Eye

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Human Anatomy & PhysiologySEVENTH EDITION

Elaine N. MariebKatja Hoehn

PowerPoint® Lecture Slides prepared by Vince Austin, Bluegrass Technical and Community College

C H

A P

T E

R

15The Special Senses

P A R T A

http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/L1Eye/m1eye.swf


Palpebrae (Eyelids)

Figure 15.1b


Lacrimal Apparatus

Figure 15.2


Extrinsic Eye Muscles

Figure 15.3a, b


Structure of the Eyeball

Figure 15.4a


Anterior Segment

Figure 15.8


Cornea Clear window in the anterior part of the eye that allows the entrance of light. It is a major part of the light-bending apparatus of the eye Covered by epithelial sheets on both faces (anterior and posterior)

External layer – stratified squamous ET – for protection; merges with the ocular conjuntiva

provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears.

This layer is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched.

Moist and being nourished by tears Deep epithelial layer – This single layer of cells is located between the

stroma and the aqueous humor. Because the stroma tends to absorb water, the endothelium's primary task is

to pump excess water out of the stroma (by having sodium pumps). Without this pumping action, the stroma would swell with water, become

cloudy, and ultimately opaque


Cornea Bowman’s layer

a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers

Stroma

Located behind the external epithelium

A thick, transparent middle layer, consisting of regularly-arranged collagen fibers

It consists primarily of water (78 percent); layered protein fibers (16 percent)

that give the cornea its strength, elasticity, and form

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg http://www.siumed.edu/~dking2/intro/IN022b.htm


Lens A biconvex, transparent, flexible, avascular structure that:

Allows precise focusing of light onto the retina

Is composed of epithelium and lens fibers

Lens is avascular because blood vessels interfere with transparency.

The lens depends entirely upon the aqueous and vitreous humors for nourishment.

Lens has 2 regions

Lens epithelium – cuboidal cells found at the anterior surface of the lens. These cells differentiate into lens fibers

Lens fibers – cells filled with the transparent protein crystallin. These cells are packed in layers and contain no nuclei.

New lens fibers are added continuously the lens enlarges, become denser, less elastic.

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://www.bu.edu/histology/p/08001loa.htm

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://www.siumed.edu/~dking2/ssb/EE011b.htm


Pupil Dilation and Constriction

Figure 15.5


Sensory Tunic: Retina A delicate two-layered

membrane

Pigmented layer – the outer layer that absorbs light and prevents its scattering

Neural layer, which contains:

Photoreceptors that transduce light energy

Bipolar cells and ganglion cells

Amacrine and horizontal cells


The Retina: Ganglion Cells and the Optic Disc Ganglion cell axons:

Run along the inner surface of the retina

Leave the eye as the optic nerve

The optic disc:

Is the site where the optic nerve leaves the eye

Lacks photoreceptors (the blind spot)


The Retina: Photoreceptors Rods:

Respond to dim light

Are used for peripheral vision

Cones:

Respond to bright light

Have high-acuity color vision

Are found in the macula lutea

Are concentrated in the fovea centralis


Functions of the retinal cell type

Horizontal cells – converge signals from several receptors: “decide” how many receptors each ganglion “see”.

Bipolar cells. Connect the receptor to ganglion cells

Amacrine cells process aspects of light information such as motion, contrast

Ganglion cells encode light information within action potentials to be processed and reconstructed by the visual cortex via the LG

RETINA PRESENTATION FROM WEBSITE

http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/L1Eye/m1eye.swf


Light

Electromagnetic radiation – all energy waves from short gamma rays to long radio waves

Our eyes respond to a small portion of this spectrum called the visible spectrum

Different cones in the retina respond to different wavelengths of the visible spectrum


Light

Figure 15.10

The wavelength is the distance between repeating units of a wave pattern


Refraction and Lenses When light passes from

one transparent medium to another its speed changes and it refracts (bends)

Light passing through a convex lens (as in the eye) is bent so that the rays converge (join) to a focal point

When a convex lens forms an image, the image is upside down and reversed right to left


Focusing Light on the Retina Pathway of light entering the eye: cornea, aqueous

humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors

Light is refracted:

At the cornea

Entering the lens

Leaving the lens

The lens curvature and shape allow for fine focusing of an image


Focusing Light on the Retina

In the normal resting state:

our ciliary muscle is relaxed

the elastic lens tends to become thick

aqueous & vitreous humour push outward on the sclerotic coat

ligaments become extended / tensed

lens pulled into a thin shape


Focusing for Distant Vision Light from a distance needs

little adjustment for proper focusing

Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.) or:

The object distance at which the eye is focused with the

eye lens in a neutral or relaxed state.

Figure 15.13a


Focusing Light on the Retina - short focal length

contraction of ciliary muscle

distance between edges of ciliary body decreases

relaxation of suspensory ligament

lens becomes thicker

focal length shortens

light rays converge earlier; image formed on retina

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://230nsc1.phy-astr.gsu.edu/hbase/vision/accom.html


Focusing for Close Vision

Close vision requires:

Accommodation – changing the lens shape by ciliary muscles to increase refractory power

Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye

Convergence – medial rotation of the eyeballs toward the object being viewed so both eye are focused on the object


Focusing for Close Vision

Figure 15.13b

LENS AND IRIS PRESENTATION FROM WEBSITE


Problems of Refraction Emmetropic eye – normal eye with light focused

properly

Myopic eye (nearsighted) – the focal point of far object is in front of the retina. Myopic people see close objects clearly but distant objects are blurred

Corrected with a concave lens

Hyperopic eye (farsighted) – the focal point is behind the retina. See far objects clear but not close ones

Corrected with a convex lens


Problems of Refraction

Figure 15.14a, b


Photoreceptors

Photoreceptors are modified neurons

They absorb light and generate chemical or electrical signals

2 cell types – rods and cones – that produce visual images

Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina)

Inner segment – facing the interior

2 segments are separated by a narrow constriction - cilium

The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal


Photoreceptors Photoreceptors are modified

neurons

They absorb light and generate chemical or electrical signals

2 cell types – rods and cones – that produce visual images

http://library.thinkquest.org/28030/physio/perceive.htm

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 15.15a, b

Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina)

Inner segment – facing the interior

2 segments are separated by a narrow constriction - cilium

The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal


Photoreception: Functional Anatomy of Photoreceptors

Photoreception – process by which the eye detects light energy

Rods and cones contain visual pigments (photopigments)

Embedded in areas of the plasma membrane that is arranged in a stack of disc-like infoldings

change shape as they absorb light

This foldings increase surface area that is available for trapping light

In rods – the discs are discontinuous while in the cones they are continuous

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://thebrain.mcgill.ca/flash/i/i_02/i_02_m/i_02_m_vis/i_02_m_vis.html


Photoreceptors - Rods functional characteristics Sensitive to dim light and best

suited for night vision

Absorb all wavelengths of visible light

Perceived input is in gray tones only

Sum of visual input from many rods feeds into a single ganglion cell

Results in fuzzy and indistinct images

http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html


Photoreceptors - Cones functional characteristics

Need bright light for activation (have low sensitivity)

Have pigments that furnish a vividly colored view

Each cone synapses with a single ganglion cell

Vision is detailed and has high resolution

http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html


Chemistry of Visual Pigments Retinal is a light-absorbing molecule

Combines with proteins called opsins to form 4 types of visual pigments

Similar to and is synthesized from vitamin A.

Vitamin A is stored in the liver and transported by the blood to the cells of the pigmented layer (local reservoir of vitamin A)

Retinal has two 3D forms/isomers:

11-cis – a bent structure when connected to opsin

all-trans – when struck by light and change the shape of opsin to its active form

Transforming fro 11-cis to all-trans is the only light dependent stage

Isomerization of retinal initiates electrical impulses in the optic nerve

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 15.15a, b

The visual pigment of rods is called rhodopsin – a deep purple pigment

Each molecule consists of two major parts - opsin and 11-cis retinal

Rhodopsin molecules are arranged in a single layer in the membrane of each of the discontinuous discs in the outer segment

Rhodopsin is formed and accumulates in the dark

Excitation of Rods


Excitation of Rods Light phase

When rhodopsin absorb light retinol is changed to its all-trans isomer

Rhodopsin breaks down into all-trans retinal + opsin (this process is called bleaching of the pigment)

Dark phase

Vitamin A oxidized to the 11-cis retinal form and combined with opsin to form rhodopsin.


CH3

C

C

HH

H2C

H2C C

C

CH3

H

CH3

C

H

C

CH3

C

H

C

H

C

H

C

CH3

C O

H

C

H

C

C

HH

H2C

H2C

H3C

C

C

C

CH3CH3

H

C

H

C

C O

C

H

C H

C

C

C HC

CH3 H CH3 H

Oxidation

Rhodopsin

Opsin

All-trans retinal

–2H

+2HReduction

Vitamin A

Regeneration ofthe pigment:Slow conversionof all-trans retinalto its 11-cis formoccurs in the pig-mented epithelium;requires isomeraseenzyme and ATP.

Dark Light

11-cis retinal

All-trans isomer

11-cis isomer

Bleaching of thepigment:Light absorptionby rhodopsintriggers a seriesof steps in rapidsuccession inwhich retinalchanges shape(11-cis to all-trans)and releasesopsin.

Figure 15.16


Excitation of Cones Visual pigments in cones are similar to rods

(retinal + opsins)

Cones are less sensitive – need more light to be activated

There are three types of cones:

Blue – wave length 420nm,

Green – wave length 530nm,

Red – 560nm

The absorption spectra overlap giving the hues - activation of more than one type of cone

Method of excitation is similar to rods but the cones need higher-intensity (brighter) light because they are less sensitive

COLOR PRESENTATION FROM WEBSITE



Phototransduction The outer segments of the photoreceptor has ligand-

regulated sodium gates that bind to cGMP on the intracellular side.

In the dark, cGMP opens the gate and permits the inflow of sodium which reduces the membrane potential from -70mv to -40mv

This depolarized current is called the dark current and it results in in continuous NT (glutamate) release by the photoreceptors in the synapse with the bipolar cells

Light stops the dark current


Phototransduction Light energy splits rhodopsin into all-trans retinal,

releasing activated opsin

The freed opsin activates the G protein transducin

Transducin catalyzes activation of phosphodiesterase (PDE)

PDE hydrolyzes cGMP to GMP and releases it from sodium channels

Without bound cGMP, sodium channels close but potassium channels in the outer segment remain open

The photoreceptor membrane hyperpolarizes, and neurotransmitter cannot be released

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html#2


Phototransduction

Figure 15.18

TRUNSDUCTION PRESENTATION FROM WEBSITE


Phototransduction

Photoreceptors do not generate action potential and neither do the bipolar cells

they generate graded potential

The ganglion cells generate action potential


Signal Transmission in the Retina


Adaptation to bright light (going from dark to light) As long as the light is low intensity, relatively little amount

of rhodopsin is bleached.

In high intensity light, rhodopsin is bleached as fast as it is re-formed

Going from dark/dim light to light - first we see white light because the sensitivity of the retina is “set” to dim light

Both rods and cones are strongly stimulated and large amounts of the pigments are broken, producing a flood of signals that are responsible for the white light

Rods system is turned off and the cones system adapts

By switching from the rod to the cone system – visual acuity is gained


Adaptation to dark

Initially we do not see nothing

Cones stop functioning in low light

Rhodopsin accumulates in the dark and retinal sensitivity is restored

When we move from a lit room to a dark room, we cannot see clearly, because:

It takes about 20-30 minutes for enough rhodopsin to reform for us to see properly


Visual Pathways Axons of retinal ganglion cells form the optic nerve

Medial fibers of the optic nerve decussate at the optic chiasm

Most fibers of the optic tracts continue to the thalamus

Other optic tract fibers end in superior colliculi in the midbrain (initiating visual reflexes)

Optic radiations travel from the thalamus to the visual cortex

Eye

Education

lens fibers lens fibers

epithelial layer

single layer of cells

new lens fibers

outer layer

tough layer

cornea bowmans layer

transparent middle layer