The EYE

Vision Dominant sense in humans Dominant sense in humans Performed by eyes, in orbits of skull Performed by eyes, in orbits of skull Surrounded by accessory structures Surrounded by accessory structures Positioned by six extrinsic muscles Positioned by six extrinsic muscles

Accessory Eye Structures Eyebrows Eyebrows Eyelids (Palpebrae) Eyelids (Palpebrae) Eyelashes Eyelashes Conjunctiva Conjunctiva Lacrimal glands/ducts Lacrimal glands/ducts Extrinsic eye muscles Extrinsic eye muscles

Functions of Accessory Structures: Eyebrows – Eyebrows – –physical protection Eyelashes – Eyelashes – –blinking reflex initiation Eyelids – Eyelids – –lubrication –contain sebaceous glands

CONJUNCTIVA CONJUNCTIVA –transparent mucous membrane –lines inner surface of eyelids –Folds over / covers eyeball surface –MAJOR LUBRICATION FUNCTION!

Conjunctivitis

The Lacrimal Apparatus (gland + ducts)!! The Lacrimal Apparatus (gland + ducts)!! –Location: superior / lateral to eyeball –Continuous lacrimal secretion ! (aka tears) –Blinking: spreads lacrimal secretions over eyeball surface –Lacrimal canals (medial to eye) drain secretion to lacrimal sac… –Lacrimal sac to nasolacrimal duct to nasal cavity!

Extrinsic Eye Muscles Extrinsic Eye Muscles –Move eyeball –Six (6) per eye! »LATERAL RECTUS »MEDIAL RECTUS »SUPERIOR RECTUS »INFERIOR RECTUS »INFERIOR OBLIQUE »SUPERIOR OBLIQUE

Reminder: Reminder: What Cranial Nerves control these muscles? III, IV, VI !!

Strabismus and Diplopia

The Eyeball Sphere of three layers (TUNICS) Sphere of three layers (TUNICS) FIBROUS TUNIC (Outermost) VASCULAR TUNIC VASCULAR TUNIC (Middle layer) (Middle layer) SENSORY TUNIC SENSORY TUNIC (Innermost) (Innermost)

Fibrous Tunic CT CT Two major Regions: Two major Regions: 1. SCLERA : »The “white” of the eye »Durable, provides shape to the eyeball ant. »Point of attachment for extrinsic eye muscles »Continuous with Dura Mater »Surrounds the Optic Nerve (II) post.

2. CORNEA »Most anterior portion of fibrous tunic »Continuous with sclera »Transparent »“window” of the eye

Corneal Transplant

Vascular Tunic blood vessels blood vessels Three Major Regions: Three Major Regions: –Choroid –Ciliary Body –Iris

1. CHOROID –Structure: »Located posteriorly »Dark brown pigmentation (melanin) –Functions: »Provides nutrition to all tunics! »Melanin absorbs light (prevents reflection)

2. CILIARY BODY –Structure: –Thick ring of : »ciliary muscles and suspensory ligaments »Ciliary processes (^ capillaries) –Functions: »Surround/control shape of lens »Secrete fluid found in anterior chamber of eyeball

3. IRIS –Structure: »Most anterior portion of Vascular Tunic »Located between cornea and lens »Flattened appearance »Contain smooth muscle fibers »Surrounds the PUPIL (central opening that allows light to enter the eye!) »Contains MELANIN ^ melanin = brown eyes ^ melanin = brown eyes melanin = green / blue /non brown eyes melanin = green / blue /non brown eyes

–Functions: »Controls diameter of the PUPIL! »Reflexive to: Amount of Light present! Amount of Light present! Distance of object being seen! Distance of object being seen! Emotional responses! Emotional responses!

Pupil Constriction (PC) or Pupil Dilation (PD)??? Reading a book 6 inches from eyes? Reading a book 6 inches from eyes? –PC Looking at a traffic sign while driving? Looking at a traffic sign while driving? –PD Solving a mathematical word problem? Solving a mathematical word problem? –PD Seeing something attractive approaching? Seeing something attractive approaching? –PD Seeing something repulsive or boring approaching? Seeing something repulsive or boring approaching? –PC Having a flashlight shown directly at face? Having a flashlight shown directly at face? –PC Entering a dark room? Entering a dark room? –PD

Sensory Tunic Aka… RETINA Aka… RETINA Structure: Structure: –2 Layers: »Outer Pigmented Layer Absorbs light/ prevents scattering & reflection Absorbs light/ prevents scattering & reflection Stores Vitamin A!! Stores Vitamin A!! »Inner Nervous Layer Contains Photoreceptor Cells! Neurons + Contains Photoreceptor Cells! Neurons + Contains Optic Nerve neuroglia! Contains Optic Nerve neuroglia!

The Eye and Vision sclera choroid retina

Retina cont’d. Function of the Retina Function of the Retina –Complex explanation: »Processes light stimuli and sends impulses to brain via the optic nerve! –Simple explanation: »ALLOWS YOU TO SEE!

Retina continued… So What about these Photoreceptor Cells? So What about these Photoreceptor Cells? –2 TYPES: »RODS  receptors for dim light and peripheral vision! »CONES  receptors for bright light and color!

Rods vs. Cones Absorb all wavelengths of visible light Absorb all wavelengths of visible light ~200 rods relay impulse to single neuron of OPTIC nerve ~200 rods relay impulse to single neuron of OPTIC nerve Widely dispersed in retina; allows for greater field of vision Widely dispersed in retina; allows for greater field of vision Absorb Red, Green, Blue wavelengths 1 cone to 1 OPTIC nerve neuron impulse ratio! Allows for better visual acuity!

Vision in a Nutshell… Rods and Cones receive light stimuli and generate impulse!! Rods and Cones receive light stimuli and generate impulse!! Impulse passed to Bipolar Neurons! Impulse passed to Bipolar Neurons! Impulse passed to Ganglion Neurons! Impulse passed to Ganglion Neurons! Action Potential generated along Action Potential generated along OPTIC NERVE!! OPTIC NERVE!!

Structures Associated with the Retina OPTIC DISC OPTIC DISC –Area where optic nerve exits the eye! –Very weak… no support from sclera! –Lacks photoreceptors! (uh-oh…) –Aka Blind Spot

Central Artery Central Artery Central Vein Central Vein –Transport blood to / from retina –Enter/leave eye through center of Optic Nerve –Form highly branched network of vessels!

The LENS Structural Characteristics: Biconvex!Transparent! Avascular! (Why??) Flexible! Held in place and changes position due to… Suspensory ligaments !!

Function of Lens: Function of Lens: –Adjusts shape to focus light on the retina!! –Divides Eye into Anterior and Posterior Chambers! Anterior and Posterior Chambers!

Lens Problems … As body ages lens becomes… As body ages lens becomes… –Thicker –More convex  –Less flexible THE RESULT OF LENS PROBLEMS…??

Internal Chambers of the Eye Lens is the DIVIDING LANDMARK!! Lens is the DIVIDING LANDMARK!! Anterior Chamber: Anterior Chamber: –Filled with Aqueous Humor »Watery, thin, clear fluid »Constantly produced and drained Posterior Chamber: Posterior Chamber: –Filled with Vitreous Humor »Thick, clear gel »Formed embryonically… lasts for lifetime

lens vitreous humor aqueous humor

CATARACTS

Glaucoma