receptors

Where are receptors located

• deep in the skin (dermis)

• In joints so organisms know which joint is changing direction

What do receptors do

Each receptors convert one form of energy into another like mechanical into generator potential (which may result in a nervous impulse)

What is generator potential

Depolarisation of a membrane produced by a receptor cell as a result of stimulus

Pacinian corpuscle structure

Sensory neurone ending containing stretch mediated sodium ion channels surrounded by layers of connective tissue called lamellae with viscous gel between layers

How is an action potential stimulated in parcinian corpuscle

Explain why a parcinian corpuscle being stimulated may not result in an action potential being generated

What are photoreceptors

Receptors found on the retina that convert light energy into generator potential in bipolar cell where photoreceptors are connected via the breakdown of pigment in response to light. A certain threshold value has to be exceeded before a generator potential is created in bipolar cell where photoreceptors are connected

What is visual acuity

Ability to distinguish between 2 objects that are close together

What is the fovea

The point on the retina directly opposite the pupil where the light is focused and receives the highest intensity of light

Rod cells

Rod cells detect light of very low intensity and cannot distinguish wavelengths of light so images are in black and white. Many rod cells connected to a single bipolar cell so low visual acuity. More rod cells at periphery absent at fovea

Cones cells

Only respond to high light intensity. Three different types each responding to different wavelengths of light so allow images to be perceived in full colour. Each of these cells is connected to a single bipolar cell so high visual acuity. Concentrated at fovea and very few at the periphery.

Compare rod and cone cells

Why do rods give low visual acuity

They detect light at low intensity so cannot distinguish different wavelengths so images are in black and white. Because a large number of rod cells are connected to a single bipolar neurone, light received by rod cells sharing the same neurone will only generate a single impulse travelling to the brain regardless of which of the rods are stimulated So brain cannot distinguish between the separate sources of light that stimulated

them.

Why do cones give high visual acuity

Because each cone is connected to its own bipolar neurone, so if 2 adjacent cone cells are stimulated, the brain receives 2 separate impulses from 2 separate

neurones The brain can therefore distinguish between the two separate sources of light that stimulated the two cone cells

Why do rods allow us to see in low light intensity

Because a number of rod cells are connected to a single bipolar neurone, (retinal convergence) generator potentials from several rod cells add together (summation), sufficient neurotransmitter released from rods collectively →much greater chance that the threshold value for action potential will be exceeded in bipolar neurone (individually rod cells would not be able to exceed threshold)