Special Senses: Eye | Physiology and Anatomy | Assignment

      

Eye

Human eye, in humans, specialized sense organ capable of receiving visual images, which are then carried to the brain. Physiological Significance of Eye  Create mental image of external world  Perception of : location, size , shape, color and texture of objects  If object moving: speed and direction  An image is formed on the retina by the refractive surfaces of the eye.  The light energy is transduced into electrical signal by the rods and the cones ( Photoreceptors).  The information needed to create the mental image is encoded by the neurons within the retina. Human eyeball  is roughly spherical, being a little flat from above downwards. The optic nerve enters the eyeball, a little inside the posterior pole, through the optic disc. 

Tunics 

From outside inward, the wall has three coats: 

1. Fibrous coat

2. Vascular coat

3. Nervous coat.

Fibrous coat: It has two parts: 

1.  Posterior (5/6ths) is opaque and called the sclera.

2. Anterior (1 / 6th) is transparent, called the cornea.

The vascular coat (uvea, or uveal tract) 

Three parts: From behind forward: 

1. The choroid-remains just behind the retina forming the posterior 5 / 6ths of the middle coat. Composed of numerous blood vessels and pigmented cells containing melanin. Hence, its colour is dark. Its function is to convert the eyeball into a dark chamber.

2. The ciliary body includes orbicularis ciliaris, ciliary processes, and ciliaris muscle.

3. The iris (vide later).

Nervous coat: The nervous coat is called the retina. It contains the photosensitive receptors, where the visual impulses are generated.

CONJUNCTIVA 

The exposed part of the eyeball is covered by a thin stratified mucous membrane which is reflected onto the inner surface of the eyelids. It is called conjunctiva. Its function is protection and lubrication. 

LACRIMAL APPARATUS 

The lacrimal gland is an almond-shaped racemose gland remaining inside the upper and outer parts of the orbit and secreting a watery fluid, the tears. Smaller accessory glands are common . The secretions are delivered into the conjunctival sac through six to ten fine ducts. The movements of the eyelids help to spread the tears over the conjunctival surface.

The tears ultimately collect into a small triangular area (lacrimal lake) at the inner angle of the eye. From here the fluid passes through puncta lacrimalia and is carried through two small lacrimal canaliculi into the lacrimal sac inside the nose, where the sac opens. The normal function of tears is to keep the exposed surface moist. Irritation or emotion increases secretion. Nerve supply: Sympathetic from superior cervical ganglion, parasympathetic from the facial. Composition of tear: Closely resembles aqueous humour.Water-98.2%; solids-1.8%; organic elements: Protein-0.67%; sugar---0.65%; NaCl---0.66%; NPN-0.05%; urea-0.03%; other mineral elements-sodium, potassium and ammonia-0.79%.

EYEBALL 

Its interior is divided into two compartments by a partition. The centre of this partition is occupied by the lens. The peripheral part, by which the lens remains attached to the wall of the eyeball, is called the suspensory ligament. The posterior compartment contains vitreous humour (vitreous body). The anterior compartment contains aqueous humour. Aqueous humour and vitreous humour create the intraocular pressure. This pressure tends to hold the eyeball round and firm . The anterior compartment is further subdivided by the iris into an anterior and a posterior chamber. Both contain aqueous humour. Intraocular pressure is 25-30 mm of Hg. It varies with general blood pressure. Pressure on the ocular veins or on eyeball from outside, disturbed drainage of aqueous humour and exposure to darkness raises the pressure.

CORNEA 

It is the round, transparent convexity in the anterior.

Parts of the eyeball. 

1. Diameters: 11 mm (vertical) x 12 mm (lateral).

2. Thickness: 0.5-1 mm. 

3. Nerve supply: They are rich and non-medullated. Only free nerve terminals-subserving pain. There are no other sensations and no other endings.

4. Nutrition-from the aqueous humour.

5. Refractive index: l.336.

Functions 

1. Allows free entry of light.

2. Acts as a refractive medium.

AQUEOUS HUMOUR 

It is a clear watery fluid occupying both the anterior and posterior chambers of the eye. It is known to be the only secretory product of epithelium covering the ciliary body. It is not a stagnant fluid but continuously circulates. The ophthalmic artery gives rise to all arterial branches and venous drainage is through the cavernous sinus and pterygoid plexus-secreted and passed over the lens through the pupil into the anterior chamber and then drained into the vascular system-the anterior ciliary veins. 

Composition: Water-98.69% and solids-1.31%. 

Further details, as compared with serum, are as 

follows: 

1. The diffusible, non-ionisable substances, viz. urea, NPN, sugar, etc. same as in serum.

2. Colloids-only traces, i.e. much less than serum.

3. Chlorides-much higher than serum.

Functions 

1. Maintains intraocular pressure and the shape of the eyeball.

2. Acts as a refractive medium.

3. Supplies nutrition to drains the metabolites from the surrounding structures.

CRYSTALLINE LENS 

It is the chief refracting medium of the eyeball. It is a transparent, elastic and biconvex lens, enclosed in a capsule. Posteriorly, it is more convex. It is circular, about 11 mm in diameter. The thickness at the centre is about 3.6-3.9 mm. Refractive index: 1.40 at the centre. It is less in the periphery. It is held in situ by the suspensory ligament.

Function 

To refract light and focus it exactly on the retina.

VITREOUS HUMOUR (VITREOUS BODY) 

It is a jelly-like material covered by a homogeneous membrane, the hyaloid membrane, and occupying the posterior compartment. It is made up of a series of lamellae arranged concentrically round the hyaloid canal. The lamellae are composed of flat cells. The spaces between the lamellae are filled up with fluid. The composition of the vitreous humour is mostly similar to that of the aqueous humour except (i) its glucose content is considerably less than that in either aqueous humour or plasma, and (ii) its concentration of pyruvic acid and lactic acid are higher than that in the aqueous humour. Retina liberates these acids greatly. Refractive index: 1.34. 

Functions 

1. Maintains shape and pressure of the eyeball

2. Acts as a refractive medium.Retinal detachment is the cause of localized liquefaction of the vitreous body and as a result a part of the retina floats in the vitreous cavity.

Control of Eye Movement 

There are six external ocular muscles for the movement of each eyeball. Their names, nerve supply and actions are: superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. Upgaze, or turning the eye upward, is primarily the work of the superior rectus muscle, with some contribution by the inferior oblique muscle.Downgaze, or turning the eye downward, is primarily the work of the inferior rectus, with some contribution by the superior oblique. Abduction, or turning the eye outward toward the ear, is primarily done by the lateral rectus. Adduction, or turning the eye inward toward the nose, is primarily done by the medial rectus. The eye is rotated medially by the superior rectus and superior oblique, and is rotated laterally by the inferior rectus and inferior oblique. In addition, the levator palpebrae superioris muscle, which is not seen on the drawing, elevates the eyelid. The extraocular muscles are innervated by three cranial nerves (CN), CN III (oculomotor nerve), CN IV (trochlear nerve), and CN VI (abducens nerve). The relationship between the cranial nerve nuclei in the brainstem, the cranial nerves, and the muscles that the nerves innervate can be visualized in the schematic below.

Co-ordination of Eye Movements 

The external ocular muscles move the eyeballs in such a way that the two images are formed on the physiologically corresponding points on the retina, so that only one visual impression is produced. These movements may be either voluntary or reflex. 

Ocular movements are of three kinds: 

1.  Those in which the eye axes move to the same side,e.g. right, left, up, or down.

2.  Those in which the axes move in opposite directions,e.g. convergence or divergence.

3. Those in which the eyeballs rotate round their axes clockwise or anticlockwise.

Nervous Control 

The co-ordinated movements of the eyeballs are controlled in several ways: Bilateral nerve supply: Nerve of one side may supply muscles on both sides. For instance, internal and inferior recti and the inferior oblique muscles of one side receive fibres from III cranial nerve nucleus of both sides. Intercommunications: Between the different parts of the III cranial nerve nucleus and between the III, IV and VI cranial nerve nuclei of the same and opposite side.

PUPIL 

Pupil, the central round aperture of the refractive system of the eye; is controlled by the iris. The iris behaves like a diaphragm. The normal size of the pupil is 3-4 mm. The size of the pupil varies with ages. It is small in newborn infant. In childhood and also in adolescence the pupils are at maximum size and in advanced age it is often miotic. Besides this, the pupil in woman is larger than that in man. If the two pupils are unequal then the condition is described as anisocoria. Anisocoria is harmless but should not be considered as normal. Unilateral or bilateral lesions may produce anisocoria.

Functions of Pupil 

There are three main functions of the pupil: 

1.  Pupil modifies the amount of light entering the eye. The amount of light that enters the eye is directly proportional to the area of the pupil. In nocturnal animals, pupil size is of great importance in permitting proper light during night and daytime.

2.  Pupil controls the depth of focus of the optical system of the eye. Smaller pupil increases the depth of focus.

3.  Acuity of vision is dependent upon pupillary size. Spherical aberrations are minimised by the reduction of the pupillary size.

IRIS

The iris is the colored part of the eye that controls the amount of light that enters into the eye. It is the most visible part of the eye. The iris lies in front of the crystalline lens and separates the anterior chamber form the posterior chamber. The iris in part of the uveal tract which includes the ciliary body that also lies behind the iris. The iris tissue makes up the pupil. The pupil is the hole in the iris in which light passes through to the back of the eye. The iris controls the pupil size. The pupil is actually located with its center a little below and slightly to the nasal side of the center of the cornea.

Functions of Iris 

1. It adjusts the amount of light falling on the retina.

2.  By cutting off the peripheral rays it helps to avoid errors of refraction (such as spherical aberration), and thus produces better definition of the image.

3. Increases the depth of focus.

Abnormalities of the Iris & Pupil

Iris and pupil disorders include:

• Aniridia - Aniridia is a genetic defect in which the person is born with an iris.

• Coloboma - An iris coloboma is a large hole in the iris

• Synechiae - Synechia is adhesions that occur between the lens and the iris

• Corectopia - Corectopia is where the pupil is off-center

• Dyscoria - Dyscoria is a disorder where the pupil is distorted or irregular and does not dilate normally

RETINA 

It is the light sensitive nervous layer situated between the choroid and vitreous. It ends just behind the ciliary body in a serrated border-the ora serrata. But the pigment layer is prolonged further onto the inner surface of the ciliary body and the iris. Opposite the pupil, lies the yellow spot (macula lutea) having a central depression (0.44 mm)-the fovea centralis. The yellow colour is due to a pigment which is bleached by light. A little medial to the macula (3.5 mm) lays the optic disc. It is a pinkish-white oval area (1.5 mm average) through which the optic nerve fibres pass out.

Functions of the Retina 

1. Vision: Retina reacts to light of wavelengths between 390 mµ and 750 mµ. Fovea due to the presence of cones is responsible for acuity of vision, bright light or daylight or photopic vision and colour vision. The peripheral retina due to preponderance of rods is responsible for dim light or twilight or scotopic vision. The duality of the retinal receptors has been collectively known as duplicity theory of vision.

2. Reflexes: It is concerned with various reflexes:

1.   Light reflex

2.  Accommodation reflex

3.  Fixation reflex

4. Visuospinal reflex, etc.

3. Tone, posture and equilibrium. Retinal impulses also help to maintain tone, posture and equilibrium.

Formation of an Image on the Retina 

Rays of light which traverse through the centre of the convex lens fall on the retina without any bend, but rays from the peripheral region of the pupil are bent back to a focus on the retina. The image that is formed in the retina is inverted. The cerebral cortex interprets the inverted image on the retina as an upright one.

Common Errors of Refraction 

The normal eye with correct refraction is called emmetropic. Emmetropic eye is capable of focussing the distant object without accommodation. Thus parallel rays from distant objects are brought to focus on the retina when the eye is at rest. Any deviation from the condition of emmetropia is called ammetropia. But refraction may be defective in a number of ways. They are briefly given below. 

Hypermetropia (Long-sightedness)

It can see distant objects but not near ones. Because, parallel rays [from distant objects: 6 metres (20 ft) or beyond] are focussed on retina, but divergent rays (from near objects) behind the retina. Two varieties: 

i.The childhood variety. Here the eyeball is very short. Because, the optical system sometimes develops much faster than the size of the eyeball. As the child grows, the eyeball becomes longer and the defect disappears (in a few cases it persists).

ii. The old age variety (between 40 and 45 age group) or presbyopia. Due to reduced power of accommodation; caused by less elasticity of the lens and weakness of the ciliary muscles. 

Remedy: Convex glasses (convergent-positive power). 

Myopia (Short-sightedness) 

It can see near objects but not distant ones. The eyeball is elongated, so that parallel rays are focussed in front of retina but divergent rays are focussed on it. 

Remedy: Concave glasses (divergent-negative power). 

Astigmatism

Astigmatism (Gr. a-, privative or negative; stigma, a point) is the condition, where the rays of light are not brought into sharp focus at the retina. It is the refractive error of the lens system of the eye due to irregular or oblong shape of the cornea (commonest) or also of the lens. 

Helmholtz's phakoscope

The curvature of an astigmatic lens along one plane is not similar with the curvature at other plane. For this reason light rays falling on one plane or on other plane of an astigmatic lens do not fall at a common focal point. In an astigmatic lens with greater curvature in vertical plane (A-C) and lesser curvature in horizontal plane (B-D) , light rays in the vertical plane are refracted more greatly than in the horizontal plane. Thus, the light rays passing through astigmatic lens do not converge on a common focal point due to the unequal curvature as well as unequal refractive power of the lens at different planes.

Remedy: Astigmatism may be corrected by a cylindrical 

lens or by combination of spherical and cylindrical lenses 

of such strength and so placed that they equalise the refrac-

tion in the meridians of the greatest and least curvature. 

Spherical Aberration 

The peripheral rays in a convex lens are focussed at a nearer point than the central rays, so that the margins of the image become blurred. In the normal eye it is corrected in two ways: 

1. The iris shuts off the peripheral rays.

2. The central portion of the lens has a higher refractive power than the peripheral portion. Hence, all the rays are brought to the same focus.

Chromatic Aberration 

Lights of different colours (e.g. of different wave lengths) undergo different degrees of refraction. Red light (shortest) is refracted least. Violet rays (longest) are refracted most. Hence, the margin of the image may show rainbow colours. The lens of the human eye has the same defect. 

It is normally rectified in two ways: 

1. The difference of refractive powers of the various refractive media of the eyeball partly rectifies it.

2. The colour fringes are ignored by the brain.

Rods and Cones 

The cones which are responsible for colour vision and the rods which cannot detect colour are not evenly distributed over the retina. Each eye contains well over 100 million rods and about 7 million cones. The cones are most densely packed in the fovea. There are no rods in the fovea and the cones themselves are finer than the cones found elsewhere. On moving out from the fovea the proportion of cones to rods gradually diminishes until at the edge of the retina no cones are found. 

Accommodation

Accommodation is the mechanism by which the eye changes refractive power by altering the shape of lens in order to focus objects at variable distances.

Far point: Position of an object when its image clearly falls on retina with no accommodation. 

Near point: Nearest point clearly seen with maximum accommodation. 

Range of accommodation: Distance between far point and near point.

Amplitude of accommodation: Dioptric power difference between rest and fully accommodated eye. – A=P-R ( A: amplitude of accommodation; P:dioptric value of near point; and R: dioptric value of far point.) 

Accommodative Convergence/Accommodation Ratio 

To view near object: Accommodation for clear retinal images, & convergence for binocular single vision. The number of prism dioptres of convergence which accompanies each dioptre of accommodation is (AC/A) ratio.  The normal range for the AC/A ratio is 3:1 to 5:1.

FIELD OF VISION 

Definition 

On looking straight ahead, with the eyeball fixed, that part of the external world which can be seen with each eye is called the visual field of that eye. 

Extent Laterally, it extends up to 104° (i.e. 14° behind the horizontal plane), on the nasal side about 65°. In front there is a cone-shaped area in which the two fields overlap and enjoy binocular vision. The visual fields for blue, red and green are progressively smaller.

BINOCULAR VISION 

Although we have two eyes, two optic nerves and two visual centres yet we do not see two objects. This phenomenon of seeing one object with two eyes is called binocular vision. The impulses set up by light rays from an object when transmitted from the two retinae are simultaneously fused at the cortex into single image. 

COLOUR VISION 

It is the ability of the eye to discriminate between different colours excited by light of different wavelengths. Colour vision is a function of the cones and thus better appreciated in photopic vision. In dim light (scotopic vision), all colours are seen grey and this phenomenon is called Purkinje shift. Colour can be specified using three properties: (1) hue, which is closely related to wavelength, and which is used to name a colour; (2) saturation, which describes the intensity of a colour; and (3) brightness, which indicates the intensity of light emitted or reflected by the surface.

Theories of Colour Vision 

There are number of theories, but none can explain the full details. The first important theory (trichromatic theory) explaining the colour vision was that of Young in the year 1801. In the middle of the same century Helmholtz elaborated colour vision in detail. 

1.Young-Helmholtz theory of colour vision (trichromatic colour theory)  

2. Granits dominator and modulator theory  

3. Hering’s opponent colour theory

 

1. Young-Helmholtz theory of colour vision (trichromatic colour theory) There are three primary colours red, green and blue. There are three types of cones with different pigments. The three pigments are: 1. Erythrolabe (Porphyropsin -- red) 2. Chlorolabe (Lodopsin-- green) 3. Cyanolabe (Cyanopsin -- blue)  Sensation of any given colour is determined by the relative frequency of impulses reaching the brain from each of the three cone systems. Colour blindness is classified based on this theory.  This theory fails to explain the black sensation as black is also considered as a colour.  This also fails to explain how the peripheral colour blind zones perceive yellow, white or grey sensations.

2. Granits dominator and modulator theory

Granit introduced micro-electrodes into the ganglion cells and investigate the sensitivity to light of various wavelengths. a) Dominator. b) Modulator cells.  a) Dominators: These respond to the whole visual spectrum. These are supposed to detect the intensity of the light but not the colour.  This is due to ‘Y’ ganglion cells. b) Modulators: These respond maximum to a narrow wavelength of light. There are three groups of modulators, blue light of wavelength 450 —470 nm green light of wavelength 520 —540 nm red yellow light of 500—600 nm. Hence the modulators are responsible for colour vision. According to the latest concept the X ganglion cells are supposed to be the modulators. Friday, February 6, 2015

3. Herring's opponent colour theory 

This is an extension of trichromatic theory and based on this theory there are four primary colours—blue, green, yellow and red. According to this theory the photochemical substances give one sensation on breakdown and a different one on resynthesis.  According to this theory, complementary colours become antagonistic to its respective primary colours.

Color blindness 

Color blindness means that you have trouble seeing red, green, or blue or a mix of these colors. It’s rare that a person sees no color at all.  Color blindness is also called a color vision problem.  A color vision problem can change your life.

TYPES OF COLOUR BLINDNESS 

1.  Trichromacy ( three colour vision ) - Normal colour vision

2. Anomalous trichomacy ( unusuall three colour vision ) 

• See all three primary colour 

•  One colour is seen weakly - Protanomaly ( l-cone defect ) red weak

•  Deuteranamoly ( M-cone defect ) green weak 

• Tritanomaly ( S-cone defect ) Blue weak

3. Dichromacy ( two colour vision)

• See only two of three primary colours

• One cone is totally disfunctional or absent

• Protanopia ( l-cone absent ) Detutranopia ( M-cone absent ) Tritanopia ( S-cone absent ) 

4. Rod monochromacy (no cones at all )

• Sees no colour only shades of grey

Test For Colour Blindness  

• Pseudoisochromatic plate test  Ishihara test  

• Transformation plate 

• Vanishing plate

TREATMENT 

There is currently no treatment. Colour filters or contact lenses can be used in some situations to enhance the brightness between some colours. For acquired colour vision deficiency, once the cause has been established and treated, your vision may return to normal. 

References

1. Human Physiology by C.C. Chatterjee

2. Textbook of Medical Physiology by A.C. Guyton and J.E Hall

3. Anatomy & Physiology by Lindsay Biga, ‎Devon Quick, ‎Sierra Dawson

4. Tortora's Principles of Anatomy and Physiology by Gerard J. Tortora, ‎Bryan H. Derrickson

5. Introduction to Human Anatomy and Physiology by Eldra Pearl Solomon

6. Introduction to Anatomy and Physiology by Susan J. Hall, ‎Michelle A. Provost-Craig, ‎William C. Rose

7. Fundamentals of Anatomy and Physiology by Frederic Martini, ‎Judi Lindsley Nath, ‎Edwin F. Bartholomew

8. Internet Sources: https://www.britannica.com/, https://en.m.wikipedia.org/, https://www.slideshare.net/