Organs of special Sense
Eye and EarVisual system
Eye is a highly specialized organ for perception of form, light and color. Eye located in protective cavities within skull called orbits. Each eye contains a protective cover to maintain its shape, a lens for focusing, photosensitive cells that respond to light, stimuli and numerous cells that process visual information. Visual impulses from sensitive cells are then conveyed to the brain via the axons in optic nerve.The photoreceptors are modified dendrites of two types of nerve cells, rod cells and cone cells. The rods are integrated into a system which is receptive to light of differing intensity; this is perceived in a form analogous to a black and white photographic image. The cones are of three functional types receptive to the colors blue, green and red and constitute a system by which colored images are seen. The rod and cone receptors and a system of integrating neurons are located in the inner layer of the eye, the retina. The remaining structures of the eye serve to support the retina or to focus images of the visual world upon the retina.
Structures of the eye
Eyeball (anterior) – section – H&E – 4x objective corneaciliary processes
anterior chamber
lens
posterior chamber
posterior cavity
ciliary body
conjunctiva
iris
Structures of the eye
Eyeball (anterior) – section – H&E – 4x objective corneaanterior chamber
lens
conjunctiva
iris
pupil
Structures of the eye
Eyeball (posterior) – section – H&E – 10x objective choroidposterior cavity
retina
sclera
Structures of the eye
Retina – section – H&E – 40x objective outer nuclear layeroptic nerve fibers
choroid
pigmented epithelium
ganglion cell layer
layer of rods and cones
outer plexiform layer
inner nuclear layer
inner plexiform layer
Layers in the eye-three distinct layers.
Corneo-scleral layer. The corneo-scleral layer forms a tough, fibroelastic capsule which supports the eye. The posterior five-sixths, the sclera, is opaque and provides insertion for the extraocular muscles. The anterior one-sixth, the cornea, is transparent and has a smaller radius of curvature than the sclera. The cornea is the principal refracting medium of the eye and roughly focuses an image onto the retina; the focusing power of the cornea depends mainly on the radius of curvature of its external surface.The corneo-scleral junction is known as the limbus and is marked internally and externally by a shallow depression. Running from the junction of the cornea and limbus the surface of the eye is covered by conjunctiva which is reflected into the eyelids
2. Uvea (vascular layer). The middle layer, the uvea or uveal tract, is a highly vascular layer which is made up of three components: the choroid, ciliary body and the iris The choroid lies between the sclera and retina in the posterior five-sixths of the eye. It provides support for the retina and is heavily pigmented, thus absorbing light which has passed through the retina. Anteriorly, the choroid merges with the ciliary body which is a circumferential thickening of the uvea lying beneath the limbus
The ciliary body surrounds the coronal equator of the lens and is attached to it by the suspensory ligament or zonule. The ciliary body contains smooth muscle, the tone of which controls the shape of the lens via the suspensory ligament. The lens is a biconvex transparent structure, the shape of which can be varied to provide fine focus of the corneal image upon the retina
The iris, the third component of the uvea, forms a diaphragm extending in front of the lens from the ciliary body so as to incompletely divide the anterior compartment into two chambers; these are known by the terms anterior and posterior chamber. The highly pigmented iris acts as an adjustable diaphragm which regulates the amount of light reaching the retina. The aperture of the iris is called the pupil
3. Retina. Inner most lining of the most posterior chambers of the eye. The posterior three quarter of the retina is the photosensitive region. It consist of rods, cones and various interneurons, cells that are stimulated by a respond to light. Retina terminate in the anterior region of the eye called ora serrate which is nonphotosensetive part of retina. This region continuous forward in the eye to line the inner part of Ciliary body and posterior region of iris.
The retina is made up of three cell types: neurons, pigmented epithelial cells and neuron support cells. The neurons are divided into three functional groups, namely photoreceptor cells (rod cells and cone cells), the cells of afferent fibers passing in the optic nerve, and a group of neurons interposed between the first two types which integrate sensory input from the photoreceptors before transmission to the cerebral cortex. The integrating neurons are further subdivided into three types: bipolar cells, horizontal cells and amacrine cells.
Bipolar cells, the most numerous of the integrating neurons, in general make direct connections between one or more photoreceptors and one or more optic tract neurons as well as with horizontal and amacrine cells. Horizontal cells have several short processes and one long process, the terminal branches of each making lateral connections between adjacent and more distant rods and cones in the outer plexiform layer. Horizontal cells also synapse with the dendrites of bipolar cells. The amacrine cells have numerous dendrites which make connections with bipolar and optic tract neurons in the inner plexiform layer, as well as making occasional feedback connections with photoreceptors in the outer plexiform layer.
Histologically, the retina is traditionally divided into 10 distinct histological layers from outer to inner
pigmented epithelial cells forming a single layer resting on Bruch's membrane which separates them from the choroid. The next layer is the photoreceptor layer made up of the rod and cone processes. thin eosinophilic structure known as the outer limiting membrane. separating them from a layer of densely packed nuclei described as the outer nuclear layer. The outer nuclear layer contains the cell bodies of the rod and cone photoreceptors. The almost featureless layer deep to this is known as the outer plexiform layer
contains synaptic connections between the short axons of the photoreceptor cells and integrating neurons, the cell bodies of which lie in the inner nuclear layer inner plexiform layer The integrating neurons make synaptic connections with dendrites of neurons whose axons form the optic tract. The cell bodies of the optic tract neurons (retinal ganglion cells) comprise the ganglion cell layer. Internal to this is the layer of afferent fibers. passing towards the optic disc to form the optic nerve. Finally, the inner limiting membrane demarcates the innermost aspect of the retina from the vitreous body
The fovea is a conical depression in the retina corresponding to the point where the visual axis of the cornea and lens meets the retina and lying about 4 mm lateral and slightly inferior to the exit of the optic nerve fibers at the optic disc. Consequently, the fovea is the area subject to the least refractory distortion. the foveal retina is modified to obtain the maximum photoreceptor sensitivity and is thus the area of the retina with the greatest visual discrimination; however, its function is poor in conditions of low light intensity. Surrounding the fovea is an ovoid yellow area about 1 mm wide called the macula lutea.
Optic nerve
The afferent fibers from the retina converge at a point medial to the fovea, the optic papilla or optic disc, fibers from the lateral quadrants sweeping above and below the macula to avoid the fovea. The fibers then penetrate the sclera through the lamina cribrosa to form the optic nerve.Conjunctiva
The conjunctiva is the epithelium which covers the exposed part of the sclera and inner surface of the eyelids. It is stratified columnar in form and for a stratified epithelium is unusual in that it contains goblet cells in the surface layers. Melanocytes are found in the basal layer. The conjunctival mucous secretions contribute to the protective layer on the exposed surface of the eye, and allow the eyelids to move freely over the eye. Beneath the conjunctival epithelium is loose vascular supporting tissue.Eyelid
Each eyelid consists of a dense fibroelastic plate, the tarsus or tarsal plate , covered externally by thin, highly folded skin and on the internal aspect by smooth conjunctiva . The skin contains scattered fine hair follicles and the underlying supporting tissue is extremely loose and devoid of fat.Within the tarsal plate lie some tarsal (Meibomian)glands oriented vertically and opening at the free margin of the eyelid via minute foramina. These glands are modified sebaceous glands each consisting of a long central duct into which open numerous sebaceous acini. Associated with the eyelashes are sebaceous glands known as the glands of Zeis and modified apocrine sweat glands known as the glands of Moll. Together, the glands of the eyelid produce an oily layer which is thought to cover the tear layer, thereby preventing evaporation of the tears
Lacrimal gland
The lacrimal gland is responsible for the secretion of tears, a watery fluid containing the antibacterial enzyme lysozyme and electrolytes of similar concentration to plasma. Histologically, the lacrimal glands are similar to the salivary glands in the lobular structure and compound tubulo-acinar form of the secretory units. The secretory cells have the typical appearance of serous (protein-secreting) cells with basally located nuclei and strongly stained granular cytoplasm. Each gland drains via a dozen or more small ducts into the superior fornix. Tears drain to the inner aspect of the eye and then into the nasal cavity via the nasolacrimal duct.
Chambers in the eye-three chambers.
Anterior chamber- space located between cornea, iris and lens. Posterior chamber-small space situated between the iris, Ciliary process, zonular fibers and lens. Viterous cahmber- large posterior space situated behind the lens, zonnular fiber and surrounded by retina. Anterior and posterior chambers are filled with a watery fluid called aqueous humor which is continuously produced by ciliar process located behind iris.aqueous humor circulate from posterior chamber to anterior chamber where it is drain into a canal at the angle of the anterior chamber, the canal of Schlemm. The vitreous chamber is filled with a gelatinous substance called vitreous body. the vitreous body contains a canal which extends from the exit of the optic nerve to the posterior surface of the lens; this hyaloid canal represents the course of the hyaloid artery which supplies the vitreous body during embryological development. The vitreous body and hyaloid canal are rarely preserved in histological preparations.
Auditory system
External Ear. The auricle (pinna) of external ear are modified cone-shaped structure composed of elastic cartilage covered by skin gathered sound waves and direct them through external auditory canal interiorly to ear drum or tympanic membrane. Elastic cartilage also forms the wall of the outer third of the canal while the inner two-thirds of the canal lie in the petrous part of the temporal bone. The canal is lined by hairy skin containing sebaceous glands and modified apocrine glands which secrete a waxy material called cerumen.2. Middle Ear. Small air filled cavity called tympanic cavity. Located in and protected by the temporal bone of skull. Tympanic membrane separate the external auditory canal from middle ear. Located in middle ear three very small bones- the auditory ossicles consisting of stapes, incus and malleus. Also in middle ear Eustachian tube which communicate the nasopharynx with middle ear and help to equalize air presuure on both sides of tympanic membrane during swallowing or blowing the nose..
The ossicles articulate with one another via synovial joints and the malleus and incus pivot on tiny ligaments which are attached to the wall of the middle ear cavity. Small slips of muscle, the tensor tympani and stapedius, pass to the midpoint of the tympanic membrane and stapes bone, respectively, and damp down excessive vibrations which might otherwise damage the delicate auditory apparatus.
3. Inner Ear. The internal ear consists of an interconnected fluid-filled membranous labyrinth lying within a labyrinth of spaces of complementary shape in the temporal bone (the osseous labyrinth). The membranous labyrinth is bound down to the walls of the osseous labyrinth in various places but in the main is separated from the bony walls by a fluid-filled space. The fluid within the membranous labyrinth is known as endolymph and the fluid in the surrounding perimembranous space is known as perilymph.
The perimembranous space is directly connected with the subarachnoid space and, like the latter, is crossed by delicate fibrous strands and lined by squamous epithelium; the perilymphatic fluid is thus similar in composition to CSF. In contrast, the membranous labyrinth is a closed system with a sac, the endolymphatic sac, lying in the subdural space of the underlying brain. The membranous labyrinth is lined by a simple epithelium except in the endolymphatic sac where the cells are columnar with morphological features suggesting that this is the site of endolymph absorption.
The osseous labyrinth may be divided into three main areas:
The vestibule. The central space of the osseous labyrinth is called the vestibule; it gives rise to three semicircular canals posteriorly, and to the cochlea anteriorly. The vestibule contains two components of the membranous labyrinth, namely the utricle and the saccule, which are connected by a short, Y-shaped duct from which arises the endolymphatic duct.The walls of the utricle and saccule each contain a specialized area of sensory receptor cells known as a macula from which axons pass into the vestibular nerve as part of sensory inputs to maintain equilibrium. Laterally, the vestibule is separated from the middle ear cavity by a thin bony plate containing two fenestrations or windows. The oval window is occluded by the base of the stapes bone and its surrounding annular ligament whereby vibrations are transmitted to the perilymph from the tympanic membrane via the ossicle chain.
The round window is closed by a membrane similar to the tympanic membrane and it is thus sometimes described as the secondary tympanic membrane. This membrane permits vibrations which have passed to sensory receptors for sound to be dissipated.
The semicircular canals. Three semicircular canals arise from the posterior aspect of the vestibule, two being disposed in vertical planes at right angles to one another and the other in a near-horizontal plane. Within each semicircular canal is a semicircular membranous duct filled with endolymph and continuous at both ends with the utricle; near one end of each semicircular membranous duct is a dilated area called the ampulla. In each ampulla, there is a ridge called the crista ampullaris containing sensory receptors with axons converging on the vestibular nerve. Together with the receptors of the maculae of the utricle and saccule, these receptors help maintain balance and equilibrium
The cochlea. The cochlea occupies a conical, spiral-shaped space in the temporal bone extending from the anterior aspect of the vestibule. The membranous component of the cochlea arises from the saccule and spirals upwards with its blind end attached at the apex of the osseous space. The membranous canal is triangular in cross-section and attached to the bony walls of the cochlea in such a manner as to divide the osseous space into three spiral compartments
The middle compartment, the scala media, contains endolymph, and the upper and lower compartments contain perilymph. At the base of the cochlea, the upper perilymph compartment is directly continuous with the perilymph of the vestibule and via this space, called the scala vestibuli, vibrations pass through the perilymph towards the apex of the cochlea. At the apex, the scala vestibuli becomes continuous with the lower perilymphatic space of the cochlear spiral via a minute hole called the helicotrema.
This lower space terminates at the secondary tympanic membrane covering the round window and 'spent' vibrations are thus dissipated; the lower perilymphatic space is therefore known as the scala tympani. The sensory receptors for sound are located in a spiral shaped structure known as the organ of Corti
Organ of Corti
Located within the cochlea Receptors = hair cells on the basilar membraneScala tympani
Scala vestibuli
Cochlea – longitudinal section – H&E – 4x objective The cochlea is a coiled structure, turning 2.75 times about a central axis (the modiolus). So, you see the same tube multiple times in the same structure.
Structures of the ear
bony cochlea
vestibule
modiolus
petrous portion of the temporal bone
Cochlea – longitudinal section – H&E – 10x objective During histological processing the delicate tectorial membrane breaks free from its attachment and curls back on itself.
Structures of the ear
spiral ganglion
scala tympani
basilar membrane
spiral organ of Corti
vestibular membrane
scala media (cochlear duct)
scala vestibuli
tectorial membrane (torn free)
Tympanic membrane
The tympanic membrane (ear drum) is a thin fibrous membrane separating the external auditory canal from the cavity of the middle ear. With the exception of a small triangular area superiorly, the pars flaccida, the membrane is tense (pars tensa), being firmly attached to the surrounding bone by a fibrocartilaginous ring. The handle of the malleus is attached to the centre of the membrane, the chain of ossicles pulling the membrane slightly inwards.The tympanic membrane is made up of three layers: an external cuticular layer, an intermediate fibrous layer and an inner mucous layer. The cuticular layer consists of thin hairless skin, the epidermis being only about 10 cells thick and the basal layer being flat and devoid of the usual epidermal ridges. The thin dermis contains plump fibroblasts and a fine vascular network
The intermediate fibrous layer consists of an outer layer of fibers radiating from the center of the membrane towards the circumference and an inner layer of fibers disposed circumferentially at the periphery. These fibers contain a large amount of type II and type III collagen, and a small amount of type I collagen representing a distinct composition especially adapted for the function of the tympanic membrane
The inner mucous layer represents a continuation of the modified respiratory-type mucous membrane lining the middle ear cavity, but in this situation it is merely a single layer of cuboidal cells devoid of cilia and goblet cells. The underlying lamina propria is thin with a blood supply separate from that of the dermis of the cuticular layer. A similar modified respiratory-type mucosa invests the ossicles, small muscles and nerves exposed to the middle ear cavity.