The Nervous System, The Eye

The Optics of Vision

1. Physical Principles of Optics
1. Slower velocity than air. Refractive Index of a transparent substance is the ratio of its velocity in air to the velocity in the transparent object.
2. Perpendicular - The direction light travels is always perpendicular to the wave front, When light passes through an angulated surface, it is bent (refracted) at some angle if the refractive indices of the two media are different. The angle depends on the refractive index of the barrier material and the angle between the two surfaces. 
1. Refractive Principles and their Application to Lenses
1.   
2. A Convex Lens - Converge
1. Light that passes the lens perimeter is refracted - bent toward the center - to make perpendicular to the wavefront. 
2.  
3. A Concave Lens - divergence
1. Light through the lens perimeter are refracted away from the midline - bent away - to make themselves perpendicular to the wave front. 
2.  
4. Focal Length of the Lens
1. Focal Length of a lens is the distance beyound a convex lens at which parallel light rays converge to a single length
2. Beyound the focal point the object - the object appears upside down, and reversed from right to left
5. The unit of measure for refractive power is the diopter. 
1. The more a lens bend light, the greater its refractive power. 
2. One Meter - A spherical lens that converges parallel light rays to a point one meter beyond the lens has a refractive power of +- one diopter; if light is bent twice as much it has a +2 diopter. 
2. The Optics of the Eye
1. Accommodation depends on a change in the shape of the lens and allow the eye to focus on a near object
1. Process of accommodation involves:
1. making the lens more convex
2. narrowing the pupil diameter, and 
3. adduction of both eyes
1. The lens is in its resting state with no tension, convex spherical. It is suspended by the inelastic zonule fibers. 
2. inactive
3. activated - When the cilliary muscle is activated - it releases tension on the zonule fibers (ligaments) and the lens becomes more convex due to the lenses own inherent elasticity. 
2. Presbyopia is the loss of accommodation by the lens
1. The ageing lens looses its elasticity and is less responsive and 
2. unable to focus on near objects.
3. Corrected with reading glasses - that magnify objects that are close.
3. The diameter of the pupil, (iris) is also a factor in accommodation 
1. The narrowing of the pupil increases the focal plain - improves the sharpness of the image.
1. Refractive Errors
1. Correction With Various Types of Eye Glasses
1. Emmertropia 
1. The normal eye, all distant objects are in sharp focus, the cilliary muscle is completely relaxed.
2. Hyperopia
1. far-sightedness, 
2. the eyeball is too short from top to bottom - causing light to be focussed behind the retina
3. Corrected with convex lenses
3. Myopia
1. Near-sightedness
2. Eye is elongated from front to back causing light to be focussed infront of the retina.
3. Corrected with concacve lenses
4. Astigmatism
1. Light rays focus on different points.
2. substantial differences in the curvature of the different planes through the eye. The curvature of the vertical plain may be less than the curvature over the horizontal plain 
3. Corrected with a cylindrical lens
5. Cataracts
1. Caused by opacity from a portion of the lense
2. Remove the lens and replace with a lens implant
6. Keratoconus
1. oddly shaped cornea with a prominent bulge on one side causing sever refractory problems that cant be corrected with a single lens.
2. Require a contact lens - helps with the bulge and may correct the vision.
2. Visual Acuity
1. Sharpest Within the Foveal Region of the Retina
1. fovea - is made up entirely out of cones
2. outside the fovea - the presence of rod photoreceptors intermixed with cones and to the linkage of some rod and cone receptors to the same ganglion cell.
3. Test Chart for visual accuity
3. Depth Perception
1. Distinguishing the Distance of an Object from the Eye
1. Knowing the size of the object - the brain calculates distance
2. Moving parallax: Close objects move faster than distant objects across the retina
3. Binocular Vision: binocular parallax - provides the ability to judge distances
2. Ophthalmoscope: to view the Retina
3. Two Types of Intraocular Fluid of the Eye
1. Vitreos Humour - lies between the lens and the retina and is more of a gelatinous body rather than a liquid. 
1. Substances can diffuse through the vitreos Humour
2. There is little movement or flow in this liquid
2. Aqueous Humour - is a watery fluid secreted by the epithelial lining of the ciliary processes on the ciliary body 
1. at a rate of 2 to 3 microliters  per minute.
2. the fluid migrates between the ligaments supporting the lens and through the pupil into the anterior chamber if the eye (between the lens and the cornea)
3. The fluid flows into the angle between the cornea and the iris, and
4. Then through the trabecular meshwork to enter the channel of Schlemm
5. The Channel of Sclemm empties directly into the extraocular veins. 
6.  
7. Intraocular pressure is normally 15mmHg (12 to 20 mmHg) - measured with a tonometer
8. Glaucoma is a condition where the intraocular pressure reaches 60 to 70 mmHg. 
1. At 20 to 30 mmHg the retinal axons are pressed and this interferes with their conduction - axonal flow in interrupted, causing permanent injury to the parent neuron
2. Compression of the central retinal artery also causes neuronal death
3. Glaucoma is treated with eyedrops that decrease the fluid secretion and increases the absorption of aqueous humour.
4. Surgery - to open the trabecular spaces or to drain the trabecular mashwork directly into the subconjectival spaces outside the eyeball. 
5.  

Receptor and Neural Function of the Retina

1. Anatomy and Function of the Structural Elements of the Retina
1. There are 10 layers or boundaries of the retina
1. Pigment layer
2. Layer of Rods and Cones
3. Outer Limit Membrane
4. Outer Nuclear Layer
5. Outer Plexiform Layer
6. Inner Nuclear Layer
7. Inner Plexiform Layer
8. Ganglionic Layer
9. Layer of Optic Nerve fibers
10. Inner Limiting Membrane - Light strikes here
11.    
2. When light passes through the lens it first encounters the Inner limiting membrane, the optic nerve fibers, and the ganglionic cell layer and then it continues through the remaining layers, to finally reach the receptors, the rods and the cones.
3. The fovea is a specialised region of the retina - region of maximal visual acuity - containing only cones.
4. Each photoreceptor - rods and cones depending on the shape of their outer segment - consists of:
1. an outer segment
2. an inner segment 
3. a nuclear region, and
4. the synaptic body or terminal.
5. rhodopsin -
1. Light sensitive photopigment
2. Rod outer segment 
6. photopsin -
1. colour-sensitive pigment
2. Cone outer Segment
7. The Rhodopsin and the Photopsin  are proteins incorporated into a stacked array of membranous discs - in the receptor outer segment.
8. The inner segment of the Rods and Cones - like other neuronal cells
9.  The synaptic body - found in the axon terminals of other neuronal cells - mitochondria and synaptic verticals.
10. The melanin pigment in the pigment layer reduces light reflection throughout the globe of the eye.
1. Albino individuals have an absence of the melanin pigment.
2. A large amount of reflection in albino eyes, albinos rarely exhibit better than 20/100 visual acuity.
3. The pigment stores large amount of Vitamin A used in the synthesis of visual pigments. 
1.      
11. The Central Retinal Artery: 
1. Blood supply to the inner most layers of the retina - ganglion axon cells to the inner nuclear layer)
2. The outermost cells receive blood from a highly vascularized  choroid, between the sclera and the retina.
12. Traumatic Retinal Detachment: (Neural Retina and the Pigment Epithelium) Because of the dual supply - a person with traumatic retinal detachment may suffer for days without functional degeneration if the retina is surgically return to its normal apposition.

1. The Photochemistry of Vision
1. Rhodopsin-Retinal Cycle and Excitation of the Rod Photoreceptors
1. Rhodopsin is decomposed by light energy
1. Retinal + Scotopsin = Rhodopsin
2. Light separates Rhodopsin into Retinal and Scotopsin Plus Electrical Impulse Transmission
2. Rhodopsin Reformation occurs
1. Retinal and or Retinol (Vit A) combine with scotopsin to form Rhodopsin
2. Vitamin A Deficiency:
1. Night Blindness occurs because the rods are maximally used in dim light conditions - and the formation of Rhodopsin is decreased due to Vitamin A deficiency
2. This condition can be reversed after an hour with a Vitamin A injection
3. MELANOPSIN - A NEW PHOTOSENSATIVE PIGMENT 
1. AXONS FROM OPSIN LIKE CELLS - MELOPSIN PROJECT TO THE SUPRACHIASMIC NUCLEUS (SCN) AND ARE CRITICAL IN THE CIRCADIAN RHYTHM

1. Rhodopsin Activation: Hyperpolarization and Rod Membrane Potential
1. 
   
2. Photochemistry, Colour Vision, Cone Photoreceptors
3. Light or Dark Adaptation: Retinal Sensitivity
1. Colour Vision
1. Colour Detection: a Three-Colour Mechanism
2. Neuronal Functions of the Retina
1. Neuronal Circuits: Six Types of Cells
2. Lateral Inhibition, Contrast and Horizontal Cells
3. Some Bipolar Cells Are Excited by Light
4. Amacrine Cells
5. Three General Classes of Ganglion Cells
6. Ganglion Cells and Continuous Activity
7. Contrast Levels and Lateral Inhibition
8. Ganglion Cell Activity: Colour Signals

Central Neurophysiology of Vision

1. The main visual pathway from the Retina to the Dorsal Lateral Geniculate Nucleus; to the Primary Visual Cortex
1. Axons from the retinal ganglion cells form the optic nerve
2. Axons from the nasal retina cross at the optic chiasm
3. Axons from the temporal retina pass through the lateral aspect of the chiasm without crossing. 
4. Axons continue posterior to the chiasm in the optic tract, 
5. majority terminate in the dorsal geniculate nucleus
6. Proceed further in the geniculocalcarine (optic) radiations, and 
7. terminate in the primary visual cortex
1. Suprchiasmic nucleus - control circadian rhythms
2. Pretectal nucleus - pupillary light reflexes
3. Superior Coliculus - control of REM
4. Ventral Lateral Geniculate Nucleus
5. 
1. Functions of the Dorsal Lateral Geniculate Nucleus - DLGN
1. 6 concentrically arranged layers
2. 
3. 
   
2. Organisation and Functions of the Visual Cortex
1. The Primary Visual Cortex has a layered Structure
1. Like all areas of the neocortex, 6 horizontally arranged layers
2. Brodmann Area 17
1.  
2. The Secondary visual cortex surrounds the primary visual cortex - Brodmann Area 18
3.  
2. There is also a vertical, columnar organisation within the V-I
3. Interspersed among these columns are the so-called colour blobs
4. Visual Signals  from two eyes remain segregated through projections from the SLGN to V-1.
5. Processing in the Primary Visual Cortex: Signals and Two Pathways In Visual Association Cortex
3. Neuronal Activity in the Occipital Cortex During Analysis of Visual Image
1. Neuronal Detection of Colour
2. Testing the Visual Fields: Perimetry
1. Effects of Lesions in the optic pathway on fields of vision
3. Eye movements and Their Control
1. Fixation involves moving the eyes to bring a discrete portion of the visual field into focus on the fovea
2. Saccadic movement of the eyes is a mechanism of successive fixation points
3. Persuit movements occur when the eyes fixate a moving target
4. Autonomic Control of accomodation and pupillary Aperture