Dark Adaptation

The bleaching reaction that occurs in the light results in a lowered amount of rhodopsin in the rods and lowered amounts of visual pigments in the cones. When a light-adapted person first enters a darkened room, therefore, sensitivity to light is low and vision is poor. A gradual increase in photoreceptor sensitivity, known as dark adaptation, then occurs, reaching maximal sensitivity in about 20 minutes. The increased sensitivity to low light intensity is partly due to increased amounts of visual pigments produced in the dark. Increased pigments in the cones produce a slight dark adaptation in the first 5 minutes. Increased rhodopsin in the rods produces a much greater increase in sensitivity to low light levels and is partly responsible for the adaptation that occurs after about 5 minutes in the dark. In addition to the increased concentration of rhodopsin, other more subtle (and less well understood) changes occur in the

Scientists have recently discovered the genetic basis for blindness in the disease dominant retinitis pigmentosa. People with this disease inherit a gene for the opsin protein in which a single base change in the gene (substitution of adenine for cytosine) causes the amino acid histidine to be substituted for proline at a specific point in the polypeptide chain. This abnormal opsin leads to degeneration of the photoreceptors.

Fox: Human Physiology, Eighth Edition

10. Sensory Physiology

Text

© The McGraw-H Companies, 2003

Chapter Ten

■ Figure 10.37 The photodissociation of rhodopsin. (a) The photopigment rhodopsin consists of the protein opsin combined with 11—a's-retinene. (b) Upon exposure to light, the retinene is converted to a different form, called all-trans, and dissociates from the opsin. This photochemical reaction induces changes in ionic permeability that ultimately result in stimulation of ganglion cells in the retina.

11-c/s-Retinene

11-c/s-Retinene

H2 CH3

In the dark

In the light m

Rhodopsin rnimimiÄ^^ /*Disc in outer rod segment

11-c/s-Retinene

Plasma membrane

Phosphodiesterase (inactive)

cGMP

cGMP

all-trans-Retinene all-trans-Retinene

Dark current stopped, causing hyperpolarization cGMP

Phosphodiesterase (active)

Dark current stopped, causing hyperpolarization cGMP

Phosphodiesterase (active)

Figure 10.38 Light striking the photoreceptors causes Na+ channels to close. Light causes the photoisomerization of 11-cis to all-trans-retinene and its dissociation from opsin. This releases a subunits of G-proteins, which activate phosphodiesterase. This activated enzyme converts cyclic GMP (cGMP) to GMP. Since the cGMP is needed to keep the Na+ channels of the plasma membrane open, conversion of cGMP to GMP causes these channels to close. This stops the dark current produced by Na+ entry in the dark, leading to hyperpolarization of the photoreceptor.

rods that ultimately result in a 100,000-fold increase in light sensitivity in dark-adapted as compared to light-adapted eyes.

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