Week 8: Eye and Ear Histology Flashcards
Outer layer (fibrous tunic) of eyeball contains
sclera and cornea
middle layer (vascular tunic) of eyeball contains
choroid, ciliary body, and iris
Sclera
- dense irregular CT
- supports and maintains shape of eye
- protects internal structures
- attachment site for extraocular muscles
cornea function
- Protects anterior surface of eye
* refracts (bends) incoming light into lens
Choroid
- CT; highly vascularized
- nourish retina
- pigment absorbs extraneous light
ciliary body
- smooth muscle + secretory epithelium
- zonular fibers to lens
- epithelium secretes aqueous humor
Iris
•sphincter pupillae and dilator pupillae + CT with central pupil
internal layer (retina) of eyeball contains
pigmented layer and neural layer
pigmented layer of retina
- absorbs extraneous light
- provides vitamin A for photoreceptors
- recycles photoreceptor products
neural layer
- neurons and glia cells
- detects incoming light
- converts light rays to nerve signals -> brain
anterior chamber of eyeball
- between cornea and iris
* filled with aqueous humor
aqueous humor
- liquid like blood plasma with less protein/glucose and more lactate/ascorbate
- produced by secretory epithelium lining ciliary body
- flows from posterior to anterior chamber via pupil
posterior chamber of eyeball
- between iris and lens
* filled with aqueous humor
vitreous cavity of eyeball
- surrounded by retina; posterior to the lens
* contains vitreous body = large transparent gelatinous mass -> hyaluronic acid
important details of cornea
- colorless
- transparent
- highly innervated
- completely avascular
5 layers of cornea
- corneal epithelium
- bowman’s layer
- corneal stroma
- descemets membrane
- corneal endothelium
corneal epithelium
- non-keratinized stratified squamous
- barrier to outside world
- nerve endings -> blink reflex
- continuous renewal from stem cells at corneal scleral junction (limbus)
bowman’s layer
- CT that supports corneal epithelium
* additional protective barrier
corneal stroma
- highly transparent layer; ~90% of thickness of cornea
* collagen fiber lattice -> highly resistant to deformations and trauma
keratocytes
- cells of the corneal stroma
* produce/maintain the collagen matrix
descemets membrane
one of the thickest basement membranes in the body
corneal endothelium
- simple squamous epithelium with impermeable junctions -> regulate influx of aqueous humor into corneal stroma
- its structural and functional integrity is vital to maintenance of a proper hydration state of stroma and corneal transparency
What percent of eye’s total focusing power is contributed by cornea?
65-75%
myopia
(near-sightedness)
•eyeball that is too long, or cornea too curved
•light rays fall short of retina -> distant objects blurry
hyperopia
(far-sightedness)
•eyeball that is too short, or a cornea too flat
•light rays are focused behind retina -> close objects blurry
presbyopia
similar to hyperopia, but is due to change in elasticity of lens
conjunctiva
thin, transparent mucosa on exposed part of sclera and lining eyelids
primary functions of conjunctiva
- lubricate anterior surface of eye
- lubricate inner eyelids -> open/close w/o friction
- protect eye
epithelia of conjunctiva
•stratified columnar with “goblet cells” that secrete mucous to add to tear film for cornea
conjunctivitis
=pink eye
•conjunctiva inflamed, usually due to bacterial infection or allergies
•inflammation increases the discharge of mucous and enlarges microvasculature of sclera -> reddish appearance
•bacterial and viral -> little effect on vision
2 layers of choroid
- choriocapillaries
2. bruch’s membrane
choriocapillaries
vascularized CT with pigment cells -> nourish outer retina and absorb scattered light
bruch’s membrane
thin CT between choriocapillaries and the pigmented layer of retina
site of Age-related macular degeneration (AMD)
choroid
AMD results from
damage to the macula that is required for central vision
Dry AMD
(nonexudative, >80%)
•diffuse or discrete deposits of lipid and protein (drusen) in Bruch’s membrane of choroid
•atrophy and degeneration of pigmented epithelium and associated photorecepters of retina -> vision loss
•no effective treatment
What may slow progression of Dry AMD?
diet, multivitamin supplements, and antioxidants
Wet AMD
(exudative, 10-15%)
•angiogenesis from the choriocapillaries -> penetrate through buch’s membrane and RPE
•vessels leak -> scarring
•treatment= injection of VEGF antagonists into vitreous cavity to inhibit angiogenesis and leaking OR laser treatment
ciliary body is anterior expansion of___
it encircles___
- choroid in the posterior chamber
* it encircles the lens at the level of the limbus
ciliary processes
ridges extending from ciliary body and lined by a double layer of epithelium
inner layer of ciliary body is highly
pigmented
cells of outer layer of ciliary body are highly
specialized for secretion of aqueous humor into the posterior chamber
trabecular meshwork
- channels found at the corneoscleral junction (limbus)
* pumps aqueous humor -> scleral venous sinus -> venous blood returning from eye
normal intraocular pressure
(about 15-22 mmHg)
•necessary for functioning optical system
•maintains smooth curvature of corneal surface
•keep photoreceptors in contact with pigmented epithelium
glaucoma
•intraocular pressure is elevated -> optic nerve constricted -> vision loss and blindness
What causes elevated intraocular pressure in glaucoma?
obstruction hampers normal drainage of aqueous humor
acute or closed-angle glaucoma
- iris tissue obstructs flow of aq. humor from post. to ant. compartment
- pushes portions of iris upward, blocking chamber angle
chronic or open-angle glaucoma
- chamber angle open, but drainage through trabecular meshwork impaired
- most common type of glaucoma that becomes more prevalent after age of 40
glaucoma versus ocular hypertension
- must have optic nerve damage and associated vision loss to be called glaucoma
- otherwise ocular hypertension
ciliary body and accommodation of lens
•zonular fibers/ suspensory ligaments extend from ciliary processes to capsule of lens
accommodation reflex
lens is flattened for far away objects and rounded for nearby objects
when ciliary muscle relaxes…
zonular fibers are put under increased tension -> lens flattens
when ciliary muscle contracts…
zonular fibers become more lax -> lens becomes more round
ciliary body innervated by
parasympathetic fibers of occulomotor nerve
posterior surface of iris has
- 2-layered epithelium continuous with covering of ciliary processes
- heavily filled with melanin
2 layers of smooth muscle in iris
- outer radial layer called dilator pupillae
* inner circular bundle near pupil called sphincter pupillae
innervation of dilator pupillae
- sympathetic to enlarge pupil
* contraction increases pupillary aperture
innervation of sphincter pupillae
- parasympathetic fibers of oculomotor nerve to constrict pupil
- contraction decreases pupillary aperture
layers of retina in order
- nerve fiber layer
- ganglion cell layer
- inner nuclear layer
- outer nuclear layer
- pigmented epithelium
retinal pigmented epithelium (RPE)
- cuboidal epithelial cells in close association with photorecepters of the retina
- convert and store retinoid
- phagocytose shed photorecepters
- absorb scattered light
- iron and fluid transport
dysfunction of RPE can result in
photoreceptor cell death and blindness
outer nuclear layer of retina
- cell bodies and nuclei of photoreceptors (rods and cones)
* generate a signal in response to a photon of light and transmit it to the neurons of inner nuclear layer
rods contain___ and are responsible for ___
- visual pigment rhodopsin
* responsible for vision in low light
cones responsible for
color vision (site of color blindness)
ganglion cell layer of retina
has neurons with very long axons that make up the nerve fiber layer, which come together to form optic nerve
inner nuclear layer consists of
- bipolar cells, amacrine cells, and horizontal cells
* all responsible to integrate signals produced by photoreceptors and transmit it to ganglion cell layer
muller glial cells
- support retinal neurons
* extend full thickness of neurosensory retina
microglial cells
- support retinal neurons
* phagocytic and other immune functions
cells that support retinal neurons
- muller glial cells
* microglial cells
production of visual image that is transmitted to the brain via the optic nerve requires
light rays to pass through full thickness of retina
optic disc
- axons from ganglion cells of retina exit eye to form the optic nerve
- retinal blood vessels enter and leave the eye
macula
•6mm oval shaped, highly pigmented spot near center of retina
fovea
- small pit in the center of macula
* responsible for high resolution/high acuity vision
ears consist of
- auditory system
* vestibular system
auricle consists of
(pinna)
•elastic cartilage covered by keratinized stratified squamous epithelium with hairs, sebaceous glands, and sweat glands
external auditory meatus
- from auricle to tympanic membrane of middle ear
- outer 1/3 is elastic cartilage that is continuous with auricle
- inner 2/3 is temporal bone
external auditory meatus lined by
stratified squamous epithelium with hairs, sebaceous glands, and ceruminous glands
cerumen
(earwax)
the combination of secretions from sebaceous glands and ceruminous glands
tympanic cavity is
an irregular space that lies within the temporal bones between the tympanic membrane and bony surface of the internal ear
anteriorly, the tympanic cavity communicates with
pharynx via the auditory tube (Eustachian)
posteriorly, the tympanic cavity communicates with
the smaller, air-filled mastoid cavities of the temporal bone
primary function of auditory (eustachian) tube
•equalize air pressure between tympanic cavity and external environment
Walls of eustachian tube…
during swallowing and yawning, walls separate to allow air to enter tympanic cavity
tympanic cavity lined with
mainly simple cuboidal epithelium, thin lamina propria -> strongly adherent to periosteum
oval and round windows
- found in the medial bony wall of the middle ear
* membrane covered regions completely devoid of bone
tympanic membrane indirectly connected to
oval window by auditory ossicles
auditory ossicles transmit
vibrations of the TM to the internal ear
tensor tympani
small skeletal muscle attached to malleus -> restrict movement to protect inner ear from loud noise
stapedius
small skeletal muscle attached to stapes -> restrict movement to protect internal ear from loud noise
internal ear located
completely within temporal bone
bony labyrinth
houses set of continuous fluid-filled epithelium lined tubes and chambers (membranous labyrinth)
vestibule
- central cavity of the bony labyrinth
* contains utricle and saccule
semicircular canals
(superior, lateral, and posterior)
•extend posteriorly from vestibule
•oriented at right angles to one another
•has ampulla at one end
what mediates sense of equilibrium
vestibule and semicircular canals
cochlea
- coils like a snail shell around a central pillar of bone containing the cochlear duct
- mediates sense of hearing
hair cells
- columnar sensory mechanoreceptors
- each contains numerous sterocilia and a kinocilium that respond to motion
- found lining utricle, saccule, and semicircular canals
kinocilium
- found in hair cells
* detects motion and conveys signals to the brain via a branch of the vestibular nerve
cochlear duct surrounded by
scala vestibuli and scala tympani, which contain perilymph
perilymph
•similar in iconic composition to CSF and extracellular fluid of other tissues, but contains little protein
What does perilymph do?
suspends and supports closed membranous labyrinth -> protect from bony labyrinth
What is cochlear duct filled with?
endolymph
endolymph
also contains few proteins, but has high K+ (150 mM) and low Na+ (16 mM), similar to that of intracellular fluid
endolymph is generated by
capillaries in the stria vascularis in the wall of the cochlear duct
endolymph is drained
from the vestibule into venous sinuses of the dura mater
perilymph emerges
emerges from microvasculature of periosteum
perilymph is drained
by a perilymphatic ducts into adjoining subarachnoid space
vestibular membrane
runs along the length of the cochlea and separates it from scala vestibuli
organ of corti
- spiral organ found in the wall that separates the cochlear duct from the scala tympani
- rest on thick basal lamina, the basilar membrane
organ of corti contains
special auditory receptors in the form of hair cells that respond to different sound frequencies
tectorial membrane
an acellular layer extending over organ of corti that has tips of tallest hair cells embedded in it
base of each hair cell is innervated by
- a branch of the cochlear division of CN VIII
* the nerve endings transmit impulses to the bipolar neurons of the spiral ganglion
how does hearing work?
sound wave collected in auricle -> vibrate TM -> moves ossicles ->fluid waves in perilymph begin at oval window and move along scala vestibuli -> causes momentary movement of vestibular and/or basilar membranes and endolymph surrounding organ of corti -> fluid displacement causes hair cells embedded in tectorial membrane to bend -> neurotransmitters released from basal portion of hair cells -> neural impulses
high-frequency sounds produce maximal movement where
nearest to the oval window
sounds of progressively lower frequency produce fluid waves that
move further along the scala vestibuli and displace the spiral organ at a point further away . from the oval window
sound of lowest frequency that can be detected produce
movement of the basilar membrane at the apex or helocotrema of the cochlea
