Eye and vision Flashcards
Vision - what it does
- Detection of light photons on a 2D sensory surface
- Transformation of 2D images to 3D
- Information segregation (depth, colour, form, and motion)
- Create a unified perception
- Pattern recognition
- Localisation
The five P’s
Remember the 5Ps: – Pain – Photophobia – Poor vision – Pus – Pupillary abnormality
Tear production
- Lacrimal gland produces and secretes tears
- Tears flow down and across eyeball
- Tears enter lacrimal canaliculi and drain into lacrimal sac
- Tears empty into nasal cavity via nasolacrimal duct
Parts of the skull
Frontal Maxilla Sphenoid Ethmoid Lacrimal Zygomatic
Anatomy of the eye
• Two cavities – Anterior segment (aqueous humor) • medium for diffusion of gases & nutrients • maintains intra-ocular pressure • recycled & regenerated
– Posterior segment (vitreous humor)
• maintains intraocular pressure
• provides structural support
• formed in embryo, lasts for life
• Three tunics (layers)
– Fibrous
– Vascular
– Neural
Clinical pearl: Glaucoma
• Aqueous humor
– Produced by ciliary body
– circulates around anterior structures (lens & cornea) – drains into venous blood via scleral venous sinus
• Glaucoma
– Failure to drain aqueous humor
– ↑↑ intra-ocular pressure, compression of retina and optic nerve
– Blindness over time (degradation of retina)
– Symptoms include seeing halos around lights, blurred vision
Layers of the eye I: Fibrous Tunic
• Sclera – White of the eye – Tough fibrous layer – Structural support – Anchor point for extrinsic muscles
• Cornea – Anterior, transparent portion of sclera – No blood vessels – Highly innervated – Epithelium heals from the limbus
Layers of the eye II: Vascular Tunic
Middle zone – Choroid • Rich in blood vessels • O2 & nutrients to retina • Melanocytes – stops light scattering
– Iris
– Ciliary body
• Coordination of lens shape
Iris and the pupillary reflex
• Controls amount of light into the eye (pupil)
• Two rings of muscle
– Pupillary constrictors (longitudinally arranged) – Pupillary dilators (radially arranged)
Controlled by ANS
Low light: SNS - pupil dilates
High light: PNS - pupilconstricts
Ciliary body
- Lens attached to ciliary body muscles via suspensory ligaments
- Contraction and relaxation of ciliary body alters lens shape
- Under ANS control
Layers of the eye III: Neural Tunic
Retina – Neural layer: • Rods and cones - light detection • Horizontal cells - signal processing • Bipolar cells - signal processing • Amacrine cells - signal processing • Ganglion cells - signal processing
– Pigmented layer • Stray light absorption • Phagocytosis of dead photoreceptors (endocytosis) • Nutrient delivery from choroid
Optic nerve (axons of ganglion cells)
The fovea
Fovea:
• Contains just cones (light passed directly to photoreceptors)
• Point of highest visual acuity
Macula:
• Area immediately surrounding fovea
• Cones, bipolar and ganglion cells
Blind spot
Optic disc = blind spot
• Point on the retina where optic nerve and major blood vessels exit.
•No rods or cones, no visual processing
Macular degeneration
– Progressive, painless loss of foveal vision.
– Build up of waste products, blood vessels grow into retina or pigment epithelium cells die.
– No cure.
Retinal detachment
– Trauma or sudden jerk of head.
– Retinal tearing or separation of pigmented and neural layers (detachment).
– Surgical reattachment if diagnosed early
Extraocular muscles
LR6, SO4 and all the rest CN3
• 2 recti – Medial – Lateral – Superior – Inferior
• 2 obliques
– Superior
– Inferior
Refraction and projection
- Refraction – bending of light rays passing through different mediums (e.g. from viewed object, through the eye).
- Cornea and lens – primary refractive structures that bend incoming light rays.
- Cornea – most contribution to total refractive ability.
- Lens – adjustable contribution of refractive strength.
Lens function in near and far vision - distant
Distant light source
- parallel rays of distant light source
- sympathetic stimulation
- relaxed ciliary muscle
- flattened, weak lens
- taut, suspensory ligaments
Lens function in near and far vision - near
Near light source
- parasympathetic stimulation
- contracted ciliary muscle
- rounded, strong lens
- slackened suspensory ligaments
Accommodation…
Accommodation: increasing the refractive power of the lens to focus a near-source image on the fovea
Refractive errors of vision - Presbyopia:
Age-related reduction in accommodation ability of lens
Refractive errors of vision - Myopia
(near sightedness) and Hyperopia (far sightedness) alter how and when accommodation is used.
Myopic (nearsighted) eye
Eyeball too long or lens too strong
Hyperopic (farsighted) eye
Eyeball too short or lens too weak
Neural Retina
Neural portion of retina consists of 3 main layers of excitable cells:
– Photoreceptor layer (outermost) contains rods and cones.
– Middle layer contains bipolar cells.
– Inner layer contains ganglion cells.
- Rodsandcones absorb light.
- Bipolar cells connect photoreceptor cells with ganglion cells.
- Axons of ganglion cells join to form optic nerve.
Photoreceptors
• Sensitivity of photo receptor type linked to its functional role (night vision vs acuity).
• Acuity dictated by extent of convergence onto ganglion cells.
– Many vs few receptors per ganglion cell
Photoreceptor structure & function
Rods and cones consist of three parts:
– Outer segment:
• Light-absorbing photopigments
– Inner segment:
• Metabolic structures
– Synaptic terminals: • Release of neurotransmitter onto bipolar cells • Complex pattern of NT release, depends on dark or light exposure of outer segment
Light detected by photopigments
– Two parts: Opsin (transmembrane protein) & retinal (derived from vitamin A)
– Retinal changes confirmation when light is absorbed
– 4 types of Opsinà7 million colours!!
Head trauma & visual dysfunction
Two streams of information processed:
– What (ventral) for colour and forms
– Where/how (dorsal) for motion & disparity
Damage to regions of occipital lobe, parietal and temporal lobes, or along any section of the neural pathway can have wide-ranging effects on visual processing and perception!
