Human Physiology, 14th edition (2016)

Sensory Physiology 299

6. Stabilization of the ion composition surrounding the pho-

toreceptors, thereby helping them to respond appropriately

to light (ion movements in the photoreceptors are shown

in fig. 10.41 )

Effect of Light on the Rods

The photoreceptors—rods and cones ( fig.  10.37 )—are acti-

vated when light produces a chemical change in molecules

of pigment contained within the membranous discs of the

outer segments of the receptor cells ( fig.  10.38 ). Each rod

contains thousands of molecules of a purple pigment known

as rhodopsin in these discs. The pigment appears purple

(a combination of red and blue) because it transmits light in

the red and blue regions of the spectrum, while absorbing

light energy in the green region. The wavelength of light that

is absorbed best—the absorption maximum —is about 500 nm

(blue-green light).

Green cars (and other green objects) are seen more easily

at night—when rods are used for vision—than are red objects.

This is because red light is not absorbed well by rhodop-

sin, and only absorbed light can produce the photochemical

reaction that results in vision. In response to absorbed light,

rhodopsin dissociates into its two components: the pigment

retinalde-hyde (also called retinene or retinal ), which is

derived from vitamin A, and a protein called opsin. This reac-

tion is known as the bleaching reaction.

Retinal (retinene) can exist in two possible configurations

(shapes)—one known as the all- trans form and one called the

Figure 10.37 Rods and

cones. ( a ) A diagram showing the

structure of a rod and a cone. ( b ) A

scanning electron micrograph of

rods and cones. Note that each

photoreceptor contains an outer and

inner segment.

Inner

segment

Outer

segment

Mitochondria

Nuclei

Synaptic

endings

Cone

cell

Rod

(a) cell (b)

Figure 10.38 Membranous discs within an outer
rod segment. The discs fill the outer segment of the rods and
contain the photopigment rhodopsin.