Question

The cornea is responsible for most of the refraction of the eye. For a normal emmetropic eye, approximately what percentage of the refraction in diopters is the typical lens accommodation in a 20 year old person relative to the refraction of the cornea?

	A.	52%
	B.	26%
	C.	4.7%
	D.	14%

Which of the following is an exception to the assertion that the only step in the visual system that requires light is the conversion of 11-cis retinal to all-trans retinal?

	A.	Color opponent P retinal ganglion cells
	B.	Motion sensitive Y ganglion cells
	C.	W retinal ganglion cells that project to the superior colliculus
	D.	Melanopsin containing ganglion cells

Answer 1

1) Option A - 52%

The total refractive power of human eye is 60 dioptre

in which 21 dioptre is contributed by the ocular lens and remaining 40 dioptre is contributed by the cornea

so the percentage of refraction by the lens relative to the refraction of the cornea will be

   refraction of len / refraction of cornea *100 = 21/40 *100 = 52 %

2) Melanopsin containing ganglion cells

here is wide variability in ganglion cell types across species. In primates, including humans, there are generally three classes of RGCs:

• W-ganglion- small, 40% of total, broad fields in retina, excitation from rods, detect direction movement anywhere in the field.
• X-ganglion- medium diameter, 55% of total, small field, color vision. Sustained response.
• Y- ganglion cells- largest, 5%, very broad dendritic field, respond to rapid eye movement or rapid change in light intensity. Transient response.

Photosensitive ganglion cells, including but not limited to the giant retinal ganglion cells, contain their own photopigment, melanopsin, which makes them respond directly to light even in the absence of rods and cones. They project to, among other areas, the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract for setting and maintaining circadian rhythms. Other retinal ganglion cells projecting to the lateral geniculate nucleus (LGN) include cells making connections with the Edinger-Westphal nucleus (EW), for control of the pupillary light reflex, and giant retinal ganglion cell