The Nervous System - The Eye

Light energy is converted to nerve impulses carried along the optic nerve to the brain

rods and cones lie in a layer outermost in the retina against the pigment epithelium and choroid layer

a layer of bipolar cells and ganglion cells lie innermost in the retina

light has to travel through thickness of the retina before it can strike and activate the rods and cones

name         structure         function

Sclera tough, white outer layer protects structures within eye and maintains shape

Choroid layer rich in blood vessels; made up of melanin absorbs light and prevents reflection inside eye

retina contains rods and cones receives light stimulus

fovea part of retina where cones are concentrated provides max visual acuity (resolution)

conjunctiva thin layer at front of eye protects surface of eye, kept moist by a film of fluid secreted by tear ducts

cornea continuous with sclera refracts light rays as they enter the eye

Iris tissue containing pigmented cells control light passing through the eye

Pupil hole in centre of iris size altered by muscles of iris to control light

Ciliary body ciliary muscles control shape of lens

lens stacks of long, narrow, transparent cells focuses rays of light onto retina

suspensory ligaments run between lens and ciliary body holds lens in place

vitreous humour gelatinous fluid behind lens maintains shape of eye

Aqeuous humour watery fluid in front of lens maintains shape of eye

Optic Nerve bundle nerve fibres carries action potentials to the brain

Rod cells elongated with outer segment specialised for receiving light.

Cones shorter, broader

part of rod or cone nearest to outside of eye is outer segment and the one closest to the inside is inner segment.

inner segment forms synapses with other cells in the retina. the inner contains the nucleus and mitochondria. proteins are formed here before passing through the connecting region to outer segment.

connecting region contains micro tubules

rod cells have visual purple pigment called rhodopsin.

three types of cone pigments sensitive to wavelength of light. (iodopsin)

there is an area where there are no light sensitive rods or cones where all the neurones collect together and the optic nerve carries info from eye to the brain known as the blind spot.

Bi polar cells have a central body from which two sets of processes arise.

process nearest rods and cones are short and branch into many endings forming synapses with a number or rods or a single cone.

other is longer and forms synapses with ganglion cells.

ganglion cells have numerous dendrites that form synapses with bipolar cells and where action potentials are first generated in the retina

amacrine cells have horizontally spreading processes rather then axons. they connect axons of bipolar cells and dendrites of ganglion cells.

Rod cells are able to work in low light intensity because it can respond t a single photon of light

rods do not detect colour which is why its difficult to tell the colour of an object at night or in the dark