Photoreceptors: Rods and Cones

Where are rods and cones found?

The retina

What type of images do rods produce?

Black and white

Why do rods detect very low light intensity?

Because many rod cells connect to one sensory neurone (retinal convergence)

How is an action potential triggered in the human retina?

• rhodopsin (pigment of rod cells) must be broken down by light energy to create a generator potential

• there is enough energy from low-intensity light to cause the breakdown

• enough pigment has to be broken down for the threshold to be met in the bipolar cell (cells that link rod cells to sensory neurone)

Why do rod cells function even at very low light intensities (2 reasons)?

1. Retinal convergence - multiple rod cells connect to one bipolar cell, which means spatial summation can occur

2. It doesn’t take a lot of light energy to break down rhodopsin

What is the advantage of retinal convergence?

• if 3 rod cells connect to one bipolar cell

• each rod cell’s rhodopsin breaks down in the low light intensity

• and collectively all of the broken down pigment results in a big enough stimulus to trigger an action potential

(spatial summation)

Why do rod cells have low visual actuity?

Multiple rod cells being connected to one bipolar cell (retinal convergence) means that the brain cannot distinguish between the separate sources of light that stimulate it

How many types of cone cells are there and what are they/ what do they do?

3

• they contain different types of iodopsin pigment - red, green and blue

• which all absorb different wavelengths of light

• depending on the proportion of each cone cell that is stimulated we perceive colour images

Why don’t you see colours when its dark?

There’s not enough light energy to break down the iodopsin and trigger an action potential

• iodopsin is only broken down if there is a high light intensity

Why do cone cells only respond to high light intensity?

• one cone cell connects to one bipolar cell

• so no spatial summation occurs

What is a positive of having one cone cells connect to one bipolar cell?

• the brain can distinguish between separate sources of light

• so cone cells give high visual acuity

• clearer, coloured vision

What is the distribution of rods and cones cells like in the retina?

It is uneven

Where are cone cells found and why?

• most cone cells are located at the fovea

• because they only respond to high light intensities

• and the fovea receives the highest light intensity as is opposite the pupil

Where are rod cells found in the retina and why?

• further away from the fovea

• because they can respond to lower light intensities