Sgdp

K+ channel

Outer segment

Golgi

Golgi

Disks containing rhodopsin

Microtubules x Mitochondria

^ Rough endoplasmic reticulum

Inner { segment

Nucleus

Nucleus

It tffl

Basal body

Synaptic body

Disks containing rhodopsin

Microtubules x Mitochondria

^ Rough endoplasmic reticulum

Basal body

Human rod cell

▲ FIGURE 13-22 Human rod cell. (a) Schematic diagram of an entire rod cell. At the synaptic body, the rod cell forms a synapse with one or more bipolar interneurons. Rhodopsin, a light-sensitive G protein-coupled receptor, is located in the flattened membrane disks of the outer segment. (b) Electron micrograph of the region j 0.5 ym j of the rod cell indicated by the bracket in (a). This region includes the junction of the inner and outer segments. [Part (b) from R. G. Kessel and R. H. Kardon, 1979, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, W. H. Freeman and Company, p. 91.]

— 90 mV) typical of neurons and other electrically active cells. As a consequence of this depolarization, rod cells in the dark are constantly secreting neurotransmitters, and the bipolar interneurons with which they synapse are continually being stimulated. The depolarized state of the plasma membrane of resting rod cells is due to the presence of a large number of open nonselective ion channels that admit Na+ and Ca2 + , as well as K+. Absorption of light by rhodopsin leads to closing of these channels, causing the membrane potential to become more negative.

The more photons absorbed by rhodopsin, the more channels are closed, the fewer Na+ ions cross the membrane from the outside, the more negative the membrane potential becomes, and the less neurotransmitter is released. This change is transmitted to the brain where it is perceived as light. Remarkably, a single photon absorbed by a resting rod cell produces a measurable response, a decrease in the membrane potential of about 1 mV, which in amphibians lasts a second or two. Humans are able to detect a flash of as few as five photons.

11-c/s-Retinal moiety

11-c/s-Retinal moiety

Lysine side chain

Rhodopsin

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