The middle, pigmented vascular layer (uvea) consists of the choroid, the ciliary body, and the iris
The choroid provides nutrients to the eye and prevents light scattering within the eye
The ciliary muscles of the ciliary body control lens shape
The iris controls the size of the pupil
The sensory layer, or retina, consists of an outer pigmented layer and an inner neural layer.
The neural layer contains the photoreceptors (rods and cones), bipolar cells, and ganglion cells.
Ganglion cell axons form the optic nerve which exits the eye via the optic disk (“blind spot”)
The outer segments of the photoreceptors contain the light-absorbing pigment membrane –bounded discs
The posterior segment of the eyeball, behind the lens, contains vitreous humor, which helps support the eyeball and keep the retina in place.
The anterior segment, anterior to the lens, is filled with aqueous humor, formed by capillaries in the ciliary processes and drained into the scleral venous sinus.
Aqueous humor is a major factor in maintaining intraocular pressure
The biconvex lens is suspended within the eye by the ciliary zonule attached to the ciliary body. It is the only adjustable refractory structure of the eye.
Physiology of Vision:
Light is made up of those wavelengths of the electromagnetic spectrum that excite the photoreceptors - ROYGBIV. Red wavelengths are the longest and have the lowest energy and violet are the shortest and the most energetic.
Light is refracted (bent) when passing from one transparent medium to another of a different density.
Concave lens disperse the light, convex lens converge the light and bring its rays to a focal point.
The greater the lens curvature, the more light bends
As light passes through the eye, it is bent by the cornea and the lens and focused on the retina.
The cornea accounts for most of the refraction.
The lens allows for active focusing for different distances
Focusing for distance vision requires no special movements of the eye structures.
Focusing for close-up vision requires accommodation (bulging of the lens), pupillary constriction, and convergence of the eyeballs.
Accommodation is the process that increases the refractory power of the lens
Pupillary constriction prevents the most divergent light rays from entering the eye. These rays would pass through the extreme edge of the lens causing blurred vision
Convergence is the medial rotation of the eyeballs by the medial rectus muscles so that each is directed toward the object being viewed
Refractory problems include presbyopia, myopia, hyperopia and astigmatism
99% of refractory problems are related to the eyeball shape, either too long of too short
Presbyopia, (old people’s vision), the eyeball is non-accommodating – the lens losing its elasticity. Having to hold a book at arm’s length
Myopia (short vision) occurs when distant objects are focused not on, but in front of, the retina. Eyeball is too long
Myopic people can see close objects without problems but distant objects are blurred. Nearsighted
Hyperiopia, (far vision) or far- sightedness. When distant objects are focused behind the retina. These people can see far objects well because the ciliary muscles contract continuously to increase the light bending power of the lens, which brings the focal point back to the retina.
Astigmatism, unequal curvatures of different parts of the cornea and/or the lens causes blurry images.
Functional Anatomy of Photoreceptors
Photoreceptors are modified neurons
Rods respond to low intensity light and provide night and peripheral vision
Cones are bright light, high discrimination receptors that provide color vision
Anything that must be viewed precisely is focused on the cone-rich fovea centralis
Rods and cones have an outer segment (receptor region) and an inner segment (connecting to the cell body)
These cells are highly vulnerable to damage and immediately begin to degenerate if the retina becomes detached.
Rods and cones contain unique visual pigments that absorb different wavelengths of light.
They also have different thresholds for activation
Rods are very sensitive and respond to very dim light ( a single photon) making them best suited for night vision and peripheral vision
They contain a single pigment so it is perceived as tones of gray
Cones need bright light, have 3 different pigments that provide a vividly colorful view
Light absorbing molecule is called Retinal which is chemically related to Vitamin A and is made from it.
Retinal combines with a proteins called Opsins to form 4 types of visual pigments.
Rods visual pigment is rhodopsin
Three types of cones all contain retinal but each has a different type of opsin. Each responding to different wavelengths of light – red, blue, green
Color blindness is the absence of one of more cones. Usually red or green, it is an X-linked trait and is more prevalent in males than females
Night blindness is a condition in which rod function is severely hampered impairing the ability to drive safely at night. Most common cause is a vitamin A deficiency which leads to rod degeneration.
Visual Pathways to Brain:
Begins with the optic nerve fibers (ganglion cell axons) from the retina.
To the optic chiasma (cross)
To the optic tracts to synapse with the thalamus neurons which connect to the primary visual cortex in the occipital lobes where seeing occurs.
Chemical Senses: Taste and Smell
Taste and smell are primitive senses that alert us to stuff to be savored or avoided
Taste (gustation)and Smell (olfaction: olfact = to smell) are chemoreceptors
Respond to chemicals in an aqueous solution
They complement each other and respond to different classes of chemicals
Taste receptors are excited by food chemicals dissolved in the saliva
Smell receptors, by the airborne chemicals that dissolve in the fluids coating the nasal membranes
Olfactory epithelium is located on the roof of the nasal cavity.
Receptor cells are ciliated (increasing surface area) bipolar neurons.
Individual neurons show a range of responsiveness to different chemicals
Smell is difficult to research because any given odor made up of hundreds of different chemicals.
Some studies have identified 1000 smell genes that are active only in the nose. Each gene encodes for a specific receptor protein
Olfactory neurons are exceptionally sensitive with some taking a few molecules to activate them.
For us to smell a particular odor, it must be volatile, that means it must be in a gaseous state as it enters the nasal cavity.
It must also dissolve in the fluid coating of the olfactory epithelium
Action potentials of olfactory nerve filaments are transmitted to the olfactory bulb where to filaments synapse with mitral cells. The mitral cells send impulses via the olfactory tract to the olfactory cortex. Fibers carrying impulses from the olfactory receptors also project to the limbic system. (our emotional, or affective (feelings) center in the brain)
Taste buds are scattered in the oral cavity and pharynx but are most abundant on the tongue papillae
Gustatory cells, the epithelial receptor cells of the taste buds, have gustatory hairs (microvilli) that serve the receptor regions.
We have 5 basic taste qualities: Sweet, Sour, Salty, Bitter and Umami (allows you to experience the “beef taste” of steak, the tang of aging cheese