Lens Flashcards
pg 179-186
What is hypermetropia?
A condition where the eyeball is too short or the lens is not strong enough, causing difficulty in focusing on near objects
Also known as farsightedness
How can hypermetropia be corrected?
By placing a convex lens in front of the eye
This is typically done using eyeglasses
What happens to light rays in hypermetropia?
They fall behind the retina
This occurs because the lens cannot sufficiently bend the light rays
What is myopia?
A condition where the eyeball is too long or the lens is too strong, causing difficulty in focusing on distant objects
Also known as nearsightedness
How can myopia be corrected?
By placing a concave lens in front of the eye
This diverges the light rays
What happens to light rays in myopia?
They fall in front of the retina
This occurs because the image cannot be focused properly on the retina
What is presbyopia?
A condition caused by the loss of lens elasticity with age, leading to difficulty in focusing on near objects
It primarily affects older individuals
What corrective lenses are often used for presbyopia?
Bifocal glasses
The upper part is for distant vision, while the lower part is for near vision
What is the primary issue with the lens in presbyopia?
The lens cannot assume a spherical shape
This affects the ability to accommodate for near vision
In what condition is a person unable to focus on a near object?
Hypermetropia
This results in the need for accommodation to see distant objects clearly
In what condition is a person unable to see a distant object clearly?
Myopia
This occurs because the image is focused in front of the retina
What happens to the nucleus of the lens with increasing age?
It becomes sclerosed and yellowish
The increasing refractive index compensates for the flattening of the lens, but increased density reduces visual acuity.
What is a cataract?
A condition where the lens becomes opaque
Senile cataract is the most common form and must be distinguished from nuclear sclerosis.
What is the cause of senile cataract?
Not known
It is characterized by the accumulation of metabolic products and osmotic balance disturbance.
What are other forms of cataract besides senile cataract?
Congenital cataracts, cataracts associated with intraocular diseases, ocular trauma, and systemic diseases
Examples of systemic diseases include diabetes mellitus and scleroderma.
What is the most commonly performed type of cataract surgery?
Phacoemulsification
This procedure involves breaking up the dense nucleus of the lens using ultrasound.
Describe the phacoemulsification procedure.
The dense nucleus is emulsified and removed by vacuum
Various methods exist for breaking up the lens, including dividing it into pieces.
What is the ‘phaco chop’ technique?
A method that splits the lens along its fracture lines using pressure from a cleaving instrument
This technique can be applied from the front or back of the lens.
Why is knowledge of the lens capsule important in cataract surgery?
It helps in understanding its consistency, dimensions, and ability to stretch for intraocular lens insertion
The resilient nature allows for capsulorrhexis procedures.
What is capsulorrhexis?
The careful removal of a circular piece of the anterior wall of the lens capsule
If done without tearing, it can be safely dilated for lens insertion.
What happens to the lens capsule after the removal of lens substance in adults?
It collapses and its diameter increases to approximately 9.5 to 10.5 mm
After implantation, it assumes an oval shape.
What is the maximum equatorial diameter of the lens capsule after lens insertion?
12.0 mm
The introduction of a lens can stretch the equator from approximately 10.5 mm to 11.9 mm.
True or False: The equatorial diameter can exceed 12.0 mm if the capsule remains intact.
False
The intact capsule cannot stretch beyond this measurement.
What happens to lens fiber cells during development?
They lose their nuclei and cytoplasmic organelles become specialized for the production of lens proteins known as crystallins.
What are the two types of crystallins found in the lens?
- Alpha
- Beta
What percentage of the lens fiber mass do crystallins constitute?
Up to 60 percent.
What causes the high refractive index of the lens?
The presence of crystallins.
What compensates for spherical and chromatic aberrations in the lens?
Differing concentrations of crystallins in different parts of the lens.
How do lens epithelial cells and lens fibers relate to each other?
They interdigitate and have numerous gap junctions, making the lens act like a syncytium.
What is the suspensory ligament of the lens also known as?
Zonule.
Where do zonule fibers arise from?
The epithelium of the ciliary processes.
How many bundles do the zonule fibers form?
About 140 bundles.
What shape does the elastic lens capsule tend to assume when the eye is at rest?
A discoid shape.
What is the role of the ciliary muscle in accommodation?
It contracts to allow the lens to assume a more globular shape for focusing on close objects.
What happens to the pupil during accommodation?
It becomes smaller.
What condition is characterized by a decreased ability to accommodate due to lens changes with age?
Presbyopia.
How does the lens change with advancing age?
It becomes denser and less elastic.
What happens to the central portion of the lens as new lens fibers are added throughout life?
It becomes less pliable and more compact.
How much larger is the lens in a 65-year-old compared to a 25-year-old?
One-third larger.
Where do lens epithelial cells transform into lens fibers?
At the equator
This is where lens mitotic activity is at a maximum.
What are the two main functions of the lens epithelium?
- Actively dividing and differentiating into lens cell fibers
- Transporting substances from the aqueous humor to the lens interior and secreting capsular material
What is the primary mass of the lens composed of?
Lens fibers
These fibers are formed by the multiplication and differentiation of lens epithelial cells.
Describe the structure of a lens fiber.
A hexagonal prism in cross-section and very long, measuring about 10 mm
How do lens fibers run in the lens?
Meridionally from the posterior to the anterior lens surface; they are U-shaped
What are the two types of sutures formed by opposing lens fibers?
- Anterior suture: erect Y shape
- Posterior suture: inverted Y shape
What term is used to describe the earliest fiber mass in the center of the lens?
Embryonic nucleus
This is followed by the fetal nucleus and the adult nucleus.
What is the area surrounding the adult nucleus called?
Lens cortex
What components can be found in the cytoplasm of lens fibers?
- Small vesicles
- Microfilaments
- Microtubules
- Occasional mitochondrion
How are lens fibers held together?
By interlocking adjacent plasma membranes
What type of interdigitation occurs between lens fibers?
- Tongue-and-groove
- Ball-and-socket
True or False: Interdigitations in the superficial zones of the lens are more complicated.
False
What may explain how deep lens fibers can survive away from a source of nourishment?
Numerous gap junctions
The process of lens cell elongation results in the movement of the nucleus. Where does the nucleus move as fibers pass deeper?
Anteriorly, taking a position anterior to the nuclei of more superficial cells
Fill in the blank: The continuous production of lens fibers leads to progressively more complicated _______ patterns.
Suture
What happens to the nucleus of a lens fiber as it elongates?
The nucleus remains intact initially but later fragments and disappears
What happens to the size of the adult nucleus over time?
It is always increasing
What are zonular fibers?
Fibers that connect the lens to the ciliary processes
What is the function of the lens capsule?
Serves as a diffusion barrier and molds the shape of the lens during accommodation
What types of compounds can the lens capsule diffuse?
Low-molecular-weight compounds
What does the lens capsule restrict?
Movement of large colloidal particles
Describe the lens epithelium.
Cuboidal and lies beneath the capsule, found only on the anterior surface
What happens to lens epithelial cells at the equator?
They elongate and form columnar cells arranged in meridional rows
The lens capsule is freely permeable to _______.
[low-molecular-weight compounds]
The chief function of the lens capsule is to _______.
[mold the shape of the lens]
True or False: The lens epithelium is found on the posterior surface of the lens.
False
What shape do lens epithelial cells take on the anterior surface?
Cuboidal
What are the refractive media of the eye?
The cornea, the aqueous humor, the lens, and the vitreous body
Where is the lens situated in the eye?
Behind the iris and the pupil, in front of the vitreous body
What shape does the lens have?
Transparent, biconvex structure
What is the measurement of the lens in adults?
Approximately 10 mm in diameter and 4 mm thick
What are the anterior and posterior poles of the lens?
Center points on its anterior and posterior surfaces
What forms the axis of the lens?
A line joining the anterior and posterior poles
What is the equator of the lens?
The marginal circumference of the lens
What keeps the lens in position?
Suspensory ligaments
What is the dioptric power of the entire eye?
About 58 diopters
How much dioptric power does the lens contribute?
About 15 diopters
How does the lens change its dioptric power?
By changing its shape to focus on distant and near objects
What happens to the range of dioptric power with age?
It is reduced, about 8 diopters by age 40 and 1 to 2 diopters by age 60
What is the refractive index of the lens in the periphery and inner zone?
About 1.36 in the periphery and 1.4 in the inner zone
How does the lens grow throughout life?
It continues to grow, measuring about 6.5 mm at birth and 10 mm in adulthood
What are the three parts of the lens?
- Elastic capsule
- Lens epithelium
- Lens fibers
What is the function of the elastic capsule of the lens?
It envelops the entire lens
Where is the capsule thickest and thinnest?
Thickest near the equator (about 20 um) and thinnest at the posterior pole (about 3 um)
What does the inner surface of the anterior part of the capsule contact?
The lens epithelium
How does the capsule appear under light and electron microscopes?
Homogeneous under light microscope; consists of 40 lamellae under electron microscope
What is the composition of each lamella in the capsule?
Type IV collagen fibrils embedded in a matrix of glycoproteins and sulfonated glycosaminoglycan
How much can the elastic capsule be stretched without tearing?
Up to about 60 percent of its circumference
