Biomedical Engineering Reference
In-Depth Information
As stated previously, the retina contains both rod cells and cone cells. There are approx-
imately 125 million rod cells and 6 million cone cells in each eye. Combined, these cells
account for more than 70% of all of the sensory receptors within the body. Rods are more
sensitive to light intensity; they do not distinguish between colors, and they help us to see
at night. Cones can distinguish between different wavelengths of light but are not that sen-
sitive to intensity and therefore do not play an important visual role when it is dark (hence
everything looks gray under low-intensity light). Each rod or cone cell is composed of an
outer segment that is composed of highly membranous discs. These discs contain the
visual pigments that consist of a light-absorbing pigment, retinal, bounded to a membrane
protein, opsin. The protein opsin varies in structure based on the photoreceptor type.
The opsin that can be found in rods combines with retinal to form the visual pigment
rhodopsin. As rhodopsin absorbs light, it experiences a conformational change that trig-
gers a signal transduction pathway, which ultimately leads to the membrane potential of
rod cells to decrease. In fact, when rod cells are in the dark, the cells are in a depolarized
state and they release an inhibitory neurotransmitter. When the membrane potential of
rod cells decreases, the rod cells stop releasing the inhibitory neurotransmitter and they
begin to send signals through the retina to the brain. Color vision occurs through the pres-
ence of three types of cone cells within the retina. Each of these cone cells has their own
type of opsin that associates with retinal to form a particular photopsin. Each type of
photopsin is best at absorbing either yellow (peak wavelength 430 nm); green (peak wave-
length 540 nm); or blue (peak wavelength 570 nm) light. Different shades of these colors
can be viewed based on the differential stimulation of each of these three types of cone
cells.
10.2 AQUEOUS HUMOR FORMATION
Aqueous humor is continually being formed and reabsorbed within the anterior and
posterior chambers. The formation and reabsorption of aqueous humor is balanced by the
volume of fluid within the eye as well as the intraocular pressure (
Section 10.5
). Aqueous
humor is formed at a rate of approximately 2
L/min within the anterior chamber. The
majority of this is secreted into the anterior chamber from the ciliary processes which are
projections from the ciliary bodies. These processes are located behind the iris where the
ciliary muscle attaches to the eye. The ciliary processes have a very large surface area asso-
ciated with them, approximately 6 cm
2
per eye. Secretory epithelial cells cover the ciliary
processes, which are highly vascularized.
Aqueous humor is actively secreted from the ciliary processes. To form aqueous humor,
sodium ions are actively (through the use of ATP) transported out of the ciliary epithelial
cells. Chloride and bicarbonate ions diffuse out of the ciliary processes, to maintain the
electrical neutrality of the intercellular space. The efflux of these ions into the intercellular
space causes an increase in the osmotic pressure of the intercellular space. To counterbal-
ance the increase in intercellular space osmotic pressure, water moves from the blood ves-
sels underlining the ciliary epithelial cells into the intercellular space. As water moves into
the intercellular space, it flows across the ciliary processes into the anterior chamber.
Other molecules, such as glucose and amino acids, can be transported within the aqueous
μ
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