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SECTION III
Central & Peripheral Neurophysiology
THE PHOTORECEPTOR
MECHANISM
ELECTRICAL RESPONSES
The potential changes that initiate action potentials in the ret-
ina are generated by the action of light on photosensitive com-
pounds in the rods and cones. When light is absorbed by these
substances, their structure changes, and this triggers a se-
quence of events that initiates neural activity.
The eye is unique in that the receptor potentials of the photo-
receptors and the electrical responses of most of the other neural
elements in the retina are local, graded potentials, and it is only in
the ganglion cells that all-or-none action potentials transmitted
over appreciable distances are generated. The responses of the
rods, cones, and horizontal cells are hyperpolarizing (Figure
12–11), and the responses of the bipolar cells are either hyperpo-
larizing or depolarizing, whereas amacrine cells produce depo-
larizing potentials and spikes that may act as generator potentials
for the propagated spikes produced in the ganglion cells.
The cone receptor potential has a sharp onset and offset,
whereas the rod receptor potential has a sharp onset and slow
offset. The curves relating the amplitude of receptor potentials to
stimulus intensity have similar shapes in rods and cones, but the
rods are much more sensitive. Therefore, rod responses are pro-
portionate to stimulus intensity at levels of illumination that are
below the threshold for cones. On the other hand, cone responses
are proportionate to stimulus intensity at high levels of illumina-
tion when the rod responses are maximal and cannot change.
This is why cones generate good responses to changes in light
intensity above background but do not represent absolute illumi-
nation well, whereas rods detect absolute illumination.
IONIC BASIS OF
PHOTORECEPTOR POTENTIALS
Na
- channels in the outer segments of the rods and cones are
open in the dark, so current flows from the inner to the outer
segment (Figure 12–12). Current also flows to the synaptic
ending of the photoreceptor. The Na
–K
pump in the inner
segment maintains ionic equilibrium. Release of synaptic
transmitter is steady in the dark. When light strikes the outer
segment, the reactions that are initiated close some of the Na
channels, and the result is a hyperpolarizing receptor poten-
tial. The hyperpolarization reduces the release of synaptic
transmitter, and this generates a signal in the bipolar cells that
ultimately leads to action potentials in ganglion cells. The ac-
tion potentials are transmitted to the brain.
PHOTOSENSITIVE COMPOUNDS
The photosensitive compounds in the rods and cones of the
eyes of humans and most other mammals are made up of a
protein called an
opsin,
and retinene
1
, the aldehyde of vitamin
A
1. The term retinene
1
is used to distinguish this compound
from retinene
2
, which is found in the eyes of some animal spe-
cies. Because the retinenes are aldehydes, they are also called
retinals.
The A vitamins themselves are alcohols and are
therefore called
retinols
(see Clinical Box 12–5)
.
RHODOPSIN
The photosensitive pigment in the rods is called
rhodopsin
(visual purple).
Its opsin is called
scotopsin.
Rhodopsin has a
peak sensitivity to light at a wavelength of 505 nm.
Human rhodopsin has a molecular weight of 41,000. It is
found in the membranes of the rod disks and makes up 90% of
the total protein in these membranes. It is one of the many recep-
tors coupled to G proteins. Retinene
1
is parallel to the surface ofFIGURE 12–11
Intracellularly recorded responses of cells in
the retina to light.
The synaptic connections of the cells are also indi-
cated. The eye is unique in that the receptor potentials of the photore-
ceptors and the electrical responses of most of the other neural
elements in the retina are local, graded potentials. The rod (R) on the
left is receiving a light flash, whereas the rod on the right is receiving
steady, low-intensity illumination. The responses of rods and horizon-
tal cells (H) are hyperpolarizing, responses of bipolar cells (B) are either
hyperpolarizing or depolarizing, and amacrine (A) cells produce depo-
larizing potentials and spikes that may act as generator potentials for
propagated spikes of ganglion cells (G).
(Reproduced with permission from
Dowling JE: Organization of vertebrate retinas. Invest Ophthalmol 1970;9:655.)R RGBABG GH