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

Detection of light 400 to 750 nm in wavelength The shortest blue The longest red 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 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

Refractive power of the lens dioptres 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? 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