Physiology lecture 23

Vision and the eye - anatomy and physiology

LOs

• Describe the anatomy of the eye and the retina

• Explain how an image is formed on the retina

• Know what is meant by the fovea and the blind spot

• Describe two types of retinal photoreceptors: rods and cones.

• Know how the photoreceptors respond to light under different conditions

• Outline the trichromatic theory of colour vision

• Explain the process of light transduction

Cross section of eye

Cornea - curved surface in front of eye - role in refracting light and protective layer - fixed

Aqueous humor - fluid that provides nutrients to eye

Pupil - can open or close to let light in

Iris - can open or close to change pupil size

Lens - adjusts size to focus light and refract at diff levels

retina - the photoreceptors sense light and colour

sclera - protective tissue and maintains eye structure

The cornea

Responsible for refracting (bending) light to bring it into focus on the retina. The cornea (not the lens!) is responsible for most of refractive power of the eye. (Laser treatment acts on cornea)

Lens

Responsible for accommodation – adjusting the refractive properties of the eye to ensure objects in focus over a wide range of distance.

Iris

Pigment determines eye colour (blue eyes are actually lack of brown pigment)

Muscles (circular and radial) controlled by autonomic ns adjusts pupil diameter according to light levels, and emotional signals - attractive people

Pupil

Opening in centre of the iris allows light entry (2(light-8 dark mm range in diameter)

Retina

Layer at the back of the eye: photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells and nerve fibres. (Retina part of brain).

Image formation

Note that the image is inverted (& left/right reversed) on retina - brain then corrects this

how large is eye’s visual field?

150 degrees fixed field of view

Role of the lens in accommodation

As object brought nearer to eye, light rays need to be bent more to keep object in focus

Lens flatter at longer distances, fatter/rounder to shorten focal length for close vision

Role of the ciliary muscles in accommodation

• Lens flattened for distant vision, rounded for near vision (below),

• Contraction of ciliary muscles allows zonulas of Zinn to slacken, lens expands and becomes more rounded.

• Accommodation weakens with age. Reading glasses needed in middle age

Emmetropia

Normal focusing

The emmetropic eye can focus light from a distant object on to the retina.

• The image at the retina is sharp (in focus)

Myopia

Short sightedness

• The myopic eye has too much focal power for its length

• Light rays converge (are focused) in front of the retina

• The image at the retina is blurred (out of focus) Image . .

• Remember that distant objects are focused closer to the lens - A myopic eye will not be able to focus on distant objects, but can focus on objects at a short distance.

Hyperopia

Long sightedness

• The hyperopic eye has too little focal power for its length

• Light rays converge (are focused) behind the retina.

• The image at the retina is blurred (out of focus) Image .

• Remember that close objects are focused further to the lens - A hyperopic eye will not be able to focus on close objects, but can focus on objects at a long distance.

The retina – macro structure

Retina viewed via an ophthalmoscope

macula - central part of retina (often affected by retinal diseases)

Middle of macula = fovea - very centre of vision

Optic disk - where optic nerve exits to brain - blind spot (lack of photoreceptors)

The foveal pit

In fovea cells, upper layers of retina pushed aside to allow light direct access to photoreceptors, region of highest acuity

Blind spot demonstration

due to lack of photoreceptors in optic disc

The retina – macro structure (3 things)

• Fovea

– Pit in centre of macula

– Central part of visual field.

– Area of maximum acuity (highest image resolution) with highest density of cone receptors, colour vision

• Blind spot or optic disk

– Region where nerve fibres and blood vessels leave the eye

– Blind in this region

– Brain “fills in” the missing area (see demonstration of blind spot)

• Blood vessels

– Delivering oxygen and nutrients to retina

Retina microstructure

light comes in from front, interacting with photoreceptors (rods and cones) connected to bipolar cells and ganglion cells - these can integrate the rods and cones signals

Horizontal connections and amacrine cells also modulate signals

Pigment layer of retina doesn’t recognise light but absorbs the light to ensure vision can occur but also to protect the photoreceptor cells

retinal organisation - pigment epithelium

– Cells at very back of retina.

– Photoreceptors embedded in this layer.

– Cells contain melanin black pigment. Absorbs light prevents scattering stray light that would affect the image.

Retinal organisation - photoreceptors

– Rods (scotopic vision - low light level)

– Cones (photopic vision – high light levels) - colour

– More rods than cones (20:1)

retinal organisation - other cells

connecting vertically and horizontally– Bipolar cells (v), horizontal cells (h), amacrine cells (h), ganglion cells – origin of an optic nerve fibre

what does light pass to reach photoreceptors?

Light must pass through blood vessels and nerve fibres to reach photoreceptors– except at fovea for maximum acuity

The retina – micro structure PRs

Two types of photoreceptors

• Rods (scotopic vision)

• Type most sensitive to light

• Sensitive to brightness

• So only detect contrast - difference between light and dark - Function at low light levels

• Bleached at high light levels

• Cones (photopic vision)

• Three types sensitive to long (red), medium (green) and short (blue) wavelengths of light

• Enable colour vision

• Responsible for highest acuity (sharpness)

• Work only at high light levels

These cells connect to pigment epithelium at outer segment and bipolar cells at synaptic terminals

Dark Adaptation

• When moving from bright to low light level (entering darkened room) initially it is hard to see: cones are not sensitive, rods are bleached.

• Gradually visual sensitivity increases as rods recover

• This process of adjustment to low light is called dark adaptation

• Note also colour loss as switch from colour vision with cones to contrast only vision with rods

Distribution of cones

fovea - cells are really small and packed

Progressively get larger and less densely packed

Distribution of rods

Further out there are rods - smaller cells in between cones

Distribution of rods and cones

Cones mostly in fovea - density decreases further away

Rods mainly absent in fovea - but peripheral mostly rods

Light transduction in photoreceptors

Light sensitive photopigments in rods and cones

– Rods - rhodopsin

– Cones - three different pigments (but wavelength (colour) sensitive)

Rhodopsin and bleaching

– Opsin (Protein) + 11-CIS-retinal (derived from Vitamin A)

– In the dark, opsin and retinal are bound together

– On light exposure retinal molecule changes shape (conformational change)

– Causes it to split from opsin:

– This process is called bleaching

Structure of Photoreceptors

diagram also shows what photoreceptors are connected to

Rods channels in dark

rods channels in light

EM spec

Colour vision: absorption spectra of cone photoreceptors

• Three cone types:

Light absorption (percent of maximum)

- short wavelength sensitive (’Blue’)

- medium wavelength sensitive(‘Green’)

- long wavelength sensitive (‘Red’)

• Each cone sensitive to range of wavelengths.

• The wavelength or colour is represented by unique ratio of outputs from the 3 different cone types.

• Allows many different colours to be represented using only 3 different receptors - the Trichromatic theory of colour vision.

Colour vision: Trichromatic theory

Each colour has unique ratio of absorption. Many colours are represented with only three cone types

what happens in absence of a cone type or no cone types?

Absence of a cone type, or a cone with an abnormal pigment leads to colour confusion or colour blindness

Summary

• The main components of the eye

• The main components of the retina

• The retinal layers

• The photoreceptors

• Light transduction

• Colour vision