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Theories about sensing Aristotle (384-322 BC) cardiocentric sensing. Biophysics of sensory receptors Galenus (129-200 AD) raised doubts about cardiocentric sensing. Cardiocentric sensing (Medieval reconstruction) Today: stimulus sensory receptors receptor potential neuron/nerve action potential central nervous system signal processing sensation fMRI recording during sensomotoric function Sensory homuncle Sensory receptors Steps of sensing OLFACTION VISION HEARING TASTE TOUCH Case of hearing Spectrum Duration Direction Physically measured sound properties Neural conductance Sensory receptor (ear) Brain (auditory center) Loudness Psychologically perceived sound effects Pitch Tone Perceived duration Perceived direction Sound recognition Sound sensation Intensity Frequency Meissner body Free axon Rod Cone Sensory receptor: Specialized sensory cell, which responds to a given stimulus (e.g., light, sound, chemicals) and relays the information to the central nervous system. Cell surface receptor (different meaning!): Proteins which specifically bind hormones, neurotransmitters and other molecules, and thus iniate specific cellular reactions. Five senses? Steps of signal transduction Most important sensory modalities (First 11: perceived modalities) Modality Receptor (physical-chemical effects) 1 Vision Rods and cones Eye 2 Hearing Hair cells Ear (organ of Corti) 3 Olfaction (smelling) Olfactory neuron mucus membrane 4 Taste Taste receptor cells Taste buds 5 Angular acceleration Hair cells Ear (semicircular canals) 6 Linear acceleration Hair cells Ear (utricle and saccule) 7 Touch, pressure Nerve endings Multiple types 8 Heat Nerve endings Multiple types 9 Pain Nerve endings Multiple types 10 Cold Free nerve endings ... 11 Joint position and motion Nerve endings Multiple types 12 Muscle length Nerve endings Muscle spindle 13 Muscle stress Nerve endings Golgi’s tendon organ 14 Arterial presure Nerve endings Sinus caroticus stretch receptors 15 Central venous pressure Nerve endings Venous, atrial stretch receptors 16 Lung stress Nerve endings Pulmonary stretch receptors 17 etc... etc... etc... Neuron Uaction sensory areas hearing and vision association association areas ASSOCIATION oten hearing Uthres ion channel Uact NERVE Sensitivity tials ten n po Uact o acti nerve from receptor to brain activation of sensory areas SENSORY CENTER STIMULUS RECEPTOR perception interpretation positive ions tial tor p ACTION POTENTIAL Central nervous system Step speech recognition p rece RECEPTOR POTENTIAL Uthres thres Ugen thres Receptor From stimulus to sensation Measured signal STIMULUS Environment Organ eV - size stimulus is sufficient for evoking action potential: • sound receptors: thermal motion of the molecules of air • light receptors: 1-2 photons 1. Modality What is coded by the action potential? Adequate stimulus Type of energy for which the receptor is most sensitive (e.g., light for the eye). • modality (type) • intensity (strength) Action potentials are identical in all nerves. How do we know, for example, whether an action potential codes for touch and not cold? • duration • localization Principle of specific sensory energies of the stimulus Sensation is determined by the stimulated cortical region! 2. Intensity 3. Duration, adaptation Which parameters carry information about stimulus strength? • frequency of action potentials • number of activated receptor cells Weber-Fechner psychophysical law Stevens’ law = const lg 0 = const 0 n = sensation strength =background intensity 0=absolute threshold intensity n=constant specific for the type of sensation n<1: compressive function (hearing, vision) Stimulus Adaptation. During constant stimulus the frequency of action potentials gradually decreases. Rapidly adapting (phasic) receptors E.g., pressure, smell, heat Slowly and partially adapting (tonic) receptors E.g., cold, pain (dental pain) n>1: expansive function (pressure, taste) Weber (1795-1878) Fechner (1801-1887) Stevens (1906-1973) Action potentials Receptor potential No adaptation Action potentials Slow adaptation Receptor potential Action potentials Receptor potential Fast adaptation Seconds 4. Localization, receptor fields Branched nerve endings define receptor fields (convergence). Such can be found in the skin (touch) and in the peripheral retina (rods). Biophysics of the eye Receptor fields with overlap Optical illusions - intensity Optical illusions – space Optical illusions – motion Optical illusions – direction Ooptical illusions point out the remarkable and unusual processing power of the visual system. Stimulus: light Stimulus: light Electromagnetic wave Transverse wave The eye is sensitive to: wavelength and amplitude (~intensity) The eye is insensitive to: phase and polarization “Receptor-organ”: eye Photoreceptors pigment epithelium light receptors plasma membrane rods sclera cones choroid vitreous humour discs membrane disc discs optic nerve fibers cytoplasm retina cones ciliary body plasma membrane t ligh zonulae ciliares cilia (suspensory ligaments) membrane disc mitochondria lens eye axis anterior chamber rods ganglion cells macula bipolar cells nucleus fovea retinal optic nerve synapse cornea opsin iris posterior chamber rod optic disc (blind spot) Optics of the eye cone rhodopsin molecule Accomodation and refractive problems flattened lens Total refractive power of eye: 62 dptr relaxed ciliary muscle bulged lens contracted ciliary muscle Optical power entering the eye (P) d P = J 2 2 J=intensity (W/m2) d=pupil diameter stretched ligaments 2 Pmax dmax = = 16 Pmin dmin dmax=8 mm dmin=2 mm Hypermetropia Astigmatism air cornea aqueous humour lens vitreous humour Refractive power of surfaces (D) D= n n' r relaxed ligaments n-n’=refractive index difference between refractive media r=radius of curvature of refractive surface Myopia Correction with cylindric lens LASIK (Laser Assisted In Situ Keratomileusis) Spatial resolution of the eye Generation of visual stimulus visual angle: the smallest angle at which two object points can be distinguished. For a healthy eye: 1’ (angular minute, 1/60 degree) visual _ acuity = 1' 100% Sensitivity of the human eye =experimental visual angle Resolution has wave optics (diffraction) and biological (receptor density) limitations. Object Image on receptors Night vision Day vision Astronomical magnitude Sensation ~2m Cloudy night sky Photoreceptor distribution in retina Clear night sky Quarter Full moon moon Twilight Sunrise sunset Cloudy day sky Direct sun exposure Properties of receptor cells blind spot Rods Rods blind spot Cones fovea optic nerve Cones Degree (˚) Rods Cones Rod Cone Stimulated by very small intensity (optimally 1 photon!) Smaller sensitivity, but functions at high intensities Saturates at average intensities No saturation Found mainly in the peripheral retina In the fovea, mainly central fovea Many rods per ganglion (convergence); greater sensitivity, smaller spatial resolution Small convergence; greater spatial resolution No color sensitivity Sensitivity to colors Photochemical reaction optical excitation all-trans-retinal In human: 3 types of receptors. Each senses different colors - absorbes at different wavelengths (R=64%, G=32%, B=2%). Relative absorption 11-cis-retinal 1 rhodopsin absorbs 1 photon 500 transducin molecules activated 500 phosphodiestherase molecules activated, and 105 cGMP molecules hydrolyzed 250 Na+-channels closed Entrance of 106-107 Na+ ions/s inhibited cell hyperpolarized (1 mV) transmitter release reduced (glutamate: inhibitory neurotransmitter). Color vision Light reflection from butterfly retina. The different receptors reflect dfferent colors. rod cones Wavelength (nm) X = rR + gG + bB Additive color mixing Stimulus: sound Biophysics of hearing Longitudinal mechanical wave (pressure wave) A Longitudinal wave Harmonic oscillation: Transverse wave y ( t ) = Asin ( ft + ) y=actual pressure; t=time f=frequency; A=amplitude =phase shift Sounds and their spectra Frequency and intensity of sounds J Intensity level: n(dB) = 10 lg 1 J2 TYMPANIC MEMBRANE RUPTURE PAIN (Fourier spectrum) pure sound MU INFRASOUND ULTRASOUND SIC HEARING Continuous spectra white noise HEARING Intensity musical sound Discrete spectra harmonics Pressure wave therapy Ultrasound diagnostics, therapy DISCOMFORT fundamental frequency SUPERSOUND HYPERSOUND SUPERSOUND Frequency domain Tu rbi dri ne ll Fourier transformation Inverse Fourier transormation Time domain Cardiac sounds SILENCE Frequency drum beat Wavelength (in air) “Receptor-organ”: ear Outer ear Middle ear Inner ear Physical schematics of the ear Semicircular canals (vestibular organ) Auricula The pressure wave of the fluid generates surface waves on the basal membrane Lever system of ossicles results transmits waves with inceased forces Ossicular muscles Outer ear Oval window Middle ear Axis of rotation of lever system Inner ear The basal membrane is narrow and taught at the beginning of the cochlea The basal membrane is wide and slack at the end of the cochlea Apex Cochlea extended (35 mm) Auditory nerve External auditory canal Eardrum Apex cochleae Auditory ossicles (malleus, incus, stapes) Round window Cochlea Eustachian tube Auditory nerve External auditory canal: resonator with quarter wavelength. The smaller surface area of the oval window, compared with that of the eardrum, results in pressure increase. Hair cells on the vibrating basal membrane are bent and excite auditory nerve endings While the oval window is pressed inside, the round window is pressed outside Pathway of atmospheric pressure equilibration Middle ear: mechanical amplifier Outer ear: sound collector Auricula Sound is steered into the external auditory canal. Auditory ossicles (malleus, incus, stapes) They amplify eardrum resonance and transmit it to the oval window. External auditory canal Conducts pressure waves towards the eardrum. More efficient in certain freqency range (2000-5000 Hz). Auditory ossicles Oval window Eardrum Amplification: due to area ratio: 17 due to lever action: 1,3 water air Total amplification: 22 (pressure increase) Eardrum Brought into resonance by sound waves. drum drum Ultrastructure of the inner ear Inner ear: sensor Vestibular organ: semicircular canals Inner hair cells. Appr. 500 Cochlea: 2.5-pitch, 35-mm-long fluid-filled channel. It is halved in length partly by an osseous, partly by a membranaeous wall, the basal membrane. Sensory organ of sound. Stapes Semicircular canals Oval window Cochlea Eardrum Basal membrane Round window Outer hair cells. Appr. 12-20000 Velocity of wave propagation Velocity of “surf” wave is smaller than that of sound (1440 m/s) Frequency “Surf” wave Apex Oval window Oval window Békésy: propagating surface waves on basal membrane Function of the organ of Corti Due to the bending of the basal membrane, hair cells become tilted and depolarized. Ape Basa x 2. Tectorial membrane becomes lifted l mem brane Inner hair cell Pivot point 3. Shear force Outer hair cells Oval window Surface curve of propagating wave Relative deflection Apex Location of propagating wave at subseqent times György Békésy Nobel-prize 1961 Distance from oval window (mm) Basal membrane at rest Pivot point 1. Basal membrane becomes distorted The frequency-dependence of the location of propagating wave maxima provide a rough frequency-discrimination. endolymph mechanical effect, stereocilia deform Tight tip link Passive mechanism (basal membrane) force cation gates open (tip link effect), K+ ions enter support cells hair cell becomes depolarized hair cell nucl. Ca2+ channels open, Ca2+ ions enter Active mechanism (outer hair cells) Base stereocilia Oscillation amplitude (log) Loose tip link Outer hair cells: amplifiers Basal membrane Distance from oval membrane (mm) Outer hair cells synapse neurotransmitter is released into synaptic cleft efferent afferent neurons afferent neuron depolarizes, frequency of action potentials increases Regenerative amplifier: positive feedback (large amplification in narrow frequency range) Apex Inner hair cells: Mechanoelectric transducers Psychoacoustics: loudness (Fletscher-Munson) Coding of acoustic information ethalon sound loudness Hson (son) w surface wave base basal membrane oval wido Frequency sensing coded locally. pain threshold Intensity level stimulus Pressure Volleyball theory Intensity loudness level Hphon (phon) Location theory Disco 120 phon apex loc Street noise 80 phon oscillatio n amplitu de Basis: 1. Weak frequency-dependence of the amplitude maxima of propagating surface waves. 2. Active amplification. 3. Frequency senitivity of afferent neurons (threshold stimulus depends on freqency). speak ing ra t (tim hair action cells Whisper 30 phon poten tials ltane o pote us action ntials (so v Ste loudness level loudness law Weber-Fechner ale (phon) mic) loudness sc rith ga (lo d cte Constru a at ld a nt e rim pe ex Intensity Intensity level es ho ld of he arin g frequency f (Hz) n) aw thr reference intensity, J0 Phon and son scales ’l ens nge e) simu Simultaneous appearance of action potentials in the auditory nerve may encode large frequencies. Loud speaking 60 phon