--- module: 027-01 language: en chapter: 27 title: "Neural Coding, Synapses, Sensory Systems, and Pain" module_title: "Foundations" source_sha256: 19e797b86c685c81d8acf22affa7b1f8d5905be17c8b0246eb1735af7590fe46 --- # Neural coding, synapses, sensory systems, and pain ## Electrical coding ### Ion gradients and permeability set resting potential ### Threshold opens voltage-gated sodium channels #### Inactivation and potassium efflux repolarise ### Refractory periods limit frequency and direction ### Myelin enables saltatory conduction #### Larger diameter also speeds conduction ### All-or-none amplitude, so intensity by rate and recruitment ### Rapid adaptation signals change, slow signals duration ## Synaptic transmission and integration ### Depolarisation opens presynaptic calcium channels #### Calcium triggers vesicle fusion and release ### Ionotropic fast, metabotropic slow and amplified ### Inhibition via chloride or potassium conductance ### Axon initial segment integrates thousands of inputs ### Presynaptic inhibition alters release ### Lateral and reciprocal inhibition sharpen contrast ### Glutamate excites, GABA inhibits in brain #### Glycine in spinal cord and brainstem ### Effect depends on receptor and circuit, not name ## Plasticity and glia ### Long-term potentiation and depression #### Receptor trafficking, kinases, genes, structure ### Supports learning, also addiction, pain, epilepsy ### Astrocytes regulate ions, clearance, blood flow ### Oligodendrocytes central, Schwann cells peripheral ### Microglia survey, inflame, prune ## Sensory organisation ### Small fields and lateral inhibition aid discrimination ### Overlapping population codes refine localisation ### Cortical maps favour important regions and reorganise ### Dorsal columns: touch, vibration, proprioception #### Ipsilateral ascent, cross in medulla ### Anterolateral system: pain, temperature, itch #### Cross near spinal entry ### Spindles sense length, tendon organs sense tension #### Sensory ataxia worsens without vision ## Vision ### Cornea refracts most, lens accommodates ### Photoreceptors hyperpolarise in light #### Rods for dim light, cones for acuity and colour ### Nasal fibres cross at chiasm #### Each optic tract carries the opposite field ### Lesion site predicts field defect pattern ### Pupillary pathway diverges before cortex #### Light response kept in cortical blindness ### Accommodation: ciliary contraction, lens rounding, convergence ## Hearing and balance ### Place coding: high at base, low at apex ### Inner hair cells output, outer hair cells amplify ### Bilateral central relay #### Unilateral central lesion rarely deafens one ear ### Conductive versus sensorineural loss ### Canals sense angular acceleration ### Utricle and saccule sense linear acceleration and tilt ### Sensory mismatch causes vertigo and nystagmus ## Taste, smell, and visceral sensation ### Five tastes via ion channels or G proteins ### Smell projects to bulb and limbic-linked cortex #### Links odour to memory and emotion ### Visceral afferents: stretch, chemistry, ischaemia ### Sparse, convergent input makes pain diffuse #### Referred pain follows shared spinal segments ## Nociception and pain ### Free nerve endings sense danger ### Injury mediators lower thresholds ### A-delta sharp and localised ### C fibres burning, aching, affective ### Pain integrates salience, fear, expectation, control #### Real without proportionate visible damage ### Descending control inhibits or facilitates #### Placebo and nocebo use genuine physiology ## Pain types and sensitisation ### Peripheral sensitisation lowers thresholds ### Central sensitisation amplifies and enlarges fields #### Innocuous input becomes painful ### Neuropathic: lesion of somatosensory system ### Nociplastic: altered processing without sufficient damage ### Treat mechanism and function, not intensity alone ## Analgesic principles ### Paracetamol: limited anti-inflammatory action #### Overdose causes hepatic injury ### Non-steroidal drugs reduce prostaglandin synthesis #### Bleeding, kidney injury, fluid, cardiovascular harm ### Local anaesthetics block sodium channels ### Opioids: sedation, respiratory depression, dependence ### Neuropathic: antidepressants, channel ligands, topicals ### Non-drug care: explanation, activity, sleep, rehab ### Multimodal care improves function, limits harm