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Neurophysiology – Dr. Hore.docx

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Physiology 3120
Tom Stavraky

Neurophysiology – Dr. Hore Neuro I – Introduction Anatomical Features • 10 neurons  10 synaptic connections • CNS = brain & spinal cord • PNS = nerve • Types of brain cells o Neurons  Information processing cells  Fire action potentials  Make synaptic connections o Glial cells  Provide support – helper cells  Major function: contribute to the well-being of neurons • Remove neurotransmitters • Removing ammonia (by-product of metabolism) • Supply glucose to neurons • Taking up excess extracellular K+ • Provide myelin sheaths for axons  10 times more glial cells than neurons Neuron Synapses • Chemical Synapse o Most common o High electrical resistance between pre- & post-synaptic cells o Electrical currents ahead of action potentials do not cross synapse o Signal which propagates electrically down is conveyed across synapse by neurotransmitters o Neurotransmitter attaches to receptor on postsynaptic neuron membrane o Slower than electrical synapses • Electrical Synapse o Neurons very close together o Cytoplasmic continuity via gap junctions o Low resistance to electrical signal o Electrical currents travel across synapse o No delay – very rapid o Bidirectional (action potential travelling from post to pre) • Synaptic plasticity o Strength of synapses can be altered Equilibrium Potentials for Representative Neurons • Representative – not exact • Example neuron: o RMP = - 70 mV o Na+ = + 60 mV o K+ = - 90 mV o Cl- = - 80 mV o Ca++ = + 120 mV • Neurons at rest o Strongly permeable to K+ o RMP is not -90 o Some Na+ leaks in  bring in +ve charge  depolarize bringing it to -70 mV • What would happen if the membrane became suddenly permeable to: o Only Cl- – membrane potential would go to -80 mV o Only K+ – membrane potential would go to -90 mV o Only Cl- & K+ – membrane potential would go to a value between equilibrium potentials of both ions i.e. approx. -85 mV o Only Na+ & K+ - somewhere between the two (experiments show value is 0 mV) EPSP & IPSP • EPSP o Transient (15 ms – doesn’t have enough time to reach 0 mV) increase in conductance to both Na+ & K+ but Na+ dominates & cell depolarizes  Long change o Takes membrane potential towards zero/threshold  If it was longer it would reach 0  Most of the time does not reach threshold – sub-threshold (if reaches threshold  AP) o At single synapse is < 1 mV in size o Total size depends on how many afferent axons were active o Opens channel where both Na+/K+ travel – Na+ in & K+ out o Na+ rushes in due to chemical & electrical gradient  depolarization  +ve charge is brought in • IPSP o Transient (15 ms) increase in conductance to Cl-, K+ or both o Produce hyperpolarization  inhibits neuron o In some neurons E Cl- be close to -70 mV o Cl- mediated IPSP may be very near RMP o Even if IPSP does not hyperpolarize it suppresses action potentials because it clamps the cell at IPSP equilibrium potential (far from AP threshold) • Produced by different transmitters Transmission at CNS Synapse vs. Transmission at NMJ • There are a variety of transmitters in CNS vs. Ach is only used at NMJ • Size of ESPS o EPSP at single central synapse is < 1 mV in size o Need for 50-100 incoming Aps to produce EPSP that summate before membrane is depolarized to threshold level & AP is triggered o At NMJ – one AP in α-MN produces an end plate potential that always reaches threshold • Central synapses can be excitatory or inhibitory vs. NMJ which is only excitation Main Signal Flow within Neuron & Action Potential • Signal flow through neuron involves receiving chemical signals at synapses by chemical transmission • Excitatory synapses o Causes Na+ to enter bringing in +ve charge o Causes current to flow to axon hillock because this region is -ve with respect to excited region o Axon hillock is region of low electrical resistance compared to axon & has a high density of V.G. Na+ channels o If membrane potential is depolarized sufficiently at axon hillock – large # of V.G. Na+ channels will be open & action potential will be initiated at axon hillock & propagate down axon • Inhibitory synapses o Positive current flow in the opposite direction – away from axon hillock • Electrical current is flow of positive charge (not electrons) • In excitable cells (neurons & muscle fibres) current flow is produced by shuffling of +ve positive ions in one directions & -ve ions in opposite direction Neuro II: Synaptic Transmission Sequence of Synaptic Transmission • Presynaptic neurons synthesize neurotransmitters which are stored in vesicles • Presynaptic 2+polarization by2+ction potential activates Ca influx at V.G. Ca channels • Ca causes exocytosis & release of neurotransmitter • Neurotransmitter acts on postsynaptic receptor to produce 3 possible effects: o Ion flow & resulting depolarization or hyperpol2+ization of postsynaptic neuron o Activation of secondary messengers through Ca influx  May produce long term biochemical & structural changes in postsynaptic neuron • Change in membrane permeability for specific ions • Changed rate of protein production o Neuromodulator (protein released with transmitter) can activate G-protein linked receptor to produce long term change in post synaptic membrane • Transmitter is removed from synapse by being transformed, diffusing away or being actively transported back to presynaptic neuron NMDA (N Methyl D Aspartate) Receptors • Class of glutamate-gated cation channels with high calcium conductance • Play a role in brain development, excitatory neurotransmission, synaptic plasticity & memory function • Ex. found in hippocampus – brain structure concerned with memory 5 Classes of Neurotransmitter or Neuromodulators • Ach – (Alzheimers) o Excitatory NMJ o Inhibitory at heart o Cerebral cortex • Biogenic Amines (Mental Health) o Dopamine – (Parkinsons, Psychoses, Motivation & Reward) o Norepinephrine & Epinephrine/Noradrenaline & Adrenaline – (Autonomic Nervous System, Depression, Sleep/Wakefulness of neurons) o 5 Hydroxy Tryptamine (Serotonin) – (Depression & Schizophrenia) o Histamine – (Arousal, Attention) • Amino Acids o Excitatory – Glutamate o Inhibitory – GABA (gamma amino butyric acid), glycine • Neuropeptides o Can act as neuromodulators  Substance which functions to modulate synaptic transmission (by activation of second messengers) • Miscellaneous o Nitric oxide (retrograde transmitter: diffuses from postsynaptic to presynaptic site), CO, steroids Main Excitatory & Inhibitory Neurotransmitters in the Brain • Most common excitatory neurotransmitter is glutamate • GABA is a widespread inhibitory transmitter Excitatory or Inhibitory depends on receptor not • Glycine acts as an inhibitory neurotransmitter, mainly at the spinal cord and brain stem • Strychnine – Antagonist o Painful muscle contractions – convulsions o Asphyxiation  death Long-Term Potentiation (LTP) • Long-lasting activity use-dependent enhancement of synaptic response • Found for synapses in hippocampus and neocortex • Persistence & other properties make it the best correlate of memory • Maintenance of LTP is probably mediated both pre- and post-synaptically through enhanced presynaptic release and increased numbers of postsynaptic receptors • Occurs mainly at excitatory synapses in the CNS which have NMDA receptors Two Possible Mechanisms: • Presynaptic 2+ o Changed property of Ca channel (mechanism unknown) o Increases Ca entry  increased transmitter release  increase in EPSP • Postsynaptic o Activation of second messengers produces  increase in excitation  increase in EPSP  Increase in sensitivity of existing ion channels  Insertion of existing ion channels into membrane  Synthesis of new ion channels Presynaptic Facilitation & Presynaptic Inhibition • Presynaptic factors: affect synaptic release via presynaptic Ca influx 2+ • Presynaptic inhibition via axo-axonic synapses decreases transmitter release by decreasing Ca influx - closure of Ca 2+ channels • Allows for selective suppression of specific inputs • Presynaptic facilitation – mediated by Ca influx Post-Synaptic Depolarization Normal normal Ca 2+entry AP  4 vesicles  1 mV 2+ Presynaptic Inhibition decreased Ca influx AP  2 vesicles  0.5 mV Presynaptic Facilitation increased Ca influx AP  8 vesicles  2 mV Spatial & Temporal Summation of Postsynaptic Potentials • Any single synapse only has a small affect in CNS – need to have summation to get post-synaptic neuron to fire an AP • Can be a huge number of synapses on a particular neuron • Spatial summation o Summing of synaptic potentials (EPSP, IPSP) at different spatial locations on a neuron • Temporal (time) summation o Summing of synaptic potentials (EPSP, IPSP) at a single synapse, due to repetitive firing of presynaptic axon over time • Duration of the postsynaptic potential (approx. 15 ms) is longer than that of the AP (approx. 1 ms) + • Speed of repolarization is an AP – due to activation of V.G. K channels • In graded potentials, membrane potential is returned to steady state levels as ions diffuse down their electrochemical gradients through “leak channels” which are far more permeable to K than Na ions • Because each neuron receives input from thousands of synapses there is constant summing of depolarization (EPSP) & hyperpolarization (IPSP) at axon hillock until threshold is reached & AP occurs • Brain works by summation Neuro III: Transduction of Environmental Information Sensory Receptors • Free nerve ending or specialized ending of a nerve cell that is particularly sensitive to detect 1 form of environmental energy o Free nerve ending  Receptor is located in terminals of the axon in the skin  No connective tissue in free nerve ending o Pacinian corpuscle  Specialized afferent receptors  Receptors located within the circle  Surrounded by connective tissue • Adequate stimulus: particular form of energy to which a receptor is most sensitive o Ex. May have a touch receptor and cold receptor  Took hammer and wacked cold receptor – may get it to discharge through injury  Electrically stimulate only afferent from cold receptor – you feel cold o Ex. Rods in the eye detect light – can be made to discharge by a punch (pressure) in the eye  Adequate stimulus is light not pressure  Brain perceives activation of the modality of the adequate stimulus  Produce light and excite rods artificially  activate receptor Conceptual Models - Transducer Mechanisms for a Mechanoreceptor, Chemoreceptor & Photoreceptor • Environmental stimuli produce changes in ion permeability of membrane of afferent nerve: o Mechanical – stretch opens ion channels o Chemical – binding with receptor results in opening of ion channel (nociceptors) o Photoreceptor – photon absorption leads to closing of ion channel 4 Characteristics of Generator (Receptor) Potentials • Process o Stimulus o Na /K channel opens o Inward rush of +ve charge (Na+) o Current flow to –ve region  Flows out node – low electrical resistance)  Doesn’t flow down axon - high electrical resistance o Depolarization at first node o Opens channels Na /K V.G. channels – AP at 1 node • Characteristics o Depolarization due to  in permeability to Na & K+ o Local, non-propagated o Graded, proportional to the magnitude of the stimulus (if reaches threshold will produce AP) o Can summate • Generator (rsteptor) potential - passively (electrotonically) conduct to 1 node of receptor to generate AP • Local current flows from the receptor membrane to 1 node where V.G. Na channels are located • If very small stimulus – sub-threshold potential – feel nothing • Results from opening of channel that is permeable to both Na + & K + + • Similar mechanism underlying EPSP, but not V.G. Na channel that underlies AP AP for a Rapidly Adapting (Dynamic) & Slowly Adapting (Tonic) Receptor to Maintained Stimulus • Slowly adapting o Single amplitude of skin indentation (intensity: frequency coding, population coding) o Eventually lose interest & stop responding o Ex. putting on a watch & and then gradually you don’t feel it • Rapidly adapting o Signal rate of skin indentation (intensity: population coding) o Movement of arm hair – don’t feel it after a minute • Pacinian corpuscle o Single stimulus -responds with 1 AP o Many stimulus (253 Hz) – responds with 253 AP/s follows frequency of stimulus o Designed to follow vibration (external environment) – most sensitive o Light stimulus vs. heavy stimulus - differences in intensity is signaled by population coding o High intensity = more corpuscles activated Example Adaptation of Somatosensory Receptors • Hair receptors – fading sensation with time of applied stimulus (ex. few seconds after movement of hair, one cannot tell if it is the new or original position) Pacinian Corpuscle is Rapidly Adapting • Because o Inner lamellae slip back in spite of maintained stimulus o Properties of ion channels on the axon membrane • If lamellae were removed o Pacinian corpuscle would still respond to touch (pressure) with some degree of rapid adaptation, but not nearly as much as before • Pacinian corpuscles are the most sensitive mechanoreceptor • Detect externally applied vibration & sense displacement of skin due to friction when hand moves across the object o Texture discrimination (rough or smooth) o Rough –fingers over surface – bumps will result in displacements on skin & corpuscle activation. • Intensity of vibration is signaled by total # of Pacinian corpuscle that are active (population coding) • Conclusion – size & shape of the objects can be recognized by different populations of receptors which respond to different sensory modalities, have different rates of adaptation, and have different thresholds Neuro IV: Somatosensory System I Nature (Quality) & Intensity (Quantity) of an Environmental Stimulus are Signaled • Quality o Specificity of receptor for one type (modality) of stimulus o Variety of receptors signal various dynamic features of stimulus – whether stimulus is stationary or moving • Quantity o Increase in intensity is signaled by (slowly adapting):  Increased frequency of discharge of a single afferent (frequency code)  Recruitment of more receptors with higher thresholds (population code) Receptors Responsible for Touch, Vibration, Temperature, Pain & Proprioception Modality Receptor Adapting Axon Type Conduction Velocity Touch Free Slowly C 1 m/s Merkel, Ruffini Slowly Aα, β Meissners, Pacinian, Hair Rapidly Aα, β Up to 80 m/s Vibration Meissners (5-100 Hz) Aα, β Pacinian (50 – 1000 Hz) Aα, β Temperature: Warm Free nerve ending C Cool Free nerve ending Αδ Pain: Fast, Sharp Free nerve (nociceptor) Αδ 20 m/s Slow, Ache Free nerve (nociceptor) C 1 m/s Itch Free nerve ** Doesn’t have to be pain – molecule can be released by damage Proprioception Muscle spindle (primary) Aα (Ia) 90 m/s Muscle spindle (secondary) Aβ (II) 50 m/s Muscle length & Golgi tendon organ Aα (Ib) tension Joint Aβ information Skin Aβ 1. Different types Free Nerve Endings – different receptors & different responses 2. Touch: 6 Receptors a. Slowly adapting – Free endings, Merkels, Ruffini b. Rapidly adapting (connective tissue structure) – Meissners, Pacinian, Hair 3. Fast/Slow Pain 4. Touch faster than pain: Aβ touch, Aδ pain 5. Proprioception Afferent & Efferent Axons: Aα, Aβ, Aγ, Aδ, C, Ia, II, III, IV • Have different diameters & conduction velocities • Fastest: bigger & myelinated Two Systems of Classification of Axons • First system was first used by sensory & axon physiologists o A alpha – alpha motoneurons, skin afferents – large, fast, myelinated o A beta – skin afferents o A gamma – fusimotor neurons o A delta – pain (fast), temperature (cold) o C – unmyelinated, pain (slow), temperature (warm), crude touch • Second system commonly used by motor physiologist o Ia – annulo spiral from muscle spindle (primary) o Ib – Golgi tendon organ o II – flower spray from muscle spindle (secondary) o III & IV – free nerve endings in muscle (group IV is unmyelinated i.e. equivalent to C fibres) Receptive Field of a Neuron • Receptive field o Area on the receptive surface (body surface, visual space, auditory space) upon which an adequate stimulation will excite or inhibit firing of a particular neuron • Smallest (diameter) receptive fields are on the tips of fingers and the lips which reflect the high density of mechanoreceptors in these regions • This can be demonstrated with two-point discrimination i.e. 2 blunt points of a divider separated by 10 mm will be detected as 2 points when applied to fingertips, but only 1 point when applied to the back • NMJ & cutaneous receptors – cannot relax receptor/muscle - no IPSP – always excitatory • Punctate Sensitivity o Sensitivity on the skin is not uniform but is punctate i.e. maximum sensitivity occurs at small spots (which overly receptors) o Demonstrated or cold on the back of the hand with a cold probe (blunt pencil tip) o Such cold spots are insensitive to the pain of a pinprick – in a specific spot there is a thermoreceptor not a pain receptor Ascending Sensory Pathways • Spinothalamic (anterolateral) o Phylogenetically old system o Contralateral o Carries pain, temperature, crude touch & itch o Large receptive fields nd o Convergence of different modalities (temperature/touch recorded by 2 order neuron) • Dorsal Column, Medial Lemniscal System o Contralateral o Only found in mammals & reaches highest development in primates o Carries: proprioception, vibration & discriminative touch – distinguish different characteristics o Mammals can perceive form of objects through active discriminative touch (different characteristics – textures) o Small receptive fields (contributes to greater sensitivity) smallest on fingertips  Labeled line • Neuron only responds to one type of receptor: Pacinian corpuscle • Modality of high frequency vibration • Comes from specificity of receptor o Neurons respond to only one modality o Faithful transmission o Fast conduction Neuro V: Somatosensory System II Organization of Neurons in the Somatosensory Cortex • Information is processed in the cortex and begins to be reconstructed • Two features of organization of neurons: o Somatotopic organization o Columnar organization Somatotopic Organization on Post-Central Gyrus (Somatosensory Cortex) • Go from medial to lateral on the cortex • Leg, trunk, arm, face • Distorted Homunculus o Size of body part represents density of innervation of body part o Ex. large area of cortex devoted to input from the fingers and lips • Evidence o Recording form a population of neurons (evoked potential) or from single neurons  Electrically stimulate sciatic nerve – record large potential in post-central gyrus (evoked potential) in the area of the leg o Lesions (strokes in humans)  Lesion in leg area in somatosensory cortex – disordered somatosensory appreciation of stimuli applied to leg o Cortical stimulation & subject reporting sensation  Applied electrical signal in finger area of cortex  don’t get movement but get tingling o Epilepsy of somatosensory cortex  Epileptic focus that starts in the hand region because there are many more receptors there – spread across cortex but starts in the hand • Modified with Experience o Present at birth o Modified by experience o Size of representation is influenced by the use of that body part o Ex. increased size of finger representation in blind people who read with braille o Phenomenon: plasticity  Occurs primarily in adjacent somatotopic regions o 1 theory is that dormant neural pathways, which are already present in adjacent regions – activated with use Cortical Column • Group (many thousands) of cortical cells that are heavily interconnected in a vertical axis (perpendicular to the cortical surface) • Proposed to be basic module of operation - especially somatosensory, visual & auditory cortex • Each neuron in a particular column in somatosensory cortex responds to the same modality of sensory input & has similar receptive field o From finger tip –corpuscle that traveled to somatosensory cortex – that particular neuron will arrive in one column o Neurons in that column will have a receptive field on that finger tip & every neuron in that finger tip will have that same modality (driven by corpuscle) o Only find a column of neurons if we recorded from the finger area of somatosensory cortex • Convergence of input occurs but is usually modality pure • Most columns in somatosensory cortex receive input from only the dorsal-column, medial-lemniscal system • Input to other columns comes from spinothalamic system – less common • Reconstruction of somatosensory stimuli begins at somatosensory cortex o Ex. analysis of properties of object being handled (smoothness, edges, temperature, direction of movement) o No receptor signals a moving stimulus – to be able to sense movement – reconstruction Body Image • Posterior parietal cortex • Effects of lesions: o Information from right goes to left – contralateral o More common in the right posterior parietal cortex o Disorder in appreciation of spatial aspects of sensory input from the left side of the body (ex. who put this arm in bed with me) o Lack of appreciation of spatial aspects of sensory input from left external space (ex. patient may leave out left side when drawing figures, or eat from the right side of the plate) Vibration of Triceps – Blindfolded Subject Touching His Nose • Receptors in the triceps that signals muscle length • Vibration on triceps tendon • Increased triceps muscle spindle Ia discharge (due to its velocity sensitivity) • Increased discharge in dorsal column medial lemniscal system afferents which conduct triceps muscle spindle discharge • Muscle is perceived to be more stretched than it really is which can create a bizarre illusion that their head is open & someone is putting their finger in • Increased discharge in arm area of somatosensory t • Increased discharge in posterior parietal cortex • Altered body image Conclusion: 1. Muscle spindle discharge contributes to sense of position of limb (which in turn contributes to body image) 2. Body image in posterior parietal cortex is constantly being updated from afferent discharge from all somatosensory receptors Neuro VI: Visual System • Visual system first deconstructs, then reconstructs visual world • Retinotopic map on visual cortex • Columnar organization • Lesions of higher areas of cortex lead to specific behavioural disorders Cell Types in Retina • Rods, cones, bipolar cells, ganglion cells, horizontal cells, amacrine cells • Horizontal cells & amacrine cells are inhibitory • OFF, ON refers to light shone on receptive field of an ON centre – OFF surround retinal ganglion cell • Shine light ON centre – rod X hyperpolarized • Shine light OFF surround – rod X depolarized Rod & Cone System • Rods: Night Vision o In peripheral part of retina (few rods in fovea) o Sensitive to faint light – i.e. can detect single photon  Due to more photopigment  Due to more convergence in which a number of rods synapse on one bipolar cell • Cones: Day Vision o Concentrated in fovea (thumb nail at arm’s length) o Have greater acuity – i.e. sharpness (less convergence) o Used in bright light, colour vision • If an image falls on retina (where the rods are), not fovea, you’ll see it better Flow Diagram – Light Transduced to Action Potentials • Note: o Light leads to hyperpolarization o Retinal ganglion cells are the first cells in the straight-through pathway which discharge APs Receptive Fields of Retinal Ganglion Cells • There are two types of retinal ganglion cells: ON centre OFF surround, & OFF centre ON surround Horizontal cells produce a depolarization on the ON centre rod  release of inhibitory transmitter  hyperpolarization of bipolar cells  no action potentials in ganglion cells Information Processing by Retina • Major feature is that the retina responds best to contrast within receptive fields • Ex. when there is a contrast between ON centre & OFF surround Receptive Field Characteristics of Simple Cells in Visual Cortex • Simple cells respond best to bars of light of a particular orientation with ON centre – OFF surround or OFF centre – ON surround • Simple cells are feature detectors – visual cortex breaks up (deconstructs) the visual world into short line segments of various orientation Organization of Visual Cortex • Retinotopic Map o Some fibres decussate, some don’t o Information from left visual space goes to right visual cortex o Information from right visual space goes to left visual cortex o Information from specific part of visual space goes to specific part of visual cortex • Columnar organization in orientation columns o Organization into ocular dominance columns & hypercubes • L & R– Ocular Dominance Column (for left eye and right eye) o Series of orientation columns through 180 degree such that every 50 microns - receptive fields change by 10 degree • Hypercube – ocular dominance columns for left & right eye for one small patch of retina • Analyses of a given section of visual field from retina of both eyes Lesion Areas of Visual Cortex • Higher cortical areas put picture back together (reconstruct visual image) • Prosopagnosia o Inability to recognize familiar faces o If you were to speak to the person, they would recognize you because there would be no damage to the auditory cortex Similarities to Somatosensory System • Visual system first deconstructs, then reconstructs visual world • Retinotopic map on visual cortex • Columnar organization • Lesions of higher areas of cortex leaf to specific behavioural disorders Neuro VII: Auditory System Overview • Detects sounds over a frequency range of 20-20,000 Hz & an intensity range of 10 with spatial resolution of 1 degree of arc • Achieved by cochlea which deconstructs sounds into specific frequencies & intensities & sends this information to the brain • In humans – most important role of hearing is to process language Diagram of Auditory System • Cochlea o Contains the sensory organs of hearing in the inner ear o Coiled structure like a snail’s shell & can be visualized uncoiled o Pressure waves are transmitted from the middle ear into a channel containing endolymph fluid • Endolymph High in K+ • Hair cells is location of most damage o Skeletal muscles in middle ear ossicles contract – dampen big sound waves coming in o If you don’t know a big sound is coming – cant contract skeletal muscles fast enough to dampen • Impedance Matching Device o Air pressure waves that vary – going towards air – specifically oval window (containing fluid) o No ossicles – 99% percent of sound would be reflected o Inertia of the endolymph impedes energy transfer from soundwaves in air to fluid in inner ear o In this case impedance means resistance of endolymph to movement o Three factors which contribute – increase pressure at cochlea 200 fold  Outer ear acts as a funnel to concentrate sound  Ossicles in the middle ear act as a lever system  Area of the tympanic membrane is greater than that of the oval window • Different frequencies of sound are transduced into action potentials ** see diagram above o Different frequencies of sound produce travelling waves which reach peaks at different regions of the basement membrane along the cochlea  High frequencies – near stapes  Low frequencies – at apex of cochlea o By this mechanism complex sounds are deconstructed into different frequency components o Basement (basilar) membrane is a mechanical frequency analyzer • Selective response of hair cells on the basilar membrane to different frequencies o Mechanisms  Mechanical properties of basilar membrane (narrow & stuff near oval window, wide & more flexible near apex) • Traveling wave reaches peak at different points along membrane for different frequencies of sound  Mechanical turning of hair cells • Length & stiffness of hair cells are different along basilar membrane
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