Sensory Systems Flashcards
SENSE ORGANS & BODY SYSTEMS
- structures w/receptors/interneurons specialised for detecting/processing particular stimuli types
- sensory organs = eye/ear
- body systems = touch
MECHANICAL SENSORY SYSTEM
MODALITIES & ADEQUATE STIMULI
- touch = contact w/ or deformation of body surface
- pain = tissue damage
- hearing = sound vibrations in air/water
- vestibular = head movement/orientation
- joint = position/movement
- muscle = tension
VISUAL SENSORY SYSTEM
MODALITIES & ADEQUATE STIMULI
- seeing = visible radiant energy
THERMAL SENSORY SYSTEM
MODALITIES & ADEQUATE STIMULI
- cold = skin temp decrease
- warmth = skin temp increase
CHEMICAL SENSORY SYSTEM
MODALITIES & ADEQUATE STIMULI
- smell = odorous substances dissolved in air/water in nasal cavity
- taste = substances in contact w/tongue
- common chemical = changes in CO2/pH/osmotic pressure
- vomeronasal = pheromones in air/water
ELECTRICAL SENSORY SYSTEM
MODALITIES & ADEQUATE STIMULI
- electroreception = difs in electrical current density
SENSORY RECEPTOR NEURONS
- specialised neurons detect internal/external stimuli of particular sensory modality
- input zone gen contains accessory structures/receptor molecules/specialised ion channels instead of dendrites
- transduction = transforming stimulus energy into neural signals transmitted to sensory interneurons
- filtering stimulus energy as have defined affinity/sensitivity range
- either spiked/non-spiking neurons
PHOTORECEPTORS
- axon terminals
- axon
- cell body w/nucleus w/DNA
- accessory structure = light-sensitive receptor molecules in membrane
MECHANORECEPTORS
- accessory structure = stereocilia w/ion-channels in membrane
- no axon/axon terminals
- input zone = axon/tissue layers (accessory structure) around unmyelinated end w/membrane ion channels
SIGNAL TRANSDUCTION IN RECEPTOR CELLS
MECHANORECEPTORS
- mechanical stimulus energy/perceived = pressure/vibration/stretch/sound
PHOTORECEPTORS
- electromagnetic stimulus energy/perceived = light
THERMORECEPTORS
- heat stimulus energy/received = temp (warm/cold)
CHEMORECEPTORS
- chemical stimulus energy = airborne/surface molecules
- stimulus perceived = odour/taste (sweet/bitter/umami/salty/sour)
NOCICEPTORS
- mechanical/thermal/chemical stimulus energy (uni/polymodal)
- stimulus perceived = pain
SIGNAL TRANSMISSION IN SENSORY SYSTEMS
TONGUE - taste receptors/sensory interneurons EYE - photoreceptor/sensory interneurons EAR - hair cells/sensory interneurons SKIN - pacinian corpuscle NOSE - olfactory receptor
REFLEX ARCS = BASIC NEURAL CIRCUITS
- reflex arcs synapse in spinal cord
- knee-jerk reflex = v fast (40ms latency) connecting sensory (receptor) neuron directly to motor neuron (monosynaptic = w/o interneurons)
- reflex arcs often more complex (polysynaptic = w/several interneurons)
SENSORY NEURONS CONNECT TO 1+ CIRCUITS
- sensory neurons (receptor neuron/interneuron) = afferent neurons
- motoneurons = efferent neurons innervating effector muscle
- spinal interneurons = mono/disynaptic somatic reflex arcs from muscle spindle -> motorneuron/other somatic reflex arcs from skin mechanoreceptors -> same motorneuron
- sensory neuron A = first order interneuron in disynaptic arc BUT also second/third order in other arcs
SENSORY SYSTEMS
receptors -> thalamic nuclei -> PSC (primary sensory cortex) -> SSC (secondary sensory cortex) -> association cortex
- good egs demonstrating main principles of how brain is organised/works
- many connect to brain
- sensory signals typically transmitted in processing step hierarchy
- info filtered/combined/enhanced as passes from 1 layer -> next in serial fashion
- each layer has networks composed of input/output neurons & as many interneurons
BRAIN CONNECTIVITY
- identified anatomically/functionally
- anatomical connection descriptions give v limited insight to functional hierarchies/info flow
SERIAL CONNECTIONS - bottom-up processing (ie. sensory organ -> primary cortex); top-down control (ie. modulation/inhibition/synchronisation)
PARALLEL STREAMS - (ie. signals diverge to dif networks)
CROSS-CONNECTIONS - (ie. modulation/inhibition/synchronisation)
MECHANORECEPTORS
- each (stretch/vibration/pain/touch) has distinct path to brain so dif skin stimulation qualities communicated to distinct brain areas
TOUCH/PAIN - diverse receptors in skin/body
POSTURE CONTROL - propioreceptors in body (muscles/joints)
HEARING - inner ear hair cells
BALANCE CONTROL - vestibular receptors in vestibular apparatus
SKIN SENSITIVITY TO MECHANICAL STIMULATION
- several somatosensory systems
- encapsulated nerve endings; long/myelinated axons
- soma located in dorsal root (spinal) ganglia of spinal cord
SMALL/LARGE NEURON RECEPTIVE FIELDS
SMALL RECEPTIVE FIELDS
- in touch-sensitive receptors
- free nerve endings
- Merkel’s disc/Meissner’s corpuscle sense innervates skin surface; sensitive to stimuli in small skin areas
WIDE/LARGE RECEPTIVE FIELDS
- Pacinian corpuscles/Ruffini’s endings innervate deeper skin layers; sensitive to stimuli over large areas
RECEPTORS TRANSMIT SIGNALS FROM SKIN -> SPINAL CORD
- pacinian corpuscle (skin/muscles detect vibration/pressure) = unipolar cell extending one axon branch to skin; other to spinal cord
- afferent projections form dorsal root (spinal) nerve; cell bodies part of dorsal root ganglion
MECHANICALLY-GATED ION CHANNELS
- vibration/pressure on skin deforms corpuscle; stretches axon tip opening mechanically-gated ion channels
- concentric tissue layers around axon tip amplify signal
SENSORY SIGNAL TRANSMISSION IN SPIKING RECEPTOR NEURON
- similar to postsynaptic neuron dendrites; receptors respond to stimulation w/graded/receptor potential
- spiking receptor neurons convert graded receptor potential -> action potentials for fast/long-distance transmission along axon
RECEPTOR NEURON RESPONSE THRESHOLDS
- receptors respond to stimulus in limited stimulus intensities range
- when sensing 2 dif low-intensity stimuli:
1. Merkel’s disc = 2 dif spike rates
2. Pacinian corpuscle = no spikes - when sensing 2 dif high-intensity stimuli:
1. Merkel’s disc = max spike rates
2. Pacinian corpuscle = 2 dif spike rates
COMBINING RECEPTOR NEURONS W/DIF SENSITIVITIES
- sensory systems can combine receptor neurons w/dif sensitivities (dif thresholds)
- useful for extending intensities range (ie. gentle touch -> strong pressure)
- graphs refer to receptor signals summation to measure intensity over large range BUT sensory systems oft also compare dif receptor signals (signal subtraction) to differentiate between stimuli
RECEPTOR ADAPTATION
- thresholds adapted over time
- receptor neuron responds to stimulus in limited stimulus intensities range; shifts threshold in limited range to code for prevailing stimulus intensities range (sensory adaptation)
- maximises coding efficiency when exposed to continuous/unchanging stimuli
- stimuli intensities range oft varies over time
STIMULATION ADAPTATION
- tonic receptors = receptor neurons showing slow response loss
- phasic receptors = receptor neurons showing fast response loss shortly after stimulation onset
RECEPTOR RESPONSE PROPERTY DIVERSITY
SMALL FIELD X TONIC - Merkel's disc (sensing texture) SMALL FIELD X PHASIC - Meissner's corpuscle (vibration) LARGE FIELD X TONIC - Ruffini's ending (sustained contact) LARGE FIELD X PHASIC - Pacinian corpuscle (initial contact)
RECEPTOR POTENTIAL -> SENSORY INFO
- ie. holding coffee cup PACINIAN CORPUSCLE (VIBRATION) = initial contact w/cup -> MEISSNER'S CORPUSCLE (TOUCH) = detects if cup slips -> MERKEL'S DISCS (TOUCH) = detects texture of cup -> RUFFINI'S ENDING (STRETCH) = hand in contact w/cup
SOMATOSENSORY PATHWAY
- segregated projections to dif brainstem/thalamus/cortex (labelled-line principle) areas
- where possible spatial stimuli location info preserved by separating projections coming from receptors in dif locations
- important to know where stimulation occurred in body (ie. if hand/back touched)/relative to body (ie. if wind blows into face/neck)
RECEPTIVE NEURON FIELDS
- receptive fields can be mapped for neurons in sensory pathway
CORTICAL ENCODING VIA SOMATOTOPIC MAPS
- primary somatosensory cortex located in postcentral gyrus in parietal lobe of brain (Brodmann areas 1/2/3a/3b)
- adjacent body regions generally encoded in adjacent cortex regions
- cortical somatotopic mao sensory segregation = fast adapting signals (from Meissner’s/Pacinian’s) & slow adapting signals (Merkel’s/Ruffini’s) remain segregated in cortex
VERTEBRATE/INVERTEBRATE SENSORY MAPS
- somatosensory map in cortex of star-nosed mole for dif tips of nose (touch organ)
EXPERIENCE DEPENDENT CORTICAL MAPS PLASTIC REORGANISATION
- cortical representations can change w/use
- owl monkey trained for several months at task w/2-4 fingers
ELBERT et al (1995) - investigated dipole strength in string players
YANG et al (1994) - region formerly stimulated by receptors in hand now responds to face/arm touch post somatosensory cortex reorganisation
STOPPING/SUPPRESSING SENSORY INPUT
- nerve cells require inhibition; receptors/conveyed signals require “switch off” times
- suppression oft involves accessory organs; structures reduce intensity/alter stimulus before it reaches receptor (ie. eyelids/ear muscles)
- can be via top-down processes (ie. brain stem sends messages to receptor cells in ear to selectively dampen sounds)
SUMMARY
- receptor neurons specialised to detect stimuli of particular sensory modalities/filter stimulus info/transform stimulus energy -> neural signals
- serial processing of receptor input = sensory processing that isn’t exclusively serial/hierarchical
- mechanoreceptors (ie. Pacinian corpuscle) transmit signals skin -> spinal cord
- info filtering/coding = receptor neurons can have dif receptive fields/response thresholds
- receptors adapt threshold over time to optimise stimuli coding; can be fast (phasic)/slow (tonic)
- sensory paths = dif receptor project to dif areas of brainstem/thalamus/cortex
- somatotropic maps = adjacent regions on body gen encoded in adjacent
