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Taste Tuesday March 3 2 15 119 PM 0 Some taste preferences are inborn such as our preference for sweetness which is satisfied by mother39s milk while bitter substances are instinctively rejected The Basic Tastes The five basic tastes are saltiness sourness sweetness bitterness and umami defined by savory taste of glutamate Each food activates a different combination of the basic tastes as well as having a distinct smell Pain sensations are essential to the hot spicy flavor of capsaicin The Organs of Taste The tongue palate pharynx and epiglottis are all involved in taste The tip of the tongue is sensitive to sweetness the back to bitterness the sides to saltiness and sourness however most of the tongue is sensitive to all basic tastes Papillae on the tongue have from one to several hundred taste buds each with 50150 m receptor cells arranged within 0 Taste buds also have basal cells surrounding the taste cells along with gustatory afferent 0 There are three types of gustatory papillae fungiform papillae foliate papillae and vallate papMae Concentrations of flavors too low will not be tasted but at the threshold concentration the stimulus evokes a perception of taste Fuguro 81 Anatomy 0 the mouth throat and nasal passages Taste l8 pnmanty a lunctton of the tongue but rogtons oi the pharynx palate and apnglottrs have some sonstttvny Notrco how the nasal passages are placed so that odors from ingested food can enter lhlough tho nose at tho pharynx theteby oasin contributing to porceptms ol flavo39 Ihvough oltnctnon Nasal cavity Palate V Tongue Pharynx Epiglottis Chapter 8The Chemical Senses Page 1 Vallate papMae Foliate papMae Fungiform papMae Taste Receptor Cells The chemically sensitive part of a taste receptor cell is its apical end near the surface They have thin extensions called microvilli that project into the taste pore a small opening on the surface of thetongue 0 Taste receptor cells form synapses with the endings of the gustatory afferent axons They also make both electrical and chemical synapses with some of the basal cells some of which synapse onto the sensory axons Cells of the taste bud undergo a constant cycle of growth death and regeneration with a lifespan of about 2 weeks 0 Depends on the influence of the sensory nerve If the nerve is cut the taste buds wi degenerate When a taste receptor is activated its membrane potential changes usually by depolarizing called the receptor potential If this is depolarizing and large enough most taste receptor cells may fire action potentials o Depolarization of the receptor membrane causes voltagegated Ca2 channels to open and Ca2 to enter the cytoplasm triggering the release of transmitter molecules which excites the postsynaptic sensory axon More than 90 of receptor cells respond to 2 basic tastes A cell39s response to chemical types depends on the particular transduction mechanisms present in the cell 9 Tll39rquoti391 I39I39 ETIMULI 5 quotmun rm55x V JUELTIQEE gm i C 1 A V 39 Iquot I I39ltf tal it l 39 firminle A 1quot I I l39 quot 39 EASE I E g D n L TER L E t x m l jml i39TisgLi iL tI1e39i ilti39quot39i39 FIBER 71 Taste Bud Structure Type cells are support cells with glialIike function Chapter 8The Chemical Senses Page 2 Type II cells have receptors and signaling effectors for bitter sweet and umami taste transduction release ATP as a neurotransmitter Type III cells may respond to sweet and sour stimuli but are not fully understood Taste P019 Edge Celli Basal Cell V 39 Intragemmal Peflgemmal Gustatoy Somatosehsow Facua N ngem nal N Mechanisms of Taste Transduction Taste transduction involves several different processes and each basic taste uses 1 of these mechanisms Tastants taste stimuli can either pass directly through ion channels salt and sour bind to and block ion channels sour or bind to Gproteincoupled receptors in the membrane bitter sweet and umami Saltiness In order to taste Na it must be in a concentration of at least 10 mM Saltsensitive cells have an Naselective channel that is common in other epithelial cells and is blocked by amiloride This channel is insensitive to voltage and remains open all the time When the concentration of Na within the mouth increases sodium will flow down its concentration gradient into the cell causing depolarization which in turn causes voltagegated sodium and calcium channels to open near the synaptic vesicles triggering the release of neurotransmitter molecules onto the gustatory afferent axon The anions of salt affect the taste of the cations because the larger an anion is the more it inhibits the salt taste of the cation as well as taking on a taste of its own Sourness Foods taste sour due to their high acidity low pH Protons H are the causative agents of acidity and sourness They affect sensitive taste receptor by permeating the amiloridesensitive sodium channels depolarizing the cell as well as by binding to and blocking Kselective channels also causing depolarization Bitterness T1R and T2R are taste receptor genes that encode for a variety of Gproteincoupled taste receptors that are similar to the GProteincoupled receptors that detect neurotransmitters Bitter substances are detected by the quot3930 different types of T2R receptors These are poison receptors The nervous system does not distinguish between one type of bitter substance from another because each bitter taste cells expresses many perhaps all of the 30 bitter receptor proteins Use a second messenger pathway When the Gprotein is activated it stimulates phospholipase C which increases the production of P3 inositol triphosphate P3 activates a special type of ion channel unique to taste cells allowing Nato enter depolarizing the taste cell causing voltage gated calcium channels to open 0 P3 can also trigger the release of Ca2 from intracellular storage sites Sweetness Use Gproteincoupled receptors formed from two proteins bound tightly together T1R2 and Chapter 8The Chemical Senses Page 3 T1R3 which activate the same second messenger system as bitter receptors Bitter receptor proteins and sweet receptor proteins are expressed in different taste cells and connect to different gustatory axons Umami Amino Acids The umami receptor is made of T1R1 and T1R3 proteins This uses the same second messenger pathway as the sweet and bitter receptors and are differentiated by the different transmission lines of the CNS Apical Paraeliu ar pathway aeo laterai Jim i fee 50 m5 Chapter 8The Chemical Senses Page 4 Salt W agooow I Na Cl K 01quot HCr How coo I Cf Cl39 WC W clooe citric add W fR CH quot i I er 0 6 0 05 a 84 42 2394 sucrm sucroseoctaacetaie I m m on 3 I K quot o t GMP sodlun sacchann denatonium benzoate N N N N Family of 30 receptors H n quot u n JHvuuUU Luuuuud T2R T1R2 T1 R3 T1R1 T1R3 Blizar sweat Urwm Central Taste Pathways From the taste buds to the primary gustatory axons into the brain stem up to the thalamus to the cerebral cortex The anterior twothirds of the tongue and the palate send axons into a branch of cranial nerve VII the facial nerve The posterior third of the tongue is innervated by a branch of cranial nerve IX the glossopharyngeal nerve The regions around the through including the glottis epiglottis and pharynx send taste axons to a branch of cranial nerve X the vagus nerve Each of these nerves enter the brain stem bundle together and synapse with the gustatory nucleus a part of the solitary nucleus in the medulla Neurons of the gustatory nucleus synapse on a subset of small neurons in the ventral posterior medial VPM nucleus a portion of the thalamus that deals with sensory information from the head 0 The VPM taste neurons sends axons to the primary gustatory cortex which is located in Brodmann39s area 36 and the insulaoperculum regions of the cortex 0 Taste pathways to the thalamus and cortex are primarily ipsilateral to the cranial nerves that supply them Gustatory nucleus cells project to a variety of brain stem regions largely in the medulla that are involved in swallowing salivation gagging vomiting digestion and respiration O Gustatory information is also distributed to the hypothalamus and parts of the basal telencephalon which are involved in the palatability of goods and the forces that motivate use to eat 0 Lesion in the hypothalamus or amygdala can cause an animal to overeat ignore food or alter its preferences for food The Neural Coding of Taste The labeled line hypothesis rationalizes that there are many specific taste receptors for basic tastes connected to separate set of axons connected to neurons in the brain that responded to one specific taste However the labels of the taste lines are uncertain rather than distinct Involves population coding in which the responses of a large number of broadly tuned neurons Chapter 8The Chemical Senses Page 5 rather than a small number of precisely tuned neurons are used to specify the properties of a particular stimulus O The brain can only distinguish between alternative tastes with a large population of taste cells with different response patterns Chapter 8The Chemical Senses Page 6 Smell Tuesday March 3 2 15 239 PM The Organs of Smell We smell with the olfactory epithelium which has three main cell types 0 Olfactory receptor cells are the site of transduction These are neurons with axons of their own that penetrate into the central nervous system 0 Supporting cells are similar to glia and help produce mucus o Basal cells are the source of new receptor cells Olfactory receptors last about 48 weeks Only a small percentage of the air breathed through the nasal passages passes over the olfactory epithelium which exudes a thin coating of mucus that is replaced about every 10 minutes 0 Odorants dissolve in the mucus layer before reaching receptor cells 0 Mucus consists of water with dissolved mucopolysaccharides a variety of proteins including antibodies enzymes and odorant binding proteins and salts The chemosensory systems in the mammalian nose include the main olfactory for general odorants the accesory olfactory vomeronasal for pheromones and the trigeminal lchemisthesisl for irritating or pungent volatiles Chapter 8The Chemical Senses Page 7 Pheromone Amygdala so O Xilzfgzn asal 4s K at A wasquot Hypothalamus 0332 epithelium Olfactory bulb Primary olfactory cortex Odorant Clquot channel ll 1 L39I39r Olfactory Receptor Neurons Olfactory receptor neurons have a single thin dendrite that ends with a small knob at the surface of the epithelium There are several long thin cilia protruding from the knob that bind to odorants and activate the transduction process 0 On the other side there is a very thin unmyelinated axon Collectively the olfactory axons constitute the olfactory nerve cranial nerve I They do not all come together as a single nerve bundle but instead small clusters of them penetrate a thin sheet of bone called the cribriform plate then course into the olfactory bulb Severance of the olfactory axons may cause anosmia the inability to smell Olfactory Transduction Odorants bind to membrane odorant receptor proteins which stimulate Gproteins Golf in the thin cilia This causes activation of adenylyl cyclase which causes formation of cyclic AMP CAMP Binding of CAMP to a specific cation channel causes the opening of cation channels and an influx of Na and Ca2 which opens Ca2activated chloride channels This in turn causes current flow and membrane depolarization Chapter 8The Chemical Senses Page 8 A Odorant Receptor Active NEW Can 2 2 gated Na Ca molecule protem ca Channel Cl channel exchanger Active adenylate 3321 0C1quot I cyclase I Second messenger Active Gprotein C 7001 Sinauor Assonates inc Activation of chloride channels amplifies the olfactory receptor potential Olfactory response may terminate due to diffusion of odorants scavenger enzymes in the mucus layer breaking them down or cAMP in the receptor cell activating other signaling pathways that end the transduction process 0 In the continuing presence of an odorant the strength of a smell usually fades due to the response of the receptor cell adapting to an odorant within about a minute 0 There are many different types of odorant receptor proteins coded for by odorant receptor genes However each olfactory receptor cell expresses very few of these receptor genes Chapter 8The Chemical Senses Page 9 Flynn 018 Speci c mapping 0 olhctory receptor 00mm onto olomuull Each glomcmlus receives Input only from We all oxpmslog I panicum receptor ptololn 90m Receptor coll expressing a particular gone In cobrcodod Olfactory epithelium 0200 meuwm Chapter 8The Chemical Senses Page 10 Figure 815 Broad tuning of single olfactory receptor cells a Each receptor cell expresses a elude olfactory receptor protein here coded by cell color and different cells are randornly scattered within a region of the epithelium b Microelectrode recordings from three cells show that each one responds to many odors but with differing preferences By measuring responses from all three cells each of the four odors can be clearly distinguished Olfactory epithelium Citrus M Peppermint Almond dLx 4 Receptor 1 Receptor 2 Receptor 3 D 02001 LupvmthMa Wllma The olfactory epithelium is organized into a few large zones each with receptor cells that express a different subset of receptor genes Uses population coding Figural elLoeetlen and Metructureoftheolfactoryeplthellumjheeplthellumeoneietsefe leyerofolfactoryreeeptor cells euppodnguihuldhesaleelcOdomtsdeeotvehthemcueWandcontaotdnolaefdnollectoryeehm unmasmmmwmmmmmwmumummtcusl Olfactory nerve Olfactory bub Crbriform piste Basal cell Crlbrllorm 9quot Olfactory Olfactory receptor cel itheliu op at supporting cal Cilia of olfactory cells Mucus layer inhaled air Chapter 8The Chemical Senses Page 11 THE OLFACTORY EPITHELIUM STRUCTURE l supporting cell y olfactory B Menco 1997 58 neuron j I basal stem 1 I Q cell A B Olfactory bulb Cribriform Olfactory e withelium 39 P Alrborne odors mm Smuum Axxm mlvs lm Chapter 8The Chemical Senses Page 12 Flaw Structured oliectory bub Axmdolbdory receptor coll penetrate thocrlbr om plate and ontqu olfactory bub AMmulbbmmodtolhcbryuoncymonWmmwmnnepho almmmm WmsmthMoquntmmmm Olfactory bulb Olfactory tract Glomerulus Secondorder olfactory neuron Cn brilorm Olfactory plate receptor cells 0200 WWIIsl WM Chapter 8The Chemical Senses Page 13 1F Engine E 1 3 il39allmgu rwurdinga mm rm allaclar mauptur call during atimul nliian Elder n39ta generate 3 Elm receptor patenniiallinthe ciliatheraceplat1mtiienlialipmpagatea dawn the dendniie anal triggem a eerie at attiia Wienlrlals within the eama at the allaclery magnet cell Finally ihe c lliain Wlanhiala bru l nail the Inceptur patain lialj pmpagaie aanlinu t ualy IEl EH39a39n lihza ul39l39ac linary I39IEFH39E EllEH1 I i Dllactary ill name all lmllh39l Ell illli lll39 q I a Z i 39 il l 1139 7 v quot L i I Aetian lfaetary pmant am reeeptar eell a l 1 i 7 l l EUWM v 39ll 7 pl DE Fl lE JM 39 m Receptor I If patential Gillie w if g Hrquot ERR lama r irl V s fgfum 23 an in ff mlv 7 ill 539 fl g9 l Ddarant 1 3 A350 IJiLWQllWI I39II739 1 39IRJ39JLI IE Central Olfactory Pathways The input layer of each olfactory bulb contains about 2000 spherical structures called glumeruli each about 50200 mm in diameter Within each the endings of about 25000 primary olfactory axons converge and terminate on the dendrites of about 100 secondorder olfactory neurons Each glomerulus receives input from only receptor cells of one particular type The output axons of the olfactory bulbs course through the olfactory tracts and project directly to several targets including the olfactory cortex of the cerebral cortex and some of its neighboring structures in the temporal lobes O The olfactory arrangement is unusually direct and produces a widespread influence on the parts of the forebrain that have roles in odor discrimination emotion motivation and certain types of memory Conscious perceptions of smell may be mediated by a path from the olfactory tubercle to the medial dorsal nucleus of the thalamus to the orbitofrontal cortex behind the eyes Spatial and Temporal Representations of Olfactory Information The brain can differentiate different chemicals due to the fact that each odor is represented by the activity of a large population of neurons the neurons responsive to particular odors may be organized into spatial maps and the timing of action potentials Olfactory Population Coding By looking at the combination of responses from cells the brain can distinguish scents from one another Olfactory Maps A sensory map is an orderly arrangement of neurons that correlates with certain features of the environment This is evident in the fact that neurons in a specific place in an olfactory bulb responds to particular odors O The smell of a particular chemical is converted into a specific map within the neural space of Chapter 8The Chemical Senses Page 14 the bulbs and the form of the map depends on the nature and concentration of the odorant Temporal Coding in the Olfactory System Temporal patterns of spiking are essential features of olfactory coding Odor information is encoded by the number temporal pattern rhythmicity and cetoce synchrony of spikes Chapter 8The Chemical Senses Page 15 Introduction Tuesday March 3 2 15 348 PM At the back of the eye is the retina which contains photoreceptors specialized to convert light energy into neural activity 0 Each eye has two overlapping retinas one specialized for low light levels and another specialized for higher light levels and for the detection of color 0 The retina is specialized to detect differences in the intensity of light falling on different parts of it Axons of retinal neurons are bundled into optic nerves which distribute visual information to several brain structures that perform different functions 0 The first synaptic relay in the pathway that serves visual perception occurs in the lateral geniculate nucleus LGN in the dorsal thalamus From here visual information ascends to the cerebral cortex where it is interpreted and remembered Chapter 9The Eye Page 16 Properties of Light Tuesday March 3 2 15 42 PM Light Light is the electromagnetic radiation that is visible to our eyes It has a wavelength frequency and amplitude The energy content is proportional to its frequency high frequency amp short wavelength has highest energy content 0 Visible light consists of wavelengths of 400700 nm Cool colors have higher energy shorter wavelength Optics Images are formed in the eye by refraction which is the bending of light rays that can occur when they travel from one transparent medium to another due to the speed of light differing in the two media 0 The greater the difference between the speed of light in the two media the greater the angle of refraction Chapter 9The Eye Page 17 The Structure of the Eye Tuesday March 3 2 15 419 PM Gross Anatomy of the Eye The pupil is the opening that allows light to enter the eye and reach the retina It occurs dark because of the lightabsorbing pigments in the retina The pupil is surrounded by the E whose pigmentation provides what we call the eye39s color contains two muscles that vary the size of the pupil The cornea covers the pupil and the iris with a glassy transparent surface The cornea is continuous with the scl the white of the eye which forms the tough wall of the eyebaH 0 There are three pairs of extraocular muscles in the sclera which move the eyeball in its socket also called the eye39s orbit O The coniectiva is in front of these muscles and is a membrane that folds back from the inside of the eyelids and attaches to the sclera The optic nerve carries axons from the retina and exists the back of the eye passes through the orbit and reaches the base of the brain near the pituitary gland Ophthalmoscopic Appearance of the Eye The retinal vessels originate from the optic disk which is also where the optic nerve fibers exit the retina O The sensation of light cannot occur here due to the lack of photo receptors nor can it occur where the large blood vessels exist because the vessels cast shadows on the retina In the middle of each retina is a darkercolored region called the macula which is responsible for central vision 0 It is yellowish in hue and relatively absent of large blood vessels The m of the retina is a dark spot about 2 mm in diameter in the center of the retina where it is thinnest 0 Above the retina is superior while below is inferior CrossSectional Anatomy of the Eye The cornea does not contain blood vessels and is nourished by the fluid behind it the agueous humor Behind the iris is a transparent suspended by ligaments zonule fibers attached to the ciliary muscles which form a ring attached to the sclera The lens divides the interior of the eye into two compartments containing slightly different fluids the aqueous humor between the cornea and the lens and the jellylike vitreous humor between the lens and the retina O The pressure of the vitreous humor keeps the eyeball spherical Chapter 9The Eye Page 18 Image Formation by the Eye Tuesday March 03 2015 446 PM Refraction by the Cornea Light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye while those that enter the center of the eye pass straight to the retina The distance from the refractive surface to the point where parallel light rays converge is called the focal distance which depends on the curvature of the cornea the tighter the curve the shorter the focal distance 0 The reciprocal of the focal distance in meters is a unit of measurement called the diopter O The cornea has a refractive power of about 42 diopters parallel light rays striking the corneal surface will be focused 0024 m behind it about the distance from cornea to retina Accommodation by the Lens The lens contributes another dozen or so diopters to the formation of a sharp image at a distant point as well as objects located closer than about 9 m from the eye Accommodation is provided by the lens changing shape in order to provide focusing power 0 During accommodation the ciliary muscles contracts and swells in size making the area inside the muscle smaller and decreasing the tension in the suspensory ligaments causing the lens to become rounder and thicker thus increasing its refractive power The Pupillary Light Reflex The pupillary light reflex involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils both Constriction of the pupil causes an increase in the depth of focus The Visual Field There is a limit to our visual field Visual Acuity The ability of the eye to distinguish two nearby points is called visual acuity which depends on several factors especially the spacing of photoreceptors in the retina and the precision of the eye39s refraction Distance across the retina is described in terms of degrees of visual angle 2020 vision is when you can recognize a letter that subtends an angle of 0083 Chapter 9The Eye Page 19 Microscopic Anatomy of the Retina Tuesday March 03 2015 513 PM The most direct pathway for visual information to exit the eye is from photoreceptors to bipolar cells to ganglion cells which fire action potentials in response to light These impulses propagate down the optic nerve to the rest of the brain Retinal processing is influenced by horizontal cells which receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors and amacrine cells which receive input from bipolar cells and project laterally to influence surrounding ganglion cells bipolar cells and other amacrine cells The only lightsensitive cells in the retina are the photoreceptors all other cells are influenced directly or indirectly by synaptic interactions The ganglion cells are the only source of output from the retina The Laminar Organization of the Retina The retina is organized in layers from the vitreous humor through the ganglion cells and bipolar cells before reaching the photoreceptors o Backwards arrangement allows for maintenance of photoreceptors and photopigments by the pigmented epithelium which also absorbs light that passes entirely through the retina minimizing the reflection of light that would blur the image The cell layers are named in reference to the middle of the eyeball The innermost layer is the ganglion cell layer the next is the inner nuclear layer contains the cell bodies of the bipolar cells the horizontal and amacrine cells next is the outer nuclear later which contains the cell bodies of the photoreceptors The final layer the layer of photoreceptor outer segments contains the lightsensitive elements of the retina which are embedded in the pigmented epithelium Photoreceptor Structure Relative response Every photoreceptor has four regions an outer segment an inner segment a cell body and a synaptic terminal 0 The outer segment contains a stack of membranous disks that contain lightsensitive photopigments which absorb light triggering changes in the photoreceptor membrane potential Rod photoreceptors have a long cylindrical outer segment containing many disks making them over 1000 times more sensitive to light than cones Cone photoreceptors have a shorter tapering outer segment with fewer membranous disks responsible for our ability to see color All rods contain the same photopigment but there are three types of cone each with a different pigment Fugurc 9 20 The spectre sensitivity at the three typos ol cono pigments 430 530 560 Blue cones Red cones Green cones Chapter 9The Eye Page 20 400 450 500 550 600 650 Wavelength nm Regional Differences in Retinal Structure The peripheral retina generally has a higher ratio of rods to cones and a higher ratio of photoreceptors to ganglion cells causing it to be more sensitive to light Good visual acuity requires a low ratio of photoreceptors to ganglion cells which is exemplified in the fovea a thinning of the retina at the center of the macula which contains only cones Chapter 9The Eye Page 21 Phototransduction Tuesday March 3 2 15 736 PM Photoreceptors convert light energy into changes in membrane potential Rods outnumber cones in the human retina by 20 to 1 Phototransduction in Rods In the photoreceptor light stimulation of the photopigment activates Gproteins which activate and effector enzyme that changes the cytoplasmic concentration of a second messenger molecule This causes a membrane ion channel to close altering the membrane potential The membrane potential of the rod outer segment is about 30 mV and depolarization is caused by the steady influx of Na through special channels in the outer segment membrane 0 The movement of positive charge across the membrane which occurs in the dark is called the dark current 0 Sodium channels are stimulated to open by cyclic guanosine monophosphate cGMP cGMP is continually produced in the photoreceptor by guanylyl cyclase keeping the Na channels open 0 Light reduces cGMP causing the Na channels to close and the membrane becomes more negative hyperpolarizing Cytoplasm lt2 Outer segment DlSkS Plasma membrane C J Dark current Inner segment Synaptic terminal Hyperpolarizing is initiated by the absorption of electromagnetic radiation by the photopigment of the stacked disks in the rod outer segments called rhodopsin O Rhodopsin39s receptor protein is called 9 and it has the seven transmembrane alpha helices typical of Gproteincoupled receptors while its prebound agonist is called retinal a derivative of vitamin A O The absorption of light causes a conformational change in retinal so that it activates the opsin called bleaching because it changes the wavelengths absorbed by the rhodopsin which stimulates a Gprotein called transducin in the disk membrane which in turn activates phosphodiesterase PDE which breaks down the cGMP that is normally present in the cytoplasm of the rod in the dark This causes Na channels to close and the membrane to hyperpolarize Phototransduction in Cones n bright sunlight cGMP levels in rods falls to the point where the response to light becomes saturated so that additional light causes no more hyperpolarization The cones in our retinas contain one of three opsins that give the photopigments different spectral sensitivities 0 Blue cones are maximally activated by light with a wavelength of about 430 nm 0 Green cones are maximally activated by light with a wavelength of about 530 nm Chapter 9The Eye Page 22 0 Red cones are maximally activated by light with a wavelength of about 560 nm Color Detection The YoungHelmholtz trichromacy theory says that the brain assigns colors based on a comparison of the readout of the three cone types When all types of cones are equally active we perceive white The peak sensitivity of the rods is to a wavelength of about 500 nm perceived as bluegreen Dark and Light Adaptation It takes about 2025 minutes to go from allcone daytime vision to allrod nighttime vision called dark adaptation The diameter of the human pupil ranges from about 28 mm Dark adaptation is primarily due to the regeneration of unbleached rhodopsin and an adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell Light adaptation takes about 510 minutes Calcium 395 Role in Light Adaptation The ability of the eye to adapt to changes in light level relies on changes in calcium concentration within the cones When first stepping into bright light from darkness the cones are maximally hyperpolarized 0 When the cGMPgated channels close the flow of Ca2 into the photoreceptor an inhibitory action is stopped and more cGMP is synthesized allowing the gates to open again 0 Also potentially affects photopigments phosphodiesterase in ways that decrease their response to light Chapter 9The Eye Page 23 Retinal Processing Tuesday March 3 2 15 756 PM Only ganglion cells fire action potentials all other cells in the retina except amacrine cells respond to with graded changes in membrane potential built from the interactions of horizontal and bipolar cells At each synaptic relay the responses are modified by the lateral connections of horizontal cells and amacrine cells Transformations in the Outer Plexiform Layer Photoreceptors release the neurotransmitter glutamate release less in the light than in the dark In the outer plexiform later each photoreceptor is in synaptic contact with two types of retinal neuron bipolar cells and horizontal cells Bipolar Cell Receptive Fields n OFF bipolar cells glutamategated cation channels mediate a classical depolarizing EPSP from the influx of Na n ON bipolar cells there are Gproteincoupled receptors that respond to glutamate by hyperpolarizing Each bipolar cell receives direct synaptic input from a cluster of photoreceptors from one at the center of the fovea to thousands in the peripheral retina Bipolar cells are also connected via horizontal cells to a circumscribed ring of photoreceptors that surrounds this central cluster The receptive field of a bipolar cell is the area of the retina that when stimulated with light changes the cell39s membrane potential and consists of a circular area of retina providing direct photoreceptor input the receptive field center and a surrounding area of retina providing input via horizontal cells called the receptive field surround O The response of a bipolar cell39s membrane potential to light in the receptive field center is opposite to that of light in the surround Depolarization of the bipolar cell due to illumination at the center versus illumination of the surround causing an antagonistic hyperpolarization Said to have antagonistic centersurround receptive fields One millimeter on the retina corresponds to a visual angle of about 35 Bipolar cell receptive field diameters range from a fraction of a degree in the central retina to several degrees in the peripheral retina Chapter 9The Eye Page 24 Retinal Output Wednesday March O4 ZOlS 554 PM The sole source of output from the retina to the rest of the brain is the action potentials arising from the ganglion cells Ganglion Cell Receptive Fields Most retinal ganglion cells have the same concentric centersurround receptive field organization as bipolar cells ONcenter and OFFcenter ganglion cells receive input from the corresponding type of bipolar cell In both types of cell the response to stimulation of the center is canceled by the response to stimulation of the surround causing most retinal ganglion cells to be not particularly responsive to changes in illumination that include both the receptive field center and the receptive field surround O Ganglion cells are mainly responsive to differences in illumination that occur within their receptive fields The center surround organization of the receptive fields leads to a neural response that emphasizes the contrast at lightdark edges Types of Ganglion Cells In the macaque monkey retina and human retina two major types of ganglion cells are distinguished 0 Large Mtype ganglion cells and smaller Ptype ganglion cells 0 P cells constitute about 90 of the ganglion cell population while M cells constitute about 5 and the remaining 5 is made up of a variety of nonMnonP ganglion cell types that are less characterized O M cells have larger receptive fields conduct action potentials more rapidly in the optic nerve and are more sensitive to lowcontrast stimuli They also respond to stimulation of their receptive field centers with a transient burst of action potentials while P cells respond with a sustained discharge as long as the stimulus is on ColorOpponent Ganglion Cells Some P cells and nonMnonP cells are sensitive to differences in the wavelength of light the majority of which are called coloropponent cells due to the fact that the response to one wavelength in the receptive field center is canceled by showing another wavelength in the receptive field surround red versus green and blue versus yellow Perceived color is based on the relative activity of ganglion cells whose receptive field centers receive input from red green and blue cones Parallel Processing In the central visual system the parallel streams of information provided by both eyes give information about depth as well as independent streams of information about light and dark that arise from the ONcenter and OFFcenter ganglion cells in each retina Also ganglion cells of both ON and OFF varieties have different types of receptive fields and response properties 0 M cells can detect subtle contrasts over their large receptive fields and are likely to contribute to lowresolution vision P cells have small receptive fields that are well suited from the discrimination of fine detail Chapter 9The Eye Page 25 Introduction Wednesday March 04 2015 615 PM The vestibular system informs the nervous system where our head and body are and how they are moving which is used to control muscular contractions Endolymph Hair bundle Chapter 11The Auditory and Vestibular Systems Page 26 The Nature of Sound Wednesday March 04 2015 620 PM Sounds are audible variations in air pressure Many sources are sound produce variation in pressure that are periodic with a frequency Our auditory system can respond to pressure waves over the range of 20 Hz to 20000 Hz A sound39s tone or m is determined by the frequency The intensity which determines the loudness we perceive of a sound wave is the difference in pressure between compressed and rarefied patches of air Chapter 11The Auditory and Vestibular Systems Page 27 The Structure of the Auditory System Wednesday March O4 ZOlS 623 PM The visual portion of the ear consists of cartilage covered by skin forming a funnel called the m which helps collect sounds from a wide area Its shape makes us more sensitive to sounds coming from ahead than from behind O The convolutions within play a role in localizing sounds The entrance to the internal ear is called the auditory canal and extends about 25 cm inside the skull before it ends at the tympanic membrane also called the eardrum 0 Connected to the medial surface of this membrane is a series of bones called ossicles located within a small airfilled chamber allowing them to transfer movements of the tympanic membrane into movements of a second membrane covering a hole in the bone of the skull called the oval window 0 Behind the oval window is the fluidfilled cochlea which contains the apparatus for transforming the physical motion of the oval window membrane into a neuronal response The basic auditory pathway looks like 0 Sound wave moves the tympanic membrane gt tympanic membrane moves the ossicles gt ossicles move the membrane at the oval window gt motion at the oval window moves fluid in the cochlea gt movement of fluid in cochlea causes a response in sensory neurons The ear is referred to as having three main divisions O The structures from the pinna to the tympanic membrane make up the outer ear the tympanic membrane and the ossicles constitute the middle ear and the apparatus medial to the oval window is the inner ear A signal generated in the inner ear is transferred to and processed by a series of nuclei in the brain stem Output is then sent to a relay in the thalamus called the medial geniculate nucleus MGN which then projects to primary auditory cortex A1 located in the temporal lobe Figure 11 3 The outer muddle and mner ear Middle ear Outer Inner ear ear Ossicles Oval window Cochlea I AuditOry Tympanic canal membrane Pinna O ZOOI Lippmcon Willam I Wimnt Chapter 11The Auditory and Vestibular Systems Page 28 The Middle Ear Wednesday March CD4 2 15 639 PM In the middle ear variations in air pressure are converted into movements of the ossicles Components of the Middle Ear The tympanic membrane is somewhat conical in shape with the point of the cone extending into the cavity of the middle ear There are three ossicles O The ossicle attached to the tympanic membrane is the malleus which forms a rigid connection with the incus which forms a flexible connection with the stapes The flat bottom portion of the stapes the footplate moves in and out like a piston at the oval window transmitting sound vibrations to the fluids of the cochlea in the inner ear The air in the middle ear is continuous with the air in the nasal cavities via the Eustachian tube which is closed by a valve Sound Force Amplification by the Ossicles The ossicles provide amplification in pressure to the fluid filled cochlea The middle ear increases pressure at the oval window by altering both the force and the surface area The ossicles act like levers and the surface area of the oval window is much smaller than that of the tympanic membrane 0 The pressure at the oval window is about 20 times greater than at the tympanic membrane The Attenuation Reflex The tensor tympani muscle is anchored to bone in the cavity of the middle ear at one end and to the malleus at the other end The stapedius muscle extends from a fixed anchor of bone and attaches to the stapes When these two muscles contract the chain of ossicles becomes more rigid and sound conduction to the inner ear is greatly diminished The onset of a loud sound triggers a neural response that causes these muscles to contract the attenuation reflex which is much greater at low frequencies that at high frequencies Function of this reflex may be to adapt the ear to continuous sound at high intensities and to protect the inner ear from loud sounds that would otherwise damage it 0 Has a delay of 50100 msec from the time that sound reaches the ear 0 Tends to make highfrequency sounds easier to discern in an environment with a lot of low frequency noise which enables us to understand speech more easily in a noisy environment Chapter 11The Auditory and Vestibular Systems Page 29 Figure 116 Tho um and Inner at 801M ohpodius much and the tensor WM muscle on mismatch ot nmiddooumonoondandtothooukbsdthoothuond Ossicles Oval window Tensor tympani muscle Tympanic Stapedius Round Cochlea membrane muscle window Vestibular Tectorial Auditory membrane Spiral ganglion Round Scala K Inner t e to window hair cells f Outer 33511 a hair cells membrane O 2001 Sinauer Associates Inc Home 117 Chapter 11The Auditory and Vestibular Systems Page 30 Figure 1 17 The throo ocoloo at tho cochlea Viowod in cross action tho cochlea contains lhroo small porde chombonJhuo chambers tho seal on pound by Robson s matron and the booilor mmbromTho organ of Com contolns the auditory Won it sits upon the builar mombm and is count by tho tocloriol mombrono Scala vestibuli Reissner s membrane Scala media Tecton al membrane Stn a vascularis Organ of Com Scala tympam Basilar membrane Chapter 11The Auditory and Vestibular Systems Page 31 ThelnnerEar Wednesday March CD4 2 15 649 PM The coclea is part of both the auditory system and the labyrinth which is part of the vestibular system Anatomy of the Cochlea The cochlea has a spiral shape a hollow tube with walls made of bone about 32 mm long and 2 mm in diameter 0 The central pillar is a conical bony structure called the modiulus 0 At the base are the oval window which is below the footplate of the stapes and the round window The cochlea is divided into three fluidfilled chambers the scala vestibuli the scala media and the scala tympani They all wrap around inside like a spiral staircase O Reissner39s membrane separates the scala vestibuli from the scala media and the basilar membrane separates the scala tympani from the scala media 0 On the basilar membrane is the organ of Corti which contains auditory receptor neurons Hanging above this is the tectorial membrane At the apex of the cochlea the scala media is closed off and the scala tympani becomes continuous with the scala vestibuli at a hole in the membranes called the helicotrema At the base of the cochlea the scala vestibuli meets the oval window and the scala tympani meets the round window The fluid in the scala vestibuli and scala tympani is called perilymph and has 7 mM K and 140 mM Na The scala media is filled with endolymph which has ionic concentrations similar to intracellular fluid 150 mM K and 1 mM Na O The difference in ion content is generated by active transport at the stria vascularis the endothelium lining one wall of the scala media The stria vascularis reabsorbs sodium and secretes potassium against their concentration gradients O The endolymph has an electricla potential about 80 mV more positive than that of the perilymph called the endochlear potential Physiology of the Cochlea Inward motion at the oval window pushes perilymph into the scala vestibuli and is accompanied by a complementary motion at the round window The Response of the Basilar Membrane to Sound The basilar membrane is wider at the apex than at the base by a factor of about 5 and the stiffness of the membrane decreases from base to apex the base about 100 times stiffer The movement of the endolymph makes the basilar membrane bend near its base starting a wave that propagates toward the apex 0 If the frequency is high the stiffer base of the membrane will vibrate greatly dissipating most of the energy causing the wave to not propagate very far The Organ of Corti and Associated Structures The auditory receptor cells convert mechanical energy into a change in membrane polarization They are located in the organ of Corti which consists of hair cells the rods of Corti and various supporting cells Auditory receptors are called hair cells because each has about 100 hairylooking stereocilia extending from its top The bending of these cilia is responsible in the transduction of sound into a neural signal Hair cells are in between the basilar membrane and a thin sheet of tissue called the reticular lamina The rods of Corti span these two membranes and provide structural support 0 Hair cells between the modiolus and the rods of Corti are called inner hair cell and cells Chapter 11The Auditory and Vestibular Systems Page 32 farther out than the rods of Corti are called outer hair cells about 1500020000 in three rows 0 The stereocilia at the tops of the air cells extend above the reticular lamina into the endolymph and their tips end either in the gelatinous substance of the tectorial membrane outer orjust below the tectorial membrane inner Hair cells form synapses on neurons whose cell bodies are located in the spiral ganglion within the modiolus They are bipolar with neurites extending to the bases and sides of the hair cells where they receive synaptic input 0 Axons from spiral ganglion enter the auditory nerve a branch of the auditoryvestibular m cranial nerve VIII which projects to the cochlear nuclei in the medulla Transduction by Hair Cells When the basilar membrane moves in response to a motion at the stapes the entire foundation supporting the hair cells moves due to the rigid connection between the basilar membrane rods of Corti reticular lamina and hair cells Actin filaments aligned on stereocilia cause them to be rigid rods that bend only at the base where they attach to the top of the hair cell Crosslink filaments make the sterocilia stick to one another so all the cilia move as a unit When the stereocilia bend in one direction the hair cell depolarizes and when they bend in the other direction the cell hyperpolarizes The receptor potential of a hair cell is saturated by the time the tips of its stereocilia have moved about 20 nm to the side The softest noise you can hear moves the stereocilia only 03 nm to each side There is a special type of cation channel the TRPAl channel on the tips of the stereocilia that are induced to open and close by the bending of stereocilia generating changes in the hair cell receptor potential 0 Each channel is connected by an elastic filament called a tip link to the wall of the adjacent cilium When straight the tension on the tip link holds the channel in a partially opened state allowing a small leak of K from the endolymph into the hair cell Displacement in one direction increases the inward K current Displacement in the opposite directing prevents inward K movement 0 The entry of K into the hair cell causes depolarization which activates voltagegated calcium channels Ca2 entry causes the release of neurotransmitters probably glutamate which activates the spiral ganglion fibers The Innervation of Hair Cells The auditory nerve consists of the axons of neurons whose cell bodies are located in the spiral gangHon The number of neurons in the spiral ganglion is estimated to be in the range of 35000 to 50000 More than 95 of the spiral ganglion neurons communicate with the relatively small number of inner hair cells and less than 5 receive synaptic input from the outer hair cells 0 One spiral ganglion fiber receives input from only one inner hair cell While each inner hair cell feeds about 10 spiral ganglion neurites One spiral ganglion fiber synapses with numerous outer hair cells Amplification by Outer Hair Cells Outer hair cells seem to act like tiny motors that amplify the movement of the basilar membrane during lowintensity sound stimuli referred to as the cochlear amplifier 0 Motor proteins found in the membranes of outer hair cells can change the length of outer hair cells They are driven by the receptor potential rather than an ATP energy source and are extremely fast The protein is called prestin which is tightly packed into the membranes of the outer hair cells 0 When motor proteins change the length of the hair cell the basilar membrane is pulled toward or pushed away from the reticular lamina and tectorial membrane When outer hair cells amplify the response of the basilar membrane the stereocilia on the inner hair cells will bend more and the increased transduction process in the inner hair cells will produce Chapter 11The Auditory and Vestibular Systems Page 33 a greater response in the auditory nerve There are about 1000 efferent fivers projecting from the brain stem toward the cochlea diverging widely and synapsing onto outer hair cells releasing acetylcholine Stimulation of these changes the shape of the outer hair cells affecting the responses of inner hair cells Figurett8 The basilar membrane in an uncoiled cochlea Although the cochlea narrows from base to apex the basilar membrane widens toward the apex The helicotrema is a hole at the apex of the basilar membrane which connects the scala veetibuli and scale tympani quot L3 7 I Uncoiled cochlea Oval window 39 E 7 quot 39 Iil39 x i Helicotrema Stapes quot 1 quot Q 39 Scala vestibuli in Basilar membrane Round window Scala tympani D 2001 Lupoincolt W llxams 8 W ik l S Figure 119 A traveling wave in the basilar membrane As the stapes moves in and out it causes endotymph to flow as shown by the arrows This generates a traveling wave in the basilar membrane l he size at the wave is magni ed about 1 million times in this illustration At this hequency 3000 Hz the fluid and membrane movement end abruptly about haltway between the base and apex Source Adapted from Noblll ammano and Ashmore 199e Fig 1 Endolymph Stapes Basilar membrane f t E g 3quot Round M window Helicotrema 639 331 remit391 warns i Jams Chapter 11The Auditory and Vestibular Systems Page 34 Figure 1110 Tbs mponss 01 lbs bssusr msmlnsns lo sound Tbs cochlss ls sgsln shown uncollsd s HIghtrsqusncy sound produces s traveling wsvs which disslpstos near 1110 nsrrow snd um bsso of tho bssllsr membrane b Low1rsqusncy sound producos s wsvs 111st Wes all the way 10 the sport 011110 bulls nnmbrsno bs1oro dlsslpstlng The bonding ol the bssllsr membrane 1 yrsst exaggerde 1or1ho purpose 01 mutation c Thou 1s s plsos code on the bullet mombrsno tor the troqusncy 111st produces the maximum amplitude de ection Frequency producing High frequency Low frequency maximum amplitude Apex wide A E and oppy 500 Hz 7 5 1 kHz Basilar 4 membrane 5 2 kHz 5 4 kHz f 8 kHz Base narrow and stiff a b 0 Chapter 11The Auditory and Vestibular Systems Page 35 Figure 1112 The organ oi Cortl The basiler membrane supports tissue that includes the inner and outer hair cells and the stitt rods 0 Cortl The tectorial membrane extends lrom the bony modlolus to cover the stereocllia that protrude from the tops oi the hair cells Tectorial membrane Stereocilia Reticular lamina Outer hair cell Spiral ganglion Basilar Rods of inner hair membrane Corti cell t O 2001 Liwinoolt Wlliarns a Wllins Auditory nerve Chapter 11The Auditory and Vestibular Systems Page 36 Modiolus Central Auditory Processes Wednesday March 145 2 15 729 PM The Anatomy of Auditory Pathways At the level of the medulla the axons innervate the dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea where the axons originated Each axon branches so that it synapses on neurons in both cochlear nuclei From cochlear nuclei to auditory cortex cells in the ventral cochlear nucleus send out axons that project to the superior olive on both sides of the brain stem Axons of these neurons ascend in the lateral lemniscus and innervate the inferior colliculus of the midbrain O The dorsal path bypasses the superior olive 0 All ascending auditory pathways converge onto the inferior colliculus no matter their path with intermediate relays O The neurons in the inferior colliculus send out axons to the medial geniculate nucleus MGN of the thalamus which projects to auditory cortex Projections and brain stem nuclei other than the ones described contribute to the auditory pathways There is also extensive feedback in the auditory pathways Also each cochlear nucleus receives input from just the one ear on the ipsilateral side all other auditory nuclei in the brains tem receive input from both ears Response Properties of Neurons in the Auditory Pathway Most spiral ganglion cells fire action potentials only in response to sound within a limited frequency range Each portion of the basilar membrane is maximally sensitive to a particular range of frequencies In the superior olive cells receive input from cochlear nuclei on both sides of the brain stem Chapter 11The Auditory and Vestibular Systems Page 37 Encoding Sound Intensity and Frequency Wednesday March CD4 2 15 741 PM Stimulus Intensity Information about sound intensity is coded in the firing rates of neurons and the number of active neurons The more intense a stimulus is the greater the vibration of the basilar membrane causing the membrane potential of the activated hair cells to be more polarized or hyperpolarized causing the nerve fibers with which the hair cells synapse to fire action potentials at greater rates 0 More intense stimuli produce movements of the basilar membrane over a greater distance which leads to the activation of more hair cells causing a broadening of the frequency range to which the fiver responds O The loudness we perceive is thought to be correlated with the number of active neurons in the auditory nerve and their firing rates Stimulus Frequency Tonotopy and Phase Locking Tonotopy Frequency sensitivity is primarily due to the mechanics of the basilar membrane Auditory nerve fibers connected to hair cells near the apical basilar membrane have low characteristic frequencies and those connected to hair cells near the basal basilar membrane have high characteristic frequencies There is a map of the basilar membrane within the cochlear nuclei called tonotopy analogous to retinotopy in the visual system The location of active neurons in auditory nuclei is one indication of the frequency of the sound but frequency must be coded in another way due to the fact that these maps do not contain neurons with low characteristic frequencies below 200 Hz and because the region of the basilar membrane is maximally displaced due to intensity as well as frequency Phase Locking The frequency of neural firing provides a lot of information about sound frequency Phase locking is the consistent firing of a cell at the same phase of a sound wave The volley principle is the idea that intermediate sound frequencies are represented by the pooled activity of a number of neurons each of which fires in a phase locked manner Occurs with sound waves up to about 4 kHz Chapter 11The Auditory and Vestibular Systems Page 38 Soundinduced vibration 7 1 Jr Ll Resting position 0 2001 Sinauer Associates Inc Chapter 11The Auditory and Vestibular Systems Page 39 gure 13 The electrical reepenee el e helr eell end itsquot eeneentr t39l tl t r tel detleetiene ef the bundle et ere The hair eell predueee graded eherrgee in the rnrnhrene petentie l s n reeeense te metrernent at the ellie ehel mettee en etrettetew extrnereee ente the preeeee ell the senetrhrr neuren get When the tiller hurtrile i5 dellerected tewerd the lengeet eillum te the right in the rellegrerttr the hair cell rile pelerlzes errel the erteijtatien et the semen neuren inereeeee B When the tiller bundle ie de tleeteel ewetr trem the lengeet eillem te the left the heir cell htrpereelarieee lees eteltetere transmitter is releeeeria enel the eeheew neurert r ee eetlen petentiele less frequently Lengeet at eilhrm A Directierl equot nmtrerrnerrt I F 1 39htletremernt Eunel le ef eille It l heir eell 39 39 39 De eezien V IDWEJ rI l l l lg Em E J i l I l I J l l eilium SENS3931quot l z 7 7 r ne39ererr eEW1e Etteitetten ef it Time Semengr neuren E I ll 39h39 l l ll A Meeement m quot et39leetiert r 39 ewey hem V Preeeee If lengeet eitium quot Em ef eerreery g i heir eell llllf n w l l l l l quotI F eeeeery A 7 t TD neuren e u a IT M J lnhthttten ef Time ISEI39IJEDW newer EKEi L ill ijquot Synapse leg frent heir cell te Reneer neetert 39 newene eyetem Chapter 11The Auditory and Vestibular Systems Page 40 A Resting sale C De ection away from longest cilium All channels closed hyperpoluizadon Chapter 11The Auditory and Vestibular Systems Page 41 A O 2001 Sinauer Associates Inc B O 2001 Sinauer Associates Inc Chapter 11The Auditory and Vestibular Systems Page 42 Mechanisms of Sound Localization Wednesday March 04 2015 754 PM Localization of Sound in the Horizontal Plane There is a relationship between location and interaural delay which is detected by specialized neurons in the brain stem Allows us to discriminate the direction of a sound source in the horizontal plan within 2 Continuous tones present more of a problem for sound localization In this situation the time at which the same phase of the sound wave reaches each ear is compared At high frequencies the brain uses the interaural intensity difference due to the sound shadow cast by your head With sounds in the range of 20 to 2000 Hz the process involves interaural time delay and from 2000 to 20000 Hz interaural intensity difference is used This is the duplex theory of sound localization The Sensitivity of Binaural Neurons to Sound Location All neurons in the cochlear nuclei are monaural neurons meaning that they only respond to sound presented to one ear At later stages of processing in the auditory system there are binaural neurons whose responses are influenced by sound at both ears In the superior olive neurons receive input from cochlear nuclei on both sides of the brain stem These cells typically have responses phase locked to lowerfrequency sound input 0 Olivary neurons receiving spikes from left and right cochlear nuclei can compute interaural time delay 0 These axons use delay lines and measure small time differences precisely 0 Many synapses of the auditory system are specially adapted for rapid operation their action potentials and EPSPs are much faster than those of most other neurons in the brain 0 Neurons in the superior olive are sensitive to the other sound location cue as well Localization of Sound in the Vertical Plane The curves of the outer ear are responsible for assessing the elevation of a source of sound The bumps and ridges produce reflections of the entering sound and the delays between the direct path and the reflected path change as a sound source move vertically The outer ear also allows higherfrequency sounds to enter the auditory canal more effectively when they come from an elevated source Owls use the same techniques that we use for horizontal localizing for vertical localizing because their ears are at different heights on their heads Chapter 11The Auditory and Vestibular Systems Page 43 Auditory Cortex Wednesday March C34 2 15 82 5 PM Axons leaving the MGN project to auditory cortex via the internal capsule in an array called the acoustic radiation Primary cortex A1 corresponds to Brodmann39s area 41 in the temporal lobe 0 Layer one contains few cell bodies and layers II and III contain mostly small pyramidal cells Layer IV where the medial geniculate axons terminate is composed of densely packed granule cells Ayers V and VI contain mostly pyramidal cells that tend to be larger than those in the superficial layers Neuronal Response Properties In tonotopic representations of A1 low frequencies are represented caudally and medially Roughly speaking there are isofrequencv bands running mediolaterally across A1 strips of neurons running across A1 contain neurons that have fairly similar characteristic frequencies Some neurons are intensity tuned giving a peak response to a particular sound intensity Some neurons are barely tuned at all Columns of cells in the auditory cortex have similar binaural interactions Other cortical area located on the superior surface of the temporal lobe respond to auditory stimuli Som are tonotopically organized while others are not The Effects of Auditory Cortical Lesions and Ablation Bilateral ablation of auditory cortex leads to deafness but deafness is more often the consequence of damage to the ears There is greater preservation of function after lesions in the auditory cortex than the visual system because both ears send output to cortex in both hemispheres O In humans the primary deficit that results from a unilateral loss of A1 is the inability to localize the source of a sound Smaller lesions can produce rather specific localization deficits Chapter 11The Auditory and Vestibular Systems Page 44 The Vestibular System Sunday March 08 2015 1212 PM The vestibular system monitors the position and movement of the head gives a sense of balance and equilibrium and helps coordinate movements of the head and eyes as well as adjustments to body posture The Vestibular Labyrinth The organs of mammalian balance and hearing both evolved from the lateral line organs which are small puts or tubes along an animal39s sides each with clusters of hairlike sensory cells whose cilia project into a gelatinous substance that is open to the water in which the animal swims O The purpose is to sense bibrations or pressure changes in the water also sometimes sensitive to temperature or electrical fields In mammals all hair cells are contained within sets of interconnected chambers called the vestibular labyrinth It includes two types of structures with different functions 0 The otolith organs detect the force of gravity and tilts of the head They are a pair of relatively large chambers called the saccule and the utricle near the center of the labyrinth O The semicircular canals are sensitive to head rotation They are three arcing structures of the labyrinth that lie in approximately orthogonal planes 0 Each structure transmits mechanical energy derived from head movement to its hair cells 0 There is a set on each side of the head and are mirror images of each other Each hair cell of the vestibular organs makes an excitatory synapse with the end of a sensory axon from the vestibular nerve a branch of the auditoryvestibular nerve cranial nerve VIII with about 20000 vestibular nerve axons on each side of the head Their cell bodies lie in Scarpa39s ganglion The Otolith Organs The saccule and utricle detect changes of head angles as well as linear acceleration of the head Each otolith organ contains a sensory epithelium called a macula which is vertically oriented within the saccule and horizontally oriented within the utricle when the head is upright The vestibular macula contains hair cells which lie among a bed of supporting cells with their cilia projecting into a gelatinous cap Movements are transduced by hair cells in the maculae when the hair bundles are deflected There are tiny crystals of calcium carbonate called otoliths that are 15 micrometers in diameter They encrust the surface of the macula39s gelatinous cap near the tips of the hair bundles and are the key to the tilt sensitivity of the macula They also have a higher density than the endolymph that surrounds them When the angle of the head changes or when the head accelerates a force is exerted on the otoliths which exerts a force in the same direction on the gelatinous cap which moves slightly and the cilia of the hair cells bend 0 Each hair cell has one especially tall cilium called the kinocilium The bending of hairs toward the kinocilium results in a depolarizing excitatory receptor potential Bending the hairs along the other direction of the kinocilium hyperpolarizes and inhibits the cell which is very direction selective The saccular maculae are oriented approximately vertically while the utricular maculae are mostly horizontal When a given head movement excites hair cells on one side of the head it will tend to inhibit hair cells in the corresponding location on the other The central nervous system by simultaneously using the information encoded by the full population of otolithic hair cells can interpret all possible linear movements The Semicircular Canals Can detect turning movements and angular acceleration The hair cells of the semicircular canals are clustered with a sheet of cells called the cri Chapter 11The Auditory and Vestibular Systems Page 45 locatedithin a bulge along the canal called the ampulla The cilia project into the gelatinous cupula which spans the lumen of the canal within the ampulla Bending of the cilia occurs when the canal is suddenly rotated about its axis like a wheel as the wall of the canal and the cupula begin to spin the endolymph tends to stay behind because of inertia While rotation excites hair cells of one canal it inhibits the hair cells of its contralateral partner canaL Vestibular axons fire at high rates even at rest so their activity can be driven either up or down depending on the direction of rotation Central Vestibular Pathways and Vestibular Reflexes Primary vestibular axons from cranial nerve V make direct connections to the vestibular nucleus on the same side of the brain stem as well as to the cerebellum 0 It also receives inputs from other parts of the brain including the cerebellum and the visual and somatic sensory systems The vestibular nucleus projects to a variety of targets above it on the brain stem and below it into the spinal cord For example axons from the otolith organs project to the lateral vestibular nucleus which then projects via the vestibulospinal tract to excite spinal motor neurons controlling muscles in the legs that help to maintain posture O Axons from the semicircular canals project to the medial vestibular nucleus which sends axons via the medial longitudinal fasciculus to excite motor neurons of trunk and neck muscles that orient the head The vestibular system makes connections to the thalamus and then to the neocortex The vestibular nuclei send axons into the ventral posterior nucleus of the thalamus which projects on regions close to the representation of the face in the primary somatosensory and primary motor areas of cortex The VestibuloOcular Reflex VOR Keeps your eyes pointed in a particular direction Each eye can be moved by a set of six extraocular muscles Because the VCR works by sensing rotations of the head it immediately commands a compensatory movement of the eyes in the opposite direction which keeps your line of sight tightly fixed on a visual target Axons from the left horizontal canal innervate the left vestibular nucleus which sends excitatory axons to the contralateral right cranial nerve V nucleus abducens nucleus Motor axons from the abducens nucleus in turn excite the lateral rectus muscle of the right eye 0 Another excitatory projection form the abducens crosses the midline back to the left side and ascends to excite the left cranial nerve nucleus which excites the right medial rectus muscle of the left eye 0 Speed is also maximized by activating inhibitory connections ot the muscles that oppose this movement Vestibular Pathology The vestibular system can be damaged in a variety of ways including toxicity of high doses of antibiotics and lesions to the vestibular labyrinths Chapter 11The Auditory and Vestibular Systems Page 46 Sensory Systems Thursday March 122 Z 15 344 PM Sensitivity of Receptors Vision rods 1 photon Hearing 02 micrometers Olfaction pheromones 1 molecule Taste 1 M sucrose There are other types of chemoreceptors in the sensory system other than olfactory and taste Cutaneous nociceptors sense irritating chemicals on the skin Muscle sensors burn during heavy exercise acidity Circulatory receptors such as oxygen and carbon dioxide receptors Digestive tract sensors act as receptors for various ingested substances Ta Ageusia is the loss of taste Dysgeusia is the inappropriate sensing of tastes Cutaneous receptors in the mouth also participate in sensing texture and temperature Gymnemic acid blocks sweet taste receptors Miraculin MCL is a glycoprotein dimer that is isolated from the red berries of the plant Richadella dulcifica 0 Has no taste at a neutral pH and elicits intense and persistent sensation of sweet when tongue is exposed to acidic pH Lecture Slide Notes Page 47