ear pdf - Dr. Rand - Muhadharaty

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Histology

Organs of special senses

THE EAR (Vestibulocochlear Apparatus)

The ear contains a series of receptors specialized for hearing

&equilibrium. The ear is divided into three parts, the external ear, the

middle ear (or tympanic cavity), and the internal ear (or labyrinth)

where the vibrations are transformed to specific nerve impulses which pass

to the central nervous system.

Fenestra Vestibuli (oval window):

Is an oval opening on the medial wall of the tympanic cavity; it is closed

by the footplate, or base, of the stapes. On the medial side of the window is

the perilymph of the scala vestibuli of the internal ear.

Fenestra Cochleae (round window):



Is a circular opening on the medial wall of the tympanic cavity below

and behind the fenestra vestibuli. It is closed by an elastic membrane

called the secondary tympanic membrane. On the medial side of this

window is the perilymph of the blind end of the scala tympani.



When the pressure of the perilymph rises as a result of medial

movement of the base of the stapes, the secondary tympanic membrane

stretches laterally and bulges into the tympanic cavity to accommodate

the increased pressure.

INTERNAL EAR (LABYRINTH):

Is situated in the petrous part of the temporal bone and consists of an

interconnected series of bony-walled chambers and passages containing

similarly shaped membranous sacs and canals. They are known as:

(1) The bony labyrinth, comprising a series of cavities within the bone.

(2) The membranous labyrinth, comprising a series of membranous sacs

and ducts contained within the bony labyrinth.

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Histology

Organs of special senses

1- Bony Labyrinth:



Consists of three parts; the vestibule, the semicircular canals, and the

cochlea. The bony walls of the vestibule and semicircular canals are

lined with several layers of flattened connective tissue cells that form

the mesothelium. From this layer, thin trabeculae, consisting of fine

fibrils and fibroblasts, extend to the outer walls of the utricle, saccule

and semicircular ducts and support these parts of the membranous

labyrinth. Blood vessels are also found in this connective tissue.



The bony labyrinth contain a tissue fluid very similar to cerebrospinal

fluid, the perilymph, in which is suspended the membranous labyrinth.

Vestibule

Is the central part of the bony labyrinth and lies posterior to the cochlea

and anterior to the semicircular canals. In its lateral wall are the fenestra

vestibuli and the fenestra cochleae. Lodged within the vestibule are the

saccule and utricle of the membranous labyrinth.

Semicircular Canals:



The three semicircular canals the superior (anterior), posterior, &

lateral open into the posterior part of the vestibule. Each canal has a

swelling at one end called the ampulla. Each canal open into the

vestibule. Lodged within the canals are the semicircular ducts. The

superior semicircular canal is positioned vertically and anteriorly and

lies at right angles to the posterior semicircular canal. The lateral canal

is set in a horizontal position.

Cochlea



This structure is a diverticulum of the saccule. It is highly specialized as

a sound receptor. The cochlea resembles a snail’s shell and contains the

cochlear duct. It opens into the anterior part of the vestibule. about

35mm in total length, makes two-and-one-half spiral turns around a

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Histology

Organs of special senses

spongy bon known as the modiolus. Each successive turn is of

decreasing radius, so that the whole structure is conical. The apex faces

anterolaterally, and the base posteromedially.



The modiolus has spaces containing blood vessels and the cell bodies

and processes of the acoustic branch of the eighth cranial nerve (spiral

ganglion). Extending laterally from the modiolus is a thin bony ridge,

the osseous spiral lamina. The modiolus has a broad base that is

situated at the bottom of the internal acoustic meatus.

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Histology

Organs of special senses

2- Membranous Labyrinth



These are membranous sacs and canals lodged within the bony

labyrinth, filled with endolymph and surrounded by perilymph.



It consists of the utricle and saccule, which are lodged in the bony

vestibule; the three semicircular ducts; which are lodged in the

semicircular canals, and the duct of the cochlea, which lies within the

bony cochlea. All these structures freely communicate with one another



The membranous labyrinth connected to the periosteum of the osseous

labyrinth by thin strands of connective tissue containing blood vessels.

Flowing between the connective tissue strands is the perilymph.

Utricle and Saccule:



The utricle and saccule are two dilated regions of the membranous

labyrinth lying within the vestibule of the inner ear and are joined by

the short utriculosaccular duct, which leads into the slender

endolymphatic duct.



Both the utricle and the saccule are filled with endolymph, (a fluid with

high K

and low Na

content) and are suspended in the perilymph of the

vestibule.



Internally, the utricle and saccule are composed of a thin sheath of

connective tissue lined with simple squamous epithelium but in each

there is a small region of highly specialized epithelium called the

macula, containing differentiated neuroepithelial cells that are

innervated by branches of the vestibular nerve.



Maculae in both locations have the same basic histologic structure and

consist of a thickening of the wall and possess two types of receptor

cells (hair cells), some supporting cells, and the afferent and efferent

nerve endings.



These receptors are sensitive to the orientation of the head in relation to

gravity or other acceleration forces. The maculae of the utricle and

saccule are disposed perpendicularly to one another. The macula

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Histology

Organs of special senses

consists of a thickened area of neuroepithelium resting on a basal

lamina and it consist of three types:

•

(1) Type I hair cells, (2) Type II hair cells, (3) supporting cells.



Type I hair cells are flask-shaped; and stains poorly; their nuclei

tending to lie lower level than those of Type II hair cells which are

cylindrical.



Each hair cell possesses on its free surface 40-80 elongated highly

specialized microvilli, or stereocilia, and a single kinocilium; the latter

has the microtubular structure of a cilium but is longer than the

microvilli.



The microvilli and the kinocilium constitute the so-called hairs

embedded in a thick gelatinous glycoprotein membrane called the

otolithic membrane (probably secreted by the supporting cells; this is

lost during histological preparation). Within the membrane are

numerous crystalline bodies called otoliths, composed of calcium

carbonate and associated with protein.



The type I & II hair cells are invested by a meshwork of dendritic

processes of afferent sensory neurons. Both cell types have efferent

nerve endings that are probably inhibitory.



The supporting cells are tall and columnar, lie on the basal lamina

between the hair cells, each possesses small microvilli on its free

surface and basally located nucleus. These cells may assist in the

nutrition of the hair cells or modify the composition of the endolymph.

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Histology

Organs of special senses

Semicircular Ducts:



Three semicircular canals arise from the vestibule of the inner ear, each

containing a membranous semicircular duct which opens at both ends

into the utricle.



At one end of each duct is a dilated portion, the ampulla, which

contains a receptor organ, called the crista ampullaris Which is an

elongated epithelial structure situated on a ridge of supporting tissue

arising from the membranous wall of the ampulla and oriented at right

angles to the direction of flow of endolymph in the semicircular duct.

Structurally, the crista is composed of Type I and Type II hair cells

supported by a single layer of columnar supporting cells which is

continuous with the simple cuboidal epithelium lining the rest of the

membranous labyrinth.



Like those of the maculae, the hair cells of the cristae have numerous

stereocilia and a single kinocilium. The stereocilia and the kinocilia of

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Histology

Organs of special senses

the hair cells are embedded in a conical form of gelatinous glycoprotein

called cupula. In contrast to the macula, the cupula does not contain

otolithic crystals.



Movement of the endolymph in the semicircular ducts, associated with

an angular movement of the head, causes the cupula to move and pull

on the hairs of the hair cells. These cells

are stimulated and in turn stimulate the

nerve endings of the vestibular division

of the eighth cranial nerve.

Duct of the Cochlea (Scala media):



The duct of the cochlea is a spirally arranged tube lying within the bony

cochlea.



It is triangular in cross section.



The sides of the triangular cross section of the duct are made up of the

following structures: (1) the basilar membrane (forming the floor of

the triangular space) (2) the stria vascularis (forming the lateral wall)

and (3) the vestibular membrane (forming the roof the space).

Walls of the Cochlear Duct:



The basilar membrane consists of a thin sheet of fibrous tissue

(collagenous and some elastic fibers). It is stretched between the

osseous spiral lamina of the modiolus and the spiral ligament laterally.

The membrane is thinnest at the base of the cochlea and becomes

progressively thicker as it spirals towards the apex.

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Histology

Organs of special senses



The highly specialized epithelium that lies on the upper surface of the

basilar membrane forms the spiral organ of Corti; the undersurface

exposed to the scala tympani, is lined by simple epithelium (flattened

mesothelial cells).

Stria vascularis : It forms the outer wall of the cochlear duct. The lining

epithelial cells are stratified cuboidal with many epithelial and

subepithelial rich plexus of capillaries, a rare example of vascularized

epithelium in the human body. It is believed to form endolymph.

Vestibular membrane (Reissner’s membrane): It forms the third wall of

the cochlear duct. It composed of two layers of flattened epithelial cells

separated by a basal lamina. The cells possess numerous microvilli and

may be involved in fluid transport.



Examination of a cross section of the cochlea shows that the basilar and

vestibular membranes divide it into three distinct portions: a scala

vestibuli above (filled with perilymph), a scala tympani below (filled

with perilymph), and the cochlear duct sometimes called scala media

(filled with endolymph).



The perilymph within the scala vestibuli is separated from the tympanic

cavity by the base of the stapes and the annular ligament at the fenestra

vestibuli (oval window).



The perilymph in the scala tympani is separated from the tympanic

cavity by the secondary tympanic membrane at the fenestra cochleae

(round window)



The helicotrema is the point at the apex of the cochlea where the

scala vestibuli and the scala tympani become continuous.

Spiral Organ Of Corti :



The spiral organ of Corti contains the sensory receptor cells for hearing

and is situated on the cochlear duct surface of the basilar membrane.



The organ of Corti consists of two types of cells, sensory (hair) cells

and support cells of several different types, including the inner and

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Histology

Organs of special senses

outer pillar cells and inner and outer phalangeal cells. The bases of

the cells are wide, and their cell bodies incline toward each other so that

the upper extremities meet to enclose the tunnel of Corti.



The tunnel of Corti is a triangular-shaped canal at the centre of the

organ, bounded on each side by a single row of tall columnar cells

called pillar cells.



On the inner aspect of the inner row of pillar cells is a single row of

flask-shaped cells (inner sensory cells) or (hair cells).



Beyond the outer row of pillar cells there are three to five rows of outer

phalangeal cells which support the same number of rows of outer

sensory (hair) cells.



The hair cells are similar in structure to the Type Ι hair cells of the

maculae and ampullae, having many long stereocilia, but they lack

kinocilia.



From the layer of border cells which cover the spiral limbus, there

extends a flap-like mass of glycosaminoglycans called the tectorial

membrane overlying the sensory cells and within which the tips of the

stereocilia are embedded.



The support cells are tall columnar cells, their function appears to be to

support the hair cells and the nerve endings near the bases of the hair

cells. Afferent and efferent nerve endings are present between the inner

and outer hair cells. The efferent nerve fibers reach the brain via the

cochlear division of the eighth cranial nerve.

Spiral Ganglion



The spiral ganglion is a spiral-shaped mass of nerve cell bodies lying in

a canal at the extremity of the osseous spiral lamina of the modiolus.

The efferent nerve fibers consists of bipolar neurons whose central

processes converge to form the cochlear division of the eighth cranial

nerve. The vestibular division of the eighth cranial nerve, which

receives sensory information from the maculae of the utricle and

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Histology

Organs of special senses

saccule and from the ampullae of the semicircular canals, has a

vestibular ganglion situated in the internal auditory meatus.