Sensory system Study Guide

📖 Core Concepts Sensory nervous system – network that converts external/internal physical signals into neural impulses for perception and interoception. Transduction – sense organs act as sensors, turning light, sound, pressure, chemicals, etc., into action potentials. Receptive field – specific area of skin, space, or stimulus that a single receptor or neuron responds to. Receptor categories – four major groups: chemoreceptors, photoreceptors, mechanoreceptors, thermoreceptors (all produce an electrical spike). Traditional vs. additional senses – classic five (vision, hearing, touch, taste, smell) plus widely accepted pain, balance, kinaesthesia, temperature. Primary sensory cortices – somatosensory (BA 1‑3), visual (V1/BA 17), auditory (BA 41‑42), olfactory (piriform, amygdala, entorhinal), gustatory (anterior insula & frontal operculum), vestibular (balance). Visual streams – dorsal “where/how” (V2→V5) vs. ventral “what” (V2→V4). Labeled‑line principle (temperature) – each thermoreceptor type (e.g., TRPV1, TRPM8) sends a dedicated signal line, keeping hot and cold pathways separate. --- 📌 Must Remember Five traditional senses: touch, taste, smell, vision, hearing. Additional accepted senses: pain, balance, kinaesthesia, temperature. Receptor types: chemoreceptor, photoreceptor, mechanoreceptor, thermoreceptor. Cone classes – S (short‑λ, blue), M (medium‑λ, green), L (long‑λ, yellow/red). Rod : cone ratio – ≈ 20 : 1 in humans (up to 1000 : 1 in nocturnal animals). Somatosensory BA – 3 receives most thalamic input; 1 & 2 receive from 3. Visual BA – V1 = BA 17; dorsal stream = “where/how”, ventral = “what”. Auditory BA – 41 = anterior transverse temporal, 42 = posterior transverse temporal. Thermoreceptor channels – TRPV1 = heat, TRPM8 = cold. Taste qualities – sour, bitter, sweet, salty, umami. Olfactory pathway – ORNs → olfactory nerve → bulb → anterior olfactory nucleus → piriform cortex (no contralateral crossing). --- 🔄 Key Processes General sensory transduction Stimulus → receptor activation → graded receptor potential → action potential → afferent pathway → primary sensory cortex. Visual information flow Retina → LGN (thalamus) → V1 (BA 17) → split into dorsal (V2→V5) and ventral (V2→V4) streams. Taste (gustatory) pathway Taste buds → cranial nerves VII, IX, X → nucleus of the solitary tract (medulla) → thalamus → gustatory cortex (anterior insula & frontal operculum). Olfactory pathway ORNs (GPCR) → olfactory nerve → olfactory bulb → anterior olfactory nucleus → piriform cortex → (amygdala & entorhinal). Thermal labeling Warm → TRPV1 → depolarization → hot line; Cool → TRPM8 → depolarization → cold line; signals stay separate up the spinal cord. --- 🔍 Key Comparisons Traditional vs. Additional senses Traditional: primarily exteroceptive; Additional: include interoceptive (pain, temperature) and proprioceptive (balance, kinaesthesia). Slowly adapting type 1 vs. type 2 mechanoreceptors SA 1: small receptive field, static stimulus → shape/roughness. SA 2: large receptive field, stretch → sustained deformation. Rapidly adapting vs. Pacinian receptors RA: small field, detects slip, low‑frequency vibration. Pacinian: large field, primary for high‑frequency vibration. Rods vs. Cones Rods: high light sensitivity, no color, dominate scotopic (dim) vision. Cones: lower sensitivity, color (S/M/L), enable photopic (bright) vision. Dorsal vs. Ventral visual streams Dorsal: “where/how” – motion, spatial location, guiding actions. Ventral: “what” – object identity, color, form. --- ⚠️ Common Misunderstandings Pain ≠ a “sixth sense” – it is a protective nociceptive system, not a modality like vision. Color blindness is not total color loss – most often a red‑green discrimination deficit, not a lack of all color perception. All mechanoreceptors are the same – they differ in adaptation rate, receptive field size, and stimulus type (static vs. vibration). Olfactory information does cross hemispheres – it does not; each bulb projects ipsilaterally only. --- 🧠 Mental Models / Intuition “Spotlight vs. Floodlight” – small receptive fields (spotlights) = fine detail (SA 1, RA), large fields (floodlights) = coarse, broad information (SA 2, Pacinian). Labeled‑line = dedicated “telephone line” – each temperature receptor type carries its own “hot” or “cold” call without mixing. Two‑stream vision = “Where’s the car?” vs. “What’s the car?” – dorsal answers location/action, ventral answers identity. --- 🚩 Exceptions & Edge Cases Thermoreceptor overlap – at moderate temperatures, both TRPV1 and TRPM8 can be minimally active, but the labeled‑line still predominates. Rod dominance varies by species – nocturnal animals may have rod : cone ratios approaching 1000 : 1, far higher than humans. Somatosensory BA 3 receives the bulk of thalamic input; damage here can spare some coarse sensation (via BA 1/2) but abolish fine discrimination. --- 📍 When to Use Which Identify visual task → use dorsal stream for motion/spatial problems, ventral stream for object identification. Assess tactile stimulus → if static shape → think SA 1; if stretch or sustained pressure → SA 2; if vibration → RA (low freq) vs Pacinian (high freq). Temperature query → ask whether the stimulus is warm (> 43 °C) → TRPV1; cool (< 25 °C) → TRPM8. Taste vs. somatosensory → flavor judgments involve gustatory cortex plus somatosensory “mouthfeel” input. --- 👀 Patterns to Recognize Small receptive field + slowly adapting → form/texture perception. Large receptive field + rapidly adapting → vibration detection. High rod:cone ratio → species adapted to low‑light environments. Ipsilateral olfactory projection → lack of cross‑hemispheric odor comparison. Dorsal stream activation → tasks requiring reaching, grasping, or navigation. --- 🗂️ Exam Traps Distractor: “There are six human senses.” – Remember pain, balance, kinaesthesia, temperature are additional but not part of the classic five; total commonly accepted = 9. Distractor: “Pacinian receptors detect low‑frequency vibration.” – They are tuned to high‑frequency vibration; RA receptors handle low‑frequency. Distractor: “TRPV1 responds to cold.” – It is a heat‑activated channel; TRPM8 is cold‑activated. Distractor: “The olfactory bulb sends fibers to the opposite hemisphere.” – Olfactory projections are ipsilateral only. Distractor: “Area 1 receives the most thalamic input.” – It’s Area 3 that receives the bulk of thalamic afferents. ---