Regulation and Integration of the Body
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The Eye and Vision
Te eye is enclosed in the bony orbit and cushioned by fat.
Accessory Structures of the Eye
Eyebrows help to shade and protect the eyes.
Eyelids protect and lubricate the eyes by reﬂex blinking. Within
the eyelids are the orbicularis oculi and levator palpebrae
superioris muscles, and modiﬁed sebaceous and sweat glands.
Te conjunctiva is a mucosa that lines the eyelids and covers the
anterior eyeball surface. Its mucus lubricates the eyeball surface.
Te lacrimal apparatus consists of the lacrimal gland (which
produces a saline solution containing mucus, lysozyme, and
antibodies), the lacrimal canaliculi, the lacrimal sac, and the
Te extrinsic eye muscles (superior, inferior, lateral, and medial
rectus and superior and inferior oblique) move the eyeballs.
Structure of the Eyeball
Te wall of the eyeball is made up of three layers. Te outermost
ﬁbrous layer consists of the sclera and the cornea. Te sclera
protects the eye and gives it shape; the cornea allows light to enter
Te middle, pigmented vascular layer (uvea) consists of the
choroid, the ciliary body, and the iris. Te choroid provides
nutrients to the eye and prevents light scattering within the eye.
Te ciliary muscles of the ciliary body control lens shape; the iris
controls the size of the pupil.
Te inner layer, or retina, consists of an outer pigmented
layer and an inner neural layer. Te neural layer contains
photoreceptors (rods and cones), bipolar cells, and ganglion cells.
Ganglion cell axons form the optic nerve, which exits via the
optic disc (“blind spot”).
Te posterior segment of the eyeball, behind the lens, contains
vitreous humor, which helps support the eyeball and keep the
retina in place. Te anterior segment, anterior to the lens, is
ﬁlled with aqueous humor, formed by capillaries in the ciliary
processes and drained into the scleral venous sinus. Aqueous
humor is a major factor in maintaining intraocular pressure.
Te biconvex lens is suspended within the eye by the ciliary
zonule attached to the ciliary body. Te lens is the only adjustable
refractory structure of the eye.
Optics and the Eye
Visible light is made up of those wavelengths of the
electromagnetic spectrum that excite the photoreceptors.
Light is refracted (bent) when passing from one transparent
medium to another of diﬀerent density. Concave lenses disperse
light; convex lenses converge light and bring its rays to a focal
point. Te greater the lens curvature, the more light bends.
As light passes through the eye, the cornea and lens bend and focus
it on the retina. Te cornea accounts for most of the refraction, but
the lens allows active focusing for diﬀerent distances.
Focusing for distant vision requires no special movements
of the eye structures. Focusing for close-up vision requires
accommodation (bulging of the lens), pupillary constriction, and
convergence of the eyeballs. Cranial nerve III controls all three
Refractory problems include presbyopia, myopia, hyperopia, and
Photoreceptors and Phototransduction
Te outer segments of the photoreceptors contain light-absorbing
visual pigment in membrane-bounded discs.
Te light-absorbing molecule retinal is combined with various
opsins to form the visual pigments. Rod visual pigment,
rhodopsin, is a combination of retinal and opsin. Te three
types of cones all contain retinal, but each has a diﬀerent type of
opsin. Each cone type responds maximally to one color of light:
red, blue, or green. Te chemistry of cone function is similar to
that of rods.
Rods respond to low-intensity light and provide night and
peripheral vision. Cones are bright-light, high-discrimination
receptors that provide color vision. Anything that must be viewed
precisely is focused on the cone-rich fovea centralis.
When struck by light, retinal changes shape (11-
) and activates opsin. Activated opsin activates transducin
(a G protein) which in turn activates PDE, an enzyme that
breaks down cGMP, allowing the cation channels to close. Tis
hyperpolarizes the receptor cells and inhibits their release of
Photoreceptors and bipolar cells generate graded potentials only;
ganglion cells generate action potentials.
During light adaptation, photopigments are bleached and rods
are inactivated; then, as cones decrease their light sensitivity,
high-acuity vision ensues. In dark adaptation, cones cease
functioning, and visual acuity decreases; rod function begins
when suﬃcient rhodopsin has accumulated.
Visual Pathways and Processing
Te visual pathway to the brain begins with the optic nerve ﬁbers
(ganglion cell axons) from the retina. At the optic chiasma, ﬁbers
from the medial half of each retina cross over and continue in
the optic tracts to the thalamus. Talamic neurons project to the
visual cortex via the optic radiation. Fibers also project from the
retina to the midbrain pretectal nuclei and the superior colliculi,
and to the suprachiasmatic nucleus of the hypothalamus.
brain. However, as we have discovered in this ﬁnal nervous
system chapter, the large and o±en elaborate sensory receptor
organs that serve the special senses are works of art in and of
Te concluding chapter of this unit describes how the body’s
functions are controlled by chemicals called hormones in a
manner quite diﬀerent from what we have described for neural