Special Senses- Chapter 8

Cataracts

A cataract is a clouding of the lens in the eye which leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry vision, halos around light, trouble with bright lights, and trouble seeing at night. This may result in trouble driving, reading, or recognizing faces. Poor vision caused by cataracts may also result in an increased risk of falling and depression.

Photoreceptors

A photoreceptor cell is a specialized type of neuron found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

Convergence of the Eye

A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. When a creature with binocular vision looks at an object, the eyes must rotate around a horizontal axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence)

Astigmatism

Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. This results in distorted or blurred vision at all distances. Other symptoms can include eyestrain, headaches, and trouble driving at night. If it occurs early in life it can result in amblyopia. The cause of astigmatism is unclear. It is believed to be partly related to genetic factors. The underlying mechanism involves an irregular curvature of the cornea or abnormalities in the lens of the eye. Diagnosis is by an eye exam.

Color Blindness

Color blindness, also known as color vision deficiency, is the decreased ability to see color or differences in color. Color blindness can make some educational activities difficult. The most common cause of color blindness is an inherited fault in the development of one or more of the three sets of color sensing cones in the eye. Males are more likely to be color blind than females, as the genes responsible for the most common forms of color blindness are on the X chromosome.

Binocular Vision

In biology, binocular vision is a type of vision in which an animal having two eyes is able to perceive a single three-dimensional image of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather than just one. Here are some of the advantages: -It gives a creature a spare eye in case one is damaged. -t can give stereopsis in which binocular disparity (or parallax) provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature. -It gives binocular summation in which the ability to detect faint objects is enhanced.

Choroid

The Choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissues, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye (at 0.2 mm), while in the outlying areas it narrows to 0.1 mm. The choroid provides oxygen and nourishment to the outer layers of the retina. Along with the ciliary body and iris, the choroid forms the uveal tract. The structure of the choroid is generally divided into four layers.

Meibomian Gland

The Meibomian glands (often written with a small m, and also called tarsal glands) are a special kind of sebaceous gland at the rim of the eyelids inside the tarsal plate, responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film. The function of these glands is to secrete oils onto the surface of the eye. Meibomian gland dysfunction is blockage or some other abnormality of the meibomian glands so they don't secrete enough oil into the tears.

Accommodation Pupillary Reflex

The accommodation reflex (or accommodation-convergence reflex) is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in vergence, lens shape and pupil size (accommodation). It is dependent on cranial nerve II (afferent limb of reflex), superior centers (interneuron) and cranial nerve III (efferent limb of reflex). The change in the shape of the lens is controlled by the ciliary muscles inside the eye. Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation

Aqueous Humor

The aqueous humour is a transparent, watery fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary epithelium, a structure supporting the lens. It fills both the anterior and the posterior chambers of the eye, and is not to be confused with the vitreous humour, which is located in the space between the lens and the retina, also known as the posterior cavity or vitreous chamber.

Ciliary body

The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The ciliary body is part of the uvea, the layer of tissue that delivers oxygen and nutrients to the eye tissues. The ciliary body joins the ora serrata of the choroid to the root of the iris.The ciliary body is a ring-shaped thickening of tissue inside the eye that divides the posterior chamber from the vitreous body. It contains the ciliary muscle, vessels, and fibrous connective tissue.

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance.

Fovea Centralis

The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina. The fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for activities where visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the parafovea belt, and the perifovea outer region

Fundus

The fundus of the eye is the interior surface of the eye opposite the lens and includes the retina, optic disc, macula, fovea, and posterior pole. The color of the fundus varies both between and within species. In one study of primates the retina is blue, green, yellow, orange, and red; only the human fundus (from a lightly pigmented blond person) is red. The major differences noted among the "higher" primate species were size and regularity of the border of macular area, size and shape of the optic disc, apparent 'texturing' of retina, and pigmentation of retina.Astigmatism is a type of refractive error in which the eye does not focus light evenly on the retina. There is also a fundus of the uterus.

Lacrimal Gland

The lacrimal glands are paired, almond-shaped exocrine glands, one for each eye, that secrete the aqueous layer of the tear film. They are situated in the upper lateral region of each orbit, in the lacrimal fossa of the orbit formed by the frontal bone. Inflammation of the lacrimal glands is called dacryoadenitis. The lacrimal gland produces tears which then flow into canals that connect to the lacrimal sac. From that sac, the tears drain through the lacrimal duct into the nose. Anatomists divide the gland into two sections. The smaller palpebral portion lies close to the eye, along the inner surface of the eyelid; if the upper eyelid is everted, the palpebral portion can be seen.

Conjunctiva

The mucous membrane that covers the front of the eye and lines the inside of the eyelids. When caused by an infection, neonatal conjunctivitis can be very serious. The most common types of neonatal conjunctivitis include: Chlamydial (or inclusion) conjunctivitis is caused by the bacterium Chlamydia trachomatis and can cause swelling of the eyelids with purulent (pus) discharge.

Optic Chiasma

The optic chiasm or optic chiasma is the part of the brain where the optic nerves partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes (lampreys and hagfishes) it is located within the brain. The optic nerve fibres on the nasal sides of each retina cross over (decussate) to the opposite side of the brain via the optic nerve at the optic chiasm (decussation of medial fibers)

Optic Disc

The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye. The ganglion cell axons form the optic nerve after they leave the eye. The optic disc represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together. The optic disc is also the entry point for the major blood vessels that supply the retina.

Optic nerve

The optic nerve, also known as cranial nerve II, is a paired nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells; it extends from the optic disc to the optic chiasma and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus

Optic Tracts

The optic tract (from the Latin tractus opticus) is a part of the visual system in the brain. It is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and superior colliculus. It is composed of two individual tracts, the left optic tract and the right optic tract, each of which conveys visual information exclusive to its respective contralateral half of the visual field. Each of these tracts is derived from a combination of temporal and nasal retinal fibers from each eye that corresponds to one half of the visual field.

Pupil

The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil. In humans the pupil is round, but other species, such as some cats, have vertical slit pupils, goats have horizontally oriented pupils, and some catfish have annular types. In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop.

Photopupillary Reflex

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict. Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

Retina

The retina is the third and inner coat of the eye which is a light-sensitive layer of tissue. The optics of the eye create an image of the visual world on the retina (through the cornea and lens), which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibres of the optic nerve

Vitreous Humor

The vitreous body is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates. It is often referred to as the vitreous humour or simply "the vitreous". The vitreous humour is a transparent, colorless, gelatinous mass that fills the space in the eye between the lens and the retina. It is surrounded by a layer of collagen called vitreous membrane separating it from the rest of the eye. it makes up four-fifths of the volume of the eyeball

Sclera

The white outer layer of the eyeball. At the front of the eye it is continuous with the cornea. The sclera, as separated from the cornea by the corneal limbus. The sclera, also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the eye containing mainly collagen and some elastic fiber.