Organs of Special Senses PDF

Organs of special senses Photoreceptor & audio-receptor systems By Laila Ferrara  TheLocation eye The eye is highly specialized organ for vision & photoreception Permits an accurate analysis...

Organs of special senses Photoreceptor & audio-receptor systems By Laila Ferrara  TheLocation eye The eye is highly specialized organ for vision & photoreception Permits an accurate analysis of the form, intensity & color of light reflected from objects Location The human eye ball is about 24 mm in diameter It is located in the orbit of skull & protected anteriorly by eyelids extraocular muscles allow the eye to move within its orbit as a conjugate gaze for both eyes; the eyes move symmetrically (in the same direction at the same time)  Structure of the eye Each eyeball includes: 1. a tough, fibrous globe to maintain its shape 2. a system of transparent tissues that refract light to focus the image 3. a layer of photosensitive cells, & a system of neurons to collect, process, and transmit visual information to the brain  The eye contains 3 compartments Anterior chamber Contain aqueous humor Posterior chamber Contain Vitreous Vitreous space humor  Sclera Posterior 5/6 of the external layer Opaque & white , relatively avascular Consists of dense CT : Collagen type I bundles Ground substance Fibroblasts Episclera is the external surface of the sclera provides sites for extraocular muscle insertion Tenon’s capsule surrounds the eye ball Tenon’s space between episclera & tenon’s capsule ( rotating movement )  Cornea Anterior 1/6 of the external layer It is convex anteriorly ( dome shaped ) Colorless, transparent & sensitive Avascular , can be successively transplanted Function: Refract light onto the lens ( focusing ), due to its transparency & curvature Protection  Structure of the cornea Consists of 5 layers: ( 3 cellular layers separated by 2 a cellular layers) 1. Epithelium 2. Bowman’s membrane 3. Stroma (substantia propria) 4. Descemet’s membrane 5. Endothelium  1. Epithelium of the cornea Stratified squamous non keratinized Formed of 5-6 layers of cells The surface cells joined by tight junctions Cells of other layers connected by desmosomes Thick basement membrane for stability & protection against infection Basal cells responsible for regeneration & turn over ( 7 days )  Protective adaptations of corneal epithelium Surface cells provided by microvilli protruding in space filled with Precorneal tear film of lipids, glycoprotein & water which keep cornea wet & prevent ulceration Absorb oxygen & nutrients from the tear film Have rich sensory nerve supply Stimulation of nerves lead to blinking of eye lid & flowing of tears Surface cells connected by tight junction  2. Bowman’s membrane anterior limiting membrane The basement membrane of the stratified epithelium Consists of randomly arranged collagen I fibers Has no cells Contributes to stability & strength Can not be regenerated  3. Stroma (substantia propria) Thick layer ( 90 % ) of the cornea thickness Consists of parallel lamella of collagen fibrils Collagen fibers within each layer will run parallel to each other but at right angles to the fibers in the next layer Flattened fibroblasts ( keratocytes ) like the wings of butterfly between the bundles Ground substance rich in glycoproteins & chondroitin sulfate  4. Decemet’s membrane posterior limiting membrane The basement membrane of the endothelium Thick & homogenous Fine collagen ( VIII ) filaments arranged in three dimensional net work  5. Endothelium Simple squamous epithelium Connected by tight junction Possess characteristic of active transport & protein synthesis cells Have limited proliferative potential(can not regenerate) Function: to keep the cornea clear by pumping the excess fluid out of the stroma Forms Descemet’s membrane Endothelium function in maintaining the transparency of the cornea through the dehydration of stroma ( active transport of Na+ & passive transport of water & Cl- ) This provide maximum transparency and optimal light refraction  Transparency of the cornea Due to : 1. Avascular 2. Dehydration state ( low water content ) 3. Regular parallel arrangement of collagen fibrils ( destructive interference ) 4. Fibrils & ground substance have the same refractive index Clinical application The shape or curvature of the cornea can be changed surgically to improve certain visual abnormalities involving the ability to focus In laser-assisted in situ keratomileusis (LASIK) surgery, the stroma reshaped by an excimer laser which vaporizes collagen and keratocytes in a highly controlled manner with no damage to adjacent cells or ECM  Corneo-scleral junction ( limbus ) It is transition zone between the cornea & sclera Highly vascularized, supplies the cornea by diffusion Contains the Schlemm's canal which drains the aqueous humor from the anterior chamber to the venous system aqueous humor has an inorganic ion composition similar to that of plasma but contains less than 0.1% protein (plasma has about 7% protein)  Changes at the limbus Corneal epithelium continuous with the epithelium of bulbar conjunctiva Bowman's membrane terminates Corneal stromal collagen fibers loses its regularity Descemet's membrane and its simple endothelium are replaced with a system of irregular endothelium-lined channels called the trabecular meshwork which penetrate the stroma and allow slow, continuous drainage of aqueous humor from the anterior cavity Changes at the limbus  flow of aqueous humor Schlemm’s canal (scleral venous sinus ) in the stroma receives aqueous humor from the trabecular meshwork & drains into the aqueous and episcleral veins of the sclera ( venous system ) Clinical application Removal of aqueous humor is of major importance in regulating intraocular pressure Factors causing impaired aqueous removal lead to glaucoma, a condition in which elevated Aqueous humor is secreted into the posterior pressure affects proper function chamber, flows toward the lens, passing between it of the retina and vision and the iris to reach the anterior chamber through the pupil  Nutrition of the cornea Cornea receives its nutrients by diffusion from aqueous humor (for the central part) & scleral vessels (for the peripheral part) Cornea obtains oxygen directly from the air & tear film  2. Middle (vascular) layer the uveal tract (uvea) Consists of 3 parts, from posterior to anterior: 1. Choroid 2. Ciliary body 3. Iris  1. Choroid Highly vascular coat It is made up of 4 layers from outer to inner: 1. Suprachoroidal lamina which is Loose CT rich in melanocytes bound to the sclera 2. Two vascular layers (large & medium vessels) 3. Inner layer is Choriocapillary layer richer than the outer layer in small BVs which nourishes the retina 4. Bruch’s membrane ( hyaline membrane) separates the choriocapillary layer from retina  Bruch’s membrane is acellular & semipermeable Bruch’s membrane made up of 5 layers: 1. Basal lamina of choriocapillary endothelial cells 2. 2 layers of collagen fibers 3. Central layer of elastic fibers sandwiched between them 4. Basal lamina of retinal pigment epithelium The abundant melanocytes give the choroid its characteristic black color & block light from entering the eye except through the pupil Choroid histology  2. Ciliary body It is an anterior expansion (thickened ring ) of the choroid at the level of lens It is triangle in shape, the apex continuous with choroid & the base faces the iris Its long base contacting the sclera, another side in contact with the vitreous body, and the third facing the posterior chamber  Histological structure of ciliary body The bulk of ciliary body is formed of smooth muscles surrounded by stroma of loose connective tissue, rich in microvasculature, elastic fibers, and melanocytes In cross section it is composed of 4 layers from internal to external: 1. Double layered epithelium 2. Stroma 3. Ciliary muscles 4. Supraciliary layer = layer of collagen fibers separates it from the sclera  Structure of ciliary body The inner surface of the ciliary body covered by 2 layers of epithelium : 1. inner pigmented is simple columnar cells rich in melanin, this is directly adjacent to ciliary body (corresponds to the pigment layer of retina) 2. outer non-pigmented ) covers the first ; the surface layer facing the posterior chamber (derived from sensory layer of retina)  Ciliary processes Extensions of ciliary body Core of CT tissue & fenestrated capillaries Covered by the 2 layers of epithelium that cover the ciliary body Zonule fibers or suspensory ligament (fibrillin & elastin ) extend from these processes & insert into the capsule of the lens The lens is anchored within the lumen of the ciliary body by this circular system of zonular fibers  Ciliary muscles Arranged in 3 directions: 1. Meridional (longitudinal) fibers 2. Radial fibers 3. Circular fibers at the inner edge of ciliary body The ciliary muscles are important in visual accommodation  Visual accommodation At rest, or gazing at distant objects the lens is under tension from zonular ligament and so it is flattened On accommodation, the ciliary muscles contract in response to parasympathetic stimulation & the tension on the zonule fibers is released Once the tension on the lens is released it assumes more convex shape (thickened or more rounded) This is suitable for focusing closer objects This is accompanied by constriction of pupil  Function of ciliary body The non-pigmented outer layer of the ciliary processes form the aqueous humor Visual accommodation  Formation of aqueous humor Cells of the non-pigmented layer have tight junctions and extensive basal infoldings characteristic of ion- transporting cells, with Na+/K+-ATPase in their lateral plasma membranes These cells actively transport fluid from the vascular stroma into the posterior chamber, thus forming the aqueous humor Aqueous humor pathway Aqueous humor is secreted into the posterior chamber, flows toward the lens, passing between it and the iris to reach the anterior chamber through the pupil The tight junction between non pigmented cells of the ciliary processes forms blood aqueous barrier that limits free movement of molecules between the ciliary body stroma & the posterior chamber  3. Iris An extension of choroid partially covers the lens with central round opining the pupil It is thin pigmented contractile circular structure analogous to the diaphragm of the camera Divides the anterior compartment into anterior & posterior chambers  Consists of : Structure of the iris 1. Anterior discontinuous, rough surface covered by discontinuous layer of pigment cells & fibroblasts (NO epithelium) 2. Stroma of loose CT : the anterior part of the stroma is highly pigmented & less vascularized & contain aggregation of smooth muscle cells form the sphincter pupillae around the pupillary margin  The posterior layer of the stroma is less pigmented & more vascular 3. Posterior smooth surface covered by double layered epithelium:  Cells of the inner (anterior) epithelium adjacent to the stroma are myoepithelial, less heavily pigmented, and comprise the dilator pupillae muscle  Cells of the outer (posterior) pigmented epithelium are very rich in melanin granules to protect the eye's interior from an excess of light iris  Function of iris 1. The heavy pigmented epithelium of the iris prevents light from entering the interior of the eye except through the pupil ( keep stray light rays from interfering with image formation ) & * Provide the Color of the eye 2. Iris muscles regulate the diameter of the pupil & the amount of light entering the eye  Iris muscles 2 muscles : 1. Dilator pupillae muscle: ( radially arranged fibers ) tongue like extensions of the outer epithelial cells (modified myoepithelial cells) { sympathetic innervation } Increase the diameter of the pupil (in dark & fear) 2. Sphincter pupillae muscle: concentric bundles at pupillary margin (smooth muscle) { parasympathetic innervation } Reduce the diameter of the pupil (in bright illumination & sleeping)  Iris muscles Dialator pupillae Sphincter pupillae muscle muscle Type Myoepithelial cells Smooth muscle Arrangement Radially near the Concentric at posterior border pupillary margin Nerve supply Sympathetic Parasympathetic Mode of action Widening the pupil Constrict the pupil (at dim light & fear) (at bright light & accommodation) Regulation of the diameter of the pupil  Color of the eye Due to the melanocytes present in the stroma of the iris & depends on the quantity of melanin The degree of the color started from blue & as the pigment increase it deepens to greenish blue, to gray, to brown Individuals with albinism have almost no pigment and the pink color of their irises is due to the reflection of incident light from the blood vessels of the stroma  The lens It is transparent biconvex structure Avascular & devoid of nerves Has great elasticity that is lost with age Used to focus light on the retina The lens is held in place by ciliary zonule The lens consists of: 1. Lens capsule 2. Subcapsular epithelium 3. Lens fibers  Lens capsule It is thick basement membrane homogenous & refractile Covers the outer surface of the lens (completely surrounds the lens, thickest near the equator) Consists mainly of collagen IV & rich in glycoprotein  Subcapsular epithelium It is a single layer of cuboidal epithelium Present only on the anterior surface of the lens The basal ends of the epithelial cells attach to the lens capsule Their apical surfaces have interdigitations that bind the epithelium to the internal lens fibers Cells at the equator responsible for growth & increasing the size of lens during normal growth (the cells divide to provide new cells that differentiate as lens fibers)  Lens fibers Thin & flattened Derived from the cells of subcapsular epithelium which loose their nuclei & organelles and become elongated & transparent The cells become filled with proteins called crystallins (increasing the index of refraction of the lens while maintaining its transparency) The lens is split into regions depending on the age of the lens fibers:  outer cortex ( younger fibers ) &  inner nucleus (older fibers ) New lens fibers, generated from the lens epithelium, are added to the outer cortex Mature lens fibers have no organelles or nuclei  Nutrition of the lens As ions, nutrients, and liquid enter the lens from the aqueous humor, epithelial cells pump ions lens receives all its out of the lens (by Na+/K+- nourishment from the ATPase pumps) to maintain the transparency of the lens aqueous humor Lens origin Unlike the rest of the eye, which is derived mostly from the neural ectoderm, the lens is derived from surface ectoderm as lens vesicle  Visual accommodation Zonhular fibers held lens in place help in focusing on near & far objects by changing the curvature of the lens Cataract: Lens age problems Lens becomes opaque & less transparent Causes: 1. Accumulation of brownish 1. Presbyopia: pigment in lens fibers Difficult accommodation for near 2. Excessive exposure to UV objects radiation Cause: reduction in lens elasticity 3. High levels of glucose in diabetes Can be corrected by wearing glasses mellitus with convex lenses  Vitreous body It is clear, colorless, transparent & gelatinous It occupies the region behind the lens & surrounded by retina posteriorly Formed of 99% of water, small amount of collagen , hyaluronic acid & few cells called hyalocytes It maintains the round shape of the eye It functions in transmitting light & allow transfer of substances from and to retina Vitreous body is stagnant ( do not replenished as aqueous humor ) It is nourished at the periphery by vessels of the retina & ciliary processes  The hyaloid membrane is a transparent membrane that encloses the vitreous humour, separating it from the retina Hyaloid canal is a small transparent canal running through the vitreous body from the optic nerve disc to the lens. It is formed by an invagination of the hyaloid membrane It facilitate changes in the volume of the lens The refractive media of the eye are transparent and formed of : Cornea Aqueous humor Lens Vitreous humor The ocular or optic parts of the eye are : Cornea Ciliary muscles & Iris Lens & retina  Retina Development : Evagination of prosencephalon forms optic vesicle Optic vesicle invaginates in the surface ectoderm forming double-walled optic cup The outer wall forms the pigment epithelium The inner wall forms the neural retina  Structure of retina Formed of 2 portions: 1. Inner optical, photosensitive or neural 2. Outer pigmented  The ora serrata Is the serrated junction between the retina and the ciliary body (termination of the retina anteriorly) The pigmented epithelium of the retina transitions into the pigmented epithelium of the ciliary body The inner portion of the retina transitions into the non-pigmented epithelium of the ciliary body  1. The pigment epithelium Columnar cells shows : 1. Connected by tight junction 2. Apical melanin granules 3. Basal ion transport activity (Basal infolding & mitochondria) 4. Apical microvilli & sheaths that envelop the tips of the photoreceptors 5. Contain lysosomes & SER Detachment of the retina occurs in this region & can be treated with laser surgery  Function of pigment epithelium Serve as an important part of the outer blood-retinal barrier (tight junction) Absorb light passing through the retina to prevent its reflection ( melanin ) Control ion homeostasis and eliminate water and metabolites Phagocytose shed components from the adjacent rods and cones ( lysosomes ) Esterification of vitamin A Isomerize & regenerate the retinoids used as chromophores by the rods and cones Remove free radicals Functions of Pigmented cells  2. The neural retina Has three major layers of neurons 1. Outer layer of photosensitive cells, the rod & cone cells 2. Intermediate layer of bipolar neurons, which connects to the rod and cone cells 3. Internal layer of ganglion cells, which synapse with the bipolar cells through their dendrites and send out axons that converge to form the optic nerve which leaves the eye and passes to the brain  Organization of optical or neural retina Formed of 9 layers: 1. Outer layer of rods & cones: contain the outer segment of rods & cones 2. Outer ( external ) limiting membrane: a zone of adhesion (tight junctions ) between rod & cone cells and glial cells (Müller cells ) 3. Outer ( external ) nuclear layer: Cell bodies & nuclei of rod & cone cells  4. Outer ( external ) plexiform layer: Synapses between photoreceptors, horizontal & bipolar cells 5. Inner nuclear layer: Nuclei of bipolar, horizontal , amacrine & Müller cells 6. Inner ( internal ) plexiform layer: Synapse between bipolar, amacrine & ganglion cells 7. Ganglion cell layer: Ganglion cells which classified into diffuse or monosynaptic ( midget ) and retinal blood vessels 8. Retinal ( optic ) nerve fibers layer : Unmylinated axons of ganglion cells, processes of Müller cells & retinal blood vessels 9. Inner limiting membrane: Basal lamina of Müller cells separates retina from vitreous body  Retinal neurons & glial cells 1. Neurons are: Rods and cons: which are polarized neurons Bipolar neurons: of 2 types (diffuse & monsynaptic) Ganglion cells: their axons form the optic nerve Horizontal cells: contact between photoreceptors Amacrine cells: contact between ganglion cells 2. Glial cells are: Müller cells are the main supporting cells (extend between the internal & external limiting membrane) Astrocytes & microglial cells  Rod & cone cells Human retina has ~120 million rods & ~ 6 million cones They are named for the shape of their outer segment Are polarized neurons (At one pole is a single photosensitive dendrite & at the other are synapses with cells of the bipolar layer ) Rods are extremely sensitive to light ;low levels of light such as dusk or night time (scotopic vision ) Cones are specialized for color vision in bright light (photopic vision)  Ultra structure of rods & cones They are polarized neurons consist of: 1. Outer segment: flattened membranous discs stacked like coins & surrounded by the plasma membrane (photosensitive) 2. Constriction & modified cilium 3. Inner segment: produce proteins (polyribsomes), metabolic & energy production (mitochondria) 4. External plexiform layer: synapses with bipolar cells  Difference Rods Cones Number 120 million 6 million Function In dim light , images of In bright light, fine black & white details & color vision Shape Taller & thinner Shorter & wider Outer segment Cylindrical Conical Visual pigment Rhodopsin Iodopsin Disks in outer segment Loose continuity with Continuous with cell cell membrane membrane Synaptic body Spherule pedicle Distribution Mainly peripheral Mainly central In fovea centralis Absent Present  The visual pigment ( photopigment ) It is contained in the outer segment ( flattened disks ) of rods & cones Rod cells contain visual purple or Rhodopsin Cones contain 3 iodopsins ( maximum sensitivity red, green or blue region of visible spectrum) Visual pigments contains a transmembrane protein, the opsin (synthesized by polyribsomes in inner segment), with a bound molecule of retinal (aldehyde form of vit. A), the light-sensitive chromophore  Vision reaction When the pigment is bleached by light it initiates the visual stimulus  Phototransduction When photons of light are absorbed by the retinal of rhodopsin, it dissociate from opsin.This activates the opsin (bleaching) This increases Ca diffusion to intracellular space of the outer segment Ca acts on the cell membrane reducing its permeability to Na result in hyperpolarization Hyperpolarization is produced which reduces the synaptic release of neurotransmitter (unlike other receptors where action potential is generated through depolarization) This change in turn depolarizes sets of bipolar neurons, which send action potentials to the various ganglion cells of the optic nerve The visual pigment is then reassembled and Ca ions are transported back into the disks  Specialized Areas of the Retina Optic disc Fovea centralis  Optic disc It is a site of exit of optic nerve (axons of ganglion cells) And a site of entry of central retinal artery & vein It is devoid of photoreceptors (insensitive to light) It is also known as blind spot, papilla of the optic nerve or optic nerve head  Fovea centralis It is a shallow depression with very thin retina It is surrounded by macula lutea or yellow spot rich in carotenoids which protect the cone cells ( filter short- wavelength light & antioxidant)  Fovea centralis consists of: Has high concentration of modified tall & narrow cone cells in the center Accumulation of bipolar & ganglion cells in the periphery Has no rod cells Has no blood vessels It is a region of extremely precise ( high ) visual acuity  Nutrition of the retina The outer part is supplied from choriocapillary layer where the tight junction between the pigmented epithelium form the outer blood retinal barrier The inner part is supplied from branches of central retinal artery where continuous type capillaries form the inner blood retinal barrier  Medical application Color blindness (inherited disease) is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions due to absence or weakness of one or more cones Ishihara color test Night blindness is due to a disorder of the cells in the retina that are responsible for vision in dim light (rods). It has many causes includes nearsightdness, glucoma, diabetes, vit A diffeciency  Accessory structures of the eye Conjunctiva, eyelids & lacrimal apparatus 1. Conjunctiva: Thin, transparent mucous membrane Formed of 2 parts: Bulbar : covers the white of the eye (st. squamous non k. epi) Palpebral: lines the internal surface of the eyelid (st. columnar epi. with goblet cells) Fornices are the conjunctiva lined the recesses between the eye lids and the eye ball  Function of conjunctiva The conjunctiva helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland. It also contributes to immune surveillance or control and helps to prevent the entrance of microbes into the eye  Eyelids Movable delicate thin skin that protect the eye Loose & elastic Contain 3 types of glands: 1. Meibomian glands: are long sebaceous glands in the tarsal plate ( fibrous tissue ) They don’t communicate with the hair follicles Create an oily layer on the tear film to prevent evaporation They open in the posterior portion of the free margin 2. Glands of Zeis: are small sebaceous , connected to follicles of eye lashes conjunctiva 3. Sweat glands of Moll: are modified sweat Tarsal gland gland unbranched simple started as skin simple spiral , empty their secretion in the follicles  Lacrimal apparatus Consists of: Lacrimal gland, canaliculi, lacrimal sac & nasolacrimal duct Lacrimal gland : is tubuloalveolar Consists of Lobes, 6-12 excretory ducts & serous columnar cells resembling the parotid acinar cells, myoepithelial cells Canaliculi is lined by st. squamous The sac & nasolacrimal duct are lined by respiratory epithelium Tears is alkaline fluid rich in lysozyme The tear film is formed of: 1.A layer of mucous formed by goblet cells of conjunctiva 2.Watery layer formed mainly by lacrimal glands 3.Lipid layer formed by tarsal glands  ( Vestibulo-cochlear apparatus ) Vestibuloauditory System Hearing & maintain equilibrium 1. External ear receives sound waves 2. Middle ear Transmission of sound waves from air to fluids of inner ear by bones 3. Inner ear Contain auditory organ for transduction of vibrations to nerve impulses Vestibular organ for equilibrium  External ear Formed of : 1. Auricle ( ear pinna ) plate of elastic cartilage covered by thin skin 2. External auditory canal (acoustic meatus) lined by st. squamous epithelium (skin) with hair follicles , sebaceous glands & ceruminous glands (modified apocrine sweet gland) in submucosa Supported by elastic cartilage ( outer part ) & temporal bone ( inner part ) 3. Tympanic membrane ( ear drum ) Formed of CT layer of collagen, elastic fibers & fibroblasts Covered externally by thin skin (epidermis) & internally by simple low cuboidal epithelium as that of middle ear Cerumen is the oily or waxy, yellowish material resulting from secretions of the ceruminous glands. It contains various proteins, saturated fatty acids, and sloughed keratinocytes and has protective, antimicrobial properties  Middle ear ( Tympanic cavity ) Air filled cavity lined with simple cuboidal epithelium Connected anteriorly to the pharynx by auditory or Eustachian tube The tube Lined by ciliated pseudo stratified columnar epithelium the walls of the tube are usually collapsed, it opens during the swallowing process, which serves to balance the air pressure in the middle ear with atmospheric pressure Posteriorly continuous with the air filled cavities of the mastoid process of temporal bone  Other boundaries of tympanic cavity Its lateral wall is the tympanic membrane Its medial wall formed by the bony wall of the inner ear & contain two membrane-covered regions devoid of bone oval & round windows Oval window closed by foot plate of stapes Round window covered by a secondary tympanic membrane  Contents of tympanic cavity 3 auditory ossicles ( articulated bones ) that transmit the mechanical vibrations of the tympanic membrane to the internal ear 1. Malleus ( hammer ) 2. Incus ( anvil ) 3. Stapes ( stirrup )  And 2 muscles ( tensor tympani & stapedius ) Are skeletal muscles insert into the malleus & stapes respectively Restricting movement of the ossicles & helping to protect the internal ear from extremely loud noises  Protection of middle ear Eustachian tube help to equalize the pressure on both sides of the tympanic membrane The shape of articulation between the ossicles The reflex contraction of stapedius & tensor tympani muscle in response to sounds of high intensity & decrease hearing sensitivity to own speech & chewing  Internal or inner ear Located within the temporal bone Consists of bony & membranous labyrinths Bony labyrinths houses the membranous labyrinths  1. Bony labyrinths A cavity in petrous bone Formed of : 1. 3 Semicircular canals ( anterior , posterior , lateral ) at right angles to one another 2. Vestibule 3. Cochlear canal  Bony labyrinths filled with perilymph perilymph is similar in ionic composition to CSF & ECF but contains little protein. Perilymph formed from the microvasculature of the periosteum and is drained by a perilymphatic duct into the adjoining subarachnoid space  2. Membranous labyrinths Are membranous ducts & sacs inside the bony labyrinth (fluid-filled, epithelium-lined tubes ) Formed of : 1. Semicircular ducts vestibular labyrinth 2. Utricle & Saccule 3. Cochlear duct ( cochlea ) Membranous labyrinths filled with endolymph Endolymph contains few proteins , high potassium and low sodium content  Sensory structures in the inner ear Specialized epithelium (neuro-epithelium) lining in the membranous labyrinths These are: 1. Organ of Corti in the cochlea for hearing 2. Maculae in the utricle and saccule & 3. Cristae in the semicircular ducts  maculae & cristae for equilibrium  Maculae Are sensory receptors contain sensory mechanoreceptors ( hair cells ) Location: 1. In the floor of the utricle 2. In the anterior wall of the saccule The macula of the saccule lies in a plane perpendicular to the macula of the utricle Function: Registration of head position & sense of linear acceleration or deceleration  Structure of maculae 1. Two types of receptor cells ( hair cells ) 2. Supporting cells 3. Afferent nerve endings (endings of the vestibular branch of the eighth cranial nerve )  Structure of Macula The supporting cells are columnar shaped cells with apical microvilli The neuroepithelium is covered with thick gelatinous glycoprotein layer with surface depositions called otoliths or otoconia Otoliths composed of calcium carbonate on a matrix of proteoglycans. Otoliths make the otolithic membrane heavier than endolymph , which facilitates bending of the kinocilia & stereocilia embedded in this membrane by gravity or movement of the head  Hair cells of the maculae 2 types: 1. Type I cells are flask shaped & have large cup-shaped afferent nerve ending at the base 2. Type II cells are cylindrical & have many afferent nerve endings Both types have: 1. One apical kinocilium ( immotile ) 2. Long sterocilia arranged in rows of graded length with the longest lie adjacent to the kinocilium Both types of hair cells, or their afferents, also have synaptic connections with efferent (from the brain) fibers that modulate the sensitivity of these mechanoreceptors  The hair cells in the maculae respond to linear acceleration, gravity, and tilt of the head Because the otoliths are heavier than endolymph when the head is tilted with respect to gravity, and when the individual is moving in a straight line and inertia causes drag on the otolithic membrane  Cristae ampullaris Are the sensory receptors in the ampullae of the semicircular ducts They function in registration of rotational movement of the head (angular acceleration) Have the same structure of maculae, but have thicker conical glycoprotein layer ( cupula ) with no otoliths  The hair cells of the cristae ampullaris detect rotational or angular movements of the head. On each side of the head these hair cells are oriented with opposite polarity, so that turning the head causes hair cell depolarization on one side & hyperpolarization on the other  Maintaining the balance ( Vestibular function ) Cupula over the crista ampullaris moves in response to the flow of fluid in the semicircular ducts This result in bending of the hair cells stereocilia Bending of steiocilia towards kinocilium increases the impulse carried by vestibular nerve Cupula moves to its normal position when uniform movement returns Macculae are sensitive to the force of gravity on the gelatinous otoliths leading to deformation of the stereocilia & generation of action potentials which are carried by vestibular nerve  Medical application Problems of the vestibular system can result in vertigo, or dizziness, a sense of the body rotating & lack of equilibrium Causes : by certain infections, drugs, or tumors near the vestibular nerve Spinning the body can also produce vertigo due to overstimulation of the cristae ampullaris of the semicircular ducts Overstimulation of the maculae of the utricle caused by repetitive changes in linear acceleration & directional changes can lead to motion sickness (seasickness)  Endolymphatic duct & sac The duct is lined with simple squamous epi. that changes to tall columnar epithelium near the sac The epithelium lining contain cells, some of them has apical microvilli for absorption of endolymph & pinocytotic vesicles for endocytosis of foreign material & cellular remnants Endolymph is drained from the vestibule into venous sinuses of dura mater by the endolymphatic duct  Cochlea Snail shaped (spiral) bony canal 35 mm in length It contains the cochlear duct in the middle It makes 2 ½ turn around the modiolus ( bony core ) The modiolus contains spiral ganglion & BVs The cochlea divided into 3 spaces: 1. Scala vestibuli 2. Scala media cochlear duct 3. Scala tympani  The cochlear duct It is a site of hearing receptor ( organ of Corti ) or spiral organ It ends at the apex of cochlea where the other 2 scalae communicate via helicotrema Scala tympani & scala vestibuli are one long tube, beginning at the oval window and ending at the round window Cochlear duct contains endolymph while other 2 scalae contain perilymph  Borders of cochlear duct Upper border is vestibular ( Reissner’s ) membrane (roof of cochlear duct ) Lateral border is Stria vascularis Lower border is Basilar membrane (floor) The vestibualr memebrane formed of: 2 layers of squamous epithelium from scala media & scala vestibuli The cells joined with tight junctions Function very high ionic gradient across the membrane (endolymph & perilymph) Stria vascularis is the lateral wall of cochlear duct It is a unique vascularized epithelium formed of ion & water transporting cells (pumping Na out & K into endolymph) Function: production & maintenance of ionic composition of endolymph Stria vascularis Organ of Corti  Basilar membrane Location: It represents the floor of the cochlear duct Extends from the spiral lamina (projection from the modiolus ) to the spiral ligament Spiral limbus is a periosteal CT extension over the osseous spiral lamina bulges into the cochlear duct & forms part of its floor The limbus is covered by columnar epithelium  The basilar membrane Formed of highly vascularized CT containing collagen & elastic fibers It is made up of 20,000 to 3o,ooo fibers Near the oval window the fibers are short & thick As moving toward the other end the fibers get longer & thinner This gives the fibers different resonant frequencies (like a tuning fork )  The organ of Corti Location: It rests on the upper surface of the basilar membrane longitudinally from spiral limbus to spiral ligament It is covered by tectorial membrane ( acellular layer rich in collagen & glyco- proteins secreted by cells of spiral limbus )  Structure of organ of Corti It is formed of mechanoreceotors (hair cells ) & supporting cells: 1. Single row of inner hair cells ( flask shaped ) 2. Three to five rows of outer hair cells (cylindrical ) 3. Supporting cells  Hair cells Both inner & outer hair cells posses surface stereocilia, IHC have one linear row of short stereocilia, while OHC have a curved row of longer stereocilia W shaped No kinocilium The tips of the tallest stereiocilia of OHC are embedded in the tectotrial membrane Both cells lie within the recess of a supporting phalangeal cells The cell bodies of the bipolar afferent neurons forms the spiral ganglia in the modiolus Function of hair cells Both hair cells have afferent & efferent nerve endings with IHC much more heavily innervated (IHC are true receptors for the sense of hearing ) OHC contracted when stimulated thus pulling on the tectorial membrane & stimulating IHC  The supporting cells 1. Pillar cells which outline the triangular space between the inner & outer hair cells ( inner tunnel ), important in sound transduction, these cells contain microtubules & keratin for stiffness 2. Hensen’s cells lateral to organ of Corti 3. Phalangeal cells surround the base of the outer & inner hair cells, almost completely enclosing each IHC but only the basal ends of the OHC Organ of Corti  Medical applications 1. Conductive hearing loss Cause: problems in the middle ear such as: Otosclerosis, in which scar-like lesions develop on the bony labyrinth near the stapes which inhibit its movement of the oval window Otitis media, Infection of the middle ear common in young children, usually progressing from an upper respiratory infection, and can reduce sound conduction due to fluid accumulation in that cavity 2. Sensorineural deafness Cause: can be congenital or acquired and due to defects in any structure or cell from the cochlea to auditory centers of the brain, but commonly involves loss of hair cells or nerve degeneration Cochlear implants  Ear parts & their functions Ear pinna determines the source of sounds & amplifies its frequency Ear drum vibration; moves back & forth in response to air pressure fluctuation (transforms the sound waves into mechanical energy ) Eustachian tube equalizes air pressure on both sides of ear drum Middle ear Pressure amplification through the ossicles Inner ear comprises both hearing & balancing organs Good luck