Photoreception - Vision

Eyelids (palpebrae) separated by the palpebral fissue Eyelashes Tarsal glands Lacrimal apparatus Vision Accessory structures of the eye

The eye can only perceive a small portion of the spectrum of electromagnetic waves

Vision In order to see an object: - 1- the pattern of the object must fall on the vision receptors (rods and cones in the retina)  accommodation - 2- the amount of light entering the eye must be regulated (too much light will “bleach out” the signals) - 3- the energy from the waves of photons must be transduced into electrical signals - 4- The brain must receive and interpret the signals

External Features and Accessory Structures of the Eye

External Structures of the Eye Conjunctiva covers most of eye Cornea is transparent anterior portion

Lacrimal Apparatus Secretions from the lacrimal gland contain lysozyme Tears form in the lacrimal glands, wash across the eye and collect in the lacrimal lake Pass through the lacrimal punctae, lacrimal canaliculi, lacrimal sac and nasolacrimal duct

The eye Three layers – Outer fibrous tunic Sclera, cornea, limbus – Middle vascular tunic Iris, ciliary body, choroid – Inner nervous tunic Retina

The Sectional Anatomy of the Eye

Internal Structures of the Eye Ciliary body – Ciliary muscles and ciliary processes, which attach to suspensory ligaments of lens Retina – Outer pigmented portion – Inner neural part Rods and cones

Sectional Anatomy of the Eye

Regulation of the Amount of Light Entering the Eye The iris controls the amount of light entering the eye cavities The contraction of radial or circular smooth muscles located within the iris permit changes in the pupil diameter

The Pupillary Muscles

Retina Retina contains rods and cones – Cones densely packed at fovea (center of the macula lutea) Retinal pathway – Photoreceptors to bipolar cells to ganglion cells, to the brain via the optic nerve Axons of ganglion cells converge at blind spot (optic disc) – Horizontal cells and amacrine cells modify the signal passed along the retinal neurons

The Organization of the Retina

Retinal structure Three cell layers: -- outer layer: photoreceptors- rods and cones -- middle layer: bipolar neurons -- inner layer: ganglion cells

The Organization of the Retina

Ciliary body and lens divide the anterior cavity of the eye into posterior (vitreous) cavity and anterior cavity Anterior cavity further divided – anterior chamber in front of eye – posterior chamber between the iris and the lens Eye Anatomy

The Circulation of Aqueous Humor

Aqueous humor circulates within the eye – diffuses through the walls of anterior chamber – passes through canal of Schlemm – re-enters circulation Vitreous humor fills the posterior cavity. – Not recycled – permanent fluid Fluids in the eye

Eye Abnormalities Glaucoma Cataract

Posterior to the cornea and forms anterior boundary of posterior cavity – Posterior cavity contains vitreous humor Lens helps focus – Light is refracted as it passes through lens – Accommodation is the process by which the lens adjusts to focus images – Normal visual acuity is 20/20 Lens

Image Formation

Accommodation It is the process of adjusting the shape of the lens so that the external image fall exactly on the retina

Accommodation

Accommodation Abnormalities

Myopia Hyperopia Astigmatism: the cornea is irregular  irregular pattern of vision Presbyopia: stiffening of the lens occurring with aging  increased difficulty with near vision

Rods – respond to almost any photon Cones – specific ranges of specificity Visual Physiology

Rods and Cones

Outer segment with membranous discs Narrow stalk connecting outer segment to inner segment Light absorption occurs in the visual pigments – Derivatives of rhodopsin Photoreceptor Structure

Phototransduction - General Photons hit the pigment of a photoreceptor    enzymes are activated in the cell which modify its state of polarization  the signals are sent to visual area of the occipital lobe of the brain through the optic nerve

Photoreception - In More Detail

Photoreception

Bleaching and Regeneration of Visual Pigments

Integration of information from red, blue and green cones Colorblindness is the inability to detect certain colors Color sensitivity

Dark adapted – most visual pigments are fully receptive to stimulation Light adapted – pupil constricts and pigments bleached. Retinal Adaptation

Large M-cells monitor rods Smaller more numerous P cells monitor cones The Visual Pathway

Convergence and Ganglion Cell Function

Vision from the field of view transfers from one side to the other while in transit Depth perception is obtained by comparing relative positions of objects from the two eyes Seeing in Stereo

Neural processing The bipolar neurons and ganglion cells process the signal In the fovea where the acuity is the highest: 1 cone  1 bipolar cell  1 ganglion cell At the periphery: many rods  1 bipolar cell … acuity is much decreased Other cells in the retina participate in signal processing

The Visual Pathways

Input to suprachiasmic nucleus affects the function of the brainstem Circadian rhythm ties to day-night cycle, and affects metabolic rates Visual Circadian Rhythm