Histology of the Eye and Visual Pathways
Competencies:
- Draw the eye with the layers of the cornea, lens, fovea and relation to the optic disc.
- Diagram the pathway from eye to LGN and to striate cortex.
- Appraise the visual deficits expected when damage due to lesions or stroke occur along various points of the visual projections to the cortex.
To master the material presented in this
lecture:
Read ...
Purves text pp 229-230, 233
Ross and Pawlina (Histology text) Chapter 24 (M1 only).
Look at the Review Questions below ...
Listen to the lecture and focus on the following points ...
- The retina forms from primitive optic vesicles that are derived from the diencephalon at
about the same time as the neural tube closes. The so-called “optic cup” comprises two
layers which ultimately form the neural retina (inner layer) and the retinal pigment
epithelium (outer layer). The epithelium of not only the retina, but also the iris and ciliary body;
as well as the sphincter and dilator pupillae muscles and the optic nerve itself are derived from neural
ectoderm. The sclera, stroma of the cornea, cililary body and iris; as well
as the choroid, extraocular muscles, coverings and connective tissue are derived from mesoderm.
- Chambers and components of the eye include:
- Anterior chamber, the space between the cornea and the iris;
- Posterior chamber, the space between the posterior iris and the lens;
- Vitreouus chamber, the space between the posterior surface of the lens and the
neural retina.
- Anterior and posterior chambers are filled with aqueous humor; the vitreous body
is filled with gelatinous vitreous humor.
- The lens is suspended from the inner surface of the ciliary body by a ring of radially
oriented fibers, the zonule of Zinn.
- The transparent cornea defines that interface where the greatest refraction of light
entering the eye occurs.
- It is comprised of layers of keratinized stroma lying
between basement membranes, a moderately thick layer of surface epithelium and
a monolayer of endothelium. The corneal epithelium is extremely sensitive to
touch and regenerates every seven days from stem cells located in the corneosclera limbus (junction).
- The corneal stroma is composed of thin lamellae consisting of parallel bundles of
collagen fibrils (keratin and chondroitin sulfate proteoglycans) with thin flattend
fibroblasts lying in between.
- The outer (Bowman’s) membrane serves as a barrier to infections – it does not
regenerate and damage leaves permanent scars. The inner (Descemet’s)
membrane can regenerate after injury. A meshwork of ligaments extend beneath
the sclera and penetrate the ciliary muscle, exerting tension on the membrane and
maintaining the shape of the cornea.
- The corneal endothelium provides for metabolic exchange between the cornea and
aqueous humor.
- In the same way that the choroid plexus synthesizes and secretes cerebral spinal fluid, the
ciliary body secretes aqueous humor into the posterior chamber of the eye.
Aqueous humor flows between the iris and lens into the anterior chamber, filters
through a trabecular meshwork and enters the canal of Schlemm which
communicates with the venous drainage of the eye. Resistance to aqueous
outflow maintains a pressure of 15 mm Hg to maintain the shape of the eye.
Blockage of the canal of Schlemm increases intraocular pressure (glaucoma), a
painful condition that can lead to retinal damage.
- Adaptation to light is accomplished by the iris, the most anterior part of the vascular coat
(uvea), which forms a contractile diaphragm in front of the lens. Constriction of
the pupil is produced by smooth muscle located in the stroma which runs around
the circumference of the pupil (constrictor pupillae muscle – parasympathetic).
Dilation is accomplished by constrictor elements extending radially that make up
the dilator pupillae muscle – sympathetic.
- While the cornea is responsible for most of the refraction of light that enters the eye, the
lens is responsible for changing the focus of the system to accommodate for
distant or near vision. At rest, the tension of the zonule suspension between the
lens and the ciliary body keeps the lens slightly flattened and focused on distant
objects. When the ciliary muscles (circumferential or parallel to the surface of the eye) contract,
they release tension on the zonules which allows the lens to round or fatten more as the
eye accommodates to near objects.
- The crystalline lens, composed of an elastic
lens capsule (collgens and proteoglycans) and lens fibers (enucleated fibers filled
with the protein crystallin). Formation of lens fibers decrease with age leading to
presbyopia (loss of elasticity). Cataracts are also common (disease, heredity,
trauma).
- The retina and optic nerve are true extensions of the central nervous system. The visual
retina extends from the zona serrata around the posterior wall of the vitrous
chamber. The neural retina is separated from the sclera by a highly vascularized
choroid layer. Eight distinct layers (not including limiting membranes) can be
defined.
- Pigment epithelium – absorb light and minimize scatter; metabolic activity of
receptor cells.
- Layer of rods and cones – receptor cells with outer (signal transduction) and
inner segment which defines the outer nuclear layer. With rods, the internal
disks of the outer segment are “pinched off” and free floating; for cones, the disks
are continuous with the external environment. The major protein of these
membranes is rhodopsin, which absorb photons and induce receptor potentials.
- Outer plexiform layer – defines synaptic layer between rods/cones and bipolar
cells.
- Inner nuclear layer – cell bodies of bipolar and horizontal cells.
- Inner plexiform layer – synapses between bipolar cells and ganglion cells.
- Ganglion cell layer – cell bodies of ganglion cells which project from retina
toward the brain.
- Nerve fiber layer – axons projecting toward optic disk and forming optic nerve.
- The visual axis is from the center of the lens
to the macula. In the center of the macula is the area
of greatest visual acuity, the fovea.
- The fovea has cones for color vision and intraretinal
synaptology to interpret a high degree of detail. Cones
need more light and are said to serve photopic vision. Thus,
the highest relative receptor density in retina is effectively
at the fovea.
- Rods, in peripheral retina, have greater sensitivity
to low intensity light (scotopic), plus a great deal of intraretinal
convergence of the signals. In effect the signal has a lower relative
receptor density.
- Thus, for the rods, the signals converge toward relatively few ganglion cells, while for the
cones, the neuronal signals diverge to a larger number of ganglion cells. Thus,
the cones are designed to analyze fine detail, and the rods to detect faint light.
Cones concentrated in the fovea (within the macula lutea – yellow spot) have a
specialized morphology and is directly in line with the visual axis (~1.5 mm
across). The retina appears to thin at that spot in order to concentrate light on the
fovea. Specialize midget bipolar cells receive input from individual foveal cones
and in turn connect to specialized midget ganglion cells.
- Given these facts, what proportion of the area in the primary visual cortex is receiving the information from the fovea?
- Ganglion cells are the only retinal neurons
that generate action potentials. Ganglion cell axons leave
the eye at the optic disc (blind spot) and carry the information
about light stimuli to the thalamus.
- The optic "nerve" is myelinated by
oligodendrocytes and reaches the optic chiasm where axons from
the nasal retina cross to the contralateral side.
- After the chiasm, the opposite "visual
field" is represented in the optic tract, nuclei (e.g. lateral
geniculate nucleus) optic radiations and visual cortex.
Lesions of the visual pathways are illustrated diagrammatically
in your text and atlas.
- Retinal fields see the opposite Visual fields.
Retinal fields remain anatomically true, so that e.g. information
from lower retina is projected to lower visual cortex, in this
example, the lingual gyrus.
- Primary visual cortex (also called, Brodmann
area 17, striate cortex, V-1) is on the banks of the calcarine
sulcus, and is organized into dominance columns that intially
process the input from the left and right eyes separately. Secondary,
and tertiary visual cortex, and more, analyze parts (e.g. where,
what, and color) of the visual signal in a parallel processing
manner.
- The primary visual cortex is supplied by the posterior cerebral artery.
Consider the Following Questions ...
- What cranial nerves are you testing when you lightly touch the sclera of the eye?
- How does increased intraocular pressure lead to retinal damage?
- Contrast the histology and functions of the fovea, optic disc and peripheral retina.
- Discuss the concepts of "convergence" and "divergence" with respect to information processing in the retina.
- Draw two cups for eyes and an X for the optic nerves, chiasm and tracts. Identify the "left visual field" in relation to both eyes and indicate which part of the retinas will receive the stimulus. Trace the signal pathways for this stimulus through your drawing.
- Describe the foveal representation on the primary visual (striate or V1) cortex, and contrast it with the non-foveal representation. What vessel supplies blood to this area of cortex?
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