Anatomy of the Eye & the 12 cranial nerves

ANATOMY OF THE EYE & THE 12 CRANIAL NERVES

I Olfactory

II Optic

III Oculomotor

IV Trochlear

V Trigeminal

VI Abducens

VII Facial

VIII Auditory (Vestiblochlear)IX Glossopharyngeal

X Vagus

XI Accessory

XII Hypoglossal

12 CRANIAL NERVES

There are 12 pairs of cranial nerves. I Olfactory

II Optic

III Oculomotor

IV Trochlear

V Trigeminal

VI AbducensVII FacialVIII Auditory (Vestiblochlear)IX GlossopharyngealX VagusXI Accessory XII Hypoglossal

Cranial Nerve Major FunctionI Olfactory Smell

II Optic Vision

III Occulomotor Eyelid and Eyeball movement

IV Trochlear Innervates superior obliqueTurns eye downward and laterally

V Trigeminal ChewingFace and Mouth touch/pain

VI Abducens Turns eye laterally

VII Facial Controls most facial expressionsSecretion of tears & salivaTaste

VIII Auditory (Vestibulocochlear) HearingEquilibrium sensation

IX Glossopharyngeal TasteSenses carotid blood pressure

X Vagus Senses Aortic blood pressureSlows heart rateStimulates digestive organsTaste

XI Spinal Accessory Controls Trapezius & SCM

XII Hypoglossal Controls the tongue (motor) MovementControls swallowing movement

ANATOMY OF THE EYE

Characteristics Measures about 1”

and is shaped as a sphere

See only anterior 1/6

Accessory Structures

Extrinsic Eye Muscles

Eyelids Conjunctiva Lacrimal apparatus

Figure 8.1

Site whereconjunctivamerges withcornea

Palpebralfissure

Lateralcommissure(canthus)

IrisEyelid

Eyebrow

EyelidEyelashes

PupilLacrimalcaruncleMedialcommissure(canthus)

Sclera(covered byconjunctiva)

ANATOMY OF THE EYE Anterior Aspects of the Eye Eyelids- provide protection Medial/Lateral Commissure (corners of

the Eye) Palpebral fissure- space b/w eyelids Eyelash- extends from the eyelid Tarsal glands- Modified sebacious gland

Produces oily secretion - lubrication Ciliary glands – modified sweat gland

Found between the eyelashes Conjunctive – lines eyelid and covers the

outer surface of the eyeball Secretes mucous

Figure 8.2a

Lacrimalgland

Conjunctiva

Anterioraspect

EyelidEyelashesTarsalglandsEyelid

Excretory ductof lacrimal gland

HOMEOSTATIC IMBALANCE OF THE EYE

Conjunctivitis Pink Eye Characteristics Inflammation of the

conjunctiva

Infectious conjunctivitis

Figure 8.2b

LacrimalglandExcretory ductsof lacrimal gland

Lacrimal canaliculusNasolacrimal ductInferior meatusof nasal cavity

Nostril

Lacrimal sac

(b)

LACRIMAL APPARATUS1. Consists of the lacrimal glands and ducts to drain

the secretions into the nasal cavity 2. Lacrimal Glands

Located above the lateral end of each eye Releases tears- dilute salt solution Tears flush the eye across the canaliculi medially into the Lacrimal Canaliculi

3. Lacrimal Canaliculia. Sends tears into the Lacrimal sacb. Lacrimal sac receives the tears

4. Naso lacrimal Duct a. empties the tears into the Nasal Cavity

LACRIMAL SECRETIONS

5. Lacrimal Secretions (Tears) contain mucuos, Antibodies and Lysozyme (destroy bacteria)

6. Nasal Mucosa Connects with the lacrimal duct system Effects of Nasal Mucosa will reach the eye

6 EXTRINSIC EYE MUSCLES

1. Attach to the outer surface of the eye 2. Produce Gross eye movment

6 EXTRINSIC EYE MUSCLES

Muscle Action Innervation

Lateral Rectus Moves eye laterally

VI (Abducens)

Medial Rectus Moves eye medially

III (Occulomotor)

Superior Rectus Elevates the eyeTurns eye Medially

III (Occulomotor)

Inferior Rectus Depresses the eyeTurns eye medially

III (Occulomotor)

Inferior Oblique Elevates the eyeTurns eye laterally

III (Occulomotor)

Superior Oblique

Depresses eyeTurns eye laterally

IV (Trochlear)

Figure 8.3a

Superioroblique muscle

Superioroblique tendon

Superiorrectus muscle

Conjunctiva

Lateral rectusmuscle

Opticnerve

Inferiorrectusmuscle

Inferiorobliquemuscle

(a)

Figure 8.3b

Trochlea

Superioroblique muscle

Medialrectus muscle

Lateralrectus muscle

Superioroblique tendon

Superiorrectus muscle

(b)

Inferiorrectus muscle

Axis atcenter ofeye

EYEBALL

1. Hollow Sphere 2. Composed of 3 layers a. Fibrous layer - Outside layer b. Vascular layer - Middle layer c. Sensory layer - Inside layer

3. Inner sphere filled with fluid called Humors4. Lens – supported upright within the cavity

a. Divides the eye into 2 chambers1. Aqueous humor – Anterior Chamber2. Vitreous Humor – Posterior chamber

LAYERS FORMING THE EYE WALL1. Fibrous Layer – outermost layer

A. Sclera – protective layer1. Thick glistening white connective tissue2. seen in the anterior as the “white of the eye”

B. Cornea

1. the central anterior portion of the fibrous layer 2. crystal clear (window of the eye) – light enters 3. many nerve endings – pain fibers 4. If touched, blinking and tearing occur 5. Exposed part of the eye

a. subject to injuryb. great ability to repair

6. only body tissue that can be transplanted without rejection

a. no blood supply – no immune system

LAYERS FORMING THE EYE WALL2. Vascular Layer (Choroid)– middle layer of the eyeball

a. Blood rich nutritive tonic that contains dark pigment b. Prevents the scattering of light inside the eye

c. Anteriorly, modified to form two smooth muscle structures

1. Ciliary body – attaches to lens by ligaments called Ciliary Zonule

2. Iris – filled with pigment (circular and radial smooth muscle)

3. Pupil – rounded opening to the ris a. Bright light and Close vision1. Pupils constrict (contraction of circular

muscles) b. Dark and far vision1. Pupils enlarge (radial fibers contract)

LAYERS FORMING THE EYE WALL3. Sensory Layer (Retina) Retina – innermost 2 layered retina

a. Extends anterior to the ciliated body b. Outer Layer – pigmented

1. Prevents the scattering of light inside the eye like the choroid

2. Acts as Phagocytesa. remove dead/ damaged receptor

cellsb. stores Vitamin Ac. Inner Layer – Neural layer - transparent

Sensory Layer (Continued) c. Inner Layer – Neural layer – transparent 1.contains millions of receptor Cells called Photoreceptors(Rods and Cones) 2. Impulse pathway travels from:

(2 neuron chain) photoreceptors bipolar cells ganglion cellsOptic Nerve 3. Leave the retina via the optic nerve and travel to the optic cortex

Figure 8.4a

Ciliary bodyCiliary zonule

CorneaIrisPupil

Lens

ScleraChoroidRetina

Fovea centralis

Optic nerve

(a)

Aqueoushumor (inanterior segment)

Scleral venous sinus(canal of Schlemm)

Optic disc(blind spot)

Central artery andvein of the retina

Vitreous humor(in posterior segment)

Figure 8.4b

Ciliary body

Iris

LensCorneaCiliary zonule

(b)

RetinaChoroidScleraFovea centralisOptic discOptic nerve

Marginof pupil

Aqueous humor(in anteriorsegment)

Vitreous humorin posterior segment

FIGURE 8.4INTERNAL ANATOMY OF THE EYE (SAGITTAL SECTION).

Ciliary bodyCiliary zonuleCorneaIrisPupil

LensScleral venous sinus(canal of Schlemm)Vitreous humor(in posterior segment)(a)

Ciliary bodyIris

LensCorneaCiliary zonule(b)

ScleraChoroidRetina

Fovea centralis

Optic nerve

Central artery andvein of the retinaOptic disc(blind spot)

RetinaChoroidScleraFovea centralisOptic discOptic nerve

Aqueoushumor (inanterior segment)

Vitreous humorin posterior segment

Marginof pupilAqueous humor(in anteriorsegment)

PHOTORECEPTOR CELLS (RODS & CONES)1. Distributed over the entire Retina except where the optic

nerve leaves the eye2. Optic disc – (the point where the optic nerve leaves the eye)

a. Blind spot3. Rods and Cones are not evenly distributed a. Rods – dense at the peripheral edge of the retina

decrease in number as you move to the center of the Retina

1. see shades of gray in dim light2. allows for peripheral vision3. night blindness – results from an

interference with rod function Causes a. Vit A deficiencyb. Leads to a deterioration of the neural retina tissuec. Vit A can help to restore function if taken

PRIOR to degeneration

PHOTORECEPTOR CELLS (CONT’D)

3. (Continued) b. Cones

1. allows us to see details and colors in bright light

2. most dense in the center of the Retina 4. Fovea Centralis a. Lateral to each blind spot

b. tiny pit containing only Cones c. area of greatest visual acuity

(sharpness)

FIGURE 8.5A THE THREE MAJOR TYPES OF NEURONS COMPOSING THE RETINA.

RodCone

(a)

Pathwayof light

Pigmentedlayer of retina

Bipolarcells

Ganglioncells

Neural Layer

2 ganlion chain

A CLOSER LOOK 8.1 VISUAL PIGMENTS—THE ACTUAL PHOTORECEPTORS

Process ofbipolar cell Synaptic

endings

Light

Light

Light

Innerfibers Rod cell

bodyRodcellbody

Conecellbody

Outerfiber

Nuclei

Mitochondria

Inne

r se

gmen

tPi

gmen

ted

laye

r

Discscontainingvisual pigments

Pigment cellnucleus

Outersegment

Melaningranules

Rhodopsin(visual purple)

Light absorptioncauses

Opsin

Retinal(visual yellow)

Releases

Bleaching ofthe pigment

Outer segment – attached to the cell body, light trapping contains visual discs to trap light. a. bleaching – results from stimulation of light, pigment regenerates b. this causes electrical changes in the photoreceptor cells-nerve impulse sent to brain

Rhodopsin (purple pigment in Rods) a. formed from union of Opsin and Retinal (modified Vit A)Kinked shape.

b. Retinal straightens when hit with light (purple color changes to yellow (bleaching)

c. Once colorless, the Retinal is now Vit A again

d. once the Vit A returns to its kinked form, it combines with Opsin to regenerate into Rhodopsin(An ATP-requiring process)

FIGURE 8.5B THE THREE MAJOR TYPES OF NEURONS COMPOSING THE RETINA.

Pigmentedlayer of retina

Neural layerof retina

Centralarteryand veinof retina Optic

disc

Opticnerve(b)

ScleraChoroid

5. Macula (macula lutea) (from Latin macula, "spot" + lutea, “yellow")

a. is an oval-shaped highly pigmented yellow spot near the center of the retina b. It has a diameter of around 1.5 mm c. defined as having two or more layers of ganglion cells d. Fovea Centralis is located near the center

a. contains the largest concentration of cone cells in the eye and is responsible for central, high resolution vision.

e. Because the macula is yellow in color it absorbs excess blue and ultraviolet light that enter the eye 1. acts as a natural sunblock (analogous to sunglasses) f. The yellow color comes from its content of lutein and Zeaxanthin

a. Zeaxanthin is found mostly at the macula

b. Lutein found in the retina. c. There is some evidence that these carotenoids protect the pigmented region from some types of macula degeneration

FIGURE 8.8 THE POSTERIOR WALL (FUNDUS) OF THE RETINA AS SEEN WITH AN OPHTHALMOSCOPE.

Foveacentralis

Macula Bloodvessels

Optic disc Retina

Lateral

Medial

Macular Degeneration

1. There is a loss of peripheral vision 2. it may go unnoticed for some time 3. damage will result in loss of central vision

3 Types of cones Each most sensitive to a particular wavelength of light

1. Blue2. Green3. Green and Red - called the red cones, only

respond to red

Intermediate colorsMultiple impulses yield a blend of colors as interpreted by the visual corex

Blue/Red Purplewhen all 3 cones are stimulated eyes

will result in White colorcolor mix occurs at the Brain Red

Yellow Green

FIGURE 8.6 SENSITIVITIES OF THE 3 CONE TYPES TO THE DIFFERENT WAVELENGTHS OF VISIBLE LIGHT.

560 nm(red cones)530 nm

(green cones)

420 nm(blue cones)

Light

abs

orpt

ion

by c

one

popu

latio

ns

380 450 500 550 600 650 700 750Wavelengths (nanometers)

Lens 1. focuses light to the Retina

2. Biconvex crystal like structure3. Held upright in position by suspensory ligaments called

(Ciliary Zonules) which attach to the Ciliary body.

Lens Divides the eye into 2 chambers: 1. Anterior (Aqueous) segment

a. contains clear fluid called Aqueous Humorb. Reabsorbed into the venous blood through

the Sclera Venous sinus or (Canal of Schlemm)

1. located at the junction of the Sclera/Cornea2. Posterior (Vitreous) segment

1. Contains clear fluid called Vitreous Humor (Body)

2. Prevents collapse of the eyeball3. Maintains intraocular pressure

4. Provides nutients for lens/cornea

LENS Imbalance Disorders

Color blindness1. The lack of all 3 cones2. Most common – lack of red or green receptorsa. 2 colors seen as one, depends on the coneb. gene for color vision on X chromosomec. sex linked – seen more in malesCataracts1. Hard opaque hazy distorted appearance2. results in blindness3. Risk factorsa. Diabetes, sunlight, smoking4. treatment – surgery, lens replacementGlaucoma 1. results from an increase of the pressure in the eye2. due to a build up of Aqueous Humor3. Test: Tonometer (puff of air)- measures the internal pressure of the eye

FIGURE 8.7 PHOTOGRAPH OF A CATARACT.

CATARACT

GLAUCOMA

OPHTHALMOSCOPIC EXAM Ophthalmoscope

Instrument that illuminates the interior of the eye

Able to view the Retina, Optic disc, Blood vessels at the Fundus, Macula, Fovea Centralis

Fundus Exam – used to detect pathology Diabetes – vascular blotches (micro aneurisms) Arteiosclerosis – copper wiring reflex

(Hypertensive Retina) Degeneration of the optic nerve and retina

See pale optic nerve – loss of axons & myelin

Diabetic Retinopathy See microanneurisms (blotches) Hard exudates (yellow) and cotton

wool spots (white)

Macular Degeneration – pigmented spotting on Retina

FIGURE 8.8 THE POSTERIOR WALL (FUNDUS) OF THE RETINA AS SEEN WITH AN OPHTHALMOSCOPE.

Foveacentralis

Macula Bloodvessels

Optic disc Retina

Lateral

Medial

Fundus photographs of the right eye (left image) and left eye (right image). The gaze is into the camera, so in each picture the macula is in the center of the image, and the optic disk is located towards the nose.

DIABETIC RETINOPATHY

HYPERTENSIVE RETINOPAHY

HYPERTENSIVE RETINOPAHY

AV nicking

COPPER WIRE REFLEX

Hypertensive retinopathy with AV nicking and mild vascular tortuosity

AV nicking

MACULAR DEGENERATION

PHYSIOLOGY OF VISIONResting Eye

Distant Objects Set for distant vision Light from over 20 feet away approaches as parallel

rays Lens does not need to change shape for focus

Closer Objects Light tends to scatter, diverge – spread out Lens must bulge more

A. Ciliary body contracts B. Allows lens to become more convex

FIGURE 8.9 RELATIVE CONVEXITY OF THE LENS DURING FOCUSING FOR DISTANT AND CLOSE VISION.

Retina

Focal pointLight from distant source(a)

Light from near source

Focal pointRetina

(b)

PHYSIOLOGY OF VISION

Accomodation The ability for the eye to focus on close objects

Real Image The image that is formed on the Retina as the result

of the light bending activity Reversed (L to R) and upside down (inverted)

Like the microscope

FIGURE 8.10 REAL IMAGE (REVERSED LEFT TO RIGHT, AND UPSIDE DOWN) FORMED ON THE RETINA.

NEAR SIGHTED VS. FAR SIGHTED Emmetropia

The ability of the eye to focus images correctly on the Retina

Harmonious vision

Near Sightedness Called Myopia Occurs when parallel rays from a distant object fail to

reach the Retina Distant objects appear blurry Nearby objects are in focus

Lens accommodates (bulges) to focus images onto the Retina Results from an eyeball that is too long

Lens that is too strong Cornea that is too curved

Correction Requires a concave lens to diverge the light rays before

entering the eye

IF I CAN’T SEE THINGS FAR AWAY, AM I NEARSIGHTED OR FARSIGHTED?

Focalplane

(a) Emmetropic eye

Correction

None required

Concave lens

Convex lens(b) Myopic eye (nearsighted)

(c) Hyperopic eye (farsighted)

NEAR SIGHTED VS. FAR SIGHTED Farsightedness

Called Hyperopia Occurs when the parallel rays focus behind the Retina

in a resting eye where the lens is flat and ciliary muscles relaxed

Results from an eveyball that is too short or a lazy lens See distant objects clearly- ciliary muscles contract Close objects blurr- lens can’t bulge enough to move

rays forward The patient will be subject to eye strain from an

overuse of the ciliary muscles

Correction Requires a convex lens to converge light rays before

they enter the eye Can see far, not close

Astigmatism When the eyeball has unequal curvatures in different

parts of the cornea Results in blurry images

Points of light not focused as points on the retina, seen as blurry lines

Require special cylindrically ground lens

VISUAL FIELDS & VISUAL PATHWAYS TO THE BRAIN

Visual Pathways Axons from the Retina exit the posterior eye via the

Optic Nerve Approach the Optic Chisma (Chism=cross) Forms fibers called the Optic Tracts

Contain medial fibers of the medial retina from the eye of the opposite sider

Contains lateral fibers of the lateral retina from the eye of the same side

Optic tract fibers synapse with neurons in the thalamus

These axons for the Optic Radiation These run to the Occipital lobe – visual cortex

They then synapse with the cortical cells This is where visual interpretation and vision occur

FIGURE 8.11 VISUAL FIELDS OF THE EYES AND VISUAL PATHWAY TO THE BRAIN.

Fixation point

Right eye Left eye

Opticchiasma

OpticnerveOptictract

Opticradiation

Occipital lobe(visual cortex)

Thalamus

VISUAL FIELDS Visual fields

Each side of the brain receives input from both eyes Lateral field from the eye-same side Medial field from the eye-opposite side

Each eye has a different view with an overlap of the visual fields

Humans have Binocular vision- two eyed Provides depth and perception Three dimensional vision

Visual fields Hemianopia – a loss of the same side of the visual field of

both eyes Results from damage to the visual cortex on one side (CVA’s) The person will not be able to see past their own visual field

from either side, depending on the sit of the CVA Accomodation will be needed for rehabilitation

EYE REFLEXES Reflexes

Both the internal and external(extrinsic) eye muscles are needed for eye function

Internal muscles – controlled by the autonomic nervous system Includes the ciliary body – changes lens shape, curvature Radial and circular muscles – control the opening of the Iris

Photopupupillary Reflex – constriction of pupils when suddenly exposed to light Accomodation Reflex – Constriction of pupils when viewing close objects

External Muscles – the rectus and oblique muscles Control eye movement Responsible for convergence (Reflexive movement)

Eyes move medially – controlled by cranial nerves III, IV and VI Provides depth and perception Three dimensional vision

Reading Requires the continuous work of both sets of muscles

Ciliary body helps to bulge the lens Circular constrictor muscles of the Iris produce the accomodation pupillary

reflex Extrinsic Muscles converge the eyes and to follow printed lines (eye strain)