4. Receptors

Resting potential

• Definition: The potential difference when a cell is at rest.

• When the receptor is not being stimulated so there’s a difference in charge between the inside and outside of the cell.

• Inside is relatively negative to the outside.

• the voltage/ potential difference is -40mv

Generator potential

• Definition: The change in potential difference due to a stimulus.

• When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell.

• A bigger stimulus is excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference - therefore a bigger generator potential is produced.

The action potential

• if the generator potential is big enough it’ll trigger an action potential.

• It is only triggered if the generator potential reaches a certain level called the threshold level.

What type of receptor is a pacinian corpuscle

mechanoreceptor - they detect mechanical stimuli, e.g. pressure and vibrations

How do pacinian corpuscles work

• When stimulated by pressure, the lamellae are deformed and press on the sensory nerve endings.

• This causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated sodium ion channels.

• The channels open and sodium ions diffuse into the cell creating a generator potential. If the generator potential reaches the threshold, it causes an action potential.

photoreceptors

receptors in the eye that detect light.

How does the eye work?

• Light enters the eye through the pupil.

• The amount of light that enters is controlled by lens onto the retina, which lines the inside of the eye.

• The retina contains photoreceptors, and the fovea contains a lot of photoreceptors.

How do photoreceptors work?

• Light enters the eye and hits the photoreceptors and is absorbed by light-sensitive optical pigments.

• Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.

• A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.

• Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.

rods

black and white vision/ monochromatic vision

found mostly in the peripheral of the retina.

cones

colour vision/ tichromatic vision

mainly found packed together in the fovea.

why are cones and rods different?

They contain different optical pigments, making them sensitive to different wavelengths of light.

Why are rods sensitive to light/ why do they work well in dim light?

Because many rods join one bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger and action potential.

Why are cones less sensitive than rods/ why do they work best in bright light?

This is because one cone joins one bipolar neurone, so it takes more light to reach the threshold and trigger an action potential.

visual acuity

the ability to tell apart points that are close together.

What type of visual acuity do rods give and why?

• low visual acuity,

• because many rods join the same bipolar neurone, which means light from two points close together can’t be told apart.

What type of visual acuity do cones give and why?

• high visual acuity

• one cone joins to one bipolar cell and are close together, so when light from two points hits two cones, two action potentials go to the brain - so you can distinguish two points that are close together as two points.