Physiology of the Eye and Retina Flashcards
1
Q
What is visible light?
A
- Visible light is electromagnetic radiation between the wavelengths of about 350 nanometers (blue) and 750 nm (red)
2
Q
What comprises the lacrimal apparatus? What do these components do?
A
- Consists of the lacrimal gland and associated ducts. Lacrimal glands secrete tears which provide an optically smooth refracting surface. They also contain antibodies and lysozyme to prevent bacterial growth. Secretion is stimulated by parasympathetic stimulation via the facial nerve (via reflex or irritation)
- Used to keep cornea clean and moist
- Tears enter the eye via excretory ducts: they exit the eye medially via a lacrimal punctum. Drain into the nasolacrimal duct
See diagram on lecture notes
3
Q
How is lacrimation (tears) stimulated?
A
parasympathetic efferents travel in the facial nerve (VII) and then branch off in the greater petrosal nerve which goes to the pterygopalatine ganglion (PPG). Post ganglionic fibres from the PPG pass into the lacrimal nerve to the lacrimal glands.
See diagram on autonomic innervation of lacrimal gland in lecture notes
4
Q
What does the eyeball consist of (3 layers of tissue)?
A
- The eye is a fluid-filled sphere enclosed by three layers of tissue:
1. The outer layer is the sclera which surrounds the eyeball. This is continuous with the dura covering the optic nerve. At the front of the eye the sclera thins and becomes transparent and is known as the cornea
2. The middle layer consists of blood vessels & connective tissue including the iris, the ciliary body and the choroid
3. The inner layer is the retina (the light sensing organ)
See diagram in lecture notes
5
Q
What is the sclera?
A
- The edge of the sclera is visible as the white of the eye. The sclera is made of tough connective tissue and is continuous with the epidural sheath of the optic nerve
- The tendons of the extraocular muscles are attached to it
6
Q
What are the iris and pupil?
A
- The pupil is the orifice that lets light into the eye; pupil diameter ranges from about 3-7 mm
- Surrounding the pupil is a sphincter muscle to that can constrict or dilate the pupil
- Eye color (brown, green, blue, etc.) dependent on amount and distribution of the pigment melanin
7
Q
What is the cornea and what are its features?
A
- The cornea is the clear bulging surface in front of the eye. It is the main refractive surface of the eye.
- Features:
o Primary refractive surface of the eye
o Index of refraction: n = 1.37
o Normally transparent and uniformly thick
o Nearly avascular
o Richly supplied with sensory unmyelinated nerve fibers from trigeminal nerve
o Sensitive to foreign bodies, cold air, chemical irritation
o Nutrition to cells from aqueous humor
o Tears maintain oxygen exchange and water content
o Tears prevent scattering and improve optical quality
8
Q
What is the conjunctiva?
A
- Is a layer of stratified columnar epithelium, goblet cells & capillaries that covers the sclera and the inside of the eyelids.
- It contains goblet cells produce mucus that mixes with the tears (this may stop the tears draining too quickly)
See diagram in notes
9
Q
How does light pass into the eye? Which humours are present in the eye? How is light focused?
A
- Light passes through the cornea into the anterior chamber of the eye which is filled with a watery fluid the aqueous humour. It then passes through the pupil, the lens and into the posterior chamber of the eye. This is the small ringlike space between the iris and the lens. It then passes through the lens and into the vitreous humour.
- Vitreous humour is a semisolid watery gel produced by cells in the ciliary body and retina,. It holds the shape of the eye constant to maintain focussing accuracy.
- The cornea contributes to the focusing of light in the eye as it is where the refractive index of the medium changes from air to transparent tissue. (this is why laser surgery works).The cornea is kept as a spherical surface by the intraocular pressure. (normally between 10 & 21 mm Hg)
10
Q
Where is visual acuity (sensitivity)? What is the blind spot? What is the focal length? What happens to the lens as we age?
A
- Visual acuity (sensitivity) is highest in the the centre of the retina, at the fovea. The place where the visual axons leave the eye to form the optic nerve is called the blind spot. (There are no photoreceptors in the blind spot)
- Focal length of eye is 28mm
- Lens becomes less flexible as less able to bulge as we get older, resulting in hypermetropia
11
Q
What is aqueous humour and where is it formed? Where does it circulate and drain to?
A
- Aqueous humour is like cerebrospinal fluid (a nearly protein-free filtrate of blood). It is formed by the ciliary body and circulates up behind the iris, through the pupil and into the anterior chamber. It drains into the canal of Schlemm (also known as the sceral venous sinus). This is a small tube or pipe that encircles the eye at the corneal-scleral junction.
See diagram in lecture notes
12
Q
How is the intraocular pressure formed? Why might high pressure occur and what might it cause?
A
- The difference between the formation and drainage of aqueous humour produces the intraocular pressure, (which can be measured by pressing on the cornea and measuring the indentation)
- If drainage of aqueous humour is blocked pressure in the anterior chamber rises. This increases pressure on the vitreous humor which in turn presses on the retina. Prolonged pressure damages the retina and causes GLAUCOMA.
13
Q
What are the two main sub-types of glaucoma?
A
- Open angle glaucoma (or chronic simple glaucoma) is a slowly progressive condition which occurs when Schlemm’s canal gradually become blocked. The angle between cornea and iris is open i.e. normal (about 40 degrees)
- Closed angle glaucoma (or acute glaucoma) occurs when the iris is pushed forward and the angle between cornea and iris is reduced. This blocks the canal of Schlemm, causing a rapid rise in pressure inside the eye. This causes extreme pain and sudden vision loss (angle may decrease to near zero - see diagram in notes)
14
Q
What is the impact of glaucoma worldwide and how might it be treated?
A
- Glaucoma is a worldwide disease affecting approximately 1–2% of the population aged over 35 years in industrial countries and is a major cause of blindness.
- Glaucoma can be treated by drugs or surgery. One surgical procedure involves an incision in the sclera at to the cornea-sclera junction (see diagram in notes). This allows the aqueous fluid to drain out on to the surface of the eye and thus lowers the pressure.
15
Q
How do drug treatments of glaucoma aim to reduce the intraocular pressure? How are they administered? What are examples of these drugs? Why aren’t these treatments used as often now?
A
- Drug treatment of glaucoma aims to reduce the intraocular pressure by either reducing the formation of aqueous humor or increasing the drainage. glaucoma medications reduce intraocular pressure. Most are applied topically to the eye
o Prostaglandin analogs increase outflow of aqueous humor through canal of Schlemm
o Beta-adrenergic receptor antagonists decrease aqueous humor production
o Alpha2-adrenergic agonists work by a dual mechanism, decreasing aqueous production and increasing outflow.
o Parasympathomimetic agonists and anticholinesterases work by contraction of the ciliary muscle, which opens the canal.
o Carbonic anhydrase inhibitors lower secretion of aqueous humor by inhibiting carbonic anhydrase in the ciliary body. - Decreasing production of aqueous humour can have adverse effects so surgery is increasingly the treatment of choice (if available)
16
Q
Why is UV light dangerous to human tissue? How can it affect the protections our eyes already have? What condition can this result in?
A
- Human tissue is easily damaged by ultraviolet light. The light catalyses the formation of free radicals in cells. In your skin, melanocyte cells produce melanin (which darkens the skin). Melanin absorbs the UV light and stops it damaging other skin cells. However light has to pass through the lens in your eye so it can’t be blocked or we would not be able to see.
- The aqueous humour contains antioxidants (vitamin C and others) to protect the lens and inner cornea against free radical damage. Its slightly alkaline nature activates the vitamin C and increases the antioxidant activity.
- If the level of antioxidants is reduced or too much uv light is absorbed, or for many other reasons, the lens can gradually become opaque and the eye is unable to see. This lens opacity is called a CATARACT
- Cataracts can develop with excess exposure to uv radiation, especially if the aqueous humour is low on antioxidants.
17
Q
How can cataract risk be reduced? How are cataracts treated? Why do cataracts look opaque? What is the impact of cataracts worldwide?
A
- There is a thought to be significantly decreased risk of cataract if there are high levels of vitamin C and E in the diet. Treatment is by surgical replacement of lens.
- In a cataract the UV light cross-links proteins in the lens and makes them opaque. The lens becomes like the white of an egg which has been cooked Cataracts develop slowly, sometimes as a by-product of other conditions such as diabetes or hypertension. These conditions may reduce the level of antioxidants in the aqueous humour. Exposure to excess UV light or microwaves are also risk factors.
- It has been estimated that age-related cataract is responsible for 48% of world blindness.
18
Q
What does normal visual acuity (detailed vision) depend on? Is foveal or peripheral retinal damage more serious for our vision? How do our eyes move at rest? How does this involve the fovea? What is the blind spot and why is it called this?
A
- Normal visual acuity (detailed vision for reading, driving etc) depends on the fovea, where the cones are most closely packed.
- Foveal damage is much more detrimental to vision than damage to the peripheral retina
- Even at rest our eyes are constantly moving back and forth. The fovea can be thought of as our ‘visual fingertip’ Our eyes scan an image so that the fovea moves repeatedly over the area of interest.
- The blind spot has no photoreceptors. It is where the optic nerve axons leave the retina. It is a circular to oval area measuring about 2 x 1.5 mm across.
See diagram in notes
