Biology of Ear and Eye
Hearing,sight,smell and taste have nerve endings outside the brain.These are in ears,nose,eyes and mouth.The incoming nerve impulses undergo complex processes of integration and coordination that result in perception of responses inside outside body.80% of perception comes from sensory stimuli.
Ear and hearing
Ear is involved in balance and hearing.It has 8th cranial nerve supply.It is the cochlear part of vestibule cochlear nerve.It is stimulated by sound wave vibrations.
The structure that form the ear are encased within petrous portion of temporal bone.
Parts of Ear
The ear is divided into
The outer ear does the function of collecting sound waves and sends them to middle ear.From here it transfers them to inner ear.Here they are converted to nerve impulses and transferred to cerebral cortex which is the hearing area.
The outer ear consists of auditory canal and auricle.
The auricle projects from side of head and is the visible part.It is fibroelastic cartilage covered with skin.It is ridged and deeply grooved.Helix is the most prominent outer ridge.
The earlobe is the soft part at lower extremity.
It is composed of adipose tissue that is fibrous and supplied richly by blood.
Auditory canal(external acoustic meatus)
Auditory canal is in the shape of 'S' ,about 2.5 cm long from auricle to ear drum.It has a cartilaginous lateral third and canal is in temporal bone. Numerous ceruminous glands and hair follicles with associated sebaceous glands are there in lateral third skin.Ceruminous glands are modified sweat glands.Foreign materials eg. dust are prevented by wax.Movements of tempo to mandibular joint during speaking and chewing massage meatus.By doing so wax is moved to exterior.The eardrum completely separates external acoustic meatus.
Middle ear (tympanic cavity)
This is an air filled cavity within temporal bone. The cavity which opens out is lined with squamous or cuboidal epithelium.
The lateral wall is formed by tympanic membrane.
The roof and floor is of temporal bone.
The posterior wall is formed by temporal bone which opens to mastoid antrum.
The medial wall is a thin layer.
Auditory ossicles are 3 small bones.They are named according to their shapes.
The malleus is a hammer shaped bone.
The incus is the middle bone.It is anvil in shape.
The stapes is the medial bone which is stirrup shaped.
Labyrinth or inner ear has hearing and balance organs.
Bony labyrinth is a cavity lined with periosteum.It encloses membranous labyrinth like a tube within a tube.A layer of watery fluid called perilymph is between bony and membranous labyrinth.
The bony labyrinth consists:
3 semicircular canals
The vestibule is nearest the middle ear.
Cochlea resembles a snail's shell.
The semicircular canals are 3 tubes.
A network of delicate tubes filled with endlymph is called membranous labyrinth. It comprises
*vestibule has utricle and saccule
Cochlea has 3compartments:
Every sound produces sound waves.The auricle concentrates waves and sends them to the auditory canal. These vibrations are transmitted through the middle ear and amplified. The footplate of stapes rocks to and fro setting up fluid waves in perilymph of Scala vestibule.Some force of these waves is transmitted along length of Scala vestibule and Scala tympani but most pressure is transmitted into cochlear duct.This causes wave motion in endolymph resulting in vibration of basilar membrane and stimulation of auditory receptor in hair cells of spiral organ.The nerve impulses are transmitted to brain in cochlear portion of vestibule cochlear nerve.The fluid wave expended into middle ear finally.The vestibule cochlear nerve transmits impulses.Because fibres cross over medulla,the auricle areas of cerebrum receive impulses from both the ears.
Sound waves have properties of volume and pitch.Different frequency sounds stimulate basilar membrane. The greater the amplitude of wave that is created in endolymph,the more is the stimulation of auditory receptors in hair cells in spiral organ enabling volume perception. Long exposure to loud noise causes loss of hearing because it damages hair cells of spiral organ.
There is no auditory function of semicircular canals. Instead head position information is provided by them.3 semicircular canals are there. They are above,behind beside the vestibule. The semicircular canals are composed of outer bony wall and inner ducts.The membranous ducts have endolymph and separated by perilymph.
The utricle is a sac and part of vestibule and 3 ducts open into it.The saccule is part of vestibule that communicates with utricle and cochlea.
In walls of utricle,ampullae and saccule are epithelial cells with minute projections called hair cells.In the hair cells are receptors which combine forming vestibule cochlear nerve.
The vestibule and semicircular canals are concerned with balance.Any head position change causes movement in endolymph and perilymph.These bend the hair cells and stimulate sensory receptors in utricle,ampullae,saccule.The nerve impulses resulting from this are transmitted by vestibular nerve.The cerebellum also receives nerve impulses from eye.
Eye and the sight
Eye is sight organ.It is supplied by optic nerve and is in orbital cavity.
It is spherical. Adipose tissue occupies space between orbital cavity and eye.
The 2 eyes are separate structurally.
Wall of eye has 3 layers of tissue:
*outer fibrous: cornea and sclera
*middle vascular having iris,ciliary body and choroid.
*inner nervous tissue:retina
Inside eyeball structures include aqueous fluid, vitreous body and lens.
Cornea and Sclera
The sclera forms the outermost layer of posterior and lateral aspects of eyeball.It consists of firm membrane and gives attachment to extrinsic muscles of eye.
Anteriorly sclera continues as clear transparent epithelial membrane,cornea.Light rays pass through cornea to reach retina.The cornea is convex anteriorly involved in refracting light rays to focus them on the retina.
Choroid lines posterior of inner surface of sclera.It is rich in blood vessels.Light enters eye through pupil,stimulates sensory receptors in retina and is absorbed by choroid.
Ciliary body is choroid's anterior continuation.. It consists of ciliary muscle and epithelial cells. Smooth muscle fibres are circular and the ciliary muscle acts like sphincter.
Lens is attached to ciliary body by suspensory ligaments.They are like spokes of wheel.Contraction and relaxation of ciliary muscle fibres attached to these ligaments control shape of lens.Epithelial cells secrete aqueous fluid into anterior segment of eye i.e. space between lens and cornea.Ciliary body is supplied by parasympathetic branches of oculomotor nerve.Stimulation causes contraction of ciliary muscle and accommodation of eye.
Iris is visible colored part of eye and extends anteriorly from ciliary body. It lies behind cornea and in front of lens.It divides anterior segment of eye into anterior chamber and posterior chamber..This contains aqueous fluid secreted by ciliary body. It is a circular body composed of pigment cells and 2 layers of smooth muscle fibres, 1 circular and other radiating.In centre is am aperture called pupil.
Iris is supplied by parasympathetic and sympathetic nerves.
Color of iris is genetically determined.
Lens is highly elastic circular biconvex body.It lies immediately behind pupil.It has fibres which are enclosed in capsule.It is suspended from ciliary body by suspensory ligament.Thickness is controlled by ciliary muscle through suspensory ligament. When ciliary muscle contracts,it moves forward,releasing pull on lens.This increases its thickness. The nearer the object being viewed,thicker the lens becomes to allow focusing.Lens bends light rays reflected by objects in front of eye.It is only structure in eye that can vary refractive power achieved by changing thickness.
Retina is innermost layer of wall of eye.It is delicate and well adapted for stimulation by light rays.It is composed of nerve cell bodies layers and their axons lying on a pigmented epithelial cells layer which attach it to choroid. Light sensitive layer consists of sensory receptor cells,rods and cones.
Retina lines eyeball and is thickest at back.Near centre of posterior part is maculalutea or yellow spot.In centre of yellow spot is a depression called fovea Centralia which has only cones.Towards anterior part of retina there are fewer cones than rods.0.5cm to nasal side of macula lutea all nerve fibres of retina converge to form optic nerve.Small area of retina where optic nerve leaves eye is blind spot.It has no light sensitive cells.