Chapter 10: Sensory Physiology

Ch 10: Sensory PhysiologyKey PointsReceptor transductionReceptive fields and perceptionPhasic and tonic receptorsDifferent somatosensory modalitiesFive special senses

Sensory Receptors - Overview are transducers → convert stimuli intograded potential (receptor potential) are of various complexity react to particular forms of stimuli• Chemoreceptors• _____________• _____________• _____________Fig 10-1

Sensory TransductionConverts Stimulus into gradedpotential = receptor potential. Howis stimulus energy transduced?Receptor potential in neuron abovethreshold APReceptor potential in non-neuralreceptors changes influence onNT release

Receptive FieldsEach 1° sensory neuron picks upinformation from a receptive fieldOften convergence onto 2° sensoryneuron summation of multiple stimuliSize of receptive field determinessensitivity to stimulus Two pointdiscrimination test (see lab)Fig 10-3Fig 10-2

SensoryPathwayStimulusSensory receptor (= transducer)_______________________CNSIntegration, perception

Sensory InfoPathways andCNS IntegrationSomatosensorycortexFig 10-4

CNS Distinguishes4 StimulusProperties Modality (nature) ofstimulus Location (lateralinhibition & populationcoding) Intensity DurationFig 10-10Somatosensory cortex

Intensity & Duration ofStimulus Intensity is encoded by # of receptors activatedand frequency of AP coming from receptor Duration is coded by duration of APs insensory neurons Sustained stimulation leads to adaptation• Tonic receptors do NOT adapt or adapt slowly• Phasic receptors adapt rapidly

Tonic Receptors vs. Phasic Receptors Slow or no adaptation Rapid adaptation Continuous signaltransmission forduration of stimulus Monitoring ofparameters that mustbe continuallyevaluated, e.g.:barorecptors__________ ? Cease firing if strengthof a continuousstimulus remainsconstant Allow body to ignoreconstant unimportantinformation, e.g.:___________?Fig 10-8

Somatic Senses Primary sensory neuronsfrom receptor to spinal cordor medulla Secondary sensoryneurons always cross over(in spinal cord or medulla) thalamus Tertiary sensory neurons somatosensory cortex(post central gyrus)Fig 10-9

Touch ReceptorsFree or encapsulated dendritic endingsIn skin and deep organs. e.g.:Pacinian corpusclesconcentric layers of c.t. large receptivefield detect vibrationopens mechanicallygated ion channelrapid adaptation receptor type?

Temperature Receptors Free dendritic endings in hypodermis Function in thermoregulation Cold receptors (< body temp.) Warm receptors (> body temp.) Test if more cold or warm receptors in lab Nociceptors Adaptation only between 20 and 40C

Nociceptors Free dendritic endings Activation by strong, noxious stimuli - Function? 3 categories:• Mechanical• Thermal (menthol and cold / capsaicin and hot)• Chemical (includes chemicals from injured tissues) May activate 2 different pathways:• Reflexive protective – integrated in spinal cord• Ascending to cortex (pain or pruritus) Fast (A) vs. slow pain (C) (review axon diameter,myelination)

Referred PainPain in organs ispoorly localizedMay be displaced ifMultiple 1° sensoryneurons convergeon single ascendingneuronFig 10-13

Fig 10-14Chemoreception: Smell and Taste2 of the five special sensesVery old (bacteria use to sense environment)Olfaction Olfactory epithelium has 1,000 differentodorant receptors Bipolar neurons continuously divide! G-protein – cAMP mediated Brain uses “population coding” todiscriminate 1,000s of odors

Gustation Organ for taste = ?See Fig 10-15Taste buds located inpapillae contain groupof taste andsupport cells

Taste Buds: Five taste sensationssour5) __________?Phasic receptors ______ adaptation!

Each tastecell issensitiveto only onetasteCa 2+ signalreleases NTFig 10-16

Sour and Salt Ligands

Special Senses: Hearing &BalanceReview Ear anatomy•outer•middle•innerHearing: organ of cortiBalance: maculae and cristae ampullaris

SoundtransmissionSound wavesTympanic membrane vibrationsOssicles transmit & amplify vibrationVia oval window to perilymph then endolymph

cont.Vibrations inendolymph stimulatesets of receptor cellsNT release of receptorcell stimulatesnearby sensoryneuronImpulse to auditorycortex of temporallobe via________________nerve

Fig 10-21Hearing Transduction= multi-step process:air waves mechanical vibrations fluidwaves chemical signals APsAt rest ~ 10% ofion channelsopenMore ionchannels open:ExcitationAll channelsclosed:Inhibition

Signal Transduction cont.At restExcitiationInhibition

Interpretation of Sound Waves:Pitch Perception Sound wave frequency expressed in Hertz(Hz) = wavelength / sec Human can hear between 20 and 20,000 Hz High pitch = high frequency Low pitch = low frequency Tone = pure sound of 1 frequency (e.g.tuning fork)

Basilar MembranePitch perception is functionof basilar membraneBM stiff near oval windowBM more flexible near distalendBrain translates location onmembrane into pitch of sound→Temporal aspects of frequencyare transformed into spatialcoding→ Spatial coding is preserved inauditory cortexCompare to Fig 10-22

Interpretation of Sound Waves:Loudness perceptionRate at which APs are fired loudness Sound Intensity Measurement:Decibel Scale (dB) starts at 0 and is logarithmic 130 dB pain threshold > 80 dB frequently or prolonged ? Examples:• noisy restaurant ~ 70 dB• rock concert ~ 120 dB

2 (3) types of Hearing LossConduction deafnessExternal or middle earMany possible etiologiesOtitis media, otosclerosis etc….Sensorineural deafness (+ central)Damage to neural structures (from receptors tocortical cells)Most common: gradual loss of receptor cells

Equilibrium= State of Balance• Utricle and saccule(otolith organs) withmaculae (sensory receptors)for linear acceleration and head position• Semicircular canals and ampullae withcristae ampullaris (sensory receptors) forrotational acceleration• Important besides inner ear: input fromvision & stretch receptors in muscle

Otolith Organs of Maculae Maculae andCristaampullarisreceptors similarto organ of cortireceptors However: gravity& accelerationprovide force tomove stereo cilia Fig 10-25

Not in bookMotion Sickness= Equilibrium disorderDue to sensory inputmismatchExample?Antimotion drugs (e.g.: Dramamine):Depression of vestibular inputs

VestibularNystagmus = Reflex movement via input fromsemicircular canals & cristae ampullaris As you rotate• eyes slowly drift in opposite direction (due to backflowof endolymph)• then rapid eye movement in direction of rotation toestablish new fixation point Continues until endolymph comes to rest Sudden stop ?

VisionReview eye anatomyespecially:Path of light througheyeballCellular layers of retinaIntrinsic eye musclesBlind spot and foveacentralis

Vision Process can be Dividedinto Three Steps1. Light enters eye, is focused bylens onto retina2. Photoreceptorstransduce lightenergy intoelectrical signal

3. Electrical signals sent along neuronalpathways are processed in visual cortex

Light Modification Pre-Retina• Amount of light is changed by alteringpupil aperture from 1.5 – 8 mm• Pupillary constriction due to ?• Dilation ?• Pupillary reflex isconsensual

The LensLight is focused bychanging lens shapeRefraction: Due to differentdensities, light waves arebent, or refracted whenpassing from onemedium into another

Accommodation: Light is focused (to keepobjects in focus) by changing lens shapeLens attached to ciliary muscle viasuspensory ligament (= zonulas)Ciliary muscle contractsSee also Fig 10-32. . . . . .Lens bulges upFig 10-31

Vision Problems Presbyopia (loss of accommodation) Myopia (near-sightedness) Hyperopia (far-sightedness) Astigmatism (asymmetry of corneaand/or lens)Test of visual acuity in lab

Compare to Fig 10-33

Important concept from 1 st part of chapter:Sensory Transduction ConvertsStimuli into Graded PotentialsStimulus energy is transduced into amembrane potential change.How can you create an excitatory orinhibitory signal?

Phototransduction atRetinaAnatomy review:Neurons organized into layers

Light = Electromagnetic EnergyWavelengthfor visiblelight: =?Someanimalscan see UVand IRwaves

Photo-ReceptorsRods Monochromaticnight time vision 1 pigment(Rhodopsin) Most numerous except in foveaCones High acuity vision & daytime color vision 3 pigmentsFig 10-38

Phototransduction forRhodopsinRetinal absorbs 1photonRhodopsin splits:Retinal is releasedfrom opsin due toconformationalchange(similar for cone pigments)= “bleaching”How does this produce AP ?Retinal = Vit.A derivativeOpsin = protein

No Light:Rhodopsin inactiveCells have membranepotential of ~ - 40mV (what does thatmean, what is it dueto ?)Fig 10-39Continuous (= tonic)NT release toadjacent bipolarcells

Light:Rhodopsin splitsActivation of transducin( = __? - protein)2 nd messenger cascadedecreases cGMPlevelsNa + channels close ?NT release decreases

120 Mio rods 6 Mio cones only 1.2 Mio axons enter optic nerve mechanism?Visual processingin visual cortexOptic nerves enterbrain at opticchiasma: somefibers cross sides right side visualfield to left sidebrain

Visual Field and BinocularVision3 vs. 2 dimensional viewFig 10-41