simple astigmatism


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Related to simple astigmatism: hyperopic astigmatism

simple astigmatism

Etymology: L, simplex, not mixed; Gk, a + stigma, point
1 simple myopic astigmatism in which one principal meridian is in focus on the retina and the other in front of it.
2 simple hyperopic astigmatism in which one meridian is focused on the retina and the other behind it.

simple astigmatism

Astigmatism along one meridian only.
See also: astigmatism

cell 

1. In biology, the basic, structural and functional units from which living organisms and tissues are built. A cell consists of a nucleus surrounded by all the cellular contents (cytoplasm) including various organelles (mitochondria, Golgi apparatus, lysosomes, ribosomes, etc.) and inclusions (glycogen, melanin, triglycerides, etc.) suspended in intracellular fluid (water, proteins, carbohydrates, lipids, inorganic and organic substances) all enclosed in a plasma membrane. There are many types of cells (blood cells, connective tissue cells, epithelial cells, muscle cells, nerve cells, secretory cells, etc.). Living cells are capable of reproduction (for body growth, wound healing, etc.) by mitotic activity.
2. In optics, a rim in a trial frame or in an optical instrument into which a lens can be placed.
A cell See M cell.
acinar cell A type of cell found within the body of the lacrimal gland. This cell lines the lumens of glands in a lobular pattern and produces a serous secretion.
amacrine cell Retinal cell located in the inner nuclear layer connecting ganglion cells with bipolar cells. Some have an ascending axon synapsing with receptors.
B cell See P cell.
basal cell See corneal epithelium.
binocular cell A cell in the visual cortex that responds to stimulation from both eyes. It may, however, show an ocular dominance for either eye. It responds more strongly when corresponding regions of each eye are stimulated by targets of similar size and orientation. See cortical column; hypercolumn.
bipolar cell Retinal cell located in the inner nuclear layer connecting the photoreceptors with amacrine and ganglion cells.
C cell A retinal ganglion cell with slow axonal conduction which sends information to the superior colliculus and to the centre involved in the control of pupillary diameter, rather than to the lateral geniculate body. There are very few such cells. Syn. Pγ cell; W cell (thus called in the cat).
Cajal's cell See astrocytes.
clump cell Large, pigmented round cells found in the pupillary zone of the iris stroma. They are considered to be macrophages containing mainly melanin granules. The number of these cells increases with age.
colour-opponent cell's Cells which respond by increasing response to light of some wavelengths and decreasing their response to others (usually complementary). If the light stimulus contains both sets of wavelengths the two responses tend to cancel each other. Two types of cells have been identified: red-green cells and blue-yellow cells. These cells are found mainly in the lateral geniculate bodies but also among retinal ganglion cells, and they form the blobs in the visual cortex. The responses of these cells support Hering's theory of colour vision. Syn. opponent-process cell (although this term also includes a cell that increases its response to white light and decreases its response to dark). See blobs; Hering's of colour vision theory.
complex cell A cell in the visual cortex whose receptive field consists of a large responsive area, approximately rectangular in shape, surrounded by an inhibitory region. The stimulus, which is usually a slit or a straight line, gives an optimum response if appropriately orientated but falling anywhere within the excitatory area. These cells tend to respond optimally to the movement of a specifically orientated slit. Many complex cells also respond better when the optimally orientated slit is moved in one direction rather than in the opposite direction. In general, complex cells show non-linear spatial summation properties. See visual area; hypercomplex cell; simple cell; receptive field; summation.
cone cell Photoreceptor of the retina which connects with a bipolar cell and is involved in colour vision and high visual acuity and which functions in photopic vision. The outer segment of the cell is conical in shape, except in the fovea centralis where it is rod-like. In the outer segment (i.e. the part closest to the pigment epithelium) are contained hollow discs (or lamellae), the membranes of which are joined together and are also continuous with the boundary membrane of the cone cell. The visual pigments are contained in these discs. There are three types of cones, each containing a different pigment sensitive to a different part of the light spectrum. They are referred to as long-wave-sensitive (or L-cones), medium-wave-sensitive (or M-cones) and short-wave-sensitive (or S-cones). There are about six million cones in the retina, with the greatest concentration in the macular area (Fig. C4). See cone pedicle; Stiles-Crawford effect; ellipsoid; foveola; macula; visual pigment; duplicity theory; photopic vision.
fixed cell See corneal corpuscle.
ganglion cell 1. Retinal cell that connects the bipolars and other cells in the inner plexiform layer with the lateral geniculate body. The axons of the ganglion cells constitute the optic nerve fibres. There are many types of ganglion cells. The two major types are: the magno (M or parasol) ganglion cells which project mainly to the magnocellular layers of the lateral geniculate bodies; and the parvo (P) ganglion cells which project to the parvocellular layers of the lateral geniculate bodies. Two types of P ganglion cells are noted: P1, which are midget cells and have small dendritic fields and P2, which have large dendritic fields. M and P cells comprise about 10% and 82% of the ganglion cells respectively. 2. One of a collection of nerve cell bodies found in a ganglion. See C cell; M cell; P cell; melanopsin.
glial cell's Cells found throughout the nervous system. They provide support and nutrition for neurons, as well as being involved in the operation of the brain, especially the fluid surrounding the neurons and their synapses. They are also believed to be involved in the reuptake of neurotransmitters from within the synaptic cleft. There are three types of glial cells: astrocytes; microglia; oligodendroglia. Syn. glia; neuroglia.
goblet cell Cell of the conjunctival epithelium which secretes mucin. See glands of Henle; mucin; xerophthalmia.
horizontal cell Retinal cell located in the inner nuclear layer which connects several cones and rods together.
hypercomplex cell A cell in the visual cortex that receives inputs from several simple and complex cells and therefore has an even more elaborate receptive field than a complex cell. It is most effectively stimulated by a stimulus of a specific size and of a specific orientation and which is moved in a specific direction. See complex cell; simple cell.
Langerhans' cell's Dendritic cells located mainly in the epidermis, mucous membranes and lymph nodes. They have surface receptors for immunoglobulin (Fc), complement (C3) and surface HLA-DR (Ia) antigen. Langerhans' cells are also found in the conjunctival epithelium and among the basal cells, mainly of the peripheral corneal epithelium. They have antigenic functions, stimulate T-lymphocytes, prostaglandin production and participate in cutaneous delayed hypersensitivity and corneal graft rejection. Extended wear of contact lenses tends to induce an increase of these cells. They are also found in histiocytic tumours.
M cell A retinal ganglion cell, mainly located in the periphery of the retina and which assists in movement perception. M cells tend to give transient responses to stimuli and to have non-linear spatial summation properties. This cell transmits information principally to the magno cells of the lateral geniculate bodies. Syn. A cell; Pαcell; Y cell (thus called in the cat).
magno cell See ganglion cell; lateral geniculate bodies.
midget cell  See ganglion cell.
Mueller's cell Neuroglial cell in the retina with its nucleus in the inner nuclear layer and with fibres extending from the external to the internal limiting membrane. These cells support the neurons of the retina and possibly assist in their metabolism. Syn. Müller cell.
orientation-specific cell A cell that responds best to specifically orientated lines. This is the case for almost all cells in the visual cortex. Examples: complex cell; simple cell. See complex cell; simple cell; receptive field.
P cell A retinal ganglion cell, mainly located in the central region of the retina and which assists in high acuity and colour vision. P cells tend to give sustained responses to stimuli and to have linear spatial summation properties. This is the most common type of ganglion cells (about 82%). This cell transmits information principally to the parvo cells of the lateral geniculate bodies. Syn. B cell; Pβcell; X cell (thus called in the cat).
parasol cell See ganglion cell.
parvo cell See ganglion cell; lateral geniculate bodies.
rod cell Photoreceptor cell of the retina which connects with a bipolar cell. It contains rhodopsin and is involved in scotopic vision. The molecules of rhodopsin are contained in about 1000 hollow discs (double lamellae or membranes), which are isolated from each other and from the boundary membrane of the rod cell. These discs are found in the outer segment (i.e. the part closest to the pigment epithelium) of the cell. There are about 100 million rod cells throughout the retina; only a small area, the foveola, is free of rods (Fig. C4). See eccentricity; ellipsoid; foveola; rhodopsin; rod spherule; duplicity theory; scotopic vision.
Schwann cell A cell whose membrane spirals around the axon with layers of myelin in between each coil, as well as being a source of the myelin sheath. The cell provides insulation to the axon. It covers about one millimetre, so that hundreds may be needed to completely cover an axon. It also allows for an increase in the speed of the nervous impulse without an increase in axonal diameter. The gaps between the segments covered by the cells are called nodes of Ranvier. See action potential.
simple cell 
A cell in the visual cortex whose receptive field consists of an excitatory and an inhibitory area separated by a straight line, or by a long narrow strip of one response flanked on both sides by larger regions of the opposite response. Responses occur only to a straight line or a narrow strip orientated approximately parallel to the boundary/ies between the two areas. In general, simple cells show linear spatial summation properties. They are presumably the first cells where the nervous impulses are processed as they enter the visual cortex. See visual area; complex cell; receptive field.
squamous cell See corneal epithelium.
W cell See C cell.
wing cell See corneal epithelium.
X cell See P cell.
Y cell See M cell.
Fig. C4 Structures of a rod and a cone cell of the retina (ELM, external limiting membrane; ONL, outer nuclear layer; OPL, outer plexiform layer)enlarge picture
Fig. C4 Structures of a rod and a cone cell of the retina (ELM, external limiting membrane; ONL, outer nuclear layer; OPL, outer plexiform layer)

Table C1 distinguishing features of the two principal types of ganglion cells of the retina
propertiesP cell (X cell)M cell (Y cell)
size of cell bodysmalllarge
dendritic spreadsmallmedium/large
receptive field sizesmallmedium/large
retinal distribution90% of these at the macula5% of these at the macula; about 13% overall
projectionLGN parvocellular layersLGN magnocellular layers
type of responsesustainedtransient
light sensitivitylowhigh
wavelength responseselective (except P cells)non-selective
spatial sensitivityfine target detaillarge target detail
temporal sensitivitylow target velocityhigh target velocity
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