special senses Flashcards
fibrous layer
The outermost coat of the eyeball, the fibrous layer, is composed of dense avascular connective tissue. It has two obviously different regions: the sclera and the cornea.
sclera
The sclera (skler′ah; “hard”), forming the posterior portion and the bulk of the fibrous layer, is glistening white and opaque. Seen anteriorly as the “white of the eye,” the tough, tendonlike sclera protects and shapes the eyeball and provides a sturdy anchoring site for the extrinsic eye muscles. Posteriorly, where the sclera is pierced by the optic nerve (cranial nerve II), it is continuous with the dura mater of the brain.
cornea
The anterior sixth of the fibrous layer is modified to form the transparent cornea, which bulges anteriorly from its junction with the sclera. The crystal-clear cornea forms a window that lets light enter the eye, and is a major part of the light-bending apparatus of the eye.
The cornea is well supplied with nerve endings, most of which are pain receptors. its capacity for regeneration and repair is extraordinary.
The cornea has no blood vessels and so it is beyond the reach of the immune system. As a result, the cornea is the only tissue in the body that can be transplanted from one person to another with little risk of rejection.
vascular layer
The vascular layer forms the middle coat of the eyeball. Also called the uvea (u′ve-ah; “grape”), this pigmented layer has three regions: choroid, ciliary body, and iris
choroid
The choroid is a blood vessel–rich, dark brown membrane (choroid = membrane) that forms the posterior five-sixths of the vascular layer. Its blood vessels nourish all eye layers. Its brown pigment, produced by melanocytes, helps absorb light, preventing it from scattering and reflecting within the eye (which would cause visual confusion). The choroid has a posterior opening where the optic nerve leaves the eye.
ciliary body
Anteriorly, the choroid becomes the ciliary body, a thickened ring of tissue that encircles the lens and has three parts:
Ciliary muscles.
Ciliary processes.
Ciliary zonule.
ciliary zonule
The ciliary zonule (suspensory ligament) extends from the ciliary processes to the lens. This halo of fine fibers encircles the circumference of the lens and helps hold it in its upright position. It also transmits tension from the ciliary muscle to the lens.
ciliary processes
Near the lens, the posterior surface of the ciliary body has radiating folds called ciliary processes, which secrete the fluid that fills the cavity of the anterior segment of the eyeball.
ciliary muscles
Ciliary muscles make up most of the ciliary body and consist of interlacing smooth muscle bundles that control lens shape.
epithelial sheets on the cornea
The external sheet, a stratified squamous epithelium that protects the cornea from abrasion, merges with the bulbar conjunctiva at the corneoscleral junction. Epithelial cells that continually renew the cornea are located here.
The deep corneal endothelium, composed of simple squamous epithelium, lines the inner face of the cornea. Its cells have active sodium pumps that maintain the clarity of the cornea by keeping its water content low.
iris
The iris, the colored part of the eye, is the most anterior portion of the vascular layer. Shaped like a flattened doughnut, it lies between the cornea and the lens and is continuous with the ciliary body posteriorly. Its round central opening, the pupil, allows light to enter the eye. The iris is made up of two smooth muscle layers with bunches of sticky elastic fibers that congeal into a random pattern before birth. Its muscle fibers allow it to act as a reflexively activated diaphragm to vary pupil size (Figure 15.5). In close vision and bright light, the sphincter pupillae (circular muscles) contracts and the pupil constricts. In distant vision and dim light, the dilator pupillae (radial muscles) contracts and the pupil dilates, allowing more light to enter. Sympathetic fibers control pupillary dilation, and parasympathetic fibers control constriction.
iris pigment
Although irises come in different colors (iris = rainbow), they contain only brown pigment. When they have a lot of pigment, the eyes appear brown or black. If the amount of pigment is small and restricted to the posterior surface of the iris, the unpigmented parts simply scatter the shorter wavelengths of light and the eyes appear blue, green, or gray. Most newborn babies’ eyes are slate gray or blue because their iris pigment is not yet developed.
inner layer of eye
The innermost layer of the eyeball is the delicate retina (ret´ĭ-nah), which develops from an extension of the brain. It contains (1) millions of photoreceptors that transduce (convert) light energy, (2) other neurons involved in processing responses to light, and (3) glia. The retina consists of two layers: an outer pigmented layer and an inner neural layer (Figure 15.6). Although the pigmented and neural layers are very close together, they are not fused. Only the neural layer of the retina plays a direct role in vision.
optic disc
The optic disc, where the optic nerve exits the eye, is a weak spot in the fundus (posterior wall) of the eye because it is not reinforced by the sclera. The optic disc is also called the blind spot because it lacks photoreceptors, so light focused on it cannot be seen. We do not usually notice these gaps in our vision because the brain uses a sophisticated process called filling in to deal with absence of input.
fundus
posterior wall of the eye
rods
Rods are our dim-light and peripheral vision receptors. They are more numerous and far more sensitive to light than cones, but they do not provide sharp images or color vision. This is why colors disappear and the edges of objects appear fuzzy in dim light and at the edges of our visual field.
cones
Cones, in contrast, are our vision receptors for bright light and provide high-resolution color vision. They look like spaceships in diagrams.
macula lutea
In the retina. Lateral to the blind spot of each eye is an oval region called the macula lutea (mak′u-lah lu′te-ah; “yellow spot”) with a minute (0.4 mm) pit in its center called the fovea centralis (see Figure 15.4). In this region, the retinal structures next to the vitreous humor are displaced to the sides. This allows light to pass almost directly to the photoreceptors rather than through several retinal layers, greatly enhancing visual acuity (the ability to resolve detail).
rods vs cones in retina
The fovea contains only cones, the macula contains mostly cones, and from the edge of the macula toward the retina periphery, cone density declines gradually. The retina periphery contains mostly rods, which decrease in density from there to the macula.
where does detailed color vision come from?
Only the foveae have a sufficient cone density to provide detailed color vision, so anything we wish to view critically is focused on the foveae. Because each fovea is only about the size of the head of a pin, only a thousandth of the entire visual field is in hard focus (foveal focus) at a given moment. Consequently, for us to visually comprehend a scene that is rapidly changing (as when we drive in traffic), our eyes must flick rapidly back and forth to provide the foveae with images of different parts of the visual field.
where does the neural retina get its blood supply from?
The neural layer of the retina receives its blood supply from two sources.
Vessels in the choroid supply the outer third (containing photoreceptors).
The inner two-thirds is served by the central artery and central vein of the retina, which enter and leave the eye through the center of the optic nerve (see Figure 15.4a). Radiating outward from the optic disc, these vessels give rise to a rich vascular network. This is the only place where small blood vessels are visible in a living person
retina pigmented layer
The outer pigmented layer, a single-cell-thick lining, is next to the choroid, and extends anteriorly to cover the ciliary body and the posterior face of the iris. These pigment cells, like those of the choroid, absorb light and prevent it from scattering in the eye. They also act as phagocytes participating in photoreceptor cell renewal (described on p. 569), and store vitamin A needed by the photoreceptor cells.
retina neural layer
The transparent inner neural layer extends anteriorly to the posterior margin of the ciliary body.
From posterior to anterior, the neural layer is composed of three main types of neurons: photoreceptors, bipolar cells, and ganglion cells. Signals are produced in response to light and spread from the photoreceptors (next to the pigmented layer) to the bipolar cells and then to the innermost ganglion cells, where action potentials are generated. The ganglion cell axons make a right-angle turn at the inner face of the retina, then leave the posterior aspect of the eye as the thick optic nerve. The retina also contains other types of neurons—horizontal cells and amacrine cells—which play a role in visual processing
ora serrata
junction between the neural layer of the retina and the posterior margin of the ciliary body
vitreous humor
a clear gel that binds tremendous amounts of water. Found in the posterior segment of the eye. Vitreous humor forms in the embryo and lasts for a lifetime.
Vitreous humor:
- Transmits light
- Supports the posterior surface of the lens and holds the neural layer of the retina firmly against the pigmented layer
- Contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles
aqueous humor
fills the anterior segment of the eye (includes the anterior and posterior chambers). It is a clear fluid similar in composition to blood plasma. Unlike the vitreous humor, aqueous humor forms and drains continually. It forms in the ciliary processes of the ciliary body, and drains at the scleral venous sinus.
Aqueous humor supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and it carries away metabolic wastes.
