Iilimbus

A. Located at the sclerocorneal junction, it contains a trabecular network and the canal of Schlcmm.

B. The limbus is involved in the flow of aqueous humor: anterior chamber—»trabecular network—»canal of Schlcmm—»aqueous veins—»episcleral veins.

C. Obstruction of flow of aqueous humor results in glaucoma.

III. RETINA. The 10 layers that constitute the retina are described and illustrated in Table

28-2 and Figure 28-1. The retina has a number of specialized areas, which include the following:

A. The optic disk is the site where optic nerve fibers converge to form the optic nerve (CN II). The optic disk lacks rods and cones and is therefore a blind spot.

B. Macula is a small yellow area (due to xanthophil pigment) located lateral to the optic disk. At the center of the macula is the fovea, which is composed exclusively of cones linked to a single ganglion cell, thereby producing the highest visual acuity and color vision.

IV. VISUAL TRANSDUCTION includes the following steps:

A. In the dark, rods have open sodium ion channels, which maintain the rod in a constant state of depolarization.

B. Rhodopsin is formed when 11-cis-retinal binds to a glycoprotein called opsin. In light, 11-cis-retinal forms the isomer all-trans-retinal, which releases from opsin and is converted to vitamin A (retinol).

C. This activates a G protein called transducin.

D. Transducin activates cyclic guanosine monophosphate (cGMP) phosphodiesterase, which hydrolyzes cGMP to GMP, thereby lowering cGMP levels.

Table 28-1

Layers of the Cornea

Layers

Characteristics

Corneal epithelium

Anterior aspect of cornea (exposed to air)

Stratified squamous epithelium (nonkeratinized)

Many free nerve endings

High capacity for regeneration

Bowman's membrane

A basement membrane

Stroma

Thickest layer of the cornea

Collagen types III and V

Descemet's membrane

A basement membrane

Corneal endothelium

Posterior aspect of cornea (exposed to aqueous humor)

Responsible for metabolic exchanges between cornea and aqueous humor

Simple squamous epithelium

E. Low cGMP levels close sodium ion channels, thereby eliciting a hyperpolarization.

F. Visual transduction is quite different from the way action potentials in muscle and nerve are generated. In muscle and nerve, stimuli open sodium ion channels, thereby eliciting a depolarization (see Chapter 6 I G and Chapter 7 I B 4).

Table 28-2

Layers of the Retina

Layers Characteristics

Table 28-2

Layers of the Retina

Layers Characteristics

10

Inner limiting membrane

Termination of Muller cells and their basement membrane

9

Optic nerve fibers

Unmyelinated axons of ganglion cells

8

Ganglion cells

Nuclei of ganglion cells

Site of action potential generation

7

Inner plexiform layer

Synapses between axons of bipolar cells and dendrites of ganglion cells

6

Inner nuclear layer

Nuclei of horizontal, bipolar, amacrine, and Muller cells

5

Outer plexiform layer

Synapses between axons of bipolar cells and dendrites of ganglion cells Retinal blood vessels may extend to this layer

4

Outer nuclear layer

Nuclei of rods and cones

3

Outer limiting membrane

Zonula adherens between rods/cones and Muller cells

2

Photoreceptor layer

Outer segment with membrane disks containing Na+ ion channels, connecting cilium, and inner segment of rods and cones

Detached retina occurs between 1 and 2

1

Pigment epithelium

Has tight junctions at apical border to form blood-retina barrier Has basal infoldings Contains melanin granules Esterifles retlnol (vitamin A) Phagocytoses shed tips of rod outer segments

Central artery of retina

Basement membrane

Ganglion cell

Muller cell

Amacrine cell

Bipolar cell Horizontal cell

Zonula adherens

Tight junction (blood-retina barriery

Melanin granules

Basal infoldings

Choroid

Optic disk

Central artery of retina

Basement membrane

Ganglion cell

Muller cell

Amacrine cell

Bipolar cell Horizontal cell

Zonula adherens

Basal infoldings

Choroid

Optic disk

Figure 28-1. Diagram of the retina. Tight junctions between the pigment epithelial cells establish a blood-retina barrier. Therefore, blood supply to the retina is from rhe central artery of the retina (a branch of the ophthalmic artery), not from vessels within the choroid. As retinal blood vessels leave the optic disk, they travel between the inner limiting membrane (layer 10) and the vitreous body. Retinal blood vessels may extend as far as the outer plexiform layer (layer 5) and play an important role in diabetic retinopathy. Müller cells act as supporting glial-type cells. Note the direction of the incident light and that it must pass through many layers of the retina before reaching the rods and cones. The asterisk indicates the site of retinal detachment; the double asterisk indicates the presence of sodium ion channels; the triple asterisk indicates the site of action potential generation.

Figure 28-1. Diagram of the retina. Tight junctions between the pigment epithelial cells establish a blood-retina barrier. Therefore, blood supply to the retina is from rhe central artery of the retina (a branch of the ophthalmic artery), not from vessels within the choroid. As retinal blood vessels leave the optic disk, they travel between the inner limiting membrane (layer 10) and the vitreous body. Retinal blood vessels may extend as far as the outer plexiform layer (layer 5) and play an important role in diabetic retinopathy. Müller cells act as supporting glial-type cells. Note the direction of the incident light and that it must pass through many layers of the retina before reaching the rods and cones. The asterisk indicates the site of retinal detachment; the double asterisk indicates the presence of sodium ion channels; the triple asterisk indicates the site of action potential generation.

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