receptors - the eye

What is the retina and where is it found?

The retina is a layer of photoreceptors cells found at the back of the eye.

What are the two types of photoreceptors located in the eye

Rods and Cones

Define sensitivity

How easily they reach action potential

Define visual acuity

The ability to tell apart points close together

Where are rod cells found?

Found in the periphery of the retina

Describe and explain what a rod cells is and what it does

Rod cells are a type of photoreceptor that contains rhodopsin and processes images in black and white

How is light detected by rod cells?

A generator potential is created when pigment rhodopsin is broken down by the light energy

They can detect light of very low intensity as many rod cells connect to one sensory neurone ( known as retinal convergence)

This means the brain cannot distinguish between the separate sources of light that stimulated it

Describe what exactly happens to pigment rhodopsin when it absorbs light.

It is broken down to form retinal and opsin

This causes an impulse to be sent down the optic nerve to the brain

It absorbs a broad range of wavelengths, and requires less light for it to be broken down (so more sensitive)

In the light:

(Rhodopsin — > retinal + opsin) —> ‘rhodopsin is bleached’

Why do rod cells have greater visual sensitivity?

Because several rod cells are connected to a single bipolar cell

(Type of spatial summation)

And so this means that even in low light intensities, threshold is more likely to be exceeded in the bipolar cell

So an impulse will be sent through the optic nerve to the brain.

Why do rod cells have less visual acuity?

Because several rod cells are connected to a single bipolar cell so (type of spatial summation)

Therefore the light hitting several points in the retina results in the same impulse being transmitted to the brain.

Where are cone cells found?

Cone cells are found in the fovea of the eye

Describe and explain what a cone cell is and what it does

Cone cells are a type of photoreceptor that contains pigment iodopsin and processes images in colour.

There are 3 types of Iodopsin pigment:

One that absorbs red light

One that absorbs Green light

One that absorbs Blue light

Explain how light is absorbed by cone cells

Iodopsin is only broken down of there is a high light intensity so the action potential can only be generated with enough light

One cone cell connects to a bipolar cell. Therefore, cones can only respond to high light intensity (which is why we can’t see colour when it’s dark)

As each fine is connected to one bipolar cell, the brain can distinguish between separate sources if light detected

Describe what exactly happens to pigment rhodopsin when it absorbs light.

Iodopsin is broken down

This causes an impulse to be sent down the optic nerve to the brain

There are 3 types if iodopsin (red,green, blue)

Iodopsin requires more light ( so it is not so much use at low light levels)

Why do cone cells have less visual sensitivity?

Because each bipolar cell is synapses to only one cone cell

(There is no spatial summation)

This means that in low light intensities threshold is not likely to be reached or exceeded in the bipolar cell so no impulse is sent through the optic nerve to the brain.

Why do cone cells have greater visual acuity

Because each bipolar cell is synapses to only one cone cell

The cones send seperate impulses to the brain.

Allowing the brain to tell apart points that are close together.