Vision and visual pathway

Electromagnetic radiation in this range of wavelengths is called visible light or simply light.

The optical spectrum is the portion of the electromagnetic spectrum that is visible to the animal eye.

The visible spectrum is approximately between 400nm to 750nm

8% of men are colour blind, it's a Y-linked disease

Both photoreceptors are found in the inner nuclear layer of the retina at the back of each eye

Bipolar and ganglion cells send information from photoreceptors to the brain are in front of the cones and rods

The human eye have 6 million cones and 120 million rods

Cones and rods are types neurons

Albinos don't have colour vision

Albinos have reduced pigment epithelium in sclera and choroid

Albinos have colour vision

Albinos have increased visual acuity, due to intra-ocular reflection of light

Retinal ganglion cells that express the photopigment melanopsin.

In the absence of rods and cones, mammals retain the ability to detect light for various non-image-forming visual functions.

Phototransduction is conducted only by cones and rods

Cones have rhodopsin and rods have fotopsin

Photosensitive ganglion cells seem to serve as photoreceptors for the photic synchronization of circadian rhythms.

Cones innervate the midbrain pupillary control center.

They project to the circadian pacemaker of the brain - the suprachiasmatic hypothalamic nucleus

Photosensitive ganglion cells contribute to the pupillary light reflex and other behavioral and physiological responses to environmental

Rods/cones - Glutamate

Amacrine cells - Inhibitory neurotransmitters (GABA, Glycine, Dopamine, Ach, Indolamine)

Horizontal cells - Excitatory neurotransmitters.

Horizontal cells - Inhibitory neurotransmitters.

Ganglion cells - the only retinal cell of the pathway transmitting signals by mean of action potential.

Amacrine cells - Occasionally action potential is recorded in.

Remaining cells of the retina - conduct their visual signals by electrotonic conduction (graded conduction).

Ganglion cells - conduct their visual signals by electrotonic conduction (graded conduction).

Rods are used to determine color, depth, and intensity.

Each particular photosensitive cell responds to monochromatic spectral stimuli

All 3 types of photoreceptors transduce light into a change in membrane potential through different signal transduction pathway.

All 3 types of photoreceptors differ in the nature of the opsin they contain and therefore in their function.

Cones are divided into red, blue and green

Visual imaging occurs in the visual cortex, occipital lobe

Fovea has the lowest visual acuity

Rods are trichromatic

Dark current - the voltage-gated sodium channels in the outer segment are opened due to cGMP bound to them.

Dark current - depolarising sodium channels to - 40mV, in dark conditions

Rods are also de-polarising in light conditions

Cones de-polarise in dark conditions

The opsin in the outer segment absorbs a photon, changing the configuration of a molecule inside the cell from the less- energetic cis-form to the more-energetic trans-form.

Night blindness can be caused by deficiency of vitamin E

The light acts as a ligand

Light is absorbed by rhodopsin or by one of the photopsins of cones, changing shape of the retina.

Each photo-activated rhodopsin triggers activation of about 100 transducins

PDE inhibits the hydrolysis of cGMP

Each transducin then activates the enzyme cGMP-specific phosphodiesterase (PDE)

cGMP was keeping the Na(Ca channels closed

A photoreceptor actually releases less neurotransmitter when stimulated by light.

Hyper-polarization means that less glutamate is released to the bipolar cell than before.

GTP provided by the inner segment powers the sodium- potassium pump.

The opening of Na+ channels hyperpolarizes the cell

Neurons of layer 4 of the visual cortex (Brodman’s area 17).

Bipolar cells, Photosensitive cells and Ganglion cells.

Horizontal and amacrine cells

Cells of the lateral geniculate body of the thalamus.

Old system - Suprachasmatic nucleus of hypothalamus – SCN (circadian rhythms)

Old System - Lateral geniculate nucleus of thalamus (LGN)

Old system - Visual cortex (4th neuronal of Brodman’s field 17)

Old system - Pretectal nuclei of the midbrain (pupillary reflexes) & Superior colliculi (rapid eye movement of both eyes)

Parvocellular portion of LGN - (shape, color, texture, fine detail).

Interlaminar path - conduct impulses from ipsilateral eye.

Magnocellular portion of LGN - (spatial organisation, movement, location)

Layers 1,4,6 - colour

Right retina - Left hemisphere, calcarine fissure

Left retina - Right hemisphere, cuneus gyrus fissure

Temporal vision crosses at the optic chiasm

Nasal vision crosses at the optic chiasm

V2, V3, VP – Motion, control of movement.

V3A , MT/V5 – Continued processing – larger visual field.

V1 – Primary visual cortex – receives input from the LGN – processing in terms on orientation edges, etc.

LO – Recognition of large objects.

V8 – Color vision.

Light, passing from one transparent medium to another, changes its speed, and bends.

Angle of refraction - The angle between the light ray and the normal, as it leaves a medium.

Each medium has its own refractive index

Angle of incidence - The angle between the light ray and the normal as it enters a medium

Strength of the light ray

The angle between the light ray and the line perpendicular to the surface separating both media

The refractive index of the media

Absence of other sources of light

Parallel light rays are bent to pass through a single point behind the lens (focal point)

Rays striking perpendicular lens surface pass the lens by bending

Convex lenses diverge light rays

Rays striking perpendicular lens surface pass the lens without being bent

In the focal length distant sources of light rays are parallell

In the focal length distant sources of light rays are converged

In the focal length the close source is parallell

Focal length - distance beyond midpoint of a convex lens, and the focal point

Reversed - Upside-down, left side-right

Inversed - Downside-up, left side-right

Each point source of light on the object comes to a separate point focus on the opposite side of the lens in line with the lens centre

Only one source of light on the object comes to a separate point focus on the opposite side of the lens in line with the lens centre

Expressed in diopters

Spherical Convex lenses = -1 power

Spherical Concave lenses = +1 power

Spherical Convex lenses = + 10 power

Interface between posterior surface of the cornea and aqueous humour is a part of the optic system

Interface on both sides of the lens are parts of the optic system

Interface between the retina and the optic nerve is a part of the optic system

Interface between air and the anterior surface of the cornea is a part of the optic system

Air = 1.0

The lens = 1.40

The cornea = 1.33

The aqueous humour = 1.38

The vitreous humour = 1.34

1/3 of all the refractive power of the eye is provided by the anterior surface of the cornea.

Total refractive power of the eye is 59 dioptres

2/3 of all the refractive power of the eye is provided by the anterior surface of the cornea.

Total refractive power of the eye is 39 dioptres

Reduced eye - a simplified design of the ocular optical system, represented as having a single refracting surface and a uniform index of refraction

Reduced eye - By close object the light rays are diverged and refraction power of the eye should be increased in order to focus eye on object and produce the image exactly on the retina.

Accommodation - By close object the light rays are diverged and refraction power of the eye should be increased in order to focus eye on object and produce the image exactly on the retina.

Accommodation - a simplified design of the ocular optical system, represented as having a single refracting surface and a uniform index of refraction

Refraction power of the eye can be increased voluntary by approximately 14 dioptres by changing the shape of the lens from moderately convexed to more convexed.

In normal conditions tension of ligaments of the lens causes its to remain spherically shaped.

Contraction of the ciliary muscle depend on parasympathetic transmission via III CN.

Contraction of smooth muscle fibers in the ciliary muscle tenses the ligaments to the lens capsule and the lens assumes a more spherical shape.

Reduced eye – refraction power 59 D

With age gets farther away

Light rays are divergent

Picks up 6 m and more distant objects.

Parallell light rays, and the point of vision comes closer with age

About 20 cm in front of eyes

Does not require accommodation

The nearest point in front of eye which can be focused on retina

Accomodation

Pupillary constriction

Pupillary dilation

Divergence of the visual axes

Distant object - parallel light rays focuses on retina

Light rays focus behind the retina

Close object - contraction of ciliary muscle provide accommodation – diverge rays are focus on retina

Light rays focus in front of the retina

Presbypia (senile farsightedness) is caused by decreased lens elasticity

Presbypia (senile farsightedness) is caused by decreased power of ciliary muscle

Astigmatism is caused by a cornea that isn't spherical

Astigmatism is corrected with concave lenses

Fixation of eye (fovea centralis)

Refraction index of medium

Astigmatism

Myopia

Optical factors (image-forming mechanisms of the eye)

Depth of vision

Retinal factors

Stimulus factors (illumination, brightness, contrast, time of exposure)

Distance is perceived by - sizes of known objects, moving parallax and stereopsis

Distance is perceived by - sizes of unknown objects and stereopsis

Distance is perceived by - relative distance, fast moving close objects and slow moving far objects

Stereopsis is dependent on mono-ocular vision - image only created on retina

Rhodopsin decompose in darkness

Rhodopsin is combined of scotopsin and retinene-1 (aldehyde of vit. A)

The only retinal neurons that transmit visual signal by means of action potential are the ganglia cells

Activated rhodopsin deactivates transducin

Photon activates an electron

Activated transducin activates rhodopsin

Activated transducin activates cGMP

Phosphodiesterase hydrolyzes cGMP (which normally causes Na-channels stay open),
and Na-channels close

Found 15 degrees temporally to visual axis

It is NOT visible through the ophtalmoscope as the optic disc.

The optic nerve leaves the eye at the point 3 mm medial to and slightly above the posterior pole of the globe

There are some photoreceptors in this area

Optic N. (CN2) - Mediates vision

Oculomotor N. (CN3) - innervates superior oblique muscle

Trochlear N. (CN4) - innervates superior oblique muscle

Trigeminal N. (CN5) - sensory portion of corneal reflex

Abducens N. (CN6) - Innervates orbicularis oculi and lacrimal glands

Light adaptation (photopic vision) - regeneration of photopigment, that in turn increases visual sensitivity

Dark adaptation (scotopic vision) - a reduction in the amount of rhodopsin, which in turn reduces visual sensitivity

Light adaptation time is 5min

Dark adaptation time is 20min

Laterally = 90 degrees

Downward = 50 degrees

Upward = 50 degrees

Medially 65-70 degrees

1, 3, 2, 4, 5

4, 2, 5, 1, 3

2, 4, 5, 1, 3

4, 5, 2, 1, 3

Axons of ganglia cells converge and leave eye as the optic nerve through optic disc.

The temporal optic fibers cross at the optic chiasm

Each optic tract fibres synapse in ventral lateral geniculate nucleus of lateral geniculate body (LGB) of the thalamus.

Geniculocalcarine fibres pass as optic radiation to the primary visual cortex in the calcarine fissure area of the medial occipital lobe (striate cortex)

Geniculate body has 6 layers, 2 main parts parvocellular and magnocellular

Parvocellular -carries signals for colour vision, texture, shape, fine detail

Parvocellular - carries signals for detection of movement, depth and flicker

Major input comes form the retina

Primary cortex; Brodmann’s area 18 (V2),19

Secondary cortex; Brodmann’s area 17 ( also known as V1)

Area V8 concern with colour vision

Dorsal and parietal pathway – motion

Ventral and temporal pathway – shape and recognition of forms and faces

Optic chiasm – right or left homonymus hemianopia (half-blindness in same side half of the visual field

Interruption of optic nerve – blindness in the eye

Occipital lesion – macular sparing (loss of peripheral vision with intact macular vision)

Optic tract - bitemporal heteronymous hemianopia (half-blindness in opposite sides of the visual field)

Afferent pathway: photoreceptors, bipolar cells, ganglion cells, optic nerve, optic tract, nuleus pretectalis and farther to Edinger-Westphal nucleus bilaterally.

Efferent pathway: from Edinger-Westphal nucleus through oculomotor nerve to ciliary ganglion and n.n. ciliares breves to papillary sphincter.

Efferent pathway: photoreceptors, bipolar cells, ganglion cells, optic nerve, optic tract, nuleus pretectalis and farther to Edinger-Westphal nucleus bilaterally.

Afferent pathway: from Edinger-Westphal nucleus through oculomotor nerve to ciliary ganglion and n.n. ciliares breves to papillary sphincter.

Slit lamp - facilitates examination of the eyelid, sclera, conjunctiva, iris, natural crystalline lens, and cornea

Ophtalmoscopy - Used to determine the health of retina and the vitreous humour

Ophtalmoscopy - facilitates examination of the eyelid, sclera, conjunctiva, iris, natural crystalline lens, and cornea

Perimetry - maps and quantifies the visual field

Slit lamp - maps and quantifies the visual field