Physiology of visual analyzer

The eyes mediate sightThe eyes mediate sight

FunctionFunction Sensory organ for sight Sensory organ for sight Detects light and converts it into neural Detects light and converts it into neural

responses that the brain interpretsresponses that the brain interprets

EYEExternal Eye Structures

The eyes are complex sense organs that have evolved from primitive light-sensitive spots on the surface of invertebrates.

Within its protective casing, each eye has a layer of receptors, a lens system that focuses light on these receptors, and a system of nerves that conducts impulses from the receptors to the brain.

Lacrimal Apparatus

Visual Pathways & Fields Objects reflect light Rays refracted by cornea, aqueous humor,

lens, vitreous body and onto retina. Light stimulus is changed to nerve

impulses, travel thru optic nerve to visual cortex in occipital lobe

Image on retina is upside down & reversed. At the optic chiasm retinal fibers cross over. Right side of brain looks at left side of world.

Sclera: a tough white layer of connective tissue that covers all of the eyeball except the cornea. Conjunctiva: external cover of the

sclera — keeps the eye moist. Cornea: transparent covering of the

front of the eye. Allows for the passage of light into

the eye and functions as a fixed lens.

Parts of the Eye

Choroid: thin, pigmented layer lining the interior surface of the sclera. Prevents light rays from scattering

and distorting the image. Anteriorly it forms the iris.

The iris regulates the size of the pupil. Retina: lines the interior surface of

the choroid. Contains photoreceptors.

Except at the optic disk (where the optic nerve attaches).

Focusing Light The lens and ciliary body divide the eye

into two cavities.

The anterior cavity is filled with aqueous humor produced by the ciliary body.

The posterior cavity is filled with vitreous humor.

The lens, the aqueous humor, and the vitreous humor all play a role in focusing light onto the retina.

** Internal Anatomy:

Outer layer: sclera it’s a protective white covering, cornea bending light rays to focused on inner retina- cornea sensitive to touch(corneal reflex)-trigeminal nerve(CN V) carries afferent sensation into brain, facial nerve(CN VII) carries efferent messages that stimulates the blink.

The Pupil is an Aperture Pupil

Opening in the center of the eyeball

Bounded by the Iris The iris controls

the size of the pupil

Opening through which light enters the eye

PupiPupill

IrisIris

Iris and Pupil

Iris = flat, round, regular, even color bilaterally.

Pupils = PERRLA Resting size norm = 3-5mm 5% population have pupils of 2 diff.

Sizes called Anisocoria

** Visual Reflexes:

# papillary light reflex: is a normal constriction of pupil when light shines on retina( a direct reflex & a consensual reflex).

Mechanism: light →retina → optic Nerve(II)__ efferent or sensory → midbrain → CN III (oculomotor) _ afferent(motor) → constriction of iris muscles for both eyes.

Direct reflex → same eye _Consensual reflex → opposite eye.

Lenses of the Eye

Cornea Crystalline Lens Primary function = focus the image

on the back of the retina

Focus

Refraction Bending of the path of a light wave

as it passes across the boundary separating two media Cause:

Change in the speed of the light wave No change in speed = no refraction!

Material A (fast) Material B (slow)

Focusing point sources of light. (a) When diverging light rays enter a dense medium at an angle to its convexsurface, refraction bends them inward.

(b) Refraction of light by the lens system. For simplicity, refraction is shown onlyat the corneal surface (site of greatest refraction) although it also occurs in the lens and elsewhere. Incoming light froma (above) and b (below) is bent in opposite directions, resulting in b' being above a' on the retina.

Emmetropia Emmetropia the refractive condition in focus on

fovea that parallel lights enter eye through refractive system without accommodation

Punctum remotum of emmetropia: infinite

Accomodation

Viewing Nearby Objects Ciliary muscles contract

Squeeze the lens into a more convex (fat) shape

Pushes cornea bulge out further = greater curvature

C-L system has a short focal length

High refractive power

Viewing Distant Objects Ciliary muscles relaxed

Lens assumes a flatter (skinnier) shape

Cornea is not pushed out = less curvature

C-L system has a long focal length

Low refractive Power

Accommodation

Accommodation

the capability that eyes change refractive

condition in order to acquire clear

near sight

ACCOMMODATION

AccommodationHelmhotz accommodation mechanism

Schachar accommodation mechanism

Ciliary muscle contractCiliary muscle contract Lens zonule relaxLens zonule relax

accommodationaccommodationElastic deformation of lens Elastic deformation of lens

See nearSee near

Ciliary muscle contractCiliary muscle contractAnte-&post-lens zonule relaxAnte-&post-lens zonule relax

accommodationaccommodationPeripheral lens Peripheral lens flattenflatten

See nearSee nearequatorial equatorial lens zonule tension lens zonule tension

Central lens Central lens projectproject

Accommodation Accommodation = diopter for far diopter for

near

Range of accommodation distance of far point — distance of near point

Myopia Myopia: the refractive condition that parallel lights enter

eye through refractive system and focus before fovea on static refraction

Punctum remotum of myopia: a point before eye

Hyperopia Hyperopia the refractive condition that parallel lights enter

eye through refractive system and focus after fovea on static refraction

Punctum remotum of myopia: a point after eye

Myopia and correction Hyperopia and correction

Hyperopia

RETINA The retina extends anteriorly

almost to the ciliary body. It is organized in 10 layers and contains the rods and cones,

which are the visual receptors, plus four types of neurons: bipolar cells, ganglion cells, horizontal cells, and

amacrine cells Neural components of the

extrafoveal portion of the retina.

C, cone; R, rod; MB, RB, and FB, midget,

rod, and flat bipolar cells; DG and MG, diffuse and

midget ganglion cells; H, horizontal cells; A, amacrine cells.

Vision Rhodopsin (retinal + opsin) is the

visual pigment of rods. The absorption of light by rhodopsin

initiates a signal-transduction pathway Receptor potential is hyperpolization .

Fig. 49.13

Vision Rods and

Cones synapse with nuerons called bipolar cells

Bipolar cells synapse with galgion cells of optic nerve

Near the posterior pole of the eye is a yellowish pigmented spot, the macula lutea. This marks the location of the

fovea centralis, a thinned-out, rod-free portion of the retina that is present in humans and other primates. In it, the

cones are densely packed, and each synapses to a single bipolar cell, which, in turn, synapses on a single ganglion

cell, providing a direct pathway to the brain. There are very few overlying cells and no blood vessels. Consequently,

the fovea is the point where visual acuity is greatest

RECEPTORS

Visual Acuity Visual acuity is the degree to which

the details and contours of objects are perceived, and it is usually defined in

terms of the shortest distance by which two lines can be separated and still be perceived as two lines. Clinically,

visual acuity is often determined by the use of the familiar Snellen letter charts viewed at a distance of 20 ft (6m).