Physiology of visual analyzer

The eyes mediate sight Function Sensory organ for sight Detects light and converts it into neural responses that the brain interprets

EYE External 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.

Parts of the Eye 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.

Prevents light rays from scattering and distorting the image. 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 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 Pupil Iris

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 Focusing point sources of light. (a) When diverging light rays enter a dense medium at an angle to its convex surface, refraction bends them inward. (b) Refraction of light by the lens system. For simplicity, refraction is shown only at the corneal surface (site of greatest refraction) although it also occurs in the lens and elsewhere. Incoming light from a (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 1．眼球前后径太短，形成轴性远视； 2．眼球前后径太长，形成轴性近视； 3．晶体向前移位，形成近视； 4．晶体向后移位，形成远视。 1．角膜前面或晶体面曲度太平(曲率半径太长)，引起弯曲性远视(或称曲率性远视)； 2．角膜前面或晶体面曲度太峻(曲率半径太短)，致成弯曲性近视(或称曲率性近视)； 3．角膜或晶体面各径向曲度不等，形成散光。 三)眼屈光系各组成因子不同轴 1．晶体倾斜 晶体半脱位或人工晶体位置倾斜，形成散光。 2．视网膜倾斜 眼屈光媒质折射率异常

Accomodation Viewing Distant Objects Viewing Nearby 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 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

Accommodation Accommodation the capability that eyes change refractive condition in order to acquire clear near sight

ACCOMMODATION

Accommodation Helmhotz accommodation mechanism Schachar accommodation mechanism Ciliary muscle contract Lens zonule relax accommodation Elastic deformation of lens See near Ciliary muscle contract Ante-&post-lens zonule relax accommodation Peripheral lens flatten See near equatorial lens zonule tension Central lens project

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

Hyperopia Myopia and correction Hyperopia and correction

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).