Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Physiology
ACCOMMODATION
Accommodation is a complex constellation of sensory neuromuscular, biophysical phenomenon, by which the overall refracting power of the eye changes rapidly to image objects at different viewing distances clearly on the retina. To accomplish this task the following strategies are required.
i)Changing the corneal curvature.
ii)Changing the distance between the cornea and retina.
iii)Placing another lens system between the cornea & the retina whose effective refracting power can be varied by changing its surface curvatures or its position within the globe.
iv)Changing the index of refraction of one or more components of the ocular media.
v)Having two or many separate optical pathways of different refractive
power.
An interesting fact is that the raccoon can accommodate upto 20 diopters by moving the lens towards the cornea without changing the lens thickness.
In humans, accommodation is brought about by altering the form of the crystalline lens and this is accomplished by contraction of the ciliary muscle. The normal eye is so constructed that, when at rest, rays of light coming from infinity are focused on the retina. The refractive indices of the ocular media, the curvature of the refractive surfaces and the position of retina is such that the rays of light entering the eyeball parallel to the optical axis are focused on the sensitive outer layer of the retina i.e. the rods & cones. By definition an emmetropic eye is one in which the retina coincides with the posterior principle focus of the optical system when the eye is at rest. Rays of light coming from a distance of 6 meters or more are considered parallel and come, therefore, to a focus on the retina of the emmetropic eye.
The region through which an object may be moved in space without causing noticeable blurring of the image is called the depth of the field. This increases considerably as the pupillary aperture is narrowed. An emmetropic eye with a pupil of 2 mm has a depth of field from infinity of about 15 metres. But if the pupil becomes 4mm it is infinity to 30 meters.
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The resolving power of the eye as an optical instrument is measured by the smallest visual angle at which two objects can be distinguished separately. This is called the minimum separable and is approximately 40 to 60 seconds of arc. In the emmetropic eye an object that subtends an angle of 60 seconds of arc has a retinal image of about 0.004 mm. An object may be seen even when it is out of focus as long as its diffusion image does not subtend an angle greater than the minimum separable (60 seconds of arc) i.e., as long as the image does not spread more than 0.004 mm. If the object is moved closer to the eye than the depth of focus permits, the blurred arc of diffusion falling on the percipient elements subtend an angle more than 60 seconds of arc. In order of obtain a clear focus under physiological conditions, these circumstances, either the dioptric power of the eye must increase or the eyeball must become large. It is obvious that the human eye cannot change its axial length under physiological condition; therefore, this change of focusing power must take place by changing the dioptric power of its refractory surface. Two objects cannot be in focus at the same time. Therefore, when the difference between the distances of each from the observer exceeds the depth of focus of the eye, the dioptric power of the eye must change.
The accommodative change whether it is one of relaxation or one of increased accommodation occurs with great precision and is generally completed in about half a second. The exact nature of the stimulus by means of which a blurred image on the retina gives rise to the accommodation reflex is not entirely known.
Anatomy of the Parts of the Eye Concerned with Accommodation
Accommodation is the result of a change in the form of the lens, brought about by the contraction of the ciliary muscles.
The Lens Capsule
It is a thin transparent membrane enclosing the lens and is composed of two layers. The outer layer is derived from the zonule and hence called the ‘ zonular lamella ’.
This becomes exfoliated in Glass Blower’s cataract and Glaucoma capsulare (Fig. 32.1).
The chief characteristic of the lens capsule is its elastic properties. The anterior capsule is much thicker than the posterior capsule and varies in thickness from the equator to the anterior pole (Fig. 32.2).
Fig. 32.1
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Fig. 32.2
Crystalline Lens and Zonular Ligaments
The Lens Substance
Anterior surface faces a layer of epithelial cells. There is no epithelium on the posterior surface. All the new fibres as they are formed arise at the equator and push the older fibres towards the centre as they form.
Growth
The lens continues to grow throughout life. There are two phases namely infancy and early childhood – rapid growth followed by a phase of slow, steady growth which continues throughout life. A loss of elasticity is associated with continued growth. Since the older fibres cannot be cast off as is usual with epithelial cells in other parts of the body, they are crowed together in the centre of the lens. Here they lose water and form a dense nucleus. This nucleus is of little physiologic or pathologic significance until the age of 30 years by which time it has become so large and dense that it interferes with the deformation of the lens during accommodation.
The growth of the lens with age is due to an increase in the thickness of the cortex.
Elasticity
The lens substance is not elastic. The posterior cortex has greater malleability than the anterior and the difference between the two becomes more marked with increase in age. The change from the soft cortex to the hard nucleus leads to an increase in refractive index of the lens. The more central fibres largely account for the refractive power of the lens.
Radii of Curvature
Anterior surface – 11 mm.
Posterior surface- 5.7 mm.
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The Zonules
The lens is supported by the ciliary body. The zonules serve as the rope and appear to be made up of separate individual fibers when viewed at with slit lamp biomicroscope.
The Ciliary Muscle
Schlemm’s has divided the ciliary muscle into three groups:
i)Meridional
ii)Radial
iii)Circular
Meridional Bundle
It is located on the scleral aspect of the muscle which is much thicker anteriorly than posteriorly.
Its origin is from the corneo-scleral junction just to the back of the Schlemm’s canal. It inserts into the choriodal coat near the posterior pole.
Radial Portion
It is interposed with the connective tissue framework.
Circular Portion (Muller’s Muscle)
This forms a circular bundle at the inner anterior aspect of the ciliary body. It acts as a sphincter muscle and on contraction, narrows the ring formed by the ciliary processes. Always poorly developed in myopic eyes and well developed in hyperopic eyes which suggest that this bundle is important in accommodation.
Innervation of the Ciliary Muscle
i)IIIrd cranial nerve.
ii)Fibres from the sympathetic nervous system.
Changes in the Eye During Accommodation (Fig. 32.3)
i)Pupil contracts during accommodation and convergence. This is synkinesis and not a true reflex in that it does not depend upon either accommodation or convergence alone for its appearance.
ii)Anterior pole of the lens moves forwards carrying the iris with it. Hence, the anterior chamber becomes slightly shallow in the centre as the anterior pole of the lens approaches the back surface of the cornea. The posterior pole does not change it position.
iii)The anterior surface of the lens becomes more convex so that its radius becomes smaller. The posterior surface increases its curvature slightly
iv)Since the posterior pole remains fixed and the anterior pole moves forward, the thickness of the lens at the centre increases.
v)As the lens increases in axial thickness, it diminishes in diameter.
vi)Change occurs in the tension of the lens capsule. The anterior capsule becomes slack and separates from the posterior capsule.
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Fig. 32.3
vii) The lens is displaced in the direction of gravity during accommodation. viii) Refractive power of the lens changes.
THEORIES OF MECHANISM OF ACCOMMODATION
Relaxation Theory of Helmholtz
Helmholtz considered that the lens was elastic and that in the normal state it was kept stretched and flattened by the tension of the suspensory ligament. In the act of accommodation, the contraction of the ciliary muscle lessened the circle formed by the ciliary processes and thus relaxed the suspensory ligament. The lens then assumed a spherical form. This is the ‘Relaxation theory of Helmholtz’.
The relaxation theory assumes that when the eye is at rest or unaccommodated, the lens is compressed in its capsule by the zonules. In the compressed form, its surfaces are curved least and the dioptric power accordingly, is at minimum. The zonule is kept constantly stretched by its attachments to the ciliary body.
In the original theory of Helmholtz, it was supposed that the elasticity of the choroid sustained this. When the ciliary muscle contracts, two things happen. The choroid is pulled forwards relaxing the tension of the zonules and the ring
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formed by the processes is narrowed by the sphincter or circular fibres of the ciliary muscle, thus making the zonules still more lax. When this occurs, the lens is freed of any compressory force and by virtue of elasticity of its capsule, it assumes the shape of a sphere. This increases the dioptric power of the eye.
It does not seem logical, however to expect the choroid to constitute a counter weight to such a continually acting force. The choroid being a richly vascular network is hardly a structure to be constantly stretched without showing pathological changes. To overcome this it was suggested that the traction of the zonules was borne chiefly by the radial and longitudinal portions of the ciliary muscles.According to Henderson, the ciliary muscle had functions mediated by different muscle bundles. The radial and longitudinal fibres maintain a constant postural activity which counterbalances the pull of the zonules, whereas sphincter or circular muscle overcomes the tension on the zonules and permits it to become slack. Thus according to him both parasympathetic and sympathetic innervations are concerned in accommodation. He considered that accommodation is accomplished by an inhibition of the postural activity of the longitudinal fibres of the ciliary muscles and by active contraction of the circular fibres. The former is due to 3rd nerve. He regarded the sympathetic as excitatory and 3rd nerve as inhibitory.
It has been shown already that during accommodation the lens is displaced in the direction of gravity and that both anterior and posterior capsules of the lens become slack, particularly the former. This is strong evidence in favour of the relaxation theory which demands that during accommodation both the surfaces of the lens become more convex.
One of the fundamental objections to the Helmholtz theory is that the choroid does not move forward when the longitudinal fibres of the ciliary muscle contract (Fig. 32.4).
Fig. 32.4
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The Tscherning Theory or Theory of Increase in Tension
Tscherning contended that in the act of accommodation the anterior surface of lens assumed, not a spherical but a hyperbolic form; and in order to account for the formation of the anterior lenticonus, he suggested that the contraction of the ciliary muscle tightened the suspensory ligament and compresses the lens against the vitreous, an action which caused its anterior surface to be bulged forwards. According to Fincham, the capsule of the lens particularly the anterior capsule does not have the same thickness throughout and he made use of this, to account for the conoidal during accommodation. If the lens capsule is elastic and exerts a compressory effect on the lens, the compression will be least where the capsule is at its thinnest. As a result, the lens bulges through. It is now generally accepted that the anterior surface of the lens becomes more convex as described by Tscherning and that during accommodation, it becomes conoidal in form.
When all the evidence for the two theories are viewed impartially, the weight of evidence is in favour of the relaxation theory. The general impression gained is that there is relaxation of the zonules during accommodation, with relaxation of the lens capsule to create greater curvature of the central surface of the lens. In addition these observations support the belief, during accommodation, the vitreous body does press on the periphery of the lens.
Aqueous Humor
The avascular structures of the anterior segment of the eye, the lens and the cornea depend upon a constant turnover of the surrounding aqueous to deliver nutrients and wash out metabolic waste products. So the important functions of the aqueous flow is to compensate the lens and the cornea for the lack of blood vessels and also to maintain the clarity of the optical media of the eye by constant production and drainage, maintaining intraocular pressure. In many respects, aqueous humor is comparable to cerebro spinal fluid. It fills the prelental space, (anterior, posterior) penetrates the gel-like vitreous humor and takes the place of the lymph that is absent within the eye as essential fluid. The combination of aqueous flow and the resistance of outflow routes ensure a relatively high pressure within the eye. The IOP is one of the highest tissue pressure in the body and may be a necessary requirement to maintain a stable form of the eye globe even when subjected to traction by the extraocular muscles.
Characteristics of Aqueous Humor
Physical Properties
Specific gravity – 1002 to 1012.
Refractive index- 1.3336 – 1.3370.
Surface tension – 72-73 dynes/cm at 18 °c
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Viscosity
Normally with little protein, the viscosity is slightly greater than that of water and considerably less than that of blood (1.029 – 1.030).
Conductivity
Conductivity is greater than that of serum but lower than that of CSF due to the latter’s greater concentration of Na+ and Cl– ions.
Osmotic Pressure
Aqueous is slightly hypertonic compared to plasma 3 – 5 mmols/li
Reaction
Slightly alkaline ( pH 7.53 ).
Chemical Composition of Aqueous
Protein 0.02 gm%. Urea 12 mg%. Amino acids 9 mg%. Creatinine 1.7 mg%.
Reducing substances 0.06%.
Organic acids – Ascorbic acid & lactic acid – higher than in plasma.
— Uric acid, pH, concentration of HCO3lower than in plasma. Hyaluronic acid present.
Inorganic acidsNa+ less than in plasma
—Clhigher than in plasma
—Ca2+, Mg2+, K+ lower than that in plasma.
Formation of Aqueous Humor
Anatomically the blood – aqueous barrier consists of
i)The anterior iris surface.
ii)The ciliary epithelium.
iii)The retina overlying the choriodal vessels anterior to the equator, where the retina is adequately nourished by choriodal circulation alone and diffusion through it into the vitreous presumably occur, and
iv)The capillary walls of the retinal vessels in the posterior half of the retina. The main source of the intra ocular fluids is from the ciliary process (non– pigmented epithelium) . Histological findings point to the ciliary body as the main source. Recently the granules have been demonstrated in the ciliary epithelium whose structure is similar to those having known secretory activity.
The process involved in the formation of aqueous are complex and more than one mechanism must operate in order to account for the various discrepancies between the distribution ratio of the major constituents of plasma and aqueous.
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Mode of entry of specific substances in the aqueous humor with regard to its composition must be influenced by the following processes:
i)Diffusion from the blood into the ciliary epithelium.
ii)Dialysis and ultra filtration.
iii)Active secretion by the ciliary epithelium into the posterior chamber (selective transport).
iv)A form of restricted leakage through spaces or pores present in the blood aqueous barrier.
Aqueous cannot be a simple plasma filtrate (diffusion) as it is slightly hypertonic to plasma.
Excess of ascorbate, chloride and sodium in the aqueous humor as compared with dialysate of plasma indicates secretory mechanism.
In examining the concentration ratios of these substances in the aqueous and plasma, the values are not compatible with the predicted values based on a distribution in accordance with GIBBS – equilibrium for a plasma dialysate. Ultra Filtration: Also cannot explain this totally (dialysis in the presence of hydrostatic pressure), since the hydrostatic pressure differential between the aqueous and capillary bed in the ciliary processes is relatively negligible.
Pinocytosis: Movement of package substances by means of vesicle formation may be considered as a form of active transport.
Active Transport: Sodium and ascorbate are actively transported from blood to aqueous by the cells of ciliary epithelium, ultrafiltration playing a secondary role. Active transport of sodium is important because it may be considered as a primary process of aqueous secretion. Other ions follow passively the Na+ ions and water is also moved from the plasma to aqueous as per osmotic considerations. The rate of Na+ transport by ciliary body is sufficient to explain the rate at which water enters the eye.
Active transport is an energy requiring process. Various ATPases may play an important role in this. Na+ activated ATPases has been found, intra cellularly in the cell membrane.
Oxidative phosphorylation is in some way connected with secretory process. Citric acid cycle may play a role in providing the energy (Fig. 32.5).
Membrane Permeability
Concentration of protein is determined by the sieve like properties of blood – aqueous barrier and by their relative molecular size.
On the other hand, penetration for lipid soluble substances is determined by their lipid solubility rather than molecular size.
Other factors affecting membrane permeability are a) Electrochemical potential on either side of the membrane b) Pore size.
Substances can move across the biologic membrane by means of simple or facilitated diffusion, filtration, active transport or a combination of all these.
In the eye, the biologic membrane forming blood – aqueous barrier in the posterior chamber is located at the site of ciliary epithelium. Capillary endothelium cannot be considered as a biologic membrane because it lacks active secretory process.
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Fig. 32.5
Circulation of Aqueous
Metabolic interchanges between aqueous and various structures like iris, cornea and lens cause the movement of metabolites only and not circulation of aqueous. This is responsible for the regional differences in the aqueous of anterior chamber and posterior chamber. E.g., Phosphate is deficient compared to plasma because retina and lens use this for their metabolic activity. Glucose; 20% lower than that of plasma as it is continuously used by lens.
Bulk Flow
Fluid from the blood in capillaries of ciliary process passes into the capillary wall to the stroma of the process and from these into the ciliary epithelium ot the posterior chamber.
Posterior chamber-Pupil-Anterior chamber-Trabecular meshwork-Episcleral veins.
During this process from the entrance to exit, components are being continuously added and removed by border structures so that the fluid leaving the eye is chemically different from that entering the eye.
Thermal Circulation
A localized and relatively unimportant movement within the anterior chamber is caused by convention currents. Aqueous flows upwards in the region of iris and downwards in the region of cornea. It is a manifestation of difference in temperature between air cooled cornea and vascularised iris. It is purely a physical phenomenon and of little physiological importance. This circulation is responsible for the deposition of cellular elements and keratic precipitates on the posterior surface of the cornea.