Page 357 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Eye Structure, Tear Fluid, lar meshwork and Schlemm’s canal. Aqueous
Aqueous Humor humor is renewed once every hour or so.
Glaucoma. Obstruction of humor drainage can
Light entering the eye must pass through the occur due to chronic obliteration of the trabecular
cornea, aqueous humor, lens and vitreous body, meshwork (open-angle glaucoma) or due to acute
which are collectively called the optical ap- block of the anterior angle (angle-closure glaucoma)
paratus, before reaching the retina and its leading to elevated intraocular pressure, pain, retinal
damage, and blindness. Drugs that decrease humor
light-sensitive photosensors (! A). This pro-
Central Nervous System and Senses must be transparent and have a stable shape The lens is held in place by the ciliary zonules
production (e.g. carbonic anhydrase inhibitors) and
duces a reduced and inverse image of the visual
induce meiosis are used to treat glaucoma.
field on the retina. All parts of the apparatus
and smooth surface to produce an undistorted
(! C). When the eye adjusts for far vision, the
image, which is the main purpose of tear fluid
zonules are stretched and the lens becomes
in case of the cornea. Tears are secreted by
flatter, especially its anterior surface (! D,
lacrimal glands located in the top outer por-
top). When looking at nearby objects (near vi-
tion of orbit and their mode of production is
sion), the zonules are relaxed due to contrac-
similar to that of saliva (! p. 236). Tears are
tion of the ciliary muscle, and the lens reas-
distributed by reflex blinking and then pass
sumes its original shape due to its elasticity
through the lacrimal puncta and lacrimal
The retina lines the interior surface of the
eyelid into the lacrimal sac and finally drain
bulbus except the anterior surface and the site
into the nasal sinuses by way of the nasolacri-
where the optical nerve (! A) exits the bulbus
12 canaliculi (or ducts) of the upper and lower (! D , bottom, and p. 346).
mal duct. Tear fluid improves the optical
via the optic papilla (! A). The fovea centralis
characteristics of the cornea by smoothing un- (! A) forms a slight depression across from
even surfaces, washing away dust, protecting it the pupillary opening. The retina consists of
from caustic vapors and chemicals, and pro- several layers, named from inside out as fol-
tects it from drying out. Tears lubricate the lows (! E): pigmented epithelium, photosen-
eyelid movement and contain lysozyme and sors (rods and cones), Cajal’s horizontal cells,
immunoglobulin A (! pp. 96ff. and 232), bipolar cells, amacrine cells, and ganglion
which help ward off infections. In addition, cells. The central processes of the ganglion
tears are a well known mode of expressing cells (n ! 10 ) exit the bulbus as the optical
6
emotions. nerve (retinal circuitry; ! p. 355ff.).
The entry of light into the eye is regulated Photosensors. Retinal rods and cones have a
by the iris (! A; p. 353 C1), which contains light-sensitive outer segment, which is con-
annular and radial smooth muscle fibers. nected to a inner segment by a thin connecting
Cholinergic activation of the sphincter muscle part (! p. 349 C1). The inner segment contains
of pupil leads to pupil contraction (miosis), and the normal cell organelles and establishes syn-
adrenergic activation of the dilator muscle of aptic contact with the neighboring cells. The
pupil results in pupil dilatation (mydriasis). outer segment of the rod cells contains ca. 800
The bulbus (eyeball) maintains its shape due membranous disks, and the plasma membrane
to its tough outer coat or sclera (! C) and in- of the outer segment of the cones is folded.
traocular pressure which is normally Visual pigments are stored in these disks and
10–21 mmHg above the atmospheric pressure. folds (! p. 348). The outer segment is continu-
The drainage of aqueous humor must balance ously regenerated; the old membranous disks
its production to maintain a constant ocular at the tip of the cell are shed and replaced by
pressure (! C). Aqueous humor is produced in new disks from the inner segment. The phago-
the ciliary process of the posterior ocular cham- cytic cells of the pigmented epithelium engulf
ber with the aid of carbonic anhydrase and ac- the disks shed by the rods in the morning, and
tive ion transport. It flows through the pupil those shed by the cones in the evening. Some
into the anterior ocular chamber and drains ganglion cells contain a light-sensitive pig-
344 into the venous system by way of the trabecu- ment (! p. 334).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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