sclera

choroid

retina

fovea

optic nerve

arteries and veins

cornea

iris

pupil

lens

aqueous hum our

ciliary m uscle

suspensory ligaments

ciliary body

vitreous hum our

eye muscle

The Retina

The Retina

• Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones.

light

to opticnerve

ganglioncells

bipolarneurones

rodcells

conecells

pigmentedretina

Rods and Cones

• The eye is made of cells that are called Rods and Cones. Cone cells are coned shaped and Rod cells are rod shaped.

Rods and cones

Light path

Inside the rod and the cone

Visual Acuity

• The Rod cells are more sensitive than the Cone cells but the Cone cells have a higher acuity than Rod cells.

Colour Vision

Rods cells have monochromatic vision and Cone cells have trichromatic vision. Cone cells see in bright light and Rod cells see in black and white and in dark light. There are three different coloured Cone cells. These are red, green and blue. There are an equal amount of coloured Cone cells in the eye. The Cone cells are all situated at the fovea.

Bipolar Cells

• Three Rod cells are connected to one bipolar cell which means that when only one of the Rod cells are activated an impulse is sent to the brain.

• One Cone cells is connected to one bipolar cell which means that the light needs activate each Cone cell to send an impulse. This is why the Cone cells have a higher acuity and why they cant function in the dark.

• This is the process by which light initiates a nerve impulse.

• The structure of a rod cell:

Visual Transduction

synapse nucleus m itochondria membrane disks

inner segment outer segment

• Detection of light is carried out on the membrane disks

• These disks contain thousands of molecules of rhodopsin (photoreceptor molecule)

• Rhodopsin consists of:

– Opsin (membrane bound protein)

– Retinal (covalently-bound prosthetic group) sensitive part• Retinal is made from vitamin A• Retinal is the light sensitive part -

– exists in 2 forms: cis and trans forms

Visual Transduction

• In the dark retinal is in the cis form.

• When it absorbs a photon of light it quickly switches to the trans form.

• This changes the shape of the opsin protein – a process called bleaching

Visual Transduction

Rhodopsin with cis retinal

Rhodopsin with trans retinal

Light- fast (s)

• The reverse reaction (trans to cis) requires an enzyme reaction and is very slow (taking a few minutes)

• This process requires ATP, as rhodopsin has to be resynthesised

Visual Transduction

Rhodopsin with cis retinal

Rhodopsin with trans retinal

Light- fast (s)

Dark - slow (mins)

Bleaching of the rhodopsin in a rod cell

Alters the permeability of the membrane to Na+

nerve impulse

sensory neurone in the optic nerve

to the brain

Visual Transduction

• Rhodopsin controls sodium channels

• Rhodopsin with cis retinal opens sodium channels (absence of light)

• Rhodopsin with trans retinal closes sodium channels (light)

Visual Transduction

In the Dark…

• In the dark the channel is open Na+ flow in can cause rod cells to depolarise.– Therefore in total darkness, the membrane of a

rod cell is polarised

• Therefore rod cells release neurotransmitter in the dark

• However the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse

In the Light…

As cis retinal is converted to trans retinal, the Na+ channels begin to close

less neurotransmitter is produced. If the threshold is reached, the bipolar cell will be

depolarised

forms an impulse which is then passed to the ganglion cells and then to the brain

Rods and Cones

Rods Cones

Outer segment is rod shaped Outer segment is cons shaped

109 cells per eye, distributed throughout the retina, so used for peripheral vision.

106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.

Good sensitivity Poor sensitivity

Only 1 type monochromatic vision

3 types (R, G & B) colour vision

Many rods connected to one bipolar cell poor acuity = poor resolution

Each cone is connected to one bipolar cell good acuity = good resolution

Colour Vision

• 3 different cone cells. Each have a different form of opsin (they have the same retinal)

• 3 forms of rhodopsin are sensitive to different parts of the spectrum– 10% red cones – 45% blue cones – 45% blue cones

• Coloured light will stimulate these 3 cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colour– Red light stimulates R cones– Yellow light stimulates R and G cones equally– Cyan light stimulates B and G cones equally– White light stimulates all 3 cones equally

• Called the trichromatic theory of colour vision

Colour Vision

• When we look at something the image falls on the fovea and we see it in colour and sharp detail.

• Objects in the periphery of our field of view are not seen in colour, or detail.

• The fovea has high density of cones.

• Each cone has a synapse with one bipolar cell and one ganglion each cone sends impulses to the brain about its own small area of the retina high visual acuity

Colour Vision

• Refers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retina

– The lens shape can be altered by suspensory ligaments and the ciliary muscles. This adjusts the focus

Accommodation

Accommodation

• Distant objects:– Light rays are almost parallel so do not

need much refraction to focus onto the retina.

– The lens therefore needs to be thin and “weak” (i.e. have a long focal length).

parallel raysfrom distant

object

rays refractedby cornea

thin lens:little furtherrefraction needed

ciliary m uscles relaxed

suspensory ligaments taught

lens pulled thin

lens viewed from back

Accommodation• Close objects:

– Light rays are likely to be diverging, so need more refraction to focus them onto the retina.

– The lens therefore needs to be thick and “strong” (i.e. have a short focal length).

diverg ing rays from

close object

rays refractedby cornea

thick lens:more refraction needed

ciliary muscles contracted

suspensory ligaments loose

lens small and thick

lens viewed from back

• Regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them

• Composed of 2 sets of muscles:– Circular and radial have opposite affects

(antagonistic)

The Iris

• By contracting and relaxing these muscles the pupil can be constricted and dilated

B r ig h t L ig h t D im L ig h t

Parasym pathe tic nerve im pulseC ircu la r m usc les con tract

R ad ial m uscles relaxPupil constricts

Less ligh t en ters eye

Sym pathe tic nerve im pulseC ircu la r m usc les relax

R adial m uscles con trac tPup il d ila tes

M ore ligh t en te rs eye

The Iris

• Is under control of the autonomic nervous system– Sympathetic Nerve pupil dilation– Parasympathtic Nerve pupil constriction– The drug atropine inhibits the

parasympathetic nerve, causing pupil to dilate

The Iris

• The iris is a good example of a reflex arc:

The Iris

stimulus

receptor

coordinator

effector

response

More Light

Rods and Cones

Brain

Iris muscles

Pupil constricts

Sensory neurone

Motor neurone

Lenses bend light in useful ways. Most devices that control light have one or more lenses in them (some use only mirrors, which can do most of the same things that lenses can do).

There are TWO basic simple lens types: Concave and Convex

What are lenses?

CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE.

CONCAVE or NEGATIVE lenses

will DIVERGE (spread out) light

rays

The correct name for farsightedness is Hyperopia. The shape of your eye does not bend light correctly, resulting in a blurred image. A convex lens is usually used to correct this problem.

Convex lens

The correct name of nearsightedness is myopia. Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. A concave lens is usually used to correct this problem.

Concave lens

Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye.

http://www.aoa.org/x6024.xml

Vision

The cornea bends or refracts the rays that pass through a round hole called the pupil.

The iris, or colored portion of the eye that surrounds the pupil, opens and closes.

The pupil gets bigger or smaller to regulate the amount of light passing through.

The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye.

The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells. The images that we see are projected onto the retina upside down.  Our brain quite simply, flips the images over so that we see things upright.

The optic nerve transmits information to the brain.

The vitreous body gives the eye its shape.