A&P Chapter 15 Special Senses

adjacent pigmented epithelium (RPE)

- absorbs excess photons - phagocytizes old discs shed from tip of outer segment

pigmented part of the retina

- absorbs light that passes though the neural part - prevents light from bouncing back and producing visual "echoes" - cells have important biochemical interactions with photoreceptors in neural part

cornea

- allows light to enter the eye (transparency and clarity) - consists primarily of a dense matrix containing multiple layers of collagen fibers, organized to not interfere with the passage of light - avascular - oxygen and nutrients obtained from tears

ciliary zonule (suspensory ligaments)

- attach to the tips of the ciliary processes - hold lens posterior to the iris and centered on the pupil - any light passing through the pupil will also pass through the lens

optic disc

- axons of ganglion cells converge at optic disc to form optic nerve - aka blind spot since lacking photoreceptors

visual pathways

- begin at photorecptors in retina - photoreceptor passes info through bipolar cell to ganglion cell - axons from about 1 million ganglion cells converge at optic disc - proceed toward diencephalon as the optic nerve (II) - two optic nerves (one from each eye) reach diencephalon at the optic chiasm - from optic chaism, along optic tracts about half of fibers go to lgn on same side of brain and about half go to lgn on opposite side - optic radiation: collection of projection fibers carrying information from each lgn to occipital cortex of cerebral hemisphere on that side - perception of visual image in visual cortex comes from integration of arriving information

ciliary muscle

- bulk of ciliary body - smooth muscular ring that projects into the interior of the eye

photoreceptors

- cells that are sensitive to light - located in the inner, neural portion of the retina - type and density varies from one portion of the retina to another - detect photons (basic units) of light - light energy also occurs as a wave - visible spectrum is 400-700 nm

lens

- consists of a concentric layer of cells surrounded by a dense fibrous capsule - capsular fibers around edge intermingle with ciliary zonule - primary function to focus the visual image on the photoreceptors by changing shape

outer segment

- contains flattened, membranous plates or discs - contain visual pigment - in cones: discs are plasma membrane infoldings and outer segment tapes to a blunt point - in rods: each disc is separate entity and outer segment forms elongated cylinder

inner segment

- contains major organelles - responsible for all cell functions other than photoreception

vascular layer (uvea)

- contains numerous blood vessels, lymphatic vessels, and the intrinsic (smooth) muscles of the eye - functions: 1) provide a route for blood vessels and lymphatics that supply tissues of the eye 2) regulate the amount of light that enters the eye 3) secreting and reabsorbing the fluid called aqueous humor that circulated within the chambers of the eye 4) controlling the shape of the lens, an essential part of the focusing process

neural part of the retina

- contains photoreceptors, supporting cells, and neurons - perform preliminary processing and integration of visual information - outmost layer contains photoreceptors - innermost layer formed by gangion cells

palpebra (eyelid)

- continuation of the skin - blinking keeps surface of eye lubricated and removes dust and debris - can close firmly to protect the delicate surface of the eye

photoreception: active state

- decrease in sodium entry reduces dark current -active transport of sodium cell continues - membrane potential drops to -70 mV - hyperpolarization of membrane decreases rate of neurotransmitter release - decreasing rate signals bipolar cell that the photoreceptor has absorbed a proton

three types of cones

- each with a different form of opsin, sensitive to different range of wavelengths (ranges overlap) 1) blue cones (16%) 2) green cones (10%) 3) red cones (74%) - all three stimulated perceived as white

conjunctiva

- epithelium covering the inner surfaces of the eyelids (palpebral conjunctiva) and the outer surface of the eye (ocular conjunctiva covers anterior outer surface) - fornix: pocket created where conjuctiva becomes continuous with the ocular conjunctiva - ocular conjunctiva is continuous with the very delicate corneal epithelium that covers the surface of the cornea

Anterior cavity

- extends from cornea to lens - filled with aqueous humor - two chambers: 1. anterior chamber (cornea to iris) 2. posterior chamber (iris to ciliary body and lens)

amacrine cells

- facilitate or inhibit communication between bipolar cells and ganglion cells - alter sensitivity of retina - play important role in adjusting to dim or brightly lit environments

horizontal cells

- facilitate or inhibit communication between photoreceptors and bipolar cells - alter sensitivity of retina - play important role in adjusting to dim or brightly lit environments

fovea in the center of the macula (fovea centralis)

- highest concentration of photoreceptors - point of sharpest vision - when you look directly at an object, its image falls on this portion of the retina - cones concentrated here

image reversal

- image consists of individual points - light from each point focused on the retina - image is inverted and reversed - brain compensates for this reversal

visual axis

- imaginary line from center of an object you are looking at directly through the center of the cornea and the center of the lens to the retina - light would strike the fovea region of the retina

conjunctivitis (pinkeye)

- inflammation of the conjunctiva - most obvious sign is redness due to the dilation of blood vessels deep to the conjunctival epithelium - possible causes: 1. pathogen infection (viral or bacterial) 2. physical, allergic, or chemical irritation of conjuctival surface

near point of vision

- inner limit of clear vision - determined by a degree of elasticity in lens - increases with age as lens becomes less elastic

inner layer (retina)

- innermost layer of the eye - consists of thin outer layer (pigmented part) that absorbs light and thick inner later (neural part) that contains photoreceptors

ora serrata

- jagged anterior edge of the neural part of the retina - outer pigmented part of the retina continues anteriorly across the posterior surface of the iris

Posterior cavity

- main volume of the eye - filled with gelatinous vitreous body - vitreous humor is fluid part of vitreous body - vitreous body helps stabilize the eye and give physical support to the retina

cones

- maximum density and fovea of macula (no rods at this location) - about 6 mil in each eye - direct correlation between visual acuity (sharpness of vision) and cone density - provide us with color vision - provide us with sharper, clearer images - require more intense light

rods

- maximum density at the periphery of the retina (few cones at this location) - about 125 mil in each eye - no discrimination among colors of light - highly sensitive, can see in dimly lit rooms, at twilight, and in pale moonlight

tarsal glands (Meibomian glands)

- modified sebaceous glands - along inner margin of the eyelids - lipid-rich secretions keep eyelids from sticking together

color blindness

- most common type is red-green color blindness - red cones are missing - unable to distinguish red light from green light - sex linked trait involving one or two pigments - 10% of all males show some color blindness - 0.67% of females are color blind - total color blindness extremely rare

eye formation in embryonic development

- optic vesicles: first indication of eye development form in the lateral walls of the prosencephalon on either side of a cavity (neurocoel) - optic cup: epidermis overlying the optic cup responds by forming a pocket that later pinches off and develops into the lens of the eye - ependymal cells of outer optic cup develop into photoreceptos - ependymal cells of inner optic cup develop into pigment cells - nerual tissue of outer layer develops into neurons, ganglion cells and specialized glial cells - embryonic cells around optic cup form connective tissue isolating neural tissues - interior chambers filled with fluid that is continuously generated and reabsorbed

nasolacrimal duct

- originates at the inferior tip of the lacrimal sac - passes through the nasolacrimal canal to deliver tears to the nasal cavity - empties into inferior meatus

fibrous layer

- outermost layer of the eye - consists of the cornea and the white sclera (continuous) - functions: 1) supports and protects 2) attachment site for the extrinsic eye muscles 3) contains the transparent cornea

lacrimal gland

- produces watery, slightly alkaline tears - tears lubricate, nourish, and oxygenate the corneal cells - secretions contain the antibacterial enzyme lysozyme and antibodies that attack pathogens

lacrimal apparatus

- produces, distributes, and removes tears - tears keep conjunctival surfaces moist and clean, reduce friction, prevent bacterial infection, provide nutrients and oxygen to portions of conjunctival epithelium

pupil constricts

- pupillary constrictor muscles form a series of concentric circles around the pupil and decrease the diameter of the pupil when they contract - increased light intensity - increased parasympathetic stimulation

pupil dialates

- pupillary dilator muscles extend radially away from the edge of the pupil, contraction enlarges pupil - decreased light intensity - increased sympathetic stimulation

photoreception: exposure to light

- retinal molecule in rhodopsin changes shape (from bent 11-cis form to more linear 11-trans form) in process called activation (opsin is activated) - opsin activates transducin, a G protein bound to the disc membrane - transducin activates enzyme phosphodiesterase (PDE) - phosphodiesterase breaks down cGMP - removing cGMP inactivates sodium channels -rate of sodium entry into cytosol decreases

color vision

- rods all contain same type of opsin - respond to blue-green wavelength of light (perceived as white) - cones contain different types of opsin, sensitive to different wavelengths of light so they have color vision

bipolar cells

- rods and cones synapse on bipolar cells - bipolar cells synapse on ganglion cells

aqueous humor (incl. functions)

- secreted by epithelial cells of ciliary processes at a rate of 1-2 µL/min - composition similar to CSF - circulates within anterior cavity, passes through pupil between chambers - also diffuses through vitreous body and across retinal surface - functions: transports nutrients and wastes, forms fluid cushion, helps retain eye shape, stabilizes position of the retina - constantly produced and recycled

crystallins

- slender, long, transparent proteins in lens - responsible for clarity and focusing power of the lens

lacrimal caruncle

- small, reddish body at medial angle of the eye - produces thick secretions that cause gritty deposits (eye boogers) sometimes appearing after a good night's sleep

depth perception

- the ability to judge depth or distance by interpreting the three dimensional relationships among objects in view - brain perceives depth perception by comparing the relative positions of objects within the images received by both eyes

ciliary body

- thickened region that bulges into the interior of the eye - suspensory ligaments extend from ciliary body to the lens, holding it in position posterior to the pupil

visual pigments

- transduce light - derivatives of rhodopsin (pigment in rods) - opsin: protein that determines wavelength absorption of pigment - retinal: pigment synthesized from vitamin A

focusing process

- two step process in the eye 1) light is refracted when it passes from air into the cornea (greatest portion of refracted) -bending occurs because of the change in density -amount of refraction at the cornea is not variable 2) light is refracted again as it passes from aqueous humor into the lens (accommodation) - bends light rays toward focal point (specific point of intersection on the retina)

choroid

- vascular layer that is covered by the sclera - extensive capillary network that delivers oxygen and nutrients to the neural tissue within the neural layer

iris

- visible through cornea - contains blood vessels, pigment cells, and layers of smooth muscle fibers - when these muscles contract they change the diameter of the pupil - body contains highly vascular, pigmented, loose connective tissue - anterior surface has no epithelial covering, instead has incomplete layer of fibroblasts and melanocytes - posterior surface is covered by a pigmented epithelium that is part of the neural tunic

scleral venous sinus (canal of Schlemm)

- where aqueous humor leaves the eye - passageway extending around eye at the level of the corneal limbus - aqueous humor flows into veins in the sclera - rate of removal should keep pace with the rate of secretion - tonometry: procedure to measure intraocular pressure (fluid pressure within the anterior chamber) - normal pressure ranges 12-21 mm Hg

photoreception: resting state

-in the dark - chemically gated sodium channels in plasma membrane in outer segment kept open - cGMP present and active - inner segment continuously pumps sodium ions out of cytosol - movement of ions called dark current - keeps resting membrane potential about -40 mV -photoreceptor continuously releases neurotransmitters across synapse to bipolar cells

infections of tarsal glands

1. styes- small, acute, painful 2. chalazions- large, chronic, painless

for close vision

Ciliary muscle contracts = ciliary body moving toward lens = reduced tension in ciliary zonule = elastic capsule of lens pulls lens into more spherical shape = increased refractive power = able to focus on nearby objects

for distant vision

Ciliary muscle relaxes = increased tension on ciliary zonule = pull at circumference of lens = lens becomes flatter = decreased refractive power = able to focus on distant objects

________ glands are modified sebaceous glands located along the margin of the eyelid that secrete a lipid-rich product.

Tarsal

For distant vision, which event is true?

The ciliary muscle relaxes.

The first step in the process of photoreception is

absorption of a photon by a visual pigment.

The ciliary muscle contracts to

adjust the shape of the lens for close vision. (more round)

Which of the following descriptions best matches the term aqueous humor? a. gelatinous fluid that fills posterior chamber b. constantly produced and recycled c. secreted in bright light d. converts to vitreous humor with age e. All of the answers are correct.

b. constantly produced and recycled

corneal limbus

border between the cornea and sclera

Which is a true process when light strikes a visual pigment? a. On absorbing light, retinal changes to a more circular shape. b. Opsin activation changes the permeability of the outer segment to potassium. c. Bleaching occurs when the rhodopsin molecule breaks down into retinal and opsin. d. The location of the stimulated horizontal cells indicates the specific portion of the retina stimulated by the arriving photons.

c. Bleaching occurs when the rhodopsin molecule breaks down into retinal and opsin.

Aqueous humor forms through active secretion by epithelial cells of which of the following? a. canal of Schlemm b. choroid c. ciliary body d. ora serrata

c. ciliary body

optic nerve (II)

carries visual information to the brain

accommodation

changes in lens shape as a result of ciliary muscle action

Pupillary muscle groups are controlled by the ANS. Parasympathetic activation causes pupillary ________, and sympathetic activation causes ________.

constriction; dilation

A ray of light entering the eye will encounter these structures in which order?

cornea - aqueous humor - lens - vitreous body - retina - choroid

In the human eye, most refraction occurs when light passes through the

cornea.

tear ducts

deliver tears from the lacrimal gland to the space behind the upper eyelid

eye color

determined by: 1) genes that influence the density and distribution of melanocytes on the anterior surface and interior of the iris 2) the density of the pigmented epithelium - Few melanocytes = light reaches pigmented layer and bounces off = blue eyes - More melanocytes = green, brown, or black eyes - Albinism (no pigment) = pale gray or blue-gray eyes 3. changing eye color: melanin turnover

focal distance

distance between the center of the lens and its focal point

inferior rectus

eye looks inferiorly

lateral rectus

eye looks laterally

medial rectus

eye looks medially (cross eyes)

superior rectus

eye looks superiorly

superior oblique

eye rolls, looks inferiorly and laterally

inferior oblique

eye rolls, looks superiorly and laterally

ciliary processes

folds of epithelium covering the ciliary muscle

The axons of which cells converge at the optic disc to form the blind spot?

ganglion cells

palpebral fissure

gap that separates the free margin of the upper and lower eyelids

eyelashes

hairs that help prevent foreign particles from reaching the eye

The vitreous body

helps to stabilize the eye and give physical support to the retina.

lateral canthus

lateral connection of the eyelids

medial canthus

medial connection of the eyelids

Pam's daughter is born cross-eyed. As she grows, the problem does not improve. Her physician suggests cutting an eye muscle to bring the eyes into a more normal position. Which muscle will the doctor cut?

medial rectus

Which structure is continuous with the very delicate corneal epithelium that covers the surface of the cornea?

ocular conjunctiva

There are three different types of cones, each one sensitive to different light energies. These cones are designated (which colors)

red, green, and blue.

In the retina, the correct sequence of neural wiring is

rod - bipolar cell - ganglion cell - thalamus

lacrimal canaliculi

small canals that connect the lacrimal puncta to the lacrimal sac

lacrimal sac

small chamber that nestles within the lacrimal sulcus of the orbit

dry macular degeneration

spots that start to die around macula, yellow???

Which eye muscle sends its tendon of origin through a pulley or trochlea before inserting on the globe?

superior oblique

photoreception: rhodopsin cannot respond to another photon until original shape of retinal regained

three step process: 1. bleaching -entire rhodopsin molecule first broken into retinal and opsin (pigment taken out) 2. retinal converted back to cis shape - requires energy (ATP) 4. reassembly - opsin and retinal are reassembled as rhodopsin - rhodopsin molecule is now ready to repeat the cycle

true or false: each photoreceptor synapses with a bipolar cell

true

lacrimal puncta

two small pores that drain the lacrimal lake

When all three cone populations are stimulated equally, we perceive

white.