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Physiology_L8, Neuronal function of retina

L8, Neuronal function of retina

Lecture video

Part 1



Part 2




Lecture data

Color Blindness:

•It is an inherited defect in color vision that is transmitted as X-linked recessive trait.
•So color blindness affects males far more
frequently than females, about 8% of all males are affected.
•individuals with normal color vision can match any spectral color by using a mixture of the three primary colors and therefore are called trichromats.

IONIC BASIS OF PHOTORECEPTOR POTENTIAL During rest (in the dark):

• The inner segments of rods and cones are
impermeable to Na" and continually pump Na from inside the cell to outside (figure 8), creating a negative potential on the inside of the entire cell.

On the other hand, Na channels in the outer segments of rods and cones are opened by the action of c-GMP on Na* channel protein that splint it in the open state. So Na* continually leak to the inside of the outer segments of rods and cones. This neutralizes much of the negativity on the inside of the entire cell.
•Therefore, the electronegativity inside the
membrane of rods and cones (about 40 mv) is far lower than that found in most sensory receptors (-70 to -80 mv).
• resting photoreceptors continually release
chemical transmitters (Glutamate) from their foot segments which lead to inhibition of bipolar cells.

On exposure to light:
• Light activates the photosensitive pigments, present in the outer segments of rods and cones (as described before).
The activated photosensitive pigment
(metarhodopsin II) acts as an enzyme that
catalyzes reactions, which hydrolyze the c-GMP. Thus removing splinting of the Na channel protein and the channels close
• The inner segments, however, continue to
pump Na outside.
• Now more Na ions leave the cell than entering it, thus the electronegativity of the inside of the cell increases, that is hyperpolarization of rods and cones. The greater the amount of light, the greater the degree of hyperpolarization.
• This hyperpolarization reduces the release
of the chemical transmitter at the foot segment, thus affecting the activities of other cells in the visual pathway (depolarization of bipolar cells)

MCQ

1_Color blindness is inherited defect that is transmitted as 

a_Y-linked recessive 
b_X-linked recessive 
c_X-linked dominant 
d_Y- linked dominant

2_Monochromates patients have

a_Lack one of the cone pigment 
b_Have only one pigment 
c_Have 2 pigments 

3_The commonest type of color blindness is

a_Monochromates
b_Dichromates
c_Anomalous trichromates
d_Achromates

4_During rest the inner segment of rods and cones is permeable to Na+

a_True 
b_False

5_The electronegativity inside the membrane of rods and cones …..than that found in most sensory receptors

a_Lower
b_Higher

6_On exposure to light , hyperpolarization ….chemical neurotransmitter (glutamate)

a_Increase
b_Decrease


7_Convergence of the two eyes is achieved by contraction of of the 

a_Medial recti muscles 
b_Lateral recti muscles 

8_Stimulation of parasympathetic part of occlumotor nucleus leads to 

a_Dilatation of the pupil 
b_Constriction of the pupil 
c_Accommodation
d_B,c

9_In pupillary light reflex pathway , fibers leave optic tract to 

a_LGB
b_Pretectal nucleus 
c_EWN

Answers:

1-b 
2-b 
3- c
4- b
5- a 
6- b
7- a
8- d
                   9-b
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