The basic structure of a Pacinian corpuscle as an example of a receptor. The creation of a generator potential on stimulation. The Pacinian corpuscle should be used as an example to illustrate the following. • Receptors only respond to specific stimuli • Stimulation of receptor membranes produces deformation of stretch-mediated sodium channels, leading to the establishment of a generator potential. Differences in sensitivity and visual acuity as explained by differences in the distribution of rods and cones and the connections they make in the optic nerve.

Pacinian corpuscle

Below is an image of a pancinian corpuscle. This is a receptor for pressure, found deep in the skin. It is made up of layers of connective tissue filled with gel and at the center is the beginning of a sensory neurone.

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When pressure is applied the layers of tissue compress and distort the neurone. This causes ion channels that are set in the membrane of the nerves axon to stretch, making them big enough for sodium ions to move through. For this reason they are called stretch-mediated ion channels.

When a nerve is at rest, the inside of the axon is negative, and the outside is positive. But as the positive sodium ions move in they change the charge, so the inside is more positive than the outside. This is known as a generator potential, and it will go on to create an action potential (the signal that sends messages to the CNS).

Rod and cone cells

In the retina of the eye there are different receptors for light. Both transduce light energy into an electrical impulse by the breaking up of a protein. However, rod cells will respond to any wavelength of light, but there are three different types of cone cells, each only responding to specific wavelengths (red, green and blue); meaning that rod cells see light and dark, and cone cells see colour.

Another difference is that each cone cell has its own bipolar neurone that connects it to a sensory neurone, but rod cells have to share. There are three rod cells to each bipolar neurone which means it is easier to reach the threshold for a generator potential, as the reception of signals in all three cells can be added together (retinal convergence). This means that rod cells see better when light is low.

Because there are three rod cells to each sensory neurone, the brain cannot tell which rod cell is being stimulated, so the image is not as sharp. Whereas cone cells each have their own attachment so visual acuity is high.