consists of 3 regions: posterior 5/6 th is the choroid , middle ciliary body , anterior iris
The choroid provides nutrients to the sclera and the sensory layer
The ciliary body ends in folds called the ciliary processes which contain blood capillaries that secrete the aqueous humor ; string-like structures extending from the ciliary processes called the ciliary zonule ( = suspensory ligament) which hold the lens in upright position in the eye
The iris is seen anteriorly as the “colored” part of the eye; the central opening in the iris called the PUPIL allows light to enter the lens; the iris contains 2 types of smooth muscle that control the size of the pupil; activation of the sympathetic nervous system and viewing distsnt objects result in pupillary dilation; activation of the parasympathetic nervous system and viewing close objects result in pupillary constriction.
Confined to the posterior wall ending at the ora serrata
Sensory layer consists of the pigmented layer and the neural layer
Pigmented layer – composed of a single layer of cells which provide nutrients to the neural layer; contain melanin which absorbs light and prevents it from scattering; contain vitamin A required for the synthesis of the light-absorbing pigment called RETINAL
Neural layer referred to as the R ETINA– extends anteriorly from the pigmented layer
Composed of 3 layers of neurons: PHOTORECEPTORS ; Bipolar neurons; Ganglion cells
PHOTORECEPTORS - abut the pigmented layer; they respond to light and generate electrical signals; 2 types of photoreceptors –
(i) rods – more numerous ( 150 million); more sensitive to light - used in dim-light and peripheral vision; provide images in shades of gray not used for color vision
(ii) cones – ( 80 million); operate in bright light, provide high- acuity color vision ;
3 types of cones – blue, green and red cones
According to the TRICHROMATIC THEORY OF VISION - several colors are seen depending on which/how many of the three types of cones are activated
Bipolar neurons – receive electrical signals from the photoreceptors and conduct the signal to the ganglion cells
Ganglion cells – neurons that receive the electrical signals from the bipolar neurons; only the axons of the ganglion cells generate and transmit action potentials
Ganglion cells are the only neurons in the retina that can generate and transmit action potentials
Bundle of axons of the ganglion cells = OPTIC NERVE (CN II)
The Optic Nerve exits the posterior wall of the eye through the OPTIC DISC = BLIND SPOT - because the optic disc lacks photoreceptors; lateral to the optic disc is the MACULA LUTEA and in its center is the FOVEA CENTRALIS ; the macula lutea contains mostly cones and the fovea centralis contains only cones and it’s used for hard focus
2 humors in the eye: Aqueous humor; Vitreous humor
Aqueous humor – filtered from blood capillaries in the ciliary processes into the anterior chamber in front of the lens; drained by the canal of Schlemm; formed and drained continually. If the rate of synthesis exceeds the rate of drainage, intraocular pressure rises causing damage to retina and the optic nerve resulting in GLAUCOMA
Function - supplies nutrients and oxygen to the lens and cornea; carries away metabolic wastes; maintains intraocular pressure to support the eyeball
Vitreous humor – gel-like fluid in the posterior segment behind the lens; formed in the embryo and lasts a lifetime.
Function – supports the posterior surface of the lens; pushes the neural layer against the pigmented layer; maintains the intraocular pressure.
Retinal detachment – the retina detaches from the pigmented layer and the vitreous humor seeps into the space – without their nutrient source, the photoreceptors in the retina die leading to blindness
Held in an upright position behind the pupil and the iris by the ciliary zonule
Composed of transparent proteins called crystallins
Function – focuses light on the retina; the flexible lens can change its shape to precisely focus light on the retina referred to as ACCOMODATION
Focusing for Distant Vision – the normal eyes are adapted for distant vision and therefore accommodation is not necessary; the far point of vision is the distance beyond which accommodation is not needed = 6m = 20ft (20/20 vision )
Focusing for Close Vision – less than 20ft; involves accommodation of the lens where the lens bulges to focus a close objects onto the retina; the near point of vision is the distance at which the lens can bulge maximally to focus the object on the retina = 10cm = 4in;
In addition, pupillary constriction occurs in close vision
Transmission of action potentials – impulse transmission
Axons of ganglion cells form the optic nerves; generate and transmit impulses via the optic nerves; medial fibers of optic nerves cross over to opposite sides at the OPTIC CHIASMA and continue on as the OPTIC TRACTS.
Impulses are transmitted to the visual reflex centers in the midbrain called the SUPERIOR COLLICULI
Impulses are transmitted to the visual relay center in the thalamus called the LATERAL GENICULATE NUCLEUS (LGN)
Impulses finally relayed to the primary visual cortex located in the occipital lobes
Mechanoreceptors – Hair Cells Sense organs - Ears
Located in the cochlear duct which contains endolymph
Rests on a flexible membrane called the BASILAR MEMBRANE
Composed of Supporting cells and HAIR CELLS , the mechanoreceptors; the apical surfaces of hair cells have stereocilia which are microvilli stiffened by actin; stereocilia are trapped in a gel-like membrane called the TECTORIAL MEMBRANE.
The afferent fibers of the cochlear nerve, a division of the vestibulocochlear nerve ( CN VIII), wrap around the bases of the hair cells
Dissolved chemical binds to gustatory hairs – results in depolarization which is transferred to the afferent fibers coiled around the gustatory cells:
chorda tympani of the facial nerve (CN VII) generates and transmits action potentials from gustatory cells in the anterior two-thirds of the tongue
Glossopharyngeal nerve (CN IX) generates and transmits action potentials from the posterior third of the tongue and the superior part of the pharynx
Vagus nerve (CN X) generates and transmits action potentials from the inferior part of the pharynx
Impulses from these 3 cranial nerves are transmitted to the SOLITARY NUCLEUS in the medulla oblongata; then to the gustatory relay center in the thalamus called the VENTRAL POSTEROMEDIAL NUCLEUS; impulse finally relayed to the primary gustatory cortex located in the Insula
Taste is 80% smell – same chemicals activate both types of chemoreceptor = olfactory cells and gustatory cells
How come food doesn't taste good when I have a cold? Because most of what we call "taste" is in fact smell, triggered by odor molecules from our food and drink. Some molecules we smell in the air, from the plate or as the fork approaches; others vaporize as we chew, then rise into the nasal passages at the back of the mouth.
Tastebuds alone can detect only sweet, sour, salty, and bitter. "If you lick a pink ice cream cone," says Donald Leopold, an otolaryngologist at Hopkins's Bayview Medical Center, "your tongue tells you it's cold and sweet and smooth, but your sense of smell tells you it's strawberry. Probably 80 percent of what you eat, you appreciate through your sense of smell." That's why if you have a cold, you could mistake a bite of onion for apple.