Thursday, December 30, 2010

Retina Layers And Blood Supply

Microscopic structure
Retina consists of 3 types of cells and their synapses
arranged (from without inward) in the following ten
layers (Fig. 11.2):
1. Pigment epithelium. It is the outermost layer of
retina. It consists of a single layer of cells
containing pigment. It is firmly adherent to the
underlying basal lamina (Bruch’s membrane) of
the choroid.
2. Layer of rods and cones. Rods and cones are the
end organs of vision and are also known as
photoreceptors. Layer of rods and cones contains
only the outer segments of photoreceptor cells
arranged in a palisade manner. There are about
120 millions rods and 6.5 millions cones. Rods
contain a photosensitive substance visual purple
(rhodopsin) and subserve the peripheral vision
and vision of low illumination (scotopic vision).
Cones also contain a photosensitive substance
and are primarily responsible for highly
discriminatory central vision (photopic vision)
and colour vision.
3. External limiting membrane. It is a fenesterated
membrane, through which pass processes of
the rods and cones.
4. Outer nuclear layer. It consists of nuclei of the
rods and cones.
5. Outer plexiform layer. It consists of connections
of rod spherules and cone pedicles with the
dendrites of bipolar cells and horizontal cells.
6. Inner nuclear layer. It mainly consists of cell
bodies of bipolar cells. It also contains cell
bodies of horizontal amacrine and Muller’s cells
and capillaries of central artery of retina. The
bipolar cells constitute the first order neurons.
7. Inner plexiform layer. It essentially consists of
connections between the axons of bipolar cells
dendrites of the ganglion cells, and processes
of amacrine cells.
8. Ganglion cell layer. It mainly contains the cell
bodies of ganglion cells (the second order
neurons of visual 7pathway). There are two
types of ganglion cells. The midget ganglioncells are present in the macular region and the
dendrite of each such cell synapses with the
axon of single bipolar cell. Polysynaptic
ganglion cells lie predominantly in peripheral
retina and each such cell may synapse with
upto a hundred bipolar cells.
9. Nerve fibre layer (stratum opticum) consists of
axons of the ganglion cells, which pass through
the lamina cribrosa to form the optic nerve. For
distribution and arrangement of retinal nerve
fibres see Figs. 9.11 and 9.12, respectively and
page 216.
10. Internal limiting membrane. It is the innermost
layer and separates the retina from vitreous. It
is formed by the union of terminal expansions
of the Muller’s fibres, and is essentially a basement membrane.
Blood supply
Outer four layers of the retina, viz, pigment
epithelium, layer of rods and cones, external limiting membrane and outer nuclear layer get
their nutrition from the choroidal vessels.
Inner six layers get their supply from the central
retinal artery, which is a branch of the ophthalmic
artery.
Central retinal artery emerges from centre of the
physiological cup of the optic disc and divides
into four branches, namely the superior-nasal,
superior-temporal, inferior-nasal and inferiortemporal.
These are end arteries i.e., they do not
anastomose with each other.
The retinal veins. These follow the pattern of the
retinal arteries. The central retinal vein drains into
the cavernous sinus directly or through the
superior ophthalmic vein. The only place where
the retinal system anastomosis with ciliary system
is in the region of lamina cribrosa.

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