214
Chapter 7
When light shines on the photoreceptors, a cascade of
events leads to hyperpolarization of the photoreceptor cell
membrane. Retinal molecules in the disc membrane assume a
new conformation induced by the absorption of energy from
photons. This stimulates an interaction between associated
opsins and another protein that is a member of the G-protein
family (see Chapter 5) called
transducin.
Transducin acti-
vates the enzyme
phosphodiesterase,
which rapidly degrades
cGMP. The decrease in cytoplasmic cGMP concentration allows
the cation channels to close, and the loss of depolarizing cur-
rent allows the membrane potential to hyperpolarize. After its
activation by light, retinal changes back to its resting shape by
way of several mechanisms that do not depend on light, but
are enzyme-mediated.
If you move from a place of bright sunlight into a dark-
ened room, a temporary “blindness” takes place until the pho-
toreceptors can undergo
dark adaptation.
In the low levels
of illumination of the darkened room, vision can only be sup-
plied by the rods, which have greater sensitivity than the cones.
During the exposure to bright light, however, the rods’ rho-
dopsin has been completely activated, making the rods insensi-
tive to light. Rhodopsin cannot respond fully again until it is
restored to its resting state, a process requiring several minutes.
Dark adaptation occurs, in part, as enzymes regenerate the ini-
tial form of rhodopsin, which can respond to light. Vitamin A
is necessary for good night vision because it is required for the
synthesis of the retinal portion of the rhodopsin.
In contrast to dark adaptation,
light adaptation
occurs
when you step from a dark place into a bright one. Initially,
the eye is extremely sensitive to light, and the visual image is
too bright and has poor contrast. However, the rhodopsin is
soon used up (“bleached” by the bright light), and the rods
become unresponsive so that only the less-sensitive cones are
operating and the image becomes less bright.
Neural Pathways of Vision
The distinct characteristics of the visual image are transmitted
through the visual system along multiple, parallel pathways.
The neural pathway of vision begins with the rods and cones.
We just described in detail how the presence or absence of
light infl
uences photoreceptor cell membrane potential, and
now we will consider how this information is encoded, trans-
mitted to the brain, and processed.
Light signals are converted into action potentials through
the interaction of photoreceptors with
bipolar cells
and
gan-
glion cells.
Photoreceptor and bipolar cells only undergo
graded responses; ganglion cells are the fi rst cells in the path-
way where action potentials can be initiated. Photoreceptors
interact with bipolar and ganglion cells in two distinct ways,
designated as “ON-pathways” and “OFF-pathways.” In both
types, photoreceptors are depolarized in the absence of light,
causing the neurotransmitter glutamate to be released onto
bipolar cells. Light striking either pathway hyperpolarizes the
photoreceptors, resulting in a decrease in glutamate release
onto bipolar cells. The key difference of the two pathways lies
in the type of glutamate receptors found on the bipolar cells,
causing them to respond exactly the opposite in the presence
and absence of light.
Glutamate released onto ON-pathway bipolar cells binds
to metabotropic receptors that cause enzymatic breakdown
of cGMP, which hyperpolarizes the bipolar cells by a mech-
anism similar to that occurring when light strikes a photo-
receptor cell.
When the bipolar cells are hyperpolarized, they
are prevented from releasing excitatory neurotransmitter onto
their associated ganglion cells. Thus, in the absence of light,
ganglion cells of the ON-pathway are not stimulated to fi re
action potentials. These processes reverse, however, when
light strikes the photoreceptors: glutamate release from photo-
receptors declines, ON-bipolar cells depolarize, excitatory
neurotransmitter is released, the ganglion cells are depolar-
ized, and action potentials propagate to the brain.
OFF-pathway bipolar cells have ionotropic glutamate
receptors that are nonselective cation channels, that depolar-
ize the bipolar cells when glutamate binds. Depolarization of
these bipolar cells stimulates them to release excitatory neu-
rotransmitter onto their associated ganglion cells, stimulating
them to fi re action potentials. Thus, the OFF-pathway gen-
erates action potentials in the absence of light, and reversal
of these processes inhibits action potentials when light does
strike the photoreceptors. The co-existence of these ON and
OFF pathways in each region of the retina greatly improves
image resolution by increasing the brain’s ability to perceive
contrast at edges or borders.
Stimulation of ganglion cells is actually more complex
than just described—a signifi cant amount of signal processing
occurs within the retina before action potentials actually travel
to the brain. Photoreceptors, bipolar cells, and ganglion cells
are interconnected by
horizontal cells
and
amacrine cells,
which pass information between adjacent areas of the retina.
Via these interactions, the ganglion cells are made to respond
differentially to the various characteristics of visual images,
such as color, intensity, form, and movement. The retina is
characterized by its very great amount of convergence; many
photoreceptors can synapse on each bipolar cell, and many
bipolar cells synapse on a single ganglion cell. The amount of
convergence varies by photoreceptor type and retinal region.
As many as 100 rod cells converge onto a single bipolar cell,
whereas in the fovea region only one or a few cone cells syn-
apse onto a bipolar cell.
The receptive fi elds in the retina have characteristics that
differ from those in the somatosensory system. If you were to
shine pinpoints of light onto the retina and at the same time
record from a ganglion cell, you would see that the receptive
eld for that cell is round. Furthermore, the response of the
ganglion cell would be either depolarization or hyperpolariza-
tion, depending on the location of the stimulus within that
single fi eld. Because of different inputs from bipolar cells to
the ganglion cell, each receptive fi eld has an inner core that
responds differently than the area surrounding the core. There
can be “ON center/OFF surround” or “OFF center/ON sur-
round” cells, so named because the responses are either depo-
larization (ON) or hyperpolarization (OFF) in the two areas
of the fi eld. The usefulness of this organization is that the
existence of a clear edge between the “ON” and “OFF” areas
of the receptive fi eld increases the contrast between the area
that is receiving light and the area around it, increasing visual
previous page 242 Vander's Human Physiology The Mechanisms of Body Function read online next page 244 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off