Visual System Retina Flashcards
What is the sclera?
Sclera: white of eye, Protect the eye from injury and serves as attachment for extra-ocular muscles
What is the iris?
Controls the size of the pupil, defines the eye color
What is the purpose of the pupil?
Pupil: Size regulates the amount of light entering the eye and reaching the retina
What is the vitreous humor?
Clear, gel-like liquid inside the eye ball
What is the ciliary body?
Releases vitreous humor, ciliary muscles change shape of lens when focusing
What are the lens?
Changes its shape to optimize focal distance of the eye to project a sharp image onto the retina
What is the retina?
Contains photoreceptors that transduce light into neural signals
What is the optic nerve?
Transmits neural impulses from retina to the brain, CN II
Describe the anatomy of the eye
The essential structures forming the eye are shown on the diagram on the right. The sclera, which is continuous to the dura mater covering of the optic nerve, forms the outer white layer of the eye ball, the retina the inner layer. Blood vessels are found at the surface of the retina. At the anterior pole of the eye ball the sclera is replaced by the transparent cornea. Aqueous humor is found in the anterior and posterior chambers of the eye, the rest of the eye ball contains vitreous humor, which has a higher viscosity.
Light enters the eye through the pupil, an aperture that is left open by the iris. The cornea and the lenses form the optic apparatus of the eye. The ciliary muscle and the zonule fibers (suspensory ligaments) take part in accommodation, producing a sharp picture of the image on the retina. Light activates the photosensitive elements of the retinal photoreceptors, which are adjacent to the retinal pigment epithelium. The optic disc region itself only contains axons of retinal ganglion cells, the output elements of the retina, but it lacks photoreceptors. As a consequence, the optic disc is responsible for the blind spot, a region inside the boundaries of the visual field, where we don’t receive visual information.
In the fovea shows, which is the central spot of the retina and which is also the location of the highest visual acuity, most layers of the retina, except retinal photoreceptors, are displaced laterally. This way the light can activate the photosensitive elements of the foveal photoreceptors without interference with other retinal structures.
Expain the exam of the ocular fundus
The back of the eye ball, the ocular fundus, can be examined using an ophthalmoscope. It has a light source that shines through the pupil of the patient and illuminates the interior of the patient’s eye ball, and an optic component which enables the examiner to see a sharp picture of the patient’s retina and associated structures.
As an alternative to an ophthalmoscope, a non-mydriatic camera can be used in retinal screening, as illustrated on the website of the Ophthalmic Photographers’ Society, where you can also see a picture of the ocular fundus. Based on this picture you should familiarize yourself with the basic interpretation of the ocular fundus and identify the fovea, the border between nasal and temporal hemiretina, and the location of the optic papilla responsible for the blind spot. You should also be able to figure out whether a picture is taken of the left or the right eye
What is the macula?
Macula (macula lutea) is a region of high visual acuity. It has a diameter of about 5 mm)
What is the purpose of the ophthalmoscope?
The optic nerve head (optic disk) can be easily identified by the radially emerging blood vessels originating in the center of the optic nerve. The fovea forms the center of the retina and is surrounded by the macula. Fovea and macula can be identified by the relative absence of large diameter blood vessels.
Looking at the back of the eye (ocular fundus) with an ophthalmoscope is important for clinical diagnosis. You will have the opportunity to perform fundal inspection on each other in one of the labs.
You can view the retina as part of the brain. The optic nerve, like the rest of the central nervous system, is covered with meninges. As a consequence, increased intracranial pressure, which can be induced by a space occupying lesion of the brain, will also affect the optic disk (optic papilla), leading to “papilledema”, an important clinical sign (see clinical correlations).
What is the blind spot?
The diagram on the right superimposes the passage of light originating from the point of fixation and from the blind spot region of the visual field on the underlying anatomical structures.
Light originating from the point of fixation is projected onto the fovea. Light originating from the blind spot region of the visual field ends up in the optic disk region of the retina. Since the optic disk region does not contain photoreceptor, the visual information of this region is lost.
In everyday life we are not aware of the existence of a blind spot, even when we close one eye (when it should become most obvious). The regions of our cerebral cortex processing visual information simply complete the picture, filling in the gap.
What is the cortical fill in?
- The cortical visual system fills in missing visual information from the blind spot
- The brain constructs a continuous visual scene
Describe the pathway of light?
Reflection
– Light rays bounce off a surface
– We need light to be reflected from objects into our eyes in order to see them
• Absorption
– Transfer of light energy to a
surface
– Basis for color vision
• Refraction
– Change in direction of light when passing from one medium (e.g. air) to another (i.e. water or glass)
– Basis for visual focusing and corrective vision (glasses)
Explain image formation by the eye
- Light rays bend (refract) when they cross from air into the curved surface of the cornea filled with aqueous humor
- Rays are bent to converge on the retina and photoreceptors in the back of the eye
- Important for clear, focused vision
- Focal distance: Distance from the refractive surface (cornea) to the point where parallel rays converge (retina)
- Refraction power of the cornea is ~ 42 diopters (= 1/0.024 m )
How does the lens help image formation?
- The lens further contributes to the refraction of light rays
- Mostly concerned in forming sharp images of objects closer than 9 m
- Accommodation occurs through changing the shape of the lens by ciliary muscles
- Near point: Contraction of ciliary muscles causes suspensory ligaments to relax
- →lens increases in curvature (‘fatter’)
- → more refractive power
• Far point: Relaxation of ciliary muscles
causes suspensory ligaments to tighten
• →lens flattens
• →less refractive power
• Accommodation by lens up to ~15 diopter
Describe the importance of the refractive power
Refraction and Image Formation
Refractive Power
When light rays pass from air into another medium, such as water (or human tissue, which also has a high water component), or a glass lens for example, they change their direction. This interaction between light and its environment is called refraction.
When incoming parallel light rays pass through a convex lens, all light rays after the passage will focus at a defined distance from the midline of the lens. This distance is called focal distance. The focal distance depends on the material and on the curvature of the lens.
Refractive power is the reciprocal value of the focal distance in meters. Its unit of measurement is called diopter, abbreviated as “D” (plural: diopters).
In the human eye, the cornea is responsible for the main refractive power of the eye of about 42 D, whereas the lens is responsibke for the modulation of refractive power (refractive plasticity)during accomodation
What is accomodation?
During far vision, light rays originating from a distant object can be considered (almost) parallel. The focus of light rays during far vision largely depends on the high refractive power of the cornea (42 D) and only a small refractive power of the flat lens (13 D).
During far vision, the ciliary muscle, which is a circular muscle around the lens, is relaxed. The diameter of this circular structure is large, causing tightening of the suspensory ligaments (zonule fibers). As a consequence, these ligaments pull on the lenses’ equator, thereby flattening it and minimizing its refractive power.
What is refractove plasticity and presbyopia?
During near vision incoming light rays can no longer be considered parallel. Therefore, a stronger refractive power is needed to focus the light rays on the retina.
This is achieved by constriction of the circular ciliary muscle, which reduces its’ diameter and relaxes the suspensory ligaments. The lens follows its own elasticity and gets a more rounded (more convex) shape, which increases its refractive power to about 26 D in a young individual. The ciliary muscle is activated by parasympathetic fibers of the autonomic nervous system (ANS). Cell bodies of the first order efferent parasympathetic neurons are located in the upper midbrain, in the Edinger-Westphal nucleus of cranial nerve III. Together with the somatic efferent fibers innervating the extraocular muscles, the preganglionic fibers originating in the midbrain form the oculomotor nerve (CN III). The preganglionic parasympathetic fibers synapse on second order neurons in the ciliary ganglion. Postganglionic fibers originating in this ganglion form the short ciliary nerves which innervate the ciliary muscle.
The variability of the refractive power of the lens between far vision (13 D) and near vision (26 D) is called refractive plasticity.
Unfortunately, the lens looses its elasticity during aging, thereby reducing the ability to focus on near objects, a condition called presbyopia.
What is visual activity?
Spatial resolution / “two-point discrimination of the visual system” • Depends on – Optical factors • Sharpness of retinal focus in the eye – Neural factors • Receptor density – Centralvsperipheral • Retina function – Retinaldetachment – Maculardegeneration • Cortical processing – Amblyopia(lazy eye) - TBI, stroke, etc.
From fovea to periphery…
Higher visual acuity in the center compared to periphery
- Blurring of image reflects the progressive loss of visual acuity with eccentricity
- The central visual field is represented by more brain area than the periphery.
Explain what is visual acuity
Visual acuity is the ability to distinguish between two nearby points. Visual acuity is high when the two-point discrimination threshold is low (high spatial resolution).
The ability to distinguish two points, A and B, can be measured in degrees of the visual angle between point A, the pupil, and point B.
Visual acuity is highly dependent on the densities of retinal photoreceptors. It is high in center of the visual field, since the density of photoreceptors in the fovea and macular region of the retina is high. In the periphery of the visual field, visual acuity is lower.
In addition to the densities of photoreceptors, visual acuity also depends on a proper function of the optical apparatus of the eye, including accommodation. When the optical apparatus fails to produce a focused (sharp) picture on the retina, the objects in the visual field appear “blurry”.
Describe the results of eye charts
• 20/20 = normal vision
• 20/800 = What patient can read at 20 ft, individual with
normal 20/20 vision can read at 800 ft = very bad eye sight!
- Ensure good ambient lighting
- Chart at prescribed distance
- Test each eye separately
- For Neuro-Exam leave glasses on
- Start from top line
• Alternatives
– Non-numericals
– Counting fingers (CF)
– Light perception (LP)
Explain hyperopia/farsightedness
Hyperopia / Farsightedness:
eyeball is too short, image focused behind retina
-> blurry vision
Correction: Convex glass lens in front of eye
Explain myopia/nearsightedness
Myopia / Nearsightedness: eyeball is too long, image focused before retina
-> blurry vision
Correction: Concave lens is placed in front of eye
Accommodation by eyeglasses is usually to about +/- 4 diopters
What are the clinical correlations of Emmetropia, Myopia and Hyperopia?
In order to produce a sharp picture of parallel rays entering the eye ball, the refractive power of the optical apparatus of the eye has to match the length of the eye ball (emmetropia). This condition is also called normal sightedness.
Aberrations from this ideal match (myopia or hyperopia) can appear during development of the eye ball. In myopia the focus of parallel light rays, as in far vision, is anterior to the photosensitive elements of the retinal photoreceptors. Objects positioned closer to the eye, as in near vision, can still be focused on the retina, even without the contribution of the usual mechanisms for near accommodation. Vision is best during near vision and the condition is therefore also called nearsightedness. In hyperopia the focus of parallel light rays would be beyond the retina, in case the ciliary muscle is relaxed and the lens has its lowest refractive power. Objects in the far distance from the eye can still be brought into focus, by activating the mechanisms for near accommodation, which increase the refractive power of the eye. Near accommodation fails for objects close to the eye and reduces the ability for near vision. Vision is best during far vision and the condition is therefore also called farsightedness. In myopia and hyperopia, vision can be corrected with lenses, or corneal surgery
How do we perform a neurological examination of visual activity?
At one point in your life you probably have seen eye charts hanging on the wall of your physician or ophthalmologist, showing different sized letters, numbers or symbols.
For bedside examination of visual acuity we have pocket-sized near cards, as shown on the right. These cards indicate visual acuity as distance equivalents. 20/20 (twenty over twenty) is normal. 20/200 indicates that a patient can read the same number at a distance of 20 feet, which a person with normal vision could read at a distance of 200 feet
What is pappilledema?
Papilledema (Optic Disk Edema)
Papilledema is an important clinical sign, which can indicate increased intracranial pressure. It is seen when inspecting the ocular fundus with an ophthalmoscope. The increased pressure compromises the venous drainage of the eye, leading to a dilation of the retinal veins. As a consequence, the optic disc is pushed forward and the disk appears white, rather than pink, as it would under normal conditions.
What is a detached retina?
The retina separates from the retinal pigment epithelium and the areas detached lose their function. A focal lesion in a defined region causes a scotoma. Laser surgery can stop the process of further separation, although the detached part of the retina does not regain its function
