Human Physiology, 14th edition (2016)

Sensory Physiology 301

way in which sensory stimuli are detected. This is because in

the dark the photoreceptors release an inhibitory neurotrans-

mitter that hyperpolarizes the bipolar neurons. Thus inhibited,

the bipolar neurons do not release excitatory neurotransmitter

to the ganglion cells. Light inhibits the photoreceptors from

releasing their inhibitory neurotransmitter and by this means

stimulates the bipolar cells, and thus the ganglion cells that

transmit action potentials to the brain.

A rod or cone contains many Na^1 channels in the plasma

membrane of its outer segment ( fig.  10.40 ), and in the dark,

many of these channels are open. As a consequence, Na^1 con-

tinuously diffuses into the outer segment and across the narrow

stalk to the inner segment. This small flow of Na^1 that occurs

in the absence of light stimulation is called the dark current,

and it causes the membrane of a photoreceptor to be somewhat

depolarized in the dark. The Na^1 channels in the outer segment

rapidly close in response to light, reducing the dark current and

causing the photoreceptor to hyperpolarize.

Cyclic GMP (cGMP) is required to keep the Na^1 channels

open, and the channels will close if the cGMP is converted into

GMP. Light causes this conversion and consequent closing of

the Na^1 channels. When a photopigment absorbs light, 11- cis-

retinene is converted into all- trans -retinene ( fig.  10.40 ) and

dissociates from the opsin, causing the opsin protein to change

shape. Each opsin is associated with over a hundred regulatory

G-proteins (chapter 6; see fig. 6.31) known as transducins,

and the change in the opsin induced by light causes the alpha

subunits of the G-proteins to dissociate. The alpha transducin

(G-protein) subunits bind to and activate an equal number of

the previously inactive cGMP phosphodiesterase enzymes,

which catalyze the conversion of cGMP into GMP. This causes

a very rapid fall in the concentration of cGMP within the nar-

row spaces of the photoreceptor outer segments, closing the

cGMP-gated Na^1 channels in the plasma membrane and inhib-

iting the dark current ( fig.  10.40 ). The absorption of a single

photon of light can block the entry of more than a million

Na^1 , thereby causing the photoreceptor to hyperpolarize and

release less inhibitory neurotransmitter. Freed from inhibition,

the bipolar cells activate ganglion cells, and the ganglion cells

transmit action potentials to the brain so that light can be per-

ceived ( fig. 10.41 ).

Cones and Color Vision

Cones are less sensitive than rods to light, but the cones pro-

vide color vision and greater visual acuity, as described in the

next section. During the day, therefore, the high light intensity

bleaches out the rods, and color vision with high acuity is pro-

vided by the cones.

Each type of cone contains retinene, as in rhodopsin, but

the retinene in the cones is associated with proteins called

photopsins. It is the three different photopsin proteins (coded

by three different genes) that give each type of cone its unique

light-absorbing characteristics. Each type of cone expresses

Figure 10.40 Light stops the dark current in photoreceptors. (1) In the dark, Na^1 enters the photoreceptors,

producing a dark current that causes a partial depolarization. (2) In the light, 11- cis -retinal is converted into all- trans -retinal. (3) This

causes G-proteins associated with the opsin to dissociate. (4) The alpha subunit binds to and activates phosphodiesterase, which

converts cyclic GMP (cGMP) into GMP. (5) As a result, the Na^1 channels close, stopping the dark current and hyperpolarizing the

photoreceptors.

Na+

Na+

In the dark In the light

Rhodopsin

Disc in outer

rod segment

Phosphodiesterase

(inactive)

G-proteins

Phosphodiesterase

(active)

αβ

γ

β

γ

α

11-cis-retinal

Dark all-trans-retinal

current

Plasma

membrane

cGMP

cGMP

GMP

Dark current

stopped, causing

hyperpolarization

1

2

3

4

5