E

mitochondria

Rods

— closed disks with rhodopsin molecules open disks with cone opsin molecules —

sodium channels on plasma membrane connecting cilium mitochondria cone photoreceptor nucleus synaptic vesicle on ribbon neurotransmitter glutamate synaptic vesicle on ribbon

Rods

Long outer segment

Big synaptic terminal

High sensitivity over 1,000 closed disks more photopigment about 40,000 rhodopsin/disk scotopic night vision

High amplification one photon stops the entry of about 107 Na+ ions

Saturates in daylight

Low temporal resolution slow response long integration time

Sensitive to scattered light poor spatial resolution

Rod system

Low acuity rods absent in central fovea highly convergent pathways

Achromatic one type of rhodopsin

Cones

Short outer segment

Huge synaptic terminal

Low sensitivity many fewer open disks less photopigment less per disk photopic day vision

Less amplification

Saturates only in intense light

High temporal resolution fast response short integration time

Sensitive to direct axial light good spatial resolution

Cone system

High acuity cones concentrated in fovea slight divergence in pathways

Chromatic (color vision) three types of cones each with different photopigment

FIGURE 3 Comparison of the cellular features of rod and cone photoreceptors. Structural and chemical differences of rods and cones make them suitable for their specialized functions: rods for scotopic vision in dim light and cones for photopic vision in bright light conditions. Humans have one class of rods and three classes of cones producing black and

Reopening of the channel requires synthesis of new cGMP. cGMP production is enhanced under increased light conditions because the calcium inhibition of guany-late cyclase has been removed and as a result sodium conductance begins to increase. This is one of the major mechanisms responsible for the phenomenon of light adaptation. The adaptation response is slower than the initial hyperpolarization response because synthesis of cGMP is slower than degradation.

Reset mechanisms also operate to maintain the necessary levels of photopigment. As with cGMP, the degradation of rhodopsin is much faster than its resynthesis. The light-stimulated conversion of rhodopsin into dissociated opsin protein and chromophore (all- trans retinal) is aided (speeded) by two other processes. A protein called arrestin binds to the opsin; in addition, opsin is phosphorylated by opsin kinase. These steps also help prevent any reassociation of opsin with the chromophore. Resynthesis of 11-cis retinal requires an isomerase enzyme found in the RPE. All-trans retinal must be released from the rod, taken up by the RPE, reisomerized, and sent back to the photoreceptor to reassociate with opsin.

It was recently discovered that opsin has a molecular structure similar to that of the family of membrane receptors that activates G-proteins. For example, it has seven hydrophobic transmembrane spanning regions and significant sequence homology with another member of this protein family, namely, the beta-adrenergic receptor. Thus, an analogy can be made between phototransduc-tion and chemical transmission. The RPE secretes a ligand (the chromophore, 11-cis retinal) that binds to a membrane receptor (opsin) located on rod photorecep-tors. The photoisomerization and removal of the ligand (by light) leads to the activation of a G-protein that regulates membrane conductance and determines neuro-transmitter output of the rod to second-order neurons. From this perspective, the cellular mechanisms used in photo-transduction are similar to those commonly used in other signaling systems, with the one unique feature being the light-sensitive properties of the ligand-receptor interaction.

Although phototransduction mechanisms in rods and cones appear to be quite similar, several significant differences are recognized. The chromophore involved (11-cis retinal) is the same in cones as in rods; however, the primary structure of the three different cone opsins differs slightly from each other and from rod opsin. These small differences in opsin structure influence the conformation of the chromophore in its bound state, which in turn establishes the specific spectral absorption properties of the four classes of photoreceptor cells.

white vision at night and color vision in the day, respectively. Other vertebrates have different combinations; for example, frogs have two of each.

A. Light / dark responses in rods

Retinal Processing B. Steps in the visual transduction cascade

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