Sense organ Ear I

Histology 2016-2017

Department of Anatomy &Histology: Dr.Rajaa Ali
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EAR I
GENERAL FEATURES
Ear is a peripheral sense organ concerned with hearing and equilibrium.
It is subdivided into three parts, the external ear, middle ear or tympanic cavity and the internal ear or labyrinth.

STRUCTURE

External Ear
The external ear consists of the auricle that collects sound waves and the external acoustic meatus that conducts these waves to the tympanic membrane (ear drum).
The auricle and the outer third of external acoustic meatus are made of elastic cartilage covered by thin skin. This skin is provided with hair, sebaceous and ceruminous (modifi ed sweat glands) glands.
The tympanic membrane separates the external ear from the middle ear. It is a trilaminar structure, lined externally by stratifi ed squamous epithelium and internally by simple cuboidal epithelium and is supported in the middle by fi brous tissue.

Middle Ear

The middle ear is a narrow air space compressed mediolaterally, occupying the petrous part of temporal bone. It contains a chain of three tiny auditory ossicles (malleus, incus and stapes) whose function is to transmit the vibrations of tympanic membrane to the perilymph of internal ear.
The middle ear also has two small muscles (tensor tympani and stapedius) attached to the ear ossicles. These muscles refl exly contract to dampen excessive vibration caused by loud noise.
Middle ear cavity communicates anteromedially with the nasopharynx through auditory tube. This helps to equalize air pressure on both surfaces of tympanic membrane.
Box 18.
Internal Ear
The internal ear consists of a bony labyrinth, comprising a series of cavities within the petrous part of temporal bone and a membranous labyrinth, comprising a series of membranous sacs and ducts present within the bony labyrinth (Fig. ).



The bony labyrinth is lined by endosteum and contains a clear fl uid, perilymph, in which the membranous labyrinth is suspended.
Fig. :General organization of labyrinth .



The membranous labyrinth is lined by simple squamous epithelium and contains endolymph. The squamous epithelial lining of the membranous labyrinth is specialised in certain regions to form receptor organs.
Bony Labyrinth
Bony labyrinth consists of three parts, vestibule, semicircular canals and bony cochlea. These structures are described, as follows:

1. Vestibule

It is the centrally situated cavity containing utricle and saccule (the larger and the smaller sac respectively) of membranous labyrinth. It communicates anteriorly with cochlea and posteriorly with three semicircular canals
through fi ve orifi ces (one being common to two of the canals).
In its lateral wall there are two openings, the fenestra vestibuli (oval window) closed by the base of stapes and fenestra cochleae (round window) closed by the secondary tympanic membrane.

2. Semicircular canals

They are three in number (superior, posterior and lateral) and are arranged at right angle to each other, so that all the three planes are represented.
Each canal has a swelling at one end called the ampulla. These canals open into the vestibule through five orifices.
The semicircular canals contain semicircular ducts.
3. Bony cochlea
It consists of a central pillar of bone, the modiolus, around which the cochlear canal makes two and a half spiral turns. Each successive turn is of decreasing radius, so that the whole structure is conical in shape, resembling
the shell of a snail. Its base is directed towards the internal acoustic meatus and is pierced by the cochlear part of the vestibulocochlear nerve.
A spiral bony shelf or ledge, the osseous spiral lamina winds round the modiolus like a thread of a screw and projects into the interior of bony canal. The endosteum over it is thickened to form limbus spiralis. The endosteum
on the opposite outer wall is thickened to form spiral ligament which projects into the interior of the canal as a prominence called crista basilaris. Extending between the osseous spiral lamina and crista basilaris is the basilar membrane.
The cochlear duct of the membranous labyrinth, which is triangular in cross section occupies the interior of cochlear canal.
In cross section, cochlea shows three compartments, the scala vestibuli above, the scala tympani below (these belong to the canal), and scala media in the middle (this belongs to the duct). The scala vestibuli and scala tympani communicate with each other at the apex of cochlea through a narrow space, the helicotrema.
The perilymph in the scala vestibuli is separated from the tympanic cavity by the base of stapes at the fenestra vestibuli.
The perilymph in the scala tympani is separated from the tympanic cavity by the secondary tympanic membrane at the fenestra cochlea (Fig. ).


Membranous Labyrinth
Membranous labyrinth is lodged within the bony labyrinth.


It consists of two sacs, the utricle and saccule besides semicircular and cochlear ducts. All these structures freely communicate with one another and form a closed system of membranous sacs and ducts.
Fig. : schematic diagram of ear , red arrows indicate the direction of vibration of perilymph

The structures are described as follows:

1. Utricle and saccule
The utricle and saccule are two membranous sacs lodged in the bony vestibule. These two sacs are connected with each other and with the saccus endolymphaticus by means of a Y-shaped utriculosaccular duct. (The saccus endolymphaticus lies beneath the dura mater on the posterior surface of petrous part of temporal bone.) The saccule is connected to the cochlear duct by ductus reuniens (Fig.). The epithelial cells lining the wall are specialized in certain regions to form sensory receptors, the maculae of utricle and saccule.
Maculae are receptors found on the lateral wall of utricle and medial wall of saccule. The macula consists of two types of cells:
namely, the sustentacular cells & hair cells.
The sustentacular cells are columnar supporting cells with microvilli on their free surface. Hair cells are fl ask shaped sensory cells lying between sustentacular cells. Hair cells are innervated by vestibular part of the vestibulocochlear nerve. Each hair cell is provided with long microvilli (stereocilia) of varying length and single nonmotile kinocilium. The stereocilia and kinocilia are embedded in a thick gelatinous plaque of glycoprotein called otolithic membrane, secreted by sustentacular cells. It contains numerous crystalline bodies called otoliths or otoconia (Fig.). Maculae are sensitive to orientation of the head in


relation to gravity or other acceleration forces. It is an organ of static balance or position sense.
Fig. : Macula