Vision. Three main parts Layer of receptors Lens and cornea system that focuses light on these receptors Nerves that conduct impulses from the receptor.

Vision

Three main parts

Layer of receptors

Lens and cornea system that focuses light on these receptors

Nerves that conduct impulses from the receptor to the brain

400 to 750 nm in wavelength

The shortest blue

The longest red

Detection of light

photoreceptors - rods and cones

Rods dim light

Cones in brighter conditions - distinguish red, green or blue light

The principal structures of the eye

Pupillary light reflex

Excess of light

Reflex for miosis – receptor - retina

afferent fibres from visual pathway

Central part – mesencephalon, area praetectalis – Edinger-Westphal nucleus (parasympathetic)

Efferent- n. oculomotorius through ncl. Ciliare

Effector – sphincter muscle of iris

Low intensity light

Reflex for mydriasis – receptor - retina

afferent fibres from visual pathway

Central part – mesencephalon, area praetectalis, RF, spinal cord C8

Efferent – sympathetic fibres from ventral horn to paravertebral cervical gangl., along a. carotis, a. ophtalmica

Effector – dilator muscle of iris

Aqueous humor is produced in the ciliary body and reabsorbed into the canal of Schlemm

Glaucoma

Open-angle

Angle-clousure

Retina

Organized in 10 layers, visual receptors rod and cones

4 tapes of neurons – bipolar, ganglion, horizontal and amacrine cells

Optic disc – place where the optic nerve leaves the eye. No rods or cones – blind spot

Macula lutea at the posterior pole of the eye – marks location of fovea centralis – rod-free, cones in high density, the greatest visual acuity

Blind spot – PC experiment Neurolab

Neural pathway

1.N – rods and cones

2.N – bipolar cells

3. N - ganglion cells

4.N – lateral geniculate body

Visual defects following cutting of pathways in various places.

Dark areas represent blindness in that visual field

The image-forming mechanism

The greater the curvature of a lens, the greater its refractive power. The refractive power of a lens is measured in dioptres

The number of dioptres being the reciprocal of the principal focal distance in meters.

Lens with a principal focal distance of 25 cm has a refractive power of ??? Dioptres

What is the refractive power of a human eye?

Refractive power of the lens

dioptres

Lens with a principal focal distance of 25 cm has a refractive power of 4 Dioptres 1/0.25 = 100/25 = 4

What is the refractive power of a human eye?

1/0.015 = 1000/15 = 66.7 dioptres in rest

Accommodation is the process by which the curvature of the lens is increased

Presbyopia

Near point of vision recede throughout the life from 9 to 80 cm

Loss of accomodation dne to increasing hardness of the lens

Correction by wearing glasses with convex lenses

Myopia (nearsightedness) – the anteroposterior diameter of the eyeball is too long, changes in refraction during development

Can be corrected by glasses with biconcave lenses which make parallel light rays diverge slightly

Hyperopia (farsightedness) the anteroposterior diameter of the eyeball is too short, parallel rays of light are brouht to a focus behind the retina

The defect can be corrected by using glasses with convex lenses, which aid the refractive power of the eye.

The photoreceptor mechanism

Sequence of events involved in phototransduction

Rhodopsin – the photosensitive pigment

Opsin (protein) and retinen (11-cis-retinen)

All trans retinen

Activation of transducin (G protein)

Activation of phosphodiesterase PDE

Decreased intacellular c GMP

Closure of Na+ channels

Hyperpolarization

Decreased release of synaptic transmitter

Production of a signal in the next neural unit in retina

Young-Helmholtz theory of color vision

Three types of photopigment

Optical illusions – PC program