Question

Choose the correct statement.
A. Photoreceptors in the human eye are depolarized during darkness and become hyperpolarized in response to the light stimulus.
B. Receptors do not produce graded potentials.
C. Nociceptors respond to changes in pressure.
D. Meissner’s corpuscles are thermoreceptors.

Answer 1

Special cells present in the eye’s retina that convert light into signals. Two types of photoreceptors are found in the human eye. They are the rods and cones. Rods function in night vision, whereas cones are capable of color vision.

Complete answer:
When the light hits either a rod cell or a cone cell, it leads to change in retinal structure. This change in structure results in activation of cGMP phosphodiesterase, closes the cation channels, preventing influx of positive ions; thus leading to membrane hyperpolarization.
In the dark, positive ions enter the photoreceptor and depolarize it.
A receptor potential is a type of graded potential, produced by the activation of a sensory receptor.
Nociceptors, also known as pain receptors, are activated by a noxious stimulus.
Meissner’s corpuscles are a type of mechanoreceptor.

Hence, the correct answer is option ‘A’.

Note:

RODS	CONES
High sensitivity	Low sensitivity
Located on edge of retina	Located in the center of retina
Slow response time	Faster response time
Respond in dim light	Respond in bright light
Black and White	Color
More in number than cones	Few in number than rods
High amplification	Less amplification
Loss causes – Night blindness	Loss causes – Color blindness

There are four types of photopigments, one in rods which is called rhodopsin and three in cones which are known as erythrolabe (red), cyanolabe (blue) and chlorolabe (green).
Rods cells have a structure that is elongated and is composed of four main regions; outer segment, inner segment, cell body and synaptic region.
A disease called retinitis pigmentosa occurs because of some genetic mutations in rhodopsin molecules.