Test 203  (Ocular physiology)

1. With regard to efferent pupillary defect:

a. the pupil reacts poorly to light and accommodation

b. anisocoria is a feature

c. the affected eye has poor distant vision

d. a lesion in the sympathetic pathway is a recognized 
    cause

e. it could be caused by topical atropine application

 
Answera.T  b.T  c.F  d.T  e.T
In efferent pupillary defect, the lesion may be anywhere along the parasympathetic pathway to the iris. The pupil typically reacts poorly or not at all to accommodation or light. Unlike afferent pupillary defect, the affected eye has normal vision. The sympathetic pathway is not involved in the pupillary reaction to light or
accommodation. 
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2. The following are true about corneal sensation:

a. the sensation is greatest at the apex and diminishes 
    towards the limbus.

b. the temporal half of the cornea is more sensitive than
    the nasal half

c. the Bonnet-Cochet aesthesiometer gives quantitative 
    measure of the degree of hypoaesthesia

d. the density of nerve endings per unit area is 30 times 
    that of the epidermis

e. the sensory nerves contains unmyelinated endings 
 

Answera.T  b.T  c.T  d.F  e.T
The density of nerve endings per unit area appears to be 300 to 400 times that of the epidermis.
 

3. The following are true about spectral sensitivity of the retina:

a. in scotopic conditions, the peak sensitivity of the eye 
    is near 500 nm

b. under photopic conditions the peak sensitivity 
    is near 555 nm

c.  in the presence of a bright yellow steady background 
     light the retina has a peak sensitivity near 440 nm to 
     a 25-Hz stimulus

d.  in the presence of a purple steady background light 
     the peak sensitivity of retina is near 540 nm to a
     25-Hz stimulus

e. in the presence of a blue-green steady background 
    light the peak sensitivity of retina is near 540 nm to 
    a 25-Hz stimulus

 
Answer
a.T   b.T  c.T  d.T e.T
Under scotopic conditions, the peak sensitivity of the eye, governed by the rod system, is near 500 nm, 
whereas under photopic conditions, the peak sensitivity, mediated by the cone system, is near 555 nm. This is called Purkinje's shift.
The blue-, green-, and red-cone function can be isolated respectively to a 25-Hz white flickering light in the presence of different coloured steady background lights that desensitize one or another cone system by
bleaching
that system (i.e., dissociating opsin from vitamin A). The blue-cone system in the presence of a bright yellow steady background light has a peak sensitivity near 440 nm to the 25-Hz stimulus. In the presence of a purple adapting light that desensitizes the blue- and red-cone
systems, the peak sensitivity is near 540 nm, thereby 
isolating green-cone function. In the presence of a blue-green steady background light that desensitizes the blue-cone and green-cone systems, the peak sensitivity is near 580 nm, thereby isolating red-cone function. 

 

. 4. Regarding retinal metabolism:

a. insulin is essential for the uptake of glucose by the retina

b. anaerobic metabolism predominates 

c. the pigment retinal epithelium stores glycogen and 
    supplies the need of the retina

d. the demand of oxygen is met entirely by the central 
    retinal artery

e. the retina accumulates lactate readily.
 

Answer
a.F  b.T  c.F  d.F  e.T
Glycogen is not stored in the retina, the glucose demand is met by the blood supply which come from the central retinal artery and the choroidal circulation. Due to the high metabolic rate, anaerobic metabolism predominates. Consequently, lactate` product of anaerobic metabolism, accumulates readily.

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5. The following conditions are required for rhodopsin regeneration:

a. NADPH

b. darkness

c. splitting of all trans-retinal from the opsin

d. retinal isomerase

e. ascorbic acid

 
Answer
a.T   b.T  c.T  d.T  e.F
Rhodopsin regeneration occurs in the dark and requires the presence of NADPH and retinal isomerase. One of the important steps of regeneration is for the all trans-retinal to be split from the opsin.

 

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