Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Uveoscleral Outflow
Aqueous flow by means of extrascleral efferents through the episclera and to the superficial scleral plexuses has occasionally been noticed but without much significance.
Vascular arrangements in the wide capillaries of the ciliary body with their high pressure head and the connection of Schlemm’s canal with venous exits for down the pressure gradient will determine the greatest inflow in the ciliary region and greatest outflow through the angle of anterior chamber while a constant incidental thermal circulation brings about a constant internal movement of aqueous humor.
Glass Rod Phenomenon
The normal intraocular pressure ranges upto 25 mm Hg (Schiotz ) with a diurnal variation of 3 to 5 mm Hg. The pressure variations are dependant on pressure changes within the episcleral veins into which the Schlemm’s canal drains; an effect that may be manifested in those eyes which have visible aqueous veins. It has been noted that these aqueous veins convey little or no aqueous during periods of the dry when intraocular pressure is rising but during the diurnal phases of decreasing intraocular pressure, they contain aqueous rather than blood; this suggests that the venous pressure enforce regulating effect on the outflow of aqueous if tributaries of aqueous vein are observed under the slit-lamp, one containing blood and the other aqueous. If the vein is occluded with a glass rod, the contents of the blood filled tributary will flow back into the aqueous filled tributary if this is under a lower pressure and vice – versa. (Glass Rod phenomenon) . This is a test of value in simple glaucoma.
Corneal Swelling and Transparency
Anatomic Basis
1.Absence of blood vessels.
2.Absence of pigments.
3.Regular arrangement of epithelial and endothelial cells.
4.The paucity of the cells in stroma.
5.Epithelial cells are not cornified.
6.Epithelial cells are covered with tears to form a regular refractive surface.
7.Epithelium, endothelium and Descemet’s membrane are transparent because they have the same refractive index.
8.Regular arrangement of collagen fibrils in the stroma.
Physiologic and Metabolic Basis
About 75% of the cornea is composed of water. Transparency is extremely sensitive to the state of corneal turgescence. Even a small increase or loss in the water content of the cornea will result in some loss of transparency.
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The morphological arrangement of the collagen fibrils within the stroma is also a basis of transparency.
Metabolic Basis
The swelling property of the stroma by imbibing water is due to
a)Lattice like and unwoven arrangement of collagen fibrils.
b)The mucopolysaccharides in the stroma.
Fluid can be imbibed from the tears fluid and aqueous humor. The metabolic activity of the epithelium and endothelium prevents the cornea from swelling by continuously extruding from it.
The energy derived from the corneal metabolism counteract the potential cornea by maintaining the deturgescing mechanism.
The intraocular pressure also contributes to the above mechanism by physically counteracting the swelling pressure. The intraocular pressure will counteract the inhibition of fluid from the tears but not from the aqueous, and this may in part account for the greater amount of swelling after damage to the endothelium than to the epithelium.
Influence of Epithelium and Endothelium
The epithelium and endothelium exert their major influence on corneal turgescence by an active rather than a passive process.
The epithelium is relatively impermeable to the passage of sodium ion, but the endothelium is 100 times permeable to sodium and other small ions.
So the sodium in the aqueous rapidly equilibrates with the tissue fluids in the cornea. Hence, a purely passive process like osmotic basis cannot maintain corneal deturgescence.
An active process driven by the metabolic energy present in the epithelium or endothelium is capable of removing the water that is entering the stroma from the fluids on the either side of the cornea.
Sodium and potassium activated ATPases present in the epithelium transports the sodium from the epithelium into the stroma.
A water pump present in the stroma removes the water and dissolved solute from stroma by a secretory process.
Pinocytosis: This may also play a role in the elimination of water from the cornea. Pinocytosis is defined as the movement of package substance by means of vesicle formation.
Membrane Permeability
This depends upon the properties within the membrane and on the characteristics of the penetrating substance.
The electrochemical potential on either side of the membrane determine the rate of diffusion of an ion.
The nature of the individual groups within a molecule, as well as its molecular weight is also important.
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A molecule containing many polar groups will be water soluble and easily traverse the stroma.
A molecule containing many non polar groups will be lipid soluble and rapidly traverse the epithelial and endothelial cell membranes.
Color Vision
Colour is a sensory phenomenon (perceptual) not a physical attribute. Human awareness of colour arises out of subjective visual experiences in which given sensations are ascribed names. The perception of color varies complexly as a function of multiple parameters, including the spectral composition of light emanating from surrounding objects, the spectral composition of light adaptation in the subject just prior to viewing any given object.
Visible Spectrum
The index of refraction for an optical medium differs according to wavelength. The index of refraction for each wavelength must be individually specified by the symbol ‘nm lambda’. This variable extent of refraction spreads a polychromatic white beam of light into its component wavelengths, a phenomenon referred to as spectral dispersion. The assay of individual wavelengths thus exposed is referred to as Visible Spectrum.
The sensation that these individual wavelengths evoke are called the spectral colours.
An object takes on a colour when it contains pigment capable of selectively reflecting or transmitting certain wavelengths of light within the visible spectrum (400-750 µm). Thus, a white object reflects most of the light rays incident upon it while a black object absorbs most of these rays. Blood is red because it contains pigments whose major absorption peaks are below 600 µm i.e., they absorb much of the violet, blue and green portions of the spectrum and transmit most of the light in the red end of the visible spectrum.
There are almost five times as many red or green cone receptors as blue due to which man sees poorly in blue light.
THEORIES OF COLOR VISION
Young Helmholtz Theory (Trichromatic)
In 1807, Thomas Young postulated the first colour vision theory known as the Trichromatic theory. He postulated that there are three sets of fibres in the retina sensitive to red, green and violet respectively. Helmholtz explained that different degree of excitation occurred in each of these sets when they were stimulated by light from various parts of the spectrum.
The peak of the violet wave is in the region of the spectrum which may be formed as blue. The term blue and violet are all rather used indiscriminately in explaining this theory.
Gradually this idea that there were three different sets of nerve fibres was changed to mean three different photoreceptors, and in general, these mean three different types of cones each with its own light sensitive material and
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absorptive characteristics. The differences in these photoreceptors were later attributed to three different visual pigments. Thus, the theory which had original anatomic basis was changed into a chemical basis. This theory assumes the presence of three separate photoreceptors in the retina each transmitting a different type of cortical visual cells which receive these impulses, but these cells have never been demonstrated.
When these three photoreceptors are stimulated i.e. when all the photo pigments of cone absorb light energy, a sensation of white results. All other sensation of colour result from unequal stimulation of these photoreceptors. The sensation of black is probably due to an active process in the retina and not due to lack of any stimulation. It is not known whether there are any linkages between these various receptors or whether they send separate messages individually to the cortex, but it is probable that the fusion of color takes place in the cortex. The phenomenon by exposing each retina to a monochromatic frequency, for example yellow may be obtained by binocular fusion of red and green showing that it can be produced by cerebral fusion. This theory also explains the phenomenon of negative after images. If a retina exposed to green light is later is exposed to white light it has no longer the ability to respond to the green wavelength. The adjacent red and violet receptors which were not stimulated, respond to the red and violet part of the spectrum of white light.
The after image produced is a mixture of two color i.e. purple. Evidence suggests that the cone vision which is responsible to a great extent for color vision is more corticalised than the rod vision. It would appear that for the color vision, cone function is dependent on the integrity of the cortex and is not entirely a retinal function.
Hering’s Theory of Color Vision
This theory assumes the presence of three photochemical receptors in the retina but they are of such a nature as to give rise to six different qualities of sensations. Visible light is supposed to breakdown a substance known as white black substance which sets up impulses in the optic nerve producing sensation of white light.
In the absence of light, this substance is synthesized into the original substance and the resynthesis produces sensation of blackness. There are two other substances red green and yellow blue, each of which produces sensation of one colour on breakdown of other on synthesis.
Photochemical substances |
Retinal process |
Sensation |
White Black |
Breakdown |
White |
|
Resynthesis |
Black |
Red Green |
Breakdown |
Red |
|
Resynthesis |
Green |
Yellow Blue |
Breakdown |
Yellow |
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Resynthesis |
Blue |
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According to this theory, the complimentary colors are nearly antagonistic to each other and when they are exhibited simultaneously to the retina neutralize each other producing a sensation of white light. Furthermore, the theory does not recognize the doctrine of specific nerve energies since it says that the same fibre can signal two different sensations to the brain. Further, according to this theory; red and green when thrown on the retina simultaneously neutralize each other and produce the sensation of white. Indeed, they produce a sensation of yellow unless blue green and blue red are selected.
Ladd Franklin’s Theory
This theory attempts a reconciliation between the above two theories and is not accepted.
Poly Chromatic Theory
Hart Ridge has postulated the presence of four secondary photoreceptors in addition to the three primary cones of the trichromatic hypothesis.
Enoch suggested a theory of wave guides whereby the cone, the diameter of which roughly corresponds to a wavelength of visible light transmits the light in the shape of patterns – “modes of transmission” depending on the wave length. If the pigments are arranged in the cones, each one will respond to a particular wave length. This is only theoretical speculation.
Four types of cones exist, three concerned with colour vision and one concerned with luminosity. The colour discrimination is based on different quality requirements of different cones. These things remain unestablished.
Binocular Vision
Binocular vision may be defined as the coordinate use of the two eyes which produce a single visual impression. (The vision is achieved by the coordinated use of both eyes so that the images which arise in each eye separately are appreciated as a single mental impression in the visual part of the cortex. There are many factors concerned with successful development and they are classified as sensory motor and central mechanisms).
Advantages of Binocular Vision
1.Optical defects present in one eye are made less obvious by the normal image of the other eye.
2.Uniocular visual field defect in one eye can be masked by the other. E.g. Blind spot, which cannot be detected in binocular visual field, since the object whose image falls on the blind area in one eye is perceived by the functioning retina in the other eye.
3.Binocular visual field is larger then either field alone.
4.Stereopsis and depth perception are made possible by binocular vision.
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Requisites for Binocular Vision
1.There must be proper fixation with each eye by the coordinated extramuscular movements which keep the object fixed by the corresponding retinal areas.
2.The visual fields of the two eyes must overlap to large extent.
3.Approximately similar images must be formed on the retina (size, shape, color intensity, etc.) When dissimilar objects are presented by a prism. retinal rivalry results in which one image followed by the other is presented to the consciousness.
4.Retina must posseses physiologically corresponding points which have common visual directions.
5.Semi decussation of fibres in the chiasma is a common attribute for binocular vision and stereopsis.
6.The eyes are coordinated by reflex activities at all times so that retinal receptors have a common visual direction receiving the same image (fusional movements).
Sensory Mechanisms
1.The factor of visual acuity value of the retinal receptors of each eye.
2.The factor of the normal correspondence of the retinal receptors of the two eyes.
3.The factor of semi – deccusations of the optic nerve fibres at the optic chiasma and the integrity of the apparent visual pathway.
4.The factor of the proprioceptive impulses of the extra ocular muscles.
The Visual Acuity Value of the Retinal Receptors of Each Eye
The development of binocular vision is dependent upon the presence of retinal receptors which have an adequate visual acuity value in each eye. The receptors of the fovea and Para fovea are represented only by cones which have a photopic type of perception ( visual perception of form and colour) whereas the receptors of the rest of the retina comprises in addition to small number of cones. Large number of rods are concerned with scotopic vision (perception of light and movement). The development of binocular vision is dependent primarily on an adequate degree of central vision and this depends on a reasonable integrity of the foveal and macular elements and of the refracting media of the eye (cornea, aqueous, lens, vitreous) and also, a certain degree of comparability of the two eyes with regard to their refraction so that “size difference” of the two retinal images is not too great to prevent their fusion. An adequate degree of peripheral vision is also essential for the development of binocular vision.
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The Factor of the Normal Correspondence of the Retinal Receptors of the Two Eyes
The significance of retinal correspondence is the fact that stimulation of corresponding retinal points results in the formation of a single mental impression. The retinal correspondence has a functional rather than anatomical basis. The two fovea may be regarded as corresponding retinal points and there are numerous other pairs of corresponding retinal points in the temporal retina of one eye and in the nasal retina of other eye which are more or less equidistant from their respective fovea.
Horopter
Binocular visual space
Binocular visual space can be divided into two regions
1.Horopter and Panum’s fusional area
2.Region of physiological diplopia.
Horopter is defined as an imaginary surface passing through point of intersection of the two visual axes and is the sum of points in space, the images of which fall on corresponding retinal areas.
When the eyes are fixed on a point (fixation points remain stationary). Horopteric surface is defined by the intersections of the visual direction of the corresponding retinal points of the two eyes. It is not a plane, but is gently curved with concavity concave towards the object.
Panum’s Area: An area just in front and behind the horopter is known as Panum’s area. An object lying in this area is also capable of stimulating the corresponding retinal receptors in both the eyes and so will be seen singly.
Visual Direction
The line through the nodal point of the eye joining the object on which the eye is fixed and the fovea, is the visual axis. The mental projection on to the space along the visual axis is the principal visual direction.
In binocular vision the eyes are always more or less converged, and when the eyes converge so that the principal visual directions intersect at the object of regard.
(Cyclopean eye – Mental correlation of the two eyes into one is called as cyclopean eye)
Objects lying in areas other than Horopter and Panum’s area are seen double. (Physiological diplopia) because it will stimulate the non corresponding retinal elements in both eyes.
If the object is in front of Panum’s area the temporal retina is stimulated on both sides. Hence, the diplopia is heteronymous diplopia or crossed diplopia.
Similarly, if the object is lying farther from the fixation object in the horopter then it will also stimulate the non corresponding retinal elements in both eyes lying in the nasal side of fovea and projected to the left of fixation object in left
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eye and to the right of fixation object in the right eye and hence produce homonymous or uncrossed diplopia.
The Factor of Semi Decussation of Optic Nerve Fibres at the Optic Chiasma
The development of binocular vision is dependent on a semi decussation of the afferent optic nerve fibres at chiasma since it enables the nerve fibres from corresponding retinal areas of the two eyes to become associated with one another ultimately in the visual area of the occipital cortex. Thus all retinal fibres from the temporal half of the fovea pass through the optic chiasma with out any decussation so that they enter ipisilateral optic tract, where as the retinal fibres from the nasal half of the retina with the nasal half of the fovea decussate in the chiasma and pass to the contralateral optic tract. It follows therefore the fibres from the corresponding retinal points (temporal retina of one eye and nasal retina of the other eye) travel in the same optic tract and terminate in the same lateral genicualte body and are relayed in the optic radiation to the striate area of the visual cortex in the occipital lobe.
The Factor of Proprioceptive Impulses from the Extraocular Muscles:
It is known that the extra ocular muscles provide the brain with sensory information of a proprioceptive nature and this is of some importance in the establishment of binocular vision.
Motor Mechanisms
The motor mechanism favouring binocular vision is concerned essentially with the maintenance of the two eyes in a correct positional relationship at rest and during movement. They may be considered in the following groups.
Anatomical Factors
Anatomical factors are concerned which determine the position of the eyes which are concerned with the structure of the bony orbit and their content with the structure of the eyes so that the eyes are able to be in the orbit in such ways that their visual axes are aligned correctly. There are several aspects of ocular development still to be completed after birth. They are 1) the retina and fovea are not fully developed at birth 2) the globe is only 73% of its adults size resulting in infantile hypermetropia. 3) the ciliary muscle is not fully developed until 3 years. 4) the medial recti are structurally more advanced muscle. By the age of 6 months structural development has progressed enough for the rudimentary formation of binocular vision to have been laid.
Physiological Factors
Physiological factors which determine the position of the eyes are of three types.
1.Postural reflexes which are independent of visual stimuli.
2.Fixation reflexes which are dependent of visual stimuli.
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3.Kinetic reflexes which are concerned with accommodation & convergence relationship.
Postural reflexes: are concerned with the maintenance of the two eyes in their correct relative positions within the orbit so that the visual axes are aligned correctly to one another despite changes in movement of the head relative to body, or of the body relative to space in the absence of the intervention of any visual stimuli. In this way the effect is subjective orientation. They are part of a subcortical involuntary reflex mechanism and are entirely unconditioned. There are two groups of postural reflexes static, and stato-kinetic reflex.
Static reflexes: are initiated by changes in the position of head relative to body. They are controlled by part of the labyrinth and by proprioceptive impulses from neck muscles. E.g., Passive turning of head to right is followed by a conjugate movement of the eye to the left i.e. in direction opposite to the movement of the head ( Doll’s head phenomenon)
Stato kinetic reflexes: Are initiated by changes in position of the head relative to space. They are controlled by part of labyrinth (semicircular canal) Fixation reflexes: Fixation reflexes are concerned with the maintenance of the two eyes in their correct position so that their visual axes are aligned correctly with one another despite changes in the movement of head relative to the body or of the body relative to space as a result of visual stimuli which reach the cortex by afferent visual pathway. They are part of cortical voluntary reflex mechanism.
a)Fixation reflex or orientation reflex and ability of each eye independently to fix a definite object.
b)Refixation Reflex: It is an elaboration of the fixation reflex and concerns the ability of the eye to retain fixation of a moving object or to change fixation from one object to another.
c)Conjugate fixation reflex: This is the application of the fixation to both eyes at the same time so that they retain fixation during the conduct of conjugate movements.
d)Disjunctive or vergence fixation reflex: This is the application of fixation reflex to both eyes at the same time so that they retain fixation during the conduct of a disjunctive movement.
Kinetic Reflexes
These are concerned with the maintenance of the two eyes in their correct relative position within the orbit as a result of a controlled accommodation – convergence relationship.
Central Mechanism
a)The factor fusion.
b)The factor of cortical motor control.
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The factor of fusion: Even though an object is viewed separately by the two eyes, the visual cortex is able to perceive a single mental impression of the object and this is due to the cortical mechanism of fusion.
Cortical Motor Control
The centres in the frontal and occipital cortex and cranial nuclei concerned in the final efferent impulses to the extraocular muscles also play an important role in the maintenance of binocular vision.
Grades of Binocular Vision
a)Simultaneous perception.
b)Fusion.
c)Stereopsis.
THE PUPILLARY PATHWAY AND REACTION
The pupils are controlled by two muscles of ectodermal origin – the sphincter and dilator. The responses of these muscles to stimuli are very rapid, delicate and are easily observed. The constrictor centre possesses the ‘tone’ and is perpetually sending out impulses to the sphincter which keep the pupil slightly contracted.
Mydriasis – abnormal dilatation. Miosis – abnormal contraction.
The Pupillary Pathways
The sphincter is supplied by cholinergic nerves of the parasympathetic system through 3rd cranial nerve. The fibres start in the EdingerWestphal nucleus in the floor of aqueduct of Sylvius. This nucleus has connections with the dilator centre as well as with the frontal and occipital cortex. Then the fibres pass through 3rd nerve as far as the orbit. Here the fibres pass into branch supplying inferior oblique leaving it by the short root of ciliary ganglion. From this they pass through short ciliary nerves piercing the sclera around the optic nerve along the short ciliary arteries. They pass forwards in the choroid and ciliary body to the iris.
The dilator pupillae is supplied by the adrenergic fibres of the cervical sympathetic nerve. The fibres commence in the hypothalamus near the constrictor centre and also has got connections with cerebral cortex.
From the hypothalamic centre the fibres pass downwards through the medulla oblongata into the lateral columns of the cord by the ventral roots of the 1st dorsal and the last two cervical nerves, enter the rami communicantes and run to the 1st thoracic or stellate ganglion. From it to the cervical sympathetic by the anterior limb of the ansa of verusuens. Then in this nerve they pass upwards to the cervical ganglion. Then pass with the carotid plexus into the skull. They run over the anterior part of the semilunar ganglion and pass into the 1st ie ophthalmic division of the 5th nervenasociliary nerve –