Visual System

Anatomy and Physiology II

BIO 232

Surface Anatomy of the Eye

Accessory Structures of the Eye

• Eyebrows - short, thick hair - superior orbital ridges

• Shade eyes, prevent falling particles or sweat from entering eyes

• Eyelids - palpebrae

• Moved via obicularis oculi, levator palpebrae superioris - upper eyelid

• Obicularis oculi - closes eyelids

• Levator palpebrae superioris - elevates upper eyelid to expose eye

• Protect eyeballs from desiccation - blink every 7 seconds

Accessory Structures of the Eye

• Commissures - angles where the eyelids come together

• Lacrimal caruncle - fleshy elevation in the medial commissure

• Sebaceous & sudoriferous (sweat) glands

• Eyelashes - airborne particle protection

Accessory Structures of the Eye

• Tarsal plates - dense connective tissue below the skin - maintain shape of the lids

• Tarsal glands - sebaceous glands in tarsal plates - so lids don’t stick

Conjunctiva

• Mucous -secreting membrane lines the interior surface of the eyelids & exposed surface of the eyeball

• Stratified squamous epithelium

Lacrimal Apparatus

• Lacrimal gland - size and shape of an almond - lacrimal fluid (tears)

• Superolateral portion of the orbit

• Contains lysozyme - antibacterial enzyme

• Series of ducts - drain secretion into the nasal cavity

Extrinsic Eye Muscles

• Eye movements controlled by six extrinsic eye muscles

• Extrinsic ocular muscles

• Four recti muscles

• Superior, inferior, lateral and medial - move in direction of name

• Two oblique muscles

• Superior & inferior - rotate eyeball on its axis

Structure of the Eyeball

• Three layers or tunics

• Outer - fibrous tunic - sclera & cornea

• Middle - vascular tunic or uvea

• Choroid, ciliary body and iris

• Inner - retina - imaging layer

Optical System Of The Eye

• Compound lens system

• Casts a reversed, much reduced image of the visual world on the retinal surface

Iris of the Eye

• Controls the amount of light entering

• Constriction of pupils - contraction of the circular fibers - parasympathetic

• Dilation - contraction of radial fibers - sympathetic

Accommodation

• Change in focus

• Far accommodation - ciliary muscle relaxed

• Near accommodation - ciliary muscle contracted

Retina

• Extension of CNS

• From neuroectoderm

• Layered structure - five types of cells

• Photoreceptors

• Bipolar Cells

• Horizontal Cells

• Amacrine Cells

• Ganglion Cells

Photoreceptors

• Rods

• Blue-green sensitive

• Nocturnal vision

• Great sensitivity

• Cones

• Color vision at all higher light intensities

Photoreceptors

• Divided into inner and outer segments

• Inner segment

• Nucleus, mitochondria & etc.

• Outer segment

• Elaborate system of stacked membranous disks

Photoexcitation of Rods

• Rhodopsin

• Photopigment

• Capable of capturing photons of light

• Arranged in a single layer on the surface of the membranous disks

• Retinal

• Light absorbing

• Opsin

• Protein

Three Dimensional Configuration of Retinal

• All trans isomer

• Straight carbon backbone

• 11-cis isomer

• Carbon chain bent at the 11th carbon

• Dark configuration of retinal

Absorption of Light by Rhodopsin

• Photoisomerization

• 11-cis to all-trans isomer

• Absorption of one photon

• 100’s of Na⁺ channels close

• This amplification via cGMP

• cGMP keeps Na⁺ channels open in the dark

Absorption of Light by Rhodopsin

• Splits rhodopsin

• Activates phosphodiesterase via transducin

• This causes hydrolysis of cGMP

• Now Na⁺ channels can close

• See hyperpolarization

Output of the Retina - Ganglion Cells

Visual Pathways

• Projection of Ganglion Cells

• Lateral Geniculate - Thalamus

• Center surround receptive fields

• Visual Cortex - striate cortex

Simple Cells of Striate Cortex

• More complex RFs

• Stimuli need linear properties

• Line, bar or edge

• Effective stimulus

• Correct retinal position

• Correct linear properties

• Specific axis of orientation

Response of Simple Cortical Neurons

How to Make a Simple Cortical Neuron

Color Vision

• Three types of cones

• Red, Green and Blue

• Involves not simply the action of the three color receptors, the cones, but interaction between them

Color Pigments