Question

1. How does the iris promote eye color as well as the dilation and constriction of the pupil?

2. What is the role of eyelashes when a foreign body invades the eye?

3. Define the terms miosis, mydriasis, accommodation, near point, and cycloplegia in relation to the eye structures and functions

4. Explain primary, secondary, and congenital glaucoma —as well as closed-angle glaucoma and open-angle glaucoma.

Answer 1

Melanin pigment present within the iris is responsible for the visual impression of human eye coloration. The change in human eye color is explained by one interval on Chromosome 15 that contains the OCA2 gene. In the brown iris there is an abundance of melanocytes and melanin in the anterior border layer and stroma whereas in the blue iris these layers contain very little melanin. As light traverses these relatively melanin‐free layers, collagen fibrils of the iris scatter the short blue wavelengths to the surface, thus a blue iris is a consequence of structure not of major differences in chemical composition. Different shades of blue, and in irises with a limited amount of melanin, different shades of grey, green and hazel, are determined by the thickness and density of the iris itself and the extent of accumulation of white collagen fibres, as well as patches of tissue loss in the anterior border layer and stroma.

Dilatation and constriction of pupil:

The iris is made up of two types of muscles ; a ring of sphincter muscles(circular) that encircles and constricts the pupil to prevent light from entering, and a set of dilator muscles(radial) that expands the pupil inorder to accomodate more light. The constriction and dilation of pupil occurs when a constriction response and dilation response occurse respectively.

Role of eyelashes when a foreign body enters the eye ;

Eyelashes are first line of defense for our eyes that keeps dust, lint and other foreign bodies from reaching the eye tissue. When a foreign body enters the eye it immediately starts the blinking reflex inorder to force clear the foreign body from the eye tissue. The follicles of eyelashes also contains glands(glands of Zeis and glands of Moll) that secretes sebus to keep the eye moist.

Miosis: miosis is excessive constriction of he pupil. Diameter of the pupil is less than 2 mm

Mydriasis: It is the excessive dilatation of the pupil.

Accomodation: It is the ability of the eye to change its focus from distant objects to near objects and vice-versa. It is the adjustment of the lens of the eye to keep an object in focus on the retina as its distance from the eye varies and is achieved by adjusting the focal length of the lens.

Near point: The near point is the closest point at which an object can be placed and still form a focused image on the retina, within the eye's accomodation range.

Cycloplegia is the paralysis of the ciliary muscle of the eye resulting in a loss of accomodation. Because of the paralysis, the curvature of the lens can no longer be adjusted(focal length cannot be change) to focus on nearby objects.