Terminology Glossary: The Retina
AMACRINE CELLS: are cells within the retinal layer that run laterally across the
retina. Specifically they span across the bipolar junction and the ganglion cell,
and are involved in lateral inhibition.
BIPOLAR CELLS: are cells within the retinal layer that collect information from
photoreceptors and then relays the information to ganglions cells.
BLEACHING: when the retinal of the pigment absorbs a photon, the pigment
reacts by breaking apart and changing its shape, as well as changing to a more
pale colour. Also known as Photoisomerisation.
CENTER-SURROUND: a type of spatial receptive field structure in which the
central region of the receptive field and the surround portion influence cell
activities in different ways e.g. ON-center, OFF-Surround.
CONCENTRIC FIELD: a receptive field divided into an inner circular region and
an outer ring-shaped region. Light falling in each of the two regions has opposite
effects on the response of the cell.
CONES: one of the two types of photoreceptors, cone receptors are short, thick,
and tapered cells that are shaped like cones and provide high-acuity colour
vision. There are three types of cones, each with their own type of photopigment.
DUPLEX RETINA THEORY: see duplicity theory.
DUPLICITY THEORY: a theory stating that the retina has functions for both
acuity and sensitivity based on the presence of two functionally different
photoreceptors in the retina. Also known as Duplex Retina Theory.
FOVEA: the small area of the retina where cones are concentrated. It is about 12 degrees at centre of vision.
GANGLION CELLS: are the inner most cells within the retinal layer that are
involved in the formation of the optic nerve. They are involved in relaying the
sensorineural signal to the brain from the other retinal cells.
HORIZONTAL CELLS: are cells within the retinal layer that run laterally across
the retina. Specifically they span across the receptors and the bipolar junction,
and are involved in lateral inhibition.
HYPERPOLARIZATION: the membrane becomes more negatively charged as a
result of membrane potential, or an action potential.
IODOPSIN: is an example of the pigment molecule in cone cells.
INNER SEGMENT: the inferior portion of the photoreceptor that contains the cell
nucleus.
L CONES: one of the types of cone receptors that contains a photopigment that
absorbs long, red colour wavelengths, approximately 558nm.
LAMELLAE: are the several hundred thin membrane plates housed in the outer
segments that contain the photopigment.
M CONES: one of the types of cone receptors that contains a photopigment that
absorbs medium, green colour wavelengths, approximately 531nm.
OPSIN: the protein component of the pigment molecule.
OPTIC DISK: commonly known as the blindspot, the location where the ganglion
nerves (i.e., optic tract) exits the retina and eye.
OPTIC NERVE: the axons of the ganglion cells that project to the cerebral cortex
in order to communicate with the brain.
OUTER SEGMENT: the superior portion of the photoreceptor that contains
lamellae.
PHOTOISOMERISATION: when the retinal of the pigment absorbs a photon, the
pigment reacts by breaking apart and changing its shape, as well as changing to
a more pale colour. Also known as Bleaching.
PHOTON: a particle of light/a light energy molecule.
PHOTOPIC: the formal term for the cone system in the retina that is specialized
for vision in bright/daylight conditions and acuity.
PHOTORECEPTORS: are contained in the inner layer to the RPE. Essentially,
they are a nerve ending, cell, or group of cells specialized to sense or receive
light.
PIGMENT: a substance in the photoreceptor that can absorb light.
RECEPTIVE FIELD: is the area of the retina over which light stimuli changes the
particular firing rate of the ganglion cell.
RETINAL: the lipid component of the pigment molecule.
RETINAL PIGMENT EPITHELIUM (RPE): the outermost layer of the retina that
is not responsive to radiant energy and does not participate in the encoding of
visual information. It also absorbs light not absorbed by the photoreceptors and
provides metabolic support to the retina.
RHODOPSIN: is an example of a pigment molecule in the rod cell.
RODS: One of the two types of photoreceptors, rod receptors are long, thin,
cylindrical cells that are shaped like rods and provide only low-acuity
monochrome vision. There is only one type of photopigment in the rod with a
maximum sensitivity at 507nm.
S CONES: one of the types of cone receptors that contains a photopigment that
absorbs short, blue colour wavelengths, approximately 419nm.
SCOTOPIC: the formal term for the rod system in the retina that is specialized for
vision in dim light conditions and light sensitivity.
TRANSDUCTION: is the process of converting an energy form into another
medium or energy form. In the retina, the transduction process is the conversion
of light energy (photons) into electrical energy.