Transcript Visionx
Photoreception - Vision Basic Structure • 70% of all sensory receptors in the body • Optic tracts contain over a million nerve fibers • Eye is sphere that is approximately 1inch in diameter • Anterior 1/6th of surface is visible • The rest is enclosed and protected by a fat pad Vision Accessory structures of the eye • Eyelids (palpebrae)- separated by the palpebral fissue and meet at the medial and lateral canthi. Medial has a fleshy elevation called the caruncle which contains sebaceous and sweat glands. Connective tissue sheets called tarsal plates where the orbicularis oculi and levator palpebrae superioris muscles run within eyelid. Orbicularis muscle encircles the eye- when it contracts eye closes, levator palpebrae muscle causes the eyelid to raise. Eyelid muscles are reflexive and blink every 3-7 seconds. • Eyelashes- follicles of the eyelash hairs are richly innervated by nerve endings, when they touch anything blinking reflex is triggered. Accessory structures of the eye cont. • Eyebrows-shade the eye from sunlight and perspiration that may trickle into eyes- orbicularis muscles of the eye cause the eyebrows to move. • Tarsal/meibomian glands- embedded in tarsal plates where their ducts open at the eyelid edge just posterior to the eyelashes. Produce oily secretion that lubricates the eyelid and eye: preventing them from sticking together. • Conjunctiva- delicate mucus membrane that lines the eyelid, blood vessels are clearly visible through this structure. Covers all of the white areas of the eye. Prevents foreign objects from penetration beyond the conjuctival sac (where contacts and medications enter the eye). Vitamin A helps to maintain this structure. • Lacrimal apparatus- consists of lacrimal gland and ducts that drain secretions and drain into the nasal cavity. Releases tears into superior part of the conjuctival sac through small excretory ducts. Blinking spreads tears. Lacrimal secretions contain mucus, antibodies, and lysozymes. External Features and Accessory Structures of the Eye • Facts about extrinsic eye muscles • -Eye muscles are among the most precisely and rapidly controlled skeletal muscles in the entire body. • - Have a high axon to muscle ratio • Movements: • -Saccades are small jerky movements allowing one to see a whole field of vision in a short time. • -Slow scanning movements allow us to follow an object or focus on one object as our head moves. • Six strap like muscles that originate from the bony orbit and insert into the outer surface of the eyeball which control movement and maintain shape. • Four of the extrinsic muscles (superior, inferior, lateral, and medial) originate from a common tendinous ring call the annular ring at the back of the orbit and run straight to their insertion on the eyeball. • Superior oblique- originates in common with rectus muscle and runs along the medial wall of the orbit • Trochlea-fibrocartilaginous loop that inserts at the superolateral aspect of the eyeball. • Inferior oblique muscle- originates from the medial orbit surace and runs laterally and obliquely to insert on the inferolateral eye surface. • Three Tunics – Outer fibrous tunic The eye • Sclera (white of eye), cornea (lets light enter eye) – Middle vascular tunic • Iris (anterior portion of uvea, contains pupil-controls amount of light), ciliary body ( muscles that control lens shape, rich in blood vessels), choroid (blood vessels feed the eye tunics, contains melanocytes to absorb light) suspensory ligaments (holds lens in position) – Inner Sensory nervous tunic • Retina (pigmented epithelial cells, absorbs light and prevent it from scattering in eye, act as phagocytes and store vitamin A. Neural layer (contains millions of photoreceptors and transduce light into energy, neuronsphotoreceptors, bipolar, and ganglion) The Sectional Anatomy of the Eye 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 Organization of the Retina 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 Retinal structure • Three cell layers: -- outer layer: photoreceptorsrods and cones -- middle layer: bipolar neurons -- inner layer: ganglion cells The Organization of the Retina Eye Anatomy • 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 Abnormalities • Glaucoma • Cataract The Circulation of Aqueous Humor Fluids in the eye • 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 Lens • 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 Visible light • 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 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 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 Visual Physiology • Rods – respond to almost any photon • Cones – specific ranges of specificity Rods and Cones Photoreceptor Structure • Outer segment with membranous discs • Narrow stalk connecting outer segment to inner segment • Light absorption occurs in the visual pigments – Derivatives of rhodopsin 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 Color sensitivity • Integration of information from red, blue and green cones • Colorblindness is the inability to detect certain colors Retinal Adaptation • Dark adapted – most visual pigments are fully receptive to stimulation • Light adapted – pupil constricts and pigments bleached. The Visual Pathway • Large M-cells monitor rods • Smaller more numerous P cells monitor cones Convergence and Ganglion Cell Function Seeing in Stereo • 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 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 Visual Circadian Rhythm • Input to suprachiasmic nucleus affects the function of the brainstem • Circadian rhythm ties to day-night cycle, and affects metabolic rates