Vision

stretch receptors called bulbous corpuscles (Ruffini corpuscles). Plexuses of nerves around hair

follicles will detect hair movement.

Stretch receptors and other somatosensory receptors are also found in joints and muscles. In

skeletal muscle, the stretch receptor that does this is called a muscle spindle. The same thing in

tendons is called a Golgi tendon organ. There are also bulbous corpuscles seen in joint capsules

that measure the amount of stretch inside the joint itself.

VISION

Vision is a special sense that relies on the transduction of light stimuli received through the

eyes. There are two eyes inside bony orbits that result in binocular vision. The eyelids are

present to help protect the eye from abrasions and the eyelashes help filter dust away from the

eyes. The palpebral conjunctiva is the inner lining of the eyelids. It extends back onto the

white part of the eye (which is called the sclera).

Figure 73 illustrates the anatomy of the eye:

Tears are created by the lacrimal gland, located beneath the lateral aspect of the nose. The

tears flow from the lacrimal duct in the medial eye to wash away foreign debris from the

surface of the eye.

There are six extraocular muscles that control the eye movements. They start in various bones

of the orbit and attach to the eyeball itself. There is the superior rectus, inferior rectus, medial

rectus, and lateral rectus on their corresponding part of the eyeball. In addition, the superior

oblique muscle rotates the eye in a medial direction and the inferior oblique muscle rotates the

eye in a lateral direction. The only other muscle in the orbit is the levator palpebrae superioris,

which elevates and retracts the upper lid. The trochlear, oculomotor, and abducens nerves all

help to rotate the eye in all directions.

The eye itself is hollow and made from three layers of tissue. The outermost layers include

either the cornea (clear in nature) and the sclera (white in nature). The cornea is transparent

which allows light enter the eye. The middle layer of the eye is called the vascular tunic. It

consists of the choroid, the ciliary body, and the iris. The choroid provides the blood supply to

the eyeball; the ciliary body is a muscular structure attached to the lens by fibers called zonule

fibers. Together, these bend the lens so that it can focus light on the retina at the back of the

eye. The iris is a smooth muscle and is the colored part of the eye. It can open or close to make

the pupil smaller or bigger. The innermost layer of the eye is the retina, also called the neural

tunic. It contains the nerve fibers necessary for vision.

There are two sections to the eyeball: the anterior and posterior cavity. The anterior portion is

the space between the cornea and lens; it includes the iris and the ciliary body. It is filled with a

thin watery fluid known as the aqueous humor. The posterior cavity is the larger part behind

the lens, containing the vitreous humor, which is thicker in nature.

There are several layers within the retina. The photoreceptors (also called the rods and cones)

will change their transmembrane potential according to light stimuli. Neurotransmitters are

then released to affect cells of the outer synaptic layer (the bipolar cells). These bipolar cells

connect each photoreceptor to a retinal ganglion cell (RGC), located in the inner synaptic layer.

The axons of these RGCs all collect at the optic disc and leave the eye as the optic nerve. There

are no photoreceptors in the optic disc, creating a “blind spot”.

The exact center of the retina is called the fovea. This part contains only photoreceptors and

lacks the overlying blood vessels and other cell layers that cover the photoreceptors in other

areas. This makes this part of the retina have the best visual acuity. Each photoreceptor is

connected to just one RCG so that the acuity can be remarkably sharp. Toward the edges of the

retina there can be 50 photoreceptors to one RCG, which reduces the sharpness significantly.

The light that reaches the retina causes chemical changes to pigmented molecules in the

photoreceptors, which activate the RCGs. There is an inner segment in a photoreceptor cell that

contains the nucleus and major organelles and an outer segment, which is specialized for

photoreception. The rods contain a stack of discs that have rhodopsin in them (which are

photosensitive). The cones have infoldings that contain photosensitive pigments. There are

three photopigments in the cones (called opsins), which are each sensitive to a different

wavelength of light.

The visual stimulus reaching a photoreceptor causes a change in the photopigment molecule,

leading to an action potential in the photoreceptor cell. These allow for the perception of light

between 380 nm and 720 nm in wavelength. The opsin pigments have a cofactor called retinal

(a hydrocarbon molecule related to vitamin A). The light hits the retinal and changes its

biochemical appearance—a process known as photoisomerization. This change in retinal along

with the opsin proteins will result in the activation of a G protein which is a transmembrane

protein that can change the membrane potential of the cell. The photoisomerization process

gets changed back via enzymatic processes.

Rods are more sensitive in low-light conditions, whereas cones, which respond to red, green,

and blue wavelengths, are more active in bright light conditions. Rods are the photoreceptors

that are active in low light and the cones are more sensitive in bright light conditions. The brain

can visualize all the different colors by comparing the activity of the different photopigments in

the cones. Low light vision tends to be relatively colorless because the rods are not color-

sensitive.