• MORPHOLOGY
• ORBITAL CAVITY
• EYELIDS
• CONJUNCTIVA
• LACRIMAL GLAND
• WALLOFTHE EYEBALL
• OUTER LAYER
• MIDDLE LAYER
• INNER LAYER
• FUNDUS OCULI
• INTRAOCULAR FLUID
• INTRAOCULAR PliESSURE
• LENS
• STRUCTURE OF THE LENS
• CHANGES IN THE LENS DURING OLD AGE
• OCULAR MUSCLES
• MUSCLES OF THE EYEBALL
• INNERVATION OF OCULAR MUSCLES
• OCULAR MOVEMENTS
• MOVEMENTS IN VERTICAL AXIS
• MOVEMENTS IN TRANSVERSE AXIS
• MOVEMENTS IN ANTEROPOSTERIOR AXIS
• SIMULTANEOUS MOVEMENTS OF BOTH EYES
• GENERAL DESCRIPTION OF THE EYE
• MORPHOLOGY
Human eyeball (bulbus oculi) is approximately globe shaped. It is flattened from above downwards. The approoximate diameter of eyeball is 24 mm anteroposteriorly and 24 mm transversely.
Eyeball is made up of two segments. The anterior part is small and transparent. It is called cornea. This forms 1/6 of the eyeball. The posterior part is larger and forms 5/6 of the eyeball. The radius of this is about 8 mm. The posterior wall of this part is lined by the light sensitive structure called retina.
The center of anterior curvature of the eyeball is called the anterior pole, and the center of posterior curvature is called the posterior pole. The line jOining the two poles is called optic axis. The line joining a point in cornea little medial to anterior pole and the fovea centralis situated lateral to posterior pole is known as visual axis. The light rays pass through the visual axis of eyeball. The optic nerve leaves the eye, little medial to the posterior pole (Fig. 165-1).
• ORBITAL CAVITY
Except for the anterior 1/6, the eyeball is situated in the bony cavity known as orbital cavity or eye socket. A thick
Sclera ------
Choroid Retina -------
Vitreous Body --------
~~.---,.-----
Optic Disc-
--
Optic Nerve ----
--------Medial Rectus
---------Ciliary Muscle
\ S
.•...... '----------- uspensory
.•.......•.............. Ligament
............... Fovea Centralis
\
\
\
\ \
Po~terior Pole
FIGURE 165-1: Structwce of the eyeball
layer of areolar tissue is interposed between the bone and eye. This serves as a cushion to protect the eyeball from external force. The eyeballs are attached to orbital cavity by the ocular muscles.
• EYEUDS
Eyelids protect the eyeball from foreign particles coming in contact with its surface and cut off the light during sleep. The eyelids can be closed voluntarily and reflexly.
The margins of eyelids have sensitive hairs called the cilia. Each cilium has a follicle, which is surrounded by a sensory nerve plexus. When the dust particle comes in contact with cilia, these sensory nerves are activated resulting in rapid blinking of eyelids. Thus, the dust particle is prevented from reaching the eyeball. There are about 100 to 150 cilia in the upper eyelid and about 50 to 75 in the lower lid. Meibomian glands and some sebaceous glands are also situated in the eyelids. These glands open into the follicles of cilia. The infection of these glands leads to the development of common eye sty.
The opening between the two eyelids is called palpebral fissure. In adults, it is about 25 mm long. Its width is about 12 to 15 mm when opened.
• CONJUNCTIVA
It is a thin mucus membrane, which covers the exposed part of the eye. After covering the anterior surface, the conjunctiva is reflected into the inner surfaces of the eyelids. The part of conjunctiva covering the eyeball is called the bulbar portion. The part covering the eyelid is called the palpebral portion. When the eyelids are closed or opened, the opposed portions of conjunctiva slid over each other. The surface of conjunctiva is lubriicated by thin film of tears secreted by lacrimal gland.
• LACRIMAL GLAND
The lacrimal gland is situated in the shelter of bone, forming the upper and outer border of wall ofthe eye socket. From the lacrimal gland, tearflowsoverthe surface of conjunctiva and drains into nose via lacrimal ducts, lacrimal sac and nasolacrimal duct. Tear is a hypertonic fluid. Due to its continuous washing and lubrication, the conjunctiva is kept in moisture and is protected from infection. Tear also
contains lysozyme, that kills bacteria. .
• WALL OF THE EYEBALL
The wall of the eyeball is composed of 3 layers namely:
I. Outer layer which includes cornea and sclera.
1. Sclera
This is formed by the white fibrous tissues and elastic fibers. The part of the sclera where it is pierced by the optic nerve is thin with perforations. It is named as lamina cribrosa.
2. Cornea
It is the anterior 1/6 of tunica fibrosa and it is transparent. The sclera overlaps cornea and appears in front as white of the eye. The diameter of cornea is about 12 mm horizontally and 11 mm vertically. Cornea is formed by 5 layers of structures, which are from front:
i. Layer of stratified epithelium
ii. Bowman's membrane or anterior elastic lamina iii. Substantia proper
iv. Descemet's layer or posterior elastic lamina v. Layer of endothelial cells.
Cornea has a refractory index of 1.376. It is very sennsitive to pain, touch, pressure and cold. Center of cornea is more sensitive to pain because of rich supply of free nerve endings. Cornea is not vascularized and therefore, derives its nourishment mainly from aqueous humor. However, during pathological conditions, cornea becomes vascularized.
The transitional part of tunica fibrosa between sclera and cornea is called limbus. It is about 1 mm width. Only at the limbus, the blood vessels are seen which form superficial marginal plexus in limbus.
• MIDDLE LAYER OR TUNICA MEDIA OR
TUNICA VASCULOSA
The tunica media completely surrounds the eyeball except for a small opening in front known as the pupil. This layer comprises from behind:
1. Choroid,
2. Ciliary body and
3. Iris.
2. Ciliary Body
The choroid is extended anteriorly up to the insertion of ciliary muscle (the level of ora serrata). In front of ora sefrata, the tunica vasculosa is thickened to form ciliary body.
. This is in the form of a ring. Its outer surface is sepaarated from the sclera by perichoroidal space. The inner surface of the ciliary body faces the vitreous body and lens. The suspensory ligaments from the lens are attached to ciliary body. The anterior surface of ciliary body faces towards the center of cornea. From the surface, the iris arises.
Ciliary body has three parts viz:
i. Orbiculus ciliaris,
ii. Ciliary body proper and iii. Ciliary processes.
i. Orbiculus ciliaris: It is continuous with choroid and it forms the posterior 2/3 of ciliary body. It is about 4 mm broad.
ii. Ciliary body proper: It is made up of two sets of ciliary muscle namely, the outer longitudinal and inner circular muscles. The ciliary muscles are innerrvated by the parasympathetic fibers of oculomotor nerve.
iii. The ciliary processes: The ciliary processes are the triangular elevations on the inner surface of the ciliary body. There are about 70 ciliary processes, projecting towards the central axis of the eye to
form radial fringes called corona ciliaris. '
3. Iris
This is the anterior most part of vascular coat. It is a thin circular diaphragm, placed in front of the lens. It has a circular opening in the center called pupil.
Iris separates the space between cornea and lens into two chambers namely, the anterior and posterior chambers. Both the chambers are communicated with each other through pupil. The lateral border of anterior chamber is angular in shape. It is called iris angle or angle of anterior chamber.
Horizontal Cells
Amacrine Cells
----.- -----. __ ._-------
Outer Plexiform Layer
------------- ------ -._-
Inner Nuclear Layer
.... - - .
Inner Plexiform Layer
-------------------- ---- .•
Layer of Ganglionic Cells
------------ -----------_ .•
Layer of Nerve Fibers
........ - - - - - ... - ... - ... - .. - -
"Internal Limiting Membrane
FIGURE 165-2: Layers of retina
• INNER LAYER OR TUNICA INTERNA
OR TUNICA NERVOSA OR RETINA
Retina extends from the margin of optic disc to just behind the ciliary body. Here, it ends abruptly as a dentated border known as ora serrata. Retina has the receptors of vision.
Layers of Retina
Retina has ten 10 layers of structures (Fig. 165-2) which are from outwards to interior:
1. Layer of pigment epithelium
2. Layer of rods and cones
3. External limiting membrane
4. Outer nuclear layer
5. Outer plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglion cell layer
9. Layer of nerve fibers
10. Internal limiting membrane.
",",,'
1. Layer of Pigment Epithelium
This is the outermost layer situated adjacent to the cornea. This is a single layer of hexagonal epithelial ce"s. The outer portion of the epithelial cells I.e., towards choroid, contains nucleus and moderate number of round pigment granules-in retina. The inner portion has plenty of needle shaped dark pigment granules. Many protoplasmic extensions arise from the inner surface of cells and pass between the rods and cones. The cytoplasmic processes also contain dark pigment granules. The pigment present in this layer is a melanin called fuscin.
2. Layer of Rods and Cones
This layer lies between pigment epithelial layer and the external limiting membrane. The rods and cones are the light sensitive portions of the visual receptor cells viz. the rod cells and the cone ceils. The receptor cells are arranged in a parallel fashion and are perpendicular to the inner surface of the eyeball.
The structure of rod cell and cone cell is explained in the next chapter.
5. Outer Plexiform Layer
This layer contains reticular meshwork formed by the terminal fibers of rods and cones and the dendrites of bipolar cells, situated in the inner nuclear layer.
6. Inner Nuclear Layer
The inner nuclear layer contains small oval shaped flattened bipolar cells. The axons of the bipolar cells synapse with dendrites of ganglionic cells in the inner plexiform layer. The dendrites synapse with fibers of rods and cones in the outer plexiform layer. This layer also contains nuclei of Muller's supporting fibers and some association neurons. The association neurons are horiizontal cells and amacrine cells.
7. Inner Plexiform Layer
Thislayer of retina consists of synapses between dendrites of ganglionic cells and axons of bipolar cells. This also contains processes from amacrine cells.
8. Ganglion Cell Layer
Multipolar cells are present in this layer. Some cells are large and are called giant ganglion cells. Other cells are smaller called midget ganglion cells. The axons from ganglion cells are in the innermost layer of the retina and these axons form the optic nerve. The dendrites of the ganglion cells synapse with axons of bipolar cells in the inner plexiform layer. This layer also contains retinal blood vessels.
9. Layer of Nerve Fibers
This is formed by nonmyelinated axons of ganglionic cells. After taking origin, the axons run horizontally to a short distance. Afterwards, the fibers converge towards the optic disc and form the optic nerve. This layer also consists of neuroglial cells, Muller's cells and retinal blood vessels.
(
I I
I
Macula with Fovea Centralis
FIGURE 165-3: Fundus oculi
10. Internal Limiting Membrane
This is a hyaline membrane formed by the opposition of expanded ends of Muller's fibers. This layer separates retina from the vitreous body.
• FUNDUS OCULI
The posterior part of interior of the eyeball is called fundus oculi or fundus (Fig. 165-3). In living subjects, the fundus is examined by ophthalmoscope. The fundus has two important structures viz:
1. Optic disc
2. Macula lutea.
• OPTIC DISC
Optic disc or optic papilla is situated near the center of the posterior wall of eyeball. It appears as a pale disc. It is formed by the convergence ofaxons from ganglion cells, while forming the optic nerve. The optic disc contains all the layers of retina except rods and cones. So, thi~ is insensitive to light i.e., the object is not seen if its image falls upon this area. Because of this, the optic disc is known as blind spot.
• MACULA LUTEA
Macula lutea or yellow spot is small yellowish area of retina. It is situated a little lateral to the optic disc. The yellow colour of this is due to the presence of a yellow pigment.
There is a minute depression called fovea centralis in the center of macula lutea. Here, all the layers of retina
subject to gain only a dim and an ill define.d impression of surroundings.
• INTRAOCULAR FLUID
The fluid in the eyeball is responsible for the maintenance of shape of eye. Two types of fluids are present in the eye viz:
1. Vitreous body and
2. Aqueous humor.
• VITREOUS BODY
This is present in the space between the lens and retina. This is a gelatinous substance. It is also known as vitreous humor. It is formed by a fine fibrillar network of proteoglycan molecules. Various substances enter vitreous body by means of diffusion.
• AQUEOUS HUMOR
Aqueous humor is a thin fluid, which fills the space between the lens and cornea. The space between lens and cornea is divided into anterior and posterior chambers by Iris. Both the chambers communicate with each other through pupil.
Properties of Aqueous Humor
Volume 0.13 ml
Reaction and pH Alkaline with apH of 7.5
Viscosity 1.029
Refractory index 1.34
Composition of Aqueous Humor
Aqueous humor consists of 98.7% water and 1.3% solids. The solids are organic and inorganic substances. The organic substances are albumin, globulin, glucose, pyruvate, lactate and urea. The inorganic substances are sodium, calcium, potassium, magnesium, chlorides, phosphates and bicarbonates.
passes through the spaces between the suspensory ligaments and pupil towards anterior chamber. In this chamber, the fluid passes into the angle between cornea and iris. From here, the fluid passes through the meshhwork of trabeculae and the canal of Schlemm, into the extraocular veins.
Functions of Aqueous Humor
1. Aqueous humor is responsible for the maintenance of shape of the eyeball.
2. It maintains the intraocular pressure.
3. It provides nutritive substances to the avascular strucctures like lens and cornea.
• INTRAOCULAR PRESSURE
The normal intraocular pressure varies between 12 to 20 mm Hg. It is measured by tonometer. When intraaocular pressure increases to about 60 to 70 mm Hg, the disease called glaucoma occurs.
In glaucoma, the vision is lost. The blindness occurs due to the compression of optic nerve fibers at the optic disc. Glaucoma occurs due to increased resistance to the drainage of aqueous humor through trabeculae. In old age, the glaucoma occurs due to the obstruction of trabeculae by fibrous structures.
• LENS
The lens of the eyeball is crystalline in nature. 't is biconvex, transparent and possesses the elastic property. The lens does not have blood supply and receives its nutrition mainly from the aqueous humor. The focal length of human lens is 44 mm and its refractory power is 23D.
• STRUCTURE OF THE LENS
The lens is formed of three components namely:
1. The capsule
2. The anterior epithelium
3. The lens substance.
" /"
Sphenoid Bone
I Lateral Rectus
(VI)
I Inferior Rectus (III)
",
" Cornea
----- Eyeball
...•..•.
\ '----Maxilla
\
Inferior Oblique
(III)
FIGURE 165-4: Extrinsic muscles of eyeball. Numbers in parenthesis indicate the cranial nerve supplying the muscle
1. The Capsule
Capsule is a highly elastic membrane covering the lens.
2. The Anterior Epithelium
It is a single layer of cuboidal epithelial cells situated beneath the capsule. At the margins, the epithelial cells are elongated. The epithelial cells give rise to the lens fibers present in the lens substance.
3. The Lens Substance
This is formed by long lens fibers derived from the anterior epithelium. The lens fibers are prismatic in nature and are arranged in concentric layers.
• CHANGES IN THE LENS DURING OLD AGE After 40 to 45 years, the lens may loose its elastic property and the amplitude of accommodation is reduced. So, the person cannot see the near objects clearly. This condition is known as presbyopia. More details of presbyopia are given in Chapter 169. In old age after 55 to 60 years, the lens may become opaque due to the accumulation of fluid and denaturation of the proteins of the lens fibers. This condition is called cataract.
• OCULAR MUSCLES
• MUSCLES OF THE EYEBALL
The muscles of the eyeball are of two types namely the intrinsic muscles and the extrinsic muscles. The intrinsic muscles are formed by the smooth muscle fibers and are controlled by the autonomic nerves. The extrinsic muscles are formed by skeletal muscles and are controllled by the somatic nerves. The intrinsic muscles of the eye are constrictor pupillae and dilator pupillae. The details of the intrinsic muscles are given in Chapter. 169.
Generally, the term ocular muscles refers to the extrinnsic muscles of the eyeball. The eyeball moves with in the orbit by six extrinsic skeletal muscles (Fig. 165-4). One end of each muscle is attached to the eyeball and the other end to the wall of orbital cavity. There are four straight (rectus) and two oblique muscles namely:
1. Superior rectus,
2. Inferior rectus,
3. Medial or internal rectus,
4. Lateral or external rectus,
5. Superior oblique and
6. Inferior oblique.
I Abduction I
I Adduction I
I Depression I
FIGURE 165-5: Diagram showing the movements of right eye. MR-Medial rectus, SO-Superior oblique, LR-Lateral rectus, to-Inferior oblique, SR-Superior rectus, IR-Inferior rectus
• INNERVATION OF OCULAR MUSCLES
The ocular muscles are innervated by three cranial nerves viz:
1. Oculomotor (third) nerve,
2. Trochlear (forth) nerve and
3. Abducent (sixth) nerve.
1. Oculomotor Nerve
The oculomotor nerve supplies the i. Superior rectus, ii. Inferior rectus, iii. Medial rectus and iv. Inferior oblique.
2. Trochlear Nerve
This cranial nerve supplies the superior oblique.
3. Abducent Nerve
The abducent nerve innervates the lateral rectus.
•. OCULAR MOVEMENTS
The eyeball moves or rotates within the orbital socket in any of the three primary axes as given below (Fig. 165-5 and Table 165-1).
• MOVEMENTS IN VERTICAL AXIS
The movements of eyeball in vertical axis or in horizontal plane are of two types viz:
TABLE 165-1: Muscles taking part in
the ocular movements
Movement Primary Secondary
Muscle Muscle
1. Abduction Lateral Superior
Rectus Oblique and
Inferior Oblique
2. Adduction Medial Superior
Rectus Rectus and
Inferior Rectus
3. Elevation Superior Inferior
Rectus Oblique
4. Depression Inferior Superior
Rectus Oblique
5. Extortion Inferior Inferior
Oblique Rectus
6. Intortion Superior Superior
Oblique Rectus
1. Abduction or Lateral Movement or
Outward Movement
This movement of the eyeball is due the contraction of mainly the lateral rectus. It is supported by the two oblique muscles.
1. Elevation or Upward Movement
This movement of the eyeball is because of the contracction of superior rectus and the inferior oblique muscles.
2. Depression or Downward Movement
This action is brought out by the inferior rectus and supeerior oblique.
• MOVEMENTS IN ANTEROPOSTERIOR AXIS The movements of the eyeball in the anteroposterior axis or in the frontal plane are called the torsion or wheel moments. The two types of torsion movements are:
1. Extorsion
During this, the eyeball is rotated in such a way that the cornea is turned upward and outward direction. This movement is due to the contraction of inferior oblique and inferior rectus.
2. IntorsIon
During intorsion, the eyeball is rotated so that, the corrnea moves in downward and inward directions. This is produced by the contraction of superior oblique and superior rectus muscles.
remain parallel. This is due to contraction of medial rectus of one eye and lateral rectus of other eye.
2. Dls/uate Movement
The movement of both eyes in the opposite direction is called the disjugate movement. There are two types of disjugate movement namely, convergence and diverrgence.
Convergence:The movement of both the eyes towards nose is called convergence. It is due to the simultaneous contraction of.medial rectus and simultaneous relaxation of lateral rectus of both eyes. The visual axes move close to each other. Convergence of eyeballs occurs during accommodation.
Divergence: The movement of both the eyes towards the temporal side is called divergence. It is due to the simultaneous contraction of lateral rectus and simultaaneous relaxation of medial rectus of both eyes. The visual axes of the eyes move away from each other.
3. Persult Movement
When the eyes are fixed on a moving object, the eyeballs also move along with the object. This type of movement is called persuit movement.
4. Saccadic Movement
When the fixation of eyes (gaze) is shifted from one object to another object, both the eyes show some quick jerky movements called saccadic movement or optokinetic movement.
-
1. ElevatIon or Upward Movement
This movement of the eyeball is because of the contracction of superior rectus and the inferior oblique muscles.
2. DepressIon or Downward Movement
This action is brought out by the inferior rectus and supeerior oblique.
• MOVEMENTS IN ANTEROPOSTERIOR AXIS The movements of the eyeball in the anteroposterior axis or in the frontal plane are called the torsion or wheel moments. The two types of torsion movements are:
1. ExtorsIon
During this, the eyeball is rotated in such a way that the cornea is turned upward and outward direction. This movement is due to the contraction of inferior oblique and inferior rectus.
2. IntorsIon
During intorsion, the eyeball is rotated so that, the corrnea moves in downward and inward directions. This is produced by the contraction of superior oblique and superior rectus muscles.
remain parallel. This is due to contraction of medial rectus of one eye and lateral rectus of other eye.
2. DlsJuate Movement
The movement of both eyes in the opposite direction is called the disjugate movement. There are two types of disjugate movement namely, convergence and diverrgence.
Convergence:The movement of both the eyes towards nose is called convergence. It is due to the simultaneous contraction of, medial rectus and simultaneous relaxation of lateral rectus of both eyes. The visual axes move close to each other. Convergence of eyeballs occurs during accommodation.
Divergence: The movement of both the eyes towards the temporal side is called divergence. It is due to the simultaneous contraction of lateral rectus and simultaaneous relaxation of medial rectus of both eyes. The visual axes of the eyes move away from each other.
3. Per suIt Movement
When the eyes are fixed on a moving object, the eyeballs also move along with the object. This type of movement is called persuit movement.
4. SaccadIc Movemen.t
When the fixation of eyes (gaze) is shifted from one object to another object, both the eyes show some quick jerky movements called saccadic movement or optokinetic movement.
-
• INTRODUCTION
• IMAGE FORMING MECHANISM
• NEURAL BASIS OF VISUAL PROCESS
• STRUCTURE OF ROD CELL
• STRUCTURE OF CONE CELL
• FUNCTIONS OF RODS AND CONES
• CHEMICAL BASIS OF VISUAL PROCESS
• RHODOPSIN
• PHOTOTRANSDUCnON
• PHOTOSENSITIVE PIGMENTS IN CONES
• DARK ADAPTATION
• LIGHT ADAPTATION
• NIGHT BLiNDNESl
• ELECTRICAL BASIS OF VISUAL PROCESS • DEFINITION
• ACUITY OF VISION
• DEFINITION
• TEST FOR VISUAL ACUITY
• INTRODUCTION
When the image of the object in environment is focused on retina, the energy in visual spectrum is converted into electrical potentials (impulses) by rods and cones of retina through some chemical reactions. The impulses from rods and cones reach the cerebral cortex through optic nerve. And, the sensation of vision is produced in cerebral cortex. Thus, process of visual sensation may be explained on the basi~ of image formation, and neural, chemical and electrical phenomena.
• IMAGE FORMING MECHANISM
While looking at an object, the light rays from the object - are refracted and brought to a focus upon retina. The image falls on the retina in an inverted position and reversed side to side. In spite of this, the object is seen in an
upright position. This is because of the role played by cerebral cortex.
The light rays are refracted by the lens and cornea.
The refractory power is measured in diopter (0). A diopter is the reciprocal of focal length expressed in meters'.
The focal length of cornea is 24 mm and refractory power is 420. The focal length of lens is 44 mm and refractory power is 230.
• NEURAL BASIS OF VISUAL PROCESS
The retina contains the light sensitive receptors or photooreceptors, which are rods and cones. There are about 6 million cones and 12 million rods in the human eye. The distribution of the photoreceptors varies in different areas of retina. Fovea has only cones and no rods. While proceeding from fovea towards the periphery of retina,
___ Inner Segment
Cell Body
" .•.•.
.•.•..••. /'
.... Nucleus'
Synaptic - -Terminal
FIGURE 166-1: Structure of visual receptors
the rods increase and the cones decrease in numbek At the periphery of the retina, only the rods are present and the cones are absent.
• STRUCTURE OF ROD CELL
Rod cells are cylindrical structures with a length of about 40 to 60 I..l and a diameter of about 21..l. Each rod is commposed of four structures namely:
1. Outer segment,
2. Inner segment,
3. Cell body and
4. Synaptic terminal.
1. Outer Segment
In rod cell, the outer segment is long, slender and gives the rod like appearance (Fig. 166-1). This segment is in close contact with the pigment epithelial cells. The outer segment of rod cell is formed by the modified cilia and it contains a pile of freely floating flat membranous discs. There are about 1000 discs in each rod. The discs in rod cells are closed structures and contain the photosensiitive pigment, the rhodopsin.
The rhodopsin is synthesized in inner seg~nts and inserted into newly formed membranous discs at the inner portion of outer segment. The new discs push the older discs towards the outer tip. The older discs are engulfed
I ne UnltH ::it::YIIIt:IIL I;:) ,",VIIIU:::;\.I'U:'U LV LII"" -,.". •• _. --~"'-""-J
means of modified cilium. The inner segment contains many types of organelles with large number of mitochonndria.
3. Cell Body
A slender fiber called rod fiber arises from the inner seggment of the rod cell and passes to the outer nuclear layer through external limiting membrane. In the outer nuclear layer, the enlarged portion of this fiber forms the cell body or rod granule that contains the nucleus.
4. Synaptic Terminal
A thick fiber arising from the cell body passes to outer plexiform layer and ends in a small and enlarged synapptic terminal or body. The synaptic terminal of the rods synapses with dendrites of bipolar cells and horizontal cells. The synaptic vesicles present in the synaptic termiinal contain the neurotransmitter, glutamate.
• STRUCTURE OF CONE CELL
Cone cell is the visual receptor with length of 35 to 40 Il and a diameter of about 5 Il. Generally, the cone cell is flask shaped. The shape and length of the cone vary in different parts of the retina. The cones in the fovea are long, narrow and almost similar to rods. In the periphery of the retina, the cones are short and broad. Those in the ora serata are still short. Like rods, cones are also formed by four parts viz:
1. Outer segment,
2. Inner segment.
3. Cell body and
4. Synaptic terminal.
1. Outer Segment
The outer segment is smaller and conical (Fig. 166-1). The outer segment of cone cell does not contain sepaarate membranous discs as in rods. In cone the infoldings of the cell membrane form the saccules, which are the counterparts of rod discs.
The photopigment ofthe cone is synthesized in the inner segment and incorporated into the folding of surface
3. Cell Body
The cone fiber arising from the inner segment is thick and it enters the inner nuclear layer through the external limitting membrane. In the inner nuclear layer, the cone fiber forms the cell body or cone granule that possesses the nucleus.
• RHODOPSIN
Rhodopsin or visual purple is the photosensitive pigment of rod cells. It is present in the membranous discs located in outer segment of rod cells.
4. Synaptic Terminal Chemistry of Rhodopsin
The fiber from the cell body of the cone leaves the inner Rhodopsin is a conjugated protein with a molecular weight
nuclear layer and enters the outer flexiform layer. Here, it
ends in the form of an enlarged synaptic terminal or body. of 40,000. It is made up of a protein called opsin and
a chromophore. The opsin present in rhodopsin is known The synaptic vesicle present in the synaptic terminal of
as scotopsin. Chromophore is a chemical substance that
cone cell also possesses the neurotransmitter, glutamate. develops colour in the cell. The chromophore present in.
-- the rod cells is called retinal. The retinal is the aldehyde of vitamin A or retinol.
Retinal is derived from food sources and it is not synthesized in the body. It is derived from the carotinoid substances like p carotene present in carrots.
Retinal is present in the form of 11-cis retinal known as retinine1• Retinine1 is different from retinine2 that is present in eyes of some animals. The significance of 111cis form of retinal is that only in this form, it can combine with scotopsin to synthesize rhodopsin.
• FUNCTIONS OF RODS AND CONES
Rods are extremely sensitive to light and have a low threshold. So, the rods are responsible for dim light vision or night vision or scotopic vision. But, rods do not take part in resolving the details and boundaries of objects (visual acuity) or the colour of the objects (colour vision). The vision by rod is black, white or in the combination of black and white namely, gray. Therefore, the coloured objects appear faded or grayish in twilight.
Cones have high threshold for light stimulus. So, the cones are sensitive only to bright light. Therefore, the cone cells are called receptors of bright light vision or photopic vision or day light vision. The cones also are responsible for acuity of vision and the colour vision.
Achromatic Interval
When an object is placed in front of a person in a dark room, he cannot see the object. When the object is slightly illuminated Le., when little light falls on the object, the person will see the object without colour. This is beecause, at this level only rods are stimulated. When, the illumination is increased further, the threshold for cones is reached. Now, the object could be seen in finer details and in colour. The interval between the threshold for rods and cones Le., the interval from when the object is first
Photochemical Changes In Rhodopsin
When retina is isolated and examined in dark, the rods appear in red because of rhodopsin. During exposure to , light, rhodopsin is bleached and the colour becomes yellow. When rhodopsin absorbs the light that falls on retina, it is split into retinine and the protein called opsin through various intermediate photochemical reactions (Fig. 16662).
The changes, which occur due to the absorption of light energy by rhodopsin, are:
1. First, rhodopsin is decomposed into bathorhodopsin
that is very unstable.
2. Bathorhodopsin is converted into lumirhodopsin.
3. Lumirhodopsin decays into metarhodopsin I.
4. Metarhodopsin I is changed to metarhodopsin II.
(Active Rhodopsin)
Retinal isomerase
Retinol isomerase
Dehydrogenase NADH2
FIGURE 166-2: Photoc~mical changes in rhodopsin. NADH2 = Reduced nicotinamide adenine dinucleotide
5. Metarhodopsin II is split into scotopsin and allretinal.
6. The all-trans retinal is converted into all-transretinol (vitamin A) by the enzyme dehydrogenase in the preesence of reduced nicotinamide adenine dinucleotide (NADHJ
Metarhodopsin is usually called the activated rhodoppsin since, it is responsible for the development of receptor potential in rod cells.
Resynthesis of Rhodopsin
Resynthesis of rhodopsin occurs in dark. First, the allltrans retinal is converted into 11-cis retinal by the enzyme retinal isomerase. 11-cis retinal immediately combines with scotopsin to form rhodopsin. The all transretinol (vitamin A) also plays an important role in the resynthesis of rhodopsin. The all-transretinol is converted into 111cis retinol by the activity of enzyme retinol isomerase. This is converted into 11-cis retinal, which combines with scotopsin to form rhodopsin. The all-trans retinol can also be reconverted into all-transretinal.
Rhodopsin can be synthesized directly from all-cis retinol (vitamin A) in the presence of nicotinamide adenine dinucleotide (NAD). However, the synthesis of rhodopsin
from 11-cis retinal (retinine) is faster than from 11-cis retinol (vitamin A).
• PHOTOTRANSDUCTION
Visual or phototransduction is the process by which the light energy causes development of receptor potential in visual receptors.
The resting membrane potential in other sensory receptor cells is usually between -70 and -90 mV. However, in the visual receptors in dark, the negativity is reduced and the resting membrane potential is aboLJl - 40 mV (a sligh! depolarization). This is because of influx of sodium ions. Normally in dark, the sodium ions are pumped out of inner segments of rod cell to extracellular fluid. However, these sodium ions leak back into the rod cells through the membrane of outer segment and reduce the electronegativity inside the rod cell (Fig. 166-3). Thus, the sodium influx maintains a slight depolarization (resting potential) up to - 40 mV. This potential is constant and it is also called the dark current.
The influx of sodium ions into the outer segment of rod cell occurs mainly because of cyclic .Quanosine monophosphate (cGMP) present in the cytop'l8sm of the cell. The cGMP always keeps the sodium channels opened.
Inner Segment
FIGURE 166-3: Maintenance of dark current (resting potential) in outer segment of rod cell
The closure of sodium channels occurs due to the reduction in cGMP. The concentration of sodium ions inside the rod cell is regulated by the sodium-potassium pump.
When light falls on retina, the rhodopsin is excited leading to the development of receptor potential in the rod cells. Following is the phototransduction cascade of receptor potential (Fig. 166-4).
1. When a photon (the minimum quantum of light energy) is absorbed by rhodopsin, the 11-cis retinal is decommposed into meta rhodopsin through few reactions mentioned earlier. The metarhodopsin II is considered as the active form of rhodopsin. It plays an important role in the development of receptor potential.
2. Metarhodopsin II activates a G-protein called transsducin that is present in rod discs.
3. The activated transducin activates the enzyme called cyclic guanosine monophosphate (cGMP) phosphoodiesterase, which is also present in the rod discs.
4. The activated cGMP phosphodiesterase hydrolyzes cGMP to 5'-GMP.
5. Now, the concentration of cGMP is reduced in the
rod cell.
6. The reduction in the concentration of cGMP immeediately causes closure of sodium channels in the membrane of visual receptors.
7. The sudden closure of sodium channels prevents the
Hyperpolarization in Receptor Cells
Reduced Release of Glutamate
Response in Bipolar and Ganglionic Cells
FIGURE 166-4: Phototransduction cascade. cGMP = Cyclic guanosine mono phosphate
entry of sodium ions leading to hyperpolarization. The potential reaches -70 to - 80 mY. This is because of sodium-potassium pump.
Thus, the process involved in receptor potential rod cells is unique in nature. When other sensory receptors are excited, the electrical response is in the form of depolarization (receptor potential). However, in visual receptors the response is in the form of hyperpolarization.
The photosensitive pigment in the cone cells is of three types namely porpyropsin, iodopsin and cyanopsin. Only one of these pigments is present in each cone. The photopigment in cone cell also is a conjugated protein made up of a protein and chromophore. The protein in cone pigment is called photopsin, which is different from scotopsin, the protein part of rhodopsin. However, the chroomophore of cone pigment is the retinal that is present in rhodopsin. Each type of cone pigment is sensitive to a particular light and the maximum response is shown at a particular light and wavelength. The details are given the Table 166-1
TABLE 166-1: Sensitivity of cone pigments
Pigment Giving response Maximum response
to at
Porpyropsin Iodopsin Cyanopsin
Red Green Blue
665 nm 535 nm 445 nm
The various processes involved in phototransduction in cone cells are similar to those in the rod cells.
• DARK ADAPTATION Definition
If a person enters a dim lighted room (darkroom) after spending a long time in bright lighted area, he is blind for sometime, Le. he can not see any object. After some time, he starts seeing the objects slowly. The process by which the person is able to see the objects in dim light is called dark adaptation. The maximum duration for dark adaptation is about 20 minutes.
Causes for Dark Adaptation
The dark adaptation is due to some changes, which occur in eyeball. The changes are:
1. Increased sensitivity of rods as a result of resynthesis of rhodopsin: The time required for dark adaptation is partly determined by the time to build up rhodopsin. In bright light, much of the pigment is being broken down. But in dim light, it may require some time for the regeneration of certain amount of rhodopsin,
cones are allowed to fUnctiOn well. I nus, u,~ lJ~r:;ur I VVt:;ClIIII~ red goggles can see well in bright lighted area and also can see the objects clearly as soon as he enters the dim lighted area.
• LIGHT ADAPTATION Definition
When a person enters a bright lighted area from a dim lighted area, he feels discomfort due to the dazzling effect of bright light. After some time, when the eyes become adapted to light, he sees the objects around him without any discomfort. This process is called light adaptation. It is the mere disappearance of dark adaptation. The maximum period for light adaptation is about 5 minutes.
Causes of Light Adaptation
There are two causes of light adaptation.
1. Reduced sensitivity of rods: During light adaptation, the sensitivity of rods decreases. This is due to the breakdown of rhodopsin.
2. Constriction of pupil: Constriction of pupil reduces quantity of light rays entering the eye.
• NIGHT BLINDNESS Definition
It is the defective dim light (scotopic) vision. It is defined as the loss of vision when light in the environment becomes dim. It is otherwise called nyctalopia.
CauseofNightSHndness
This is due to the deficiency of vitamin A, which is essential for the function of rods. The deficiency of vitamin A occurs because of any of the following causes.
1. The diet containing less amount of vitamin A
2. Decreased absorption of vitamin A from the intestine.
Vitamin A deficiency causes defective cone function.
Prolonged deficiency leads to anatomical changes in rods and cones, and finally the degeneration of other retinal layers occurs. So, retinal function can be restored, only
Method of Recording ERG
ERG is recorded by using a galvanometer or a suitable recording device. The recording electrode is placed on the cornea of eye in its usual forward up looking position. The indifferent electrode is placed over any moist surface of body, like inside the mouth.
10
B
>e
I II 4
'tl
f
161 f
Cessation of Light Stimulus
FIGURE 166--5: Electroretinogram
f
Light Stimulus
• ACUITY OF VISION
• DEFINITION
The ability of the eye to determine the precise shape and details of any object is called visual acuity or acuity of vision. It is also defined as the ability to recognize the separateness of two objects placed together. Cones ofthe retina are responsible for acuity of vision. Visual acuity is highly exhibited in fovea centralis, which contains only cones. It is greatly reduced during the refractory errors.
• TEST FOR VISUAL ACUITY
Acuity of vision is tested for distant vision as well as near vision. If there is any difficulty in seeing the distant object or the near object, the defect is known as error of refraction. "The refractive errors are described separately in Chapter 171.
Distant Vision
Snellen's chart is used to test the acuity of vision for distant vision in the diagnosis of refractive errors of the eye.
Near Vision
Jeagers chart is used to test the visual acuity for near
vision. .
• DEFINITION
• BINOCULAR AND MONOCULAR VISION
• DIVISIONS OF VISUAL FIELD
• CORRESPONDING RETINAL POINTS
• DIPLOPIA
• BLiNDSPOT
• VISUAL FIELD AND RETINA
• MAPPING OF VISUAL FIELD
• DEFINITION
The part of the external world seen by one eye when it is fixed in one direction is called visual field of that eye. According to Traquir, the visual field is described as" island of vision surrounded by a sea of blindness".
• BINOCULAR AND MONOCULAR VISION
• BINOCULAR VISION
In man and some animals, the eyeballs are placed in front ofthe head. So, the visual fields of both the eyes overlap. That is, a portion of the external world is seen by both the eyes. This type of vision is called binocular vision.
• MONOCULAR VISION
In some animals like dog, rabbit and horse, the eyeballs are present at the sides of head. So, the visual fields of both eyes overlap to a very small extent. This type of vision is called monocular vision.
• DIVISIONS OF VISUAL FIELD
The visual field of human eye has an angle of 160° in horizontal meridian and 135° in vertical meridian. The visual filed is divided into four parts viz:
1. Temporal field
2. Nasal field
3. Upper field and
4. Lower field.
• TEMPORAL AND NASAL FIELDS
The visual field of each eye can be divided into two unequal parts namely, outer or temporal part or field and the inner or nasal part by a vertical line passing through the fixation point. The fixation point is the meeting point of visual axis with the object.
The temporal part of visual field extends up to about 100° but the nasal part extends only up to 60° because it is restricted by nose.
• UPPER AND LOWER FIELDS
The visual field of each eye is also divided into an upper part or field and a lower part or field by a horizontal line passing through the fixation point. The extent of the upper field is about 60° as it is restricted by upper eyelid and orbital margin. The extent of lower field is about 75°. This is restricted by cheek. Thus, the visual field is restricted in all the sides except in the temporal part.
• CORRESPONDING RETINAL POINTS
In the binocular vision, the objects are seen by both eyes and, the points of retina in both eyes on which the light rays from the object fall are called corresponding retinal points. The two images developed on retina of
Diplopia means double vision. While looking at an object, if the eyeballs are directed in such a way that the light rays from the object do not fall upon the corressponding point on the retina of both eyes, a double vision occurs, i.e. one single object is seen as two.
Causes of Diplopia
1. Paralysis or weakness of ocular muscles causes permanent diplopia.
2. In alcoholic intoxication, the unbalanced actions of ocular muscles produce temporary diplopia.
Experimental Diplopia
Diplopia can also occur in the following experimental conditions:
1. Applying pressure from the outer side of one eye and thus displacing the eye from its normal position.
2. By holding an object like pen or pencil vertically in front of face at about 5 cm from the root of nose. It is not possible to converge the eyeballs sufficiently, and the
• VISUAL FIELD AND RETINA
The light rays from different halves of each visual field do not fall on the same halves of the retina. The light rays from temporal part of visual field of an eye fall on the nasal half of retina of that eye. Similarly, the light rays from nasal part of visual field fall on the temporal half of retina of the same side.
• MAPPING OF VISUAL FIELD
. The shape and extent of visual field is mapped out by means of an instrument called perimeter and this techniique is called perimetry. The visual field can also be deterrmined by Bjerrum's screen or by confrontation test.
Diplopia means double vision. While looking at an object, if the eyeballs are directed in such a way that the light rays from the object do not fall upon the corressponding point on the retina of both eyes, a double vision occurs, i.e. one single object is seen as two.
Causes of Diplopia
1. Paralysis or weakness of ocular muscles causes permanent diplopia.
2. In alcoholic intoxication, the unbalanced actions of ocular muscles produce temporary diplopia.
Experimental Diplopia
Diplopia can also occur in the following experimental conditions:
1. Applying pressure from the outer side of one eye and thus displacing the eye from its normal position.
2. By holding an object like pen or pencil vertically in front of face at about 5 cm from the root of nose. It is not possible to converge the eyeballs sufficiently, and the
• VISUAL FIELD AND RETINA
The light rays from different halves of each visual field do not fall on the same halves of the retina. The light rays from temporal part of visual field of an eye fall on the nasal half of retina of that eye. Similarly, the light rays from nasal part of visual field fall on the temporal half of retina of the same side.
• MAPPING OF VISUAL FIELD
. The shape and extent of visual field is mapped out by means of an instrument called perimeter and this techniique is called perimetry. The visual field can also be deterrmined by Bjerrum's screen or by confrontation test.
• INTRODUCTION
• VISUAL RECEPTORS
• FIRST ORDER NEURONS
• SECOND ORDER NEURONS
• THIRD ORDER NEURONS
• CONNECTIONS OF VISUAL RECEPTORS TO OPTIC NERVE
• PRIVATE PATHWAY
• DIFFUSE PATHWAY
• COURSE OF VISUAL PATHWAY
• OPTICNERVE
• OPTIC CHIASMA
• OPTIC TRACT _
• LATERAL GENICULATE BODY
• OPTIC RADIATION
• VISUAL CORTEX
• APPLIED PHYSIOLOGY- EFFECTS OF LESION AT
DIFFERENT LEVELS OF VISUAL PATHWAY
• INTRODUCTION
The retinal impulses are carried to visual center in cerebral cortex by the nervous pathway called visual pathway or optic pathway.
In binocular vision, the light rays from temporal (outer) half of visual field fall upon the nasal part of corresponding retina. The rays from nasal (inner) half of visual field fall upon the temporal part of retina.
• VISUAL RECEPTORS
Rods and cones, which are present in the retina of eye, form the visual receptors. Fibers from the visual receptors synapse with dendrites of bipolar cells of inner nuclear layer of retina.
• FIRST ORDER NEURONS
First order neurons (primary neurons) are bipolar cells in the retina. Axons from the bipolar cells synapse with dendrites of ganglionic cells.
• SECOND ORDER NEURONS
Second order neurons (secondary neurons) are the ganglionic cells in ganglionic cell layer of retina. The axons of the ganglionic cells form optic nerve. The optic nerve leaves the eye and terminates in lateral geniculate body. \
• THIRD ORDER NEURONS
The third order neurons are in the lateral geniculate body. Fibers arising from this reach the visual cortex.
• CONNECTIONS OF VISUAL RECEPTORS TO OPTIC NERVE
There are two pathways between the visual receptors and optic nerve namely:
1. Private pathway
2. Diffuse pathway.
A number of cones and rods are connected with a polysynaptic bipolar cell. The bipolar cells are connected to diffused ganglionic cells. So, there is great overlapping. This type of pathway is present outside the fovea.
• COURSE OF VISUAL PATHWAY The visual pathway consists of:
1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Lateral geniculate body
5. Optic radiation
6. Visual cortex.
• 1. OPTIC NERVE
It i~ formed by the axons of ganglionic cells (Fig. 168-1). Optic nerve leaves the eye through optic disc. The fibers from temporal part of retina are in lateral part of the nerve and carry the impulses from nasal half of visual field of same eye. The fibers from nasal part of retina are in medial part of the nerve and carry the impulses from temporal half of visual field of same eye.
• 2. OPTIC CHIASMA
Here, the medial fibers of each optic nerve cross the midline and join the uncrossed lateral fibers of opposite side to form the optic tract (Fig. 168-1).
• 3. OPTIC TRACT
After forming optic chiasma, all the fibers run backward and outward toward the cerebral peduncle. While reachhing the peduncle, the fibers pass between tuber cinereum and anterior perforated substance. Then, the fibers turn around the peduncle to reach the lateral geniculate body. Here, many fibers synapse while some fibers just pass through this and run towards superior colliculus. Fibers from fovea do not enter superior colliculus.
Some fibers from fovea of each side pass through optic tract of same side and others through optic tract of opposite side. Due to crossing of medial fibers in
• 4. LATERAL GENICULATE BODY
The lateral geniculate body forms the subcortical center for visual sensation. Many fibers from optic tract end in lateral geniculate body, which is in thalamus. From here, the geniculocalcarine tract (optic radiation) arises. This tract is the last relay of visual pathway.
Some of the fibers from optic tract do not synapse in lateral geniculate body but, pass through it and termiinate in any of the other 2 centers namely,
i. The superior colliculus: This is concerned with reflex movements of eyeballs and head in response to optic stimulus
ii. Supraoptic nucleus of hypothalamus: It is concerned with the retinal control of hypophysis in animals. But in man, it does not play any important role.
• 5. OPTIC RADIATION
Fibers from lateral geniculate body pass through internal capsule and form optic radiation. Optic radiation ends in visual cortex. The fibers between lateral geniculate body and visual cortex are also called geniculocalcarine fibers.
• 6. VISUAL CORTEX
The primary cortical center for vision is called visual cortex that is located on the medial surface of OCCipital lobe. It forms the walls and lips of calcarine fissure in the medial
surface of occipital lobe. \
There is definite localization of retinal projections upon visual cortex. In fact, the point to point projection of retina upon visual cortex is well established. The peripheral retinal representation occupies the anterior part of visual cortex. The macular representation occupies the posterior
part of visual cortex near the occipital pole. "
Areas of Visual Cortex
The areas of visual cortex are:
i. Primary visual area-area 17
ii. Visual association area-area 18 iii. Occipital eye field-area 19.
Lateral Geniculate Body
, .
Left Eye ---
__ ~ Right Eye
Optic Nerve·---------
______ Temporal Fibers
Optic Chiasma ------------
Optic Tract --------
---- Superior Colliculus
Optic Radiation
Visual Cortex .••••.
FIGURE 168-1: Visual Pathway
Functions of Areas of Visual Cortex
i. Primary visual area-Area 17 is concerned with perception of visual impulses.
ii. Visual association area-Area 18 is concerned with interpretation of visual impulses.
iii. Occipital eye field-Area 19 is concerned with movement of eyes.
• APPLIED PHYSIOLOGY-EFFECTS OF LESION AT DIFFERENT LEVELS OF VISUAL PATHWAY
The injury to any part of optic pathway causes visual defect and the nature of defect depends upon the location and extent of injury. The loss of vision in one visual field is known as anopia. Loss of vision in one half of visual
f) ()
.
FIGURE 168-2: Types of hemianopia
() f)-
field is called hemianopia (Figs 168-2 and 168-3). Hemianopia is classified into two types:
1. Homonymous hemianopia
2. Heteronymous hemianopia.
1. Homonymous Hemianopia
Homonymous hemianopia means loss of vision in the same halves of both the visual fields. Loss of vision in right half of visual field of both eyes is known as right homonymous hemianopia. Similarly left homonymous hemianopia means loss of vision in left half of visual field of both eyes.
2. Heteronymous Hemianopia
Heteronymous hemianopia means loss of vision in opposite halves of visual field. For example, binasal hemianopia means loss of vision in right half of left visual field and left half of right visual field (nasal half of both visual fields). Bitemporal hemianopia is the loss of sight
Bitemporal Hemianopia
Binasal Hemianopia
in left side of left visual field and right side of right visual field (temporal half of both visual fields).
Effects of Lesion of Optic Nerve
The lesion in one optic nerve will cause total blindness or anopia in the corresponding visual field. Lesion may occur due to increased intracranial pressure.
Effects of Lesion of Optic Chiasma
The nature of defect depends upon the fibers involved.
i. Pressure on uncrossed lateral fibers by aneurysmal dilatation of carotid artery causes blindness in the temporal part of retina of same side i.e. the retina cannot receive light stimulus from the objects in nasal half of same visual field. So, the hemianopia developed is called left or right nasal hemianopia.
ii. If lateral fibers of both sides are affected, the vision is lost in nasal half of both visual fields causing binasal hemianopia. This occurs due to dilated third ventricle, which forces the angle of chiasma
,., "-..-/
Left Eye Right Eye e
B 0
A
C C () 0
F D 0 ()
C+D () ()
E () ()
F () ()
G () ()
A. B. C. D.
C + D.
E. F. G.
FIGURE 168-3: Effects of lesions of optic pathway
Dark shade in circles indicates blindness
Lesion of left optic nerve-Total blindness of left eye Lesion of right optic nerve-Total blindness of right eye
Lesion of lateral fibers in left side of optic chiasma-Left nasal hemianopia Lesion of lateral fibers in right side of optic chiasma-Right nasal hemianopia Lesion of lateral fibers in both sides of optic chiasma-Binasal hemianopia Lesion of medial fibers in optic chiasma -Bitemporal hemianopia
Lesion of left optic radiation-Right homonymous hemianopia
Lesion of right optic radiation-Left homonymous hemianopia
against carotid arteries. This can also occur due to dilatation of carotid artery on both sides.
iii. If the nasal, i.e. crossed fibers are affected due to pituitary tumor, bitemporal hemianopia occurs.
Effects of Lesion of Optic Tract, Lateral . Geniculate Body and Optic Radiation
The lesion of optic tract or lateral geniculate body or optic radiation causes homonomous hemianopia. In the right
• INTRODUCTION
• LIGHT REFLEX
• DIRECT LIGHT REFLEX
• INDIRECT LIGHT REFLEX
• PATHWAY FOR LIGHT REFLEX
• CILIOSPINAL REFLEX
• ACCOMMODATION
• DEFINITION
• MECHANISM OF ACCOMMODATION
• ACCOMMODATION REFLEX
• PATHWAY FOR ACCOMMODATION REFLEX
• RANGE AND AMPLITUDE OF ACCOMMODATION
• PRESBYOPIA
• INTRODUCTION
Pupillary reflexes are the reflexes in which, the size of pupil is altered. Pupillary reflexes are:
1. Light reflex,
2. Ciliospinal reflex and
3. Accommodation reflex.
• LIGHT REFLEX
When light is flashed into the eyes, it causes constriction of pupil. This is known as light reflex. Light reflex is of two types namely:
1. Direct light reflex
2. Indirect light reflex.
• DIRECT LIGHT REFLEX
When light is thrown into one eye, the constriction of pupil occurs in that eye. It is known as direct light reflex.
• INDIRECT LIGHT REFLEX
If light is flashed into one ~ye, the constriction of pupil occurs in the opposite eye even though no light falls on
that eye. This is known as indirect or consensual light reflex.
• PATHWAY FOR LIGHT REFLEX
When light falls on the eye, the cones are stimulated. The afferent impulses pass through the optic nerve, optic chiasma and optic tract. At the midbrain level few fibers get separated from the optic tract and synapse on the neurons of pretectal nucleus, which lies close to the superior colliculus. From pretectal nucleus, the impulses pass to the Edinger - Westphal nucleus (parasympathetic nucleus) of oculomotor nerve (third cranial nerve). From here the impulses are carried by the preganglionic fibers to the ciliary ganglion. From the ciliary ganglion, the posttganglionic fibers pass through the short ciliary nerves, reach the eyeball and supply the constrictor pupillae muscle of iris (Fig. 169-1).
The reason for the consensual light reflex is that, some of the fibers from pretectal nucleus from one side cross to the opposite side and end on the opposite Edinger- . Westphal nucleus.
FIGURE 169-1: Pathway for light reflexes
• CILIOSPINAL REFLEX
The stimulation of skin over the neck causes dilatation of pupil. This is called the ciliospinal reflex. It is due to the contraction of dilator pupillae muscle. The pathway for this reflex is similar to that of light reflex up to the midbrain. Beyond that, the impulses pass via sympathetic nerve fibers and reach the dilator pupillae.
• ACCOMMODATION
• DEFINITION
Accommodation is the adjustment of the eye to see either near or distant objects clearly. It is the process, by which light rays from near objects or distant objects are brought
mese rays snoUia De rerractea (convergeo) to a greater extent. There are 3 possible ways by which, accommoodation can be brought about.
1. The retina must be moved towards or away from the lens. This could be done by shortening or elongation of eyeball. So, the divergent, parallel or convergent rays can be focused accurately. This mechanism is present only in some molluscs and not in human beings.
2. The lens must be moved towards or away from the retina. This is done in photography. This mechanism exists only in some fishes.
3. The convexity of lens must be altered, so that the refractory power of lens is altered. This mechanism is present in human eye. The mechanism was first suggested by Young and later supported by Helmholtz (Fig. 169-2).
Young-Helmholtz Theory
This describes how the curvature of lens can be increased and thereby, the refractive power of lens is enhanced. In resting condition i.e., during distant vision, lens is flat due to the traction of suspensory ligaments. Suspensory
Corneoirls Junction ,
I I I I
Ciliary Muscle
I I
I Ciliary Process
I I
I I
I I
I I
I
I
I
I I I I
I I I I
In Relaxed Condition
I I
I I J
Suspensory
I Ligaments Choroid
During Accommodation
FIGURE 169-2: Accommodation
Purkinje-Sanson Images
The Purkinje-Sanson images are used to demonstrate the change in convexity of lens during accommodation for near vision. A subject is made to sit in a darkroom. A lighted candle is held in front. One eye is opened and the other eye is closed. Three images of the flame can be seen in the opened eye (Fig. 169-3).
First image is upright and bright. It shines from the surface of cornea, which acts as a mirror. Second image is upright but dim. This is reflected from the anterior convex surface of the lens. Third image is inverted and small. This is formed by posterior surface of the lens, which acts as a concave mirror.
When the person looks at a distant object, the second image reflected from anterior surface of the lens is near the third image from posterior surface. During accommoodation for near vision, no change occurs either in first image or the third image. But, the second image becomes smaller and moves towards the first image.
Thus, the increased convexity of the anterior surface of lens during accommodation for near vision is evident by the change in the size and position of second image.
Other Adjustments in Eyeball During Accommodation
Besides increase in anterior curvature of the lens, two more adjustments are made in the eyeball during accommodation for near vision viz:
1. Convergence of both eyeballs: This is necessary to bring the retinal images on to the corresponding points and
2. Constriction of pupil: This is necessary to:
a. Increase the visual acuity by reducing lateral chroomatic and spherical aberrations,
During Near Vision
FIGURE 169-3: Purkinje-Sanson images
b. Reduce the quantity of light entering eye and
c. Increase the depth of focus through more central part of lens as its convexity is increased.
• ACCOMMODATION REFLEX
Accommodation is a reflex action. When a person looks at a near object after seeing a far object, three adjusttments are made in the eyeballs:
1. Convergence of the eyeballs,
2. Constriction of the pupil and
3. Increase in the anterior curvature of the lens.
Convergence of the eyeballs occurs because of conntraction of the medial recti. The constriction of pupil is due to the contraction of sphincter pupillae of iris. Conntraction of the ciliary muscle causes increase in the anteerior curvature of the lens. Thus, the accommodation
Afferent Pathway
Visual impulses from retina pass through the optic nerve, optic chiasma, optic tract, lateral geniculate body and optic radiation to visual cortex (area 17) of occipital lobe. From here, the association fibers carry the impulses to
frontal lobe (Fig. 169-4).
Center
The center for accommodation lies in frontal eye field (area 8) that is situated in the frontal lobe of cerebral cortex.
Efferent Pathway
1. Efferent fibers to ciliary muscle and sphincter pupillae From area 8, the corticonuclear fibers pass via internal capsule to the Edinger-Westphal nucleus of III cranial nerve. From here, the preganglionic fibers pass through the third cranial nerve to the ciliary ganglion. The postganglionic fibers pass via the short ciliary nerves and supply the ciliary muscle and the sphincter pupillae.
2. Efferent fibers to medial rectus
Some of the fibers from frontal eye field terminate in the somatic motor nucleus of oculomotor nerve. The fibers from the motor nucleus supply the medial rectus.
• RANGE AND AMPLITUDE OF ACCOMMODATION The farthest point from the eye at which the object can be seen is called far point or punctum remotum. In the normal eye it is infinite, i.e. at a distance beyond 6 meters or 20 feet. It is limited only by the size of object, clearness of the atmosphere and the curvature of earth.
The nearest point from eye at which the object is seen clearly is called near point or punctum proximum. It is about 7 to 40 cm, depending upon the age. Distance between far point and near point is called Range of accommodation.
• "~ .~" U~L1Y
the unit for focal length is 1 meter or 100 cm. The refractory power is expressed as diopter (D). For example, in an emmetropic i.e., normal eye, if the near point is 10 cm, the dynamic refraction is
1 meter 100 cm
p=--- =---= 100
10 cm 10 cm
In the emmetropic eye since the far point is at infinite distance, the static refraction is taken as zero.
Now the amplitude of accommodation is,
=P-R
= 10 - 0
= 100
_ Amplitude of AccommodatIon at DIfferent Ages Amplitude of accommodation varies with age. The amplitude of accommodation at different age groups is:
10 Years = 11.0 0
20 Years = 9.5 0
30 Years = 7.5 D
40 Years = 5.5, D
50 Years = 2.0 D
60 Years = 1.2 D
70 Years = 1.0 D
• PRESBYOPIA
In old age, the amplitude of accommodation is reduced , as the near point is away from the eye. This condition
is called presbyopia. Causes for this are:
1. Reduced elasticity of lens due to physical changes in lens and its capsule. So, the anterior curvature is not increased during near vision.
2. Reduced convergence of eyeballs due to the conncomitant weakness of ocular muscles in old age. More details about presbyopia are given in Chapter 171.
• INTRODUCTION
• VISIBLE SPECTRUM AND SPECTRAL COLOURS
• SPECTRAL COLOURS
• EXTRASPECTRALCOLOURS
• PRIMARY COLOURS
• COMPLEMENTARY COLOURS
• THEORIES OF COLOUR VISION
• THOMAS YOUNG'S TRICHROMATIC THEORY
• HELMHOLTZ TRICHROMATIC THEORY
• GRANIT'S MODULATOR AND DOMINATOR THEORY
• HARTRIDGE'S POLYCHROMATIC THEORY
• HERING'S THEORY OF OPPOSITE COLOURS
• RETINAL AREAS SENSITIVE TO COLOUR
• CONTRAST EFFECTS
• SIMULTANEOUS CONTRAST
• SUCCESSIVE CONTRAST
• AFTER IMAGE
• POSITIVE AFTER IMAGE
• NEGATIVE AFTER IMAGE
• APPLIED PHYSIOLOGY-COLOUR BLINDNESS
• CLASSIFICATION OF COLOUR BLINDNESS
• TESTS FOR COLOUR BLINDNESS
• INTRODUCTION
The human eye can recognize about 150 different colours in the visible spectrum. The discrimination and appreciaation of colours depend upon the ability of receptors in retina.
• VISIBLE SPECTRUM AND
SPECTRAL COLOURS
• SPECTRAL COLOURS
When the sunlight or white light is passed through a glass prism, it is separated into different colours. The series of coloured light produced by the prism is called the visible spectrum. The colours forming the spectrum
are called the spectral colours. The spectral colours are red, orange, yellow, green, blue, indigo and violet (ROYGI;3IV or VIBGYOR). In the spectrum the colours occupy the position according to their wavelengths. Wavelength is the distance between two identical points in the wave of light energy. Accordingly, red has got the maximum wavelength of about 8,000 A and the violet has got the minimum wave length of about 3000 A. The light rays longer than the red are called infrared rays or the heat waves and the rays shorter than violet are called the ultraviolet rays. But, these two extraordinary types of rays do not evoke the sensation of vision. The refraction of the spectral colours by the prism also depends on the wavelengths. Red is refracted less and violet is
• EXTRASPECTRAL COLOURS
The colours other than those present in visible spectrum are called the extraspectral colours. These colours are formed by the combination of two or more spectral colours. For example, purple is the combination of violettand red. Pink is the combination of red and white.
• PRIMARY COLOURS
The primary colours are those, which when combined together can produce the white. The primary colours are red, green and blue. These three colours in equal proportion give white.
• COMPLEMENTARY COLOURS
When two colours are mixed or combined in right prooportion, white is produced. Such two colours are called complimentary colours. Examples of complementary colours are red and greenish blue; orange and cyan blue; yellow and indigo blue; violet and greenish yellow; and purple and green.
• THEORIES OF COLOUR VISION
Many theories are available to explain the mechanism of perception of colour vision. However, most of the theories are not accepted universally. Following are the important theories
1. Thomas Young's trichromatic theory
2. Helmholtz trichromatic theory
• 2. HELMHOLTZ TRICHROMATIC THEORY Helmholtz substituted sensitive filaments of nerve for the cones. The sensitive filaments of nerves give response selectively to one orthe other of the three primary colours. This is also called Young-Helmholtz theory.
• 3. GRANIT'S MODULATOR AND
DOMINATOR THEORY
Granit observed that the ganglionic cells of retina are stimulated by the whole of the visual spectrum. He studied the action potentials in ganglionic cells stimulated by light and obtained some sensitivity curves by using the different wavelengths of light both in light adapted and dark adapted eyes. On the basis of the sensitivity curves, he classified the ganglionic cells into two groups namely, Dominators and Modulators.
Dominators
The dominators are responsible for brightness of light. Dominators are further divided into two types.
i. Dominators for cones, which respond in light adappted eye and a broad sensitivity curve is produceC\ with the maximum response around the waveelengths 55 A.
ii. Dominators for rods, which respond in dark adapted eye and in the sensitivity curve the maximum response is given at the wavelengths of 500 A.
Modulators
The modulators are responsible for different colour sennsations. There are three types of modulators:
i. Modulators of blue, which are stimulated by lights with wave lengths of 450 to 470 A.
• 4. HARTRIDGE'S POLYCHROMATIC TliEORY According to this theory, there are seven types of recepptors in retina of man. All the seven receptors are grouped in the three units.
A tricolour unit consisting of receptors for orange, green and blue.
Second Unit
A dicolour unit with receptors for yellow and blue colours. Receptors for yellow and blue are complementary to each other.
Third Unit
Another dicolour unit with red and blue-green receptors.
• 5. HERING'S THEORY OF OPPOSITE
COLOURS
According to Hering, there are three photochemical subbstances in retina. Each substance causes the sensation of a particular colour by its breakdown or resynthesis.
First Substance
It is white-black substance. Breakdown of it, causes sensation of white and resynthesis causes sensation of black.
Second Substance
It is yellow-blue substance. Breakdown of it causes sensaation of yellow and resynthesis causes sensation of blue.
Third Substance
It is red-green substance. Its breakdown causes senn_ sation of red. Resynthesis causes sensation of green.
This theory can explain the successive contrast and after images but not the simultaneous sensation of antagonistic colours.
• CONTRAST EFFECTS
The contrast effects are of two types, which are:
1. The simultaneous contrast and
2. The successive contrast.
• SIMULTANEOUS CONTRAST
When black is placed against white or white against black, these two colours set one another off i.e., the black looks blacker arid the white looks whiter. Similarly, the g~en is enhanced by red and red by green. This is known as simultaneous contrast. The maximum effect of the simultaneous contrast is obtained when the compleementary colours are paired. The reason for simultaneous contrast is that, the stimulation of an area of retina by one colour modifies the response in the surrounding or neighboring areas. It increases the sensitivity to other colours in the surrounding receptors. The action is probably due to horizontal cells.
• SUCCESSIVE CONTRAST
When a person looks at a green object after looking at a bright red, the green object appears to be more greener. There is an increase in the sensitiveness to the complementary colour. This phenomenon is called successive contrast.
Reason forthis is that, the stimulation of ah area of retina modifies its sensitiveness to the successive stimuli. Thus, there is an increase in the sensitiveness to the second colour.
• AFTER IMAGE
After looking at a bright object, if the eyes are closed, the image remains more distinct for sometime and then fades away gradually. This phenomenon where the retention of image occurs even after the stoppage of
• 2. NEGATIVE AFTER IMAGE
After looking at bright object, if the eyes are fixed on white surface (instead of closing or fixing on a black surface), the after image appears in the complementary colour. This is called negative after image. The reason for the negative after image is the persistence of activity in retina, even after the particular stimulus ceases to
act.
• APPLIED PHYSIOLOGY-
COLOUR BLINDNESS
The failure to appreciate one or more colour is called colour blindness. It is common in 8% of males and only in 0.4% of females, as mostly the colour blindness is an inherited sex linked recessive character. It also occurs due to injury or disease of retina.
• CLASSIFICATION OF COLOUR BLINDNESS The classification of colour blindness is on the basis of Young-Helmholtz trichromatic theory. It is as follows:
1. Monochromatism,
2. Dichromatism and
3. Trichromatism.
1. Monochromatism
In this condition, the subject cannot appreciate any colour. Monochromatism is very rare. The persons with monoochromatism are called monochromats. The retina of monochromats is totally insensitive to colour and they see the whole spectrum in different shades of gray. So, their vision is similar to black and white photography.
Types of Monochromatism
There are two types of monochromatism called:
i. Rod monochromatism and
ii. Cone monochromatism.
2. Dichromatism
In this condition, the subject can appreciate only two colours. Persons with this defect are called dichromats. They can match the entire spectrum of colours by only two primary colours because, the receptors for third colour are defective. The defects are classified into three groups.
i. Protanopia
ii. Deuteranopia and iii. Tritanopia
i. Protanopia
In this the defect is in the receptor of first primary colour-red. So, the red colour cannot be appreciated. The persons having protanopia are known as protonopes. The protanope uses blue and green to match the colours. Thus, he confuses red with green.
ii. Deuteranopia
In this, the defect is in the second receptor i.e., green receptor, and the deuteranope uses blue and red colours and he cannot appreciate green colour.
iii. Tritanopia
In this defect, the third receptor i.e., blue receptor is defective and the tritanope uses red and green colour and he cannot appreciate blue colour.
3. Trichromatism
The persons with this defect are called trochromats The persons with this defect are able to perceive all th three colours but the intensity of one of the primary colour cannot be appreciated very much. Even the dark shade look dull for them. Trichromatism is classified into thre
types as:
ii. Deuteranomaly
The perception for green is less.
• AMETROPIA
• MYOPIA OR SHORT SIGHTEDNESS
• HYPERMETROPIA OR LONG SIGHTEDNESS •. ANISOMETROPIA
• ASTIGMATISM
• PRESBYOPIA
• AMETROPIA
The eye with normal refractive power is called emmetropic and the condition is called emmetropia. Any deviation in the refractive power from normal condition is callecJ.. ametropia and the eye is called ametropic. There are two forms of ametropia viz:
1. Myopia and
2. Hypermetropia.
• MYOPIA OR SHORT SIGHTEDNESS
In emmetropia the far point is infinite. In myopia, the near vision is normal but the far point is not infinite i.e., it is at definite distance (Fig. 171-1). In extreme conditions, it may be only a few centimeters away from the eye (Myo = half closed; ops = eye).
Cause of Myopia
In myopia, the refractive power of the lens is usually normal. But, the anteroposterior diameter of the eyeball is abnormally long. Therefore, the image is brought to a focus a little in front of retina. In other words, the lens is too strong for the length of eyeball. The light rays, after coming to a focus, disperse again so, a blurred image is formed upon retina.
Correction of Myopia
In myopic eye, in order to form a clear image on the retina, the light rays entering the eye must be divergent
and not parallel. Thus, the myopic eye can be corrected by using concave lens. The concave lens diverges the light rays before entering the eye.
• HYPERMETROPIA OR LONG SIGHTEDNESS
In this defect, the distant vision is normal but, the near vision is affected (metras = measure).
Cause of Hypermetropia
Hypermetropia is due to reduced anteroposterior diameter of the eyeball. So, even though the refractive power of the lens is normal, the light rays are not converged enough to form a clear image on retina i.e., the light rays are brought to a focus behind retina. This causes a blurred image of near objects. Hypermetropia can occur in c~ildhood, if the eyeballs fail to develop to the corre~t size. In old age, hypermetropia occurs due to absorption of water.
Correction of Hypermetropia
Hypermetropia is corrected by using convex lens that converges the light rays before entering the eye.
• ANISOMETROPIA
In this, there is a difference between the refractive power of both eyes. This is corrected by using different appropriate lens for each eye.
-@
Hyper Metropla
=====8
Myopia
After Correction
After Correction
FIGURE 171-1: Errors of refraction
• ASTIGMATISM
It is the common optical defect. The light rays are not brought to a sharp point upon retina. This defect is present in all eyes. When it is moderate, it is known as physiological astigmatism. When it is well marked, it is considered abnormal. For example, the stars appear as small dots of light to a person with normal eye. But in astigmatism, the stars are seen as radiating short lines of light (A = not; stigma = point).
• CAUSE OF ASTIGMATISM
The light rays pass through all meridians of a lens. In a normal eye, lens has approximately same curvature in all meridians. So, the light rays are refracted almost equally in all meridians and brought to a focus.
If the curvature is different in different meridianssvertical, horizontal, oblique, the refractive power is also different in different meridians. The meridian with greater curvature refracts the light rays more strongly than the other meridians. So, these light rays are brought to a focus in front of the rays, which pass through other meridians. Such irregularity of curvature of lens causes
astigmatism.
• TYPES OF ASTIGMATISM Astigmatism is of two types:
1. Regular astigmatism and
2. Irregular astigmatism.
1. Regular Astigmatism
In this, the refractive power is unequal in different meridians but in one single meridian, it is uniform
throughout.
2. Irregular Astigmatism
In this, the refractive power is unequal not only in diffeerent meridians, but also in different points of same
meridian.
• CORRECTION OF ASTIGMATISM
Astigmatism is corrected by using cylindrical glass lens having the convexity in the meridians corresponding to that of lens of eye having a lesser curvature Le., if the horizontal curvature of lens is less, the person should use cylindrical glass lens with the convexity in horizontal meridian.
,
• EXTERNAL EAR
• AURICLE
• EXTE;RNAL AUDITORY MEATUS
• MIDDLE EAR •
• TYMPANIC CAVITY
• TYMPANIC MEMBRANE
• AUDITORY OSSICLES
• MUSCLES ATTACHED TO AUDITORY OSSICLES
• AUDITORY TUBE
• INTERNAL EAR
• COCHLEA
• COMPARTMENTS OF SPIRAL CANAL OF COCHLEA
• ORGAN OF CORTI
• EXTERNAL EAR
The ear consists of 3 parts namely, External ear, Middle ear and Internal ear (Fig. 172-1). The external ear is formed by two parts called:
1. Auricle or pinna and
2. External auditory meatus.
• AURICLE OR PINNA
The auricle or pinna of the external ear consists of fibrocartilaginous plate covered by connective tissue and skin. The plate is characteristically folded and ridged. The skin covering the plate is thin and contains many fine hairs and sebaceous glands. On the posterior surface of the auricles, many sweat glands are present.
In many animals, the auricle can be turned to locate the source of sound or, the auricle can be folded to avoid unwanted sound. But in man, the extrinsic and intrinsic muscles of auricles are rudimentary. The depression of auricle, which forms the orifice of external auditory meatus, is called concha.
• EXTERNAL AUDITORY MEATUS
The external auditory meatus is a slightly curved canal with a length of about 55 mm. The meatus consists of 2 parts viz:
1. The outer cartilaginous part and
2. The inner bony part.
1. The Outer Cartilaginous Part
This is formed by cartilage. It is continuous with that , of auricle. The skin covering this is thick and contains stiff hairs, which prevent the entrance of foreign partiicles.
Large sebaceous glands and cereminous glands are also present in skin covering this portion. These glands are coiled and tubular in nature and open on the surface of the skin. The columnar epithelial cells of these glands contain brown pigment granules and fat droplets. The secretions of cereminous glands, sebaaceous glands and the desquamated epithelial cells form the ear wax.
- Eustachian Tube \ \ L _Middle Ear
\ Tympanic Membrane
External Auditory Meatus
Auricle/
FIGURE 172-1: Diagram showing the structure of ear
2.· The Inner Bony Part
The inner part of the external auditory meatus is covered by skin, which adheres closely to the periosteum. Only sebaceous glands are present here. Small fine hairs are present on the superior wall of the canal. The skin covering this portion is continuous with cuticular layer of tympanic. membrane.
• MIDDLE EAR
The middle ear consists of the tympanic cavity with audiitory ossicles, two small muscles and the auditory tube .
• TYMPANIC CAVITY
The tympanic cavity or tympanum is a small, narrow, laterally compressed chamber, situated within the temporal bone. The tympanic cavity is separated from external auditory meatus by tympanic membrane. The tympanic cavity contains the auditory ossicles.
• TYMPANIC MEMBRANE
-,The tympanic membrane is a semitransparent structure separating middle ear from the external auditory meatus. ThE}periphery of the membrane is fixed to the tympanic sulcus in the surrounding bony ring by means of the fibroocartilage (Fig. 172-2).
Stapes
Tympanic Membrane
FIGURE 172-2: Tympanic membrane and auditory ossicles
Structure of Tympanic Membrane
The tympanic membrane is formed by 3 layers.
1. Cuticular layer: The skin layer which is the continuation of the skin of auditory meatus.
2. The fibrous layer with collagenous fibers.
3. Inner mucus layer or tympanic mucosa composed of single layer of squamous epithelial cells.
• AUDITORY OSSICLES
The auditory ossicles are the three miniature bones, which are arranged in a chain extending across the middle ear from the tympanic membrane to oval window (Fig. 172-2). The three ossicles are:
1. Malleus,
2. Incus and
3. Stapes.
1. Italleus
It is otherwise called hammer. It has a head, neck and handle. The handle or manubrium is attached to the tympanic membrane. The head or capitullum articulates with the body of incus.
2. Incus
Incus is also known as anvil. This looks like a premolar tooth. Incus has a body, one long process and one short process. Anterior surface of the body articulates with head of malleus. The short process is attached to a ligament. The long process runs parallel to handle of malleus. The tip of this process is like a knob, called lenticular process that articulates with stapes.
3. Stapes
This is also called stirrup. This is the smallest ossicle. It has a head, neck, anterior crus, posterior crus and a foot plate. Head articulates with incus. Foot plate fits into the oval window.
• MUSCLES ATTACHED TO AUDITORY OSSICLES There are two skeletal muscles attached to the ossicles. which are:
1. Tensor tympani and
2. Stapedius.
1. Tensor Tympani
It lies in a canal above the auditory tube. Its tendon is attached to manubrium of malleus. This muscle is supplied by mandibular division of trigeminal nerve. When tensor tympani contracts, it pulls the malleus inwards and this prevents the outward movement of tympanic membrane.
2. Stapedius
It lies in a conical bony cavity on the posterior wall of the tympanic cavity. Its tendon is inserted into the posterior surface of neck of stapes. This muscle is supplied by facial nerve. When stapedius contracts, it pulls the neck
of stapes backwards and reduces the movement of foot plate against the fluid in cochlea.
Tympanic Reflex
A loud sound causes reflex contraction of both the muscles in middle ear, tensor tympani and stapedius after a latent period of 40 to 80 milliseconds. This is called the tympanic reflex. The tympanic reflex protects the tympanic membrane from being ruptured by loud sound. It also prevents fixation of foot plate of stapes, against oval window during exposure to loud sound.
Tympanic reflex protects the cochlea also. The contraction of tensor tympani and stapedius during exposure to loud sound develops stiffness of the auditory ossicles so that, the transmission of sound is decreased. This helps to protect the cochlea from the damage by loud sound.
• AUDITORY TUBE
The auditory tube or Eustachian tube is the flattened canal leading from the anterior wall of the middle ear to the nasopharynx. Its upper part is surrounded by bony wall and the lower part is surrounded by fibrocartilaginous
_plate.
This tube connects the middle ear with posterior part of nose and forms the passage of air between middle ear and atmosphere. Thus, the pressure on both sides of tympanic membrane is equalized.
• INTERNAL EAR
The internal ear or labyrinth contains the sense organs of hearing and equilibrium. The sense organ for hearing is the cochlea. And, the sense organ for equilibrium is the vestibular apparatus, which includes the semicircular canals and otolith organ.
The labyrinth is a membranous structure, enclosed by a bony labyrinth in petrous part of temporal bone.
• COCHLEA
Cochlea is a coiled structure like a snail's shell (cochlea = snail's shell). It consists of two structures viz:
1. Central conical axis formed by spongy bone called modiolus and
2. Bony canal or tube which winds around this modiolus.
In man, the canal makes two and a half turns, starting from the base of the cochlea and ends at the top (apex) of cochlea. The end of the canal is called cupula. The base of the modiolus forms the bottom of internal audiitory meatus through which the cochlear nerve fibers pass
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