sensorineural loss distorts speech ### Tuning forks screen, audiometry quantifies ### Low frequency by timing, high by intensity ### Sudden unilateral sensorineural loss is urgent ### Vestibulo-ocular reflex stabilises vision #### Central lesions may not suppress with fixation ## Pain as protective inference ### Nociception encodes threat, pain is experience #### Anaesthesia blocks pain despite injury #### Sensitisation sustains pain after danger ### Peripheral sensitisation makes tissue tender ### Central sensitisation reduces inhibition ### Attention, fear, sleep, mood modify pain #### Graded safe experience updates predictions ## Neuropathic and nociplastic patterns ### Neuropathic distribution fits anatomy #### Positive signs: burning, shocks, allodynia #### Negative signs: reduced sensation ### Nociplastic: pathology does not explain extent #### Widespread sensitivity, fatigue, poor sleep ### Mechanisms often overlap in one patient ### Match treatment to dominant mechanism ### Opioid benefit shown by function and harm review ## Plasticity, formulation, and reassessment ### Cortical maps change after amputation or injury #### Phantom sensation reflects central representation ### Mirror therapy and graded motor imagery ### Plasticity is neither always good nor limitless ### Formulation names pathway, signs, amplification ### Separate urgent new injury from network change ### Examination is the baseline for testing #### Reassess by the same method after intervention ### Function can improve without lower intensity