Receptors

The potential difference when a cell is at rest is called its resting potential. 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 therefore altering the potential difference of the cell. this change in potential difference due to a stimulus is called the generator potential.

A bigger stimulus excites the membrane more causing a bigger movement of ions and a bigger change in potential difference this produces a bigger generator potential. If this generator potential is big enough an action potential is produced an action potential is an electrical impulse along a neurone. 

• Pacinian corpuscles are mechanoreceptors - they detect mechanical stimuli such as pressure or vibrations.
• Pacinian corpuscles contain the end of a sensory neurone. Called the sensory nerve ending.
• the sensory nerve ending is wrapped in loads of layers of connective tisue called lamallae.
• If the pacinian corpuscle is stimulated the lamallae deform and press on the sensory nerve ending.
• this causes deformation of stretch mediated sodium channels in the sensory neurones cell membrane. the sodium ion channels open and sodium ions diffuse into the cell creating a generator potential.
• If this generator potential reaches the threshhold it triggers an action potential.

Sensitivity:

• Rods are very sensetive to light this is because many rods join one neurone so many weak generator potentials combine to reach the threshold and trigger an action potential.

• Cones are less sensitive than rods this is because one cone is joined to one neurone so therefore it takes more light to reach the threshold and trigger an action potential

Visual acuity: - visual acuity is the ability to tell apart points that are close together.

• Rods give low visual acuity because many rods join the same neurone, which means that light from two objects close together cannot be told apart.
• Cones give high visual acuity because cones are close together and one cones joins one neurone. When light from two points hits two cones, two action potentials (one from each cone) go to the brain. this is so that you can distinguish two points that are close together as two separate points.