Dr. Reem A.R. Alsaad Medicine College of university Babylon Photoreceptor and Audioreceptor Systems (Organs of Special Senses) The Photoreceptor System The Eye :- The eye is a complex and highly developed photosensitive organ that permits an accurate analysis of the form, light intensity, and color reflected from objects. The eyes are located in protective bony structures of the skull, the orbits

Dr. Reem A.R. Alsaad
Medicine College of university Babylon
Photoreceptor and Audioreceptor Systems
(Organs of Special Senses)
The Photoreceptor System
The Eye :-
The eye is a complex and highly developed photosensitive organ that permits an accurate analysis of the form, light intensity, and color reflected from objects.
The eyes are located in protective bony structures of the skull, the orbits.

Each eye is composed of 3 layers:-
1- an external layer that consists of the sclera and the cornea.
2- a middle layer (the vascular layer)-consisting of the choroid, ciliary body, and iris.
3- an inner layer of nerve tissue, the retina, which consists of an outer pigment epithelium and an inner retina proper.

Note :- the retina proper communicates with the cerebrum through the optic nerve and extends forward to the ora serrata.




The eye contains 3 compartments:-
1- the anterior chamber: which occupies the space between the cornea and the iris and lens.

2- The posterior chamber: between the iris, ciliary process,
zonular attachments, and lens.

3- The vitreous space: which lies behind the lens and zonular attachments and is surrounded by the retina.


Both the anterior and posterior chambers contain fluid called aqueous humor. The vitreous space is filled with a gelatinous substance called the vitreous body.

The eye layers

1-External layer, or (Tunica fibrosa):
The opaque white posterior five sixths of the external layer of the eye is the sclera. The sclera consists of tough, dense connective tissue. The external surface of the sclera. The




episclera is connected to a dense layer of connective tissue called Tenon s capsule. Between Tenon s capsule and the sclera is Tenons space, so the eyeball can make rotating movements.
Between the sclera and the choroid is the suprachoroidal lamina, a thin layer of loose connective tissue. The sclera is relatively avascular.


The anterior one sixth- the cornea- is colorless and transparent.
The cornea is consists of 5 layers:-
1- epithelium (stratified squamous nonkeratinized, 5-6 layers). The surface corneal cells show microvilli protruding into the space filled by the precorneal tear film. The cornea has one of the richest sensory nerve supplies of any eye tissue.
2- Bowman s membrane (thick homogeneous layer of collagen fibers).
3- Stroma : the stroma is formed of many layers of paralled collagen bundles and fibroblasts. The stroma is avascular area.
4- Descemet s membrane is a thick homogeneous structure composed of fine collagenous filaments.
5- Endothelium is a simple squamous epithelium. The corneal endothelium and epithelium are responsible for maintaining the transparency of the cornea.


The corneoscleral junction, or limbus, is an area of transition from the transparent collagen bundles of the cornea to the white opaque fibers of the sclera.
It is highly vascularized, and its blood vessels assume an important role in corneal inflammatory processes.
The cornea, an avascular structure.
Schlemm s canal, which drains fluid from the anterior chamber of the eye and communicates externally with the venous system.

2-Middle, or vascular, layer:-

The middle layer of the eye consists of 3 parts :
- choroid
- ciliary body
- iris






choroid :-
the choroid is a highly vascularized coat, with loose connective tissue. Melanocytes are a abundant and give it its characteristic black color. It has a function in nutrition of the retina, and damage to this tissue causes serious damage to the retina.
The choroid is bound to the sclera by the suprachoroidal lamina, a loose connective tissue rich in melanocytes.
Ciliary body
The ciliary body, an anterior expansion of the choroid at the level of the lens, is a continuous ring that lies at the inner surface of the anterior portion of the sclera, it forms a triangle .
One of its faces is in contact with the vitreous body, one with the sclera, and the third with the lens and the posterior chamber of the eye.
The ciliary body is loose connective tissue surrounding the ciliary muscle.
Ciliary processes
The ciliary processes are ridge like extensions of the ciliary body. They have a loose connective tissue core and capillaries and are covered by two simple epithelial layers. From the ciliary processes emerge fibers (zonule fibers) that insert into the lens.





Iris
The iris is an extension of the choroids that partially covers the lens, leaving a round opening in the center called the pupil.
The anterior surface is irregular and rough, with grooves and ridges and it is formed of
A-layer of pigment cells
B-poorly vascularized connective tissue
C- rich in blood vessels in loose connective tissue
The posterior surface is smooth and covered by 2 layers of epithelium
-the inner epithelium is pigmented with melanin granules
-the outer epithelial cells have radially directed tongue like extensions, creating the dilator papillae muscle of the iris.







Lens
The lens is a biconvex structure characterized by great elasticity, a feature that is lost with age as the lens hardens. The lens has 3 components:-
1-Lens capsule :- the lens is enveloped by a thick (10-20)µm homogeneous, retractile ,carbohydrate-rich capsule coating the outer surface of epithelial cells

2-Sub-capsular epithelium :-consists of a single layer of cuboidal epithelial cells that is present only on the anterior surface of the lens.

3-Lens fibers :- are elongated and appear as thin , flattened structures .They are highly differentiated cells derived from cells of the sub-capsular epithelium.


2-Vitreous Body
The vitreous body occupies the region of the eye behind the lens. It is a transparent gel that consists of water (99%), collagen, and hyaluronic acid.

3-Retina
The retina, the inner layer of the globe, consists of 2 portions. The posterior portion is photosensitive, the anterior part is constitutes the inner lining of the ciliary body and the posterior part of the iris


Accessory Structures of the Eye
1-Conjunctiva:-
The conjunctiva is a thin, transparent mucous membrane that covers the anterior portion of the eye up to the cornea and the internal surface of the eyelids.
Conjunctiva (mucous membrane:-
1. stratified columnar epithelium with goblet cells
2- lamina propria is loose connective tissue
2-Eyelids:-
Eyelids are movable folds of tissue that protect the eye. The skin of the lids is loose and elastic.
The 3 types of glands are :-
1. the Meibomian (sebaceous glands)
2. the glands of Moll (sweat glands)
3. Zeis glands (sebaceous glands)




3-lacrimal Apparatus:-
The lacrimal apparatus consists of :-
1.one of the lacrimal gland
2. canaliculi
3. lacrimal sac
4. nasolacrimal duct.

The lacrimal gland is a tear-secreting gland located in the anterior superior temporal portion of the orbit.



The lacrimal gland is a tubuloalveolar gland that has distended lumens and is composed of column-shaped cells of the serous type, resembling the parotid acinar cells.
The lacrimal glands secrete a fluid rich in lysozyme, an enzyme that hydrolyzes the cell walls of bacteria, facilitating their destruction
The second lecture .
Hearing :The Audioreceptor system
The Ear(Vestibulocochlear Apparatus)
The functions of the vestibulochlear apparatus are related to equilibrium and hearing .The organ consists of three parts:
1-External ear : which receives sound waves ,it contains three components: -
A- The auricle (pinna) consists of an irregularly shaped plate of elastic cartilage covered by tightly adherent skin on all sides.
B-The external auditory meatus is a somewhat flattened canal extending from the surface into the temporal bone. its internal limit is the tympanic membrane. A stratified squamous epithelium continuous with the skin lines the canal .Hair follicles , sebaceous glands and the ceruminous glans ( type of modified sweat gland)are found in the sub-mucosa.
The ceruminous gland:-are a coiled tubular glands that produce the cerumen –or earwax a brownish ,semisolid mixture of fats and waxes.
C-The tympanic membrane(eardrum) its external surface is covered with a thin layer of epidermis , and its inner surface is covered with simple cuboidal epithelium continuous with the lining of the tympanic cavity .between the two epithelial coverings is a tough connective tissue layer composed of collagen and elastic fibers and fibroblasts, the tympanic membrane is the structure that transmit sound waves to the ossicles of the middle ear.
2- Middle ear :in which sound waves are transmitted from air to bone and by bone to the internal ear, middle ear or tympanic cavity is an irregular space that lies in the interior with the pharynx via the auditory tube (Eustachian tube)and posteriorly with the air-filed cavities of the mastoid process of the temporal bone .The middle ear is lined with simple squamous epithelium resting on a thin lamina propria .the epithelium that lines the middle ear is gradually transformed into ciliated pseudostratified columnar epithelium,
-Audoitory tube :-the tube opens during the process of swallowing , blancing the pressure of the air in the middle ear with atmospheric pressure.
The medial bony wall of the middle ear are two membrane covered oblong regions devoid of bone are the
1-Oval window:-have the Mastoid cavites
2- Auditory ossicles: the consists from
-Malleus
-Incus
-Stapes
The malleus inserts itself into the tympanic membrane and the stapes into the membrane of the oval window , these bones are articulated by synovial joints and , like all structures of this cavity , are covered with simple squamous epithelium .. in the middle ear small muscles that insert themselves into the malleus and stapes .they have a function in regulating sound conduction.

3- Internal ear :which these vibrations are transduced to specific nerve impulses that pass via the acoustic nerve to the vestibular organ , which maintains equilibrium. It consists from two part

1- Labyrinth it consists from
-bony labyrinth
-membranous labyrinth:- it giving rise to two specialized regions of the membranous labyrinth :
a-the utricle: the semicircular ducts originate from utricle
b-saccule : whereas the elaborate cochlear duct is formed from the utricle.
The caccule and the utricle are composed of a thin sheath of connective tissue lined with simple squamous epithelium .
c-semicircular ducts
d-endolymphatic duct and sac.

2-Cochlea :it derived from the saccule is highly specialized as a sound receptor , it is about 35 mm long and is surrounded by specialized perilymphatic spaces .The cochlea histological sections divided into three complaints
1-Scala vestibule:it consists from two layers of squamous epithelium , one derived from the scala media and the other from the lining of the scala vestibule, cells of both layers are joined by means of extensive tight juunctions that help preserve the very high ionic gradients across this membrane
-the stria vascularis :-is an unusual vascularized epithelium located in the lateral wall of the cochlear duct , it consists of cell that have many deep inholdings of their basal plasma membranes where numerous, mitochondria are located, these characteristics indicate that they are ion-and water –transporting cells.
2-scala media : the structure of the internal ear that containts special auditory receptors is called the organ of Corti ;
Corti organ :-it is contains hair cells pond to different sound frequencies .It consists from three layers
-basilar membrane:- it thick layer of ground substance , it supporting cells and two types of hair cells can be distinguished
-outer hair cells
-inner hair cells
The most characteristic feature of these cells is the W-shaped (outer hair cells ) or linear (inner hair cells).Of the supporting cells , the pillar cells should be singled out for special mention .Pillar cells contain a large number of microtubules that seems to impart stiffness to these cells. They outline a triangular space between the outer and inner hair cells , the inner tunnel this structure is important in sound transduction.
3- scala tympani




3- scala tympani