Receptors - AL only (3.6.1.2)

Sensory receptors are specific to one type of stimulus, for example, mechanical pressure. This ensures the brain can accurately identify and interpret different types of stimuli.

Sensory receptors transduce (convert) energy from the stimulus into a generator potential.

The mechanism of action of a Pacinian corpuscle is:

• Pressure applied to the skin deforms the layers of gel and connective tissue surrounding the sensory neurone ending.
• This deformation stretches the membrane of the sensory neurone, causing stretch–mediated sodium ion channels to open.
• Sodium ions diffuse into the neurone, causing depolarisation of the membrane and production of a generator potential.
• If the generator potential reaches the threshold, voltage–gated sodium channels open.
• An action potential is generated and travels along the sensory neurone to the central nervous system.

The retina is made up of three layers of cells:

• Rod and cone cells: photoreceptors that detect light.
• Bipolar neurone cells: help transmit visual information to the brain.
• Ganglion neurones: which bundle together to form the optic nerve.

Each layer has a synapse with the next.

Within the retina, there are many receptors, including rod and cone cells.

Rods and cones are transducers that convert light energy into electrical impulses.

Rod and cone cells contain light–sensitive pigments which are broken down by light, producing a generator potential.

Rod cells are more numerous than cones and are more sensitive to light. Rhodopsin, the pigment found in rod cells, is broken down easily by light.

Rod cells can detect light but cannot distinguish between different wavelengths. They can detect low light intensity, but only in black and white.

Multiple rod cells converge into each bipolar neurone, meaning that the combined activation of rod cells can lead to the depolarisation of the bipolar cell.

Given that information from many rod cells is combined, it is difficult to distinguish between two closely spaced points, resulting in lower visual acuity.

There is a higher concentration of cone cells in the fovea, where light is focused by the lens.

Cone cells are less sensitive to light because the pigment they contain, iodopsin, is broken down less easily.

There are three types of cone, each with a different iodopsin, each able to detect different wavelengths of light. Together, these enable colour vision.

There is typically only one cone cell connected to each bipolar cell. As a result, signals from individual cone cells remain separate, allowing the brain to distinguish between two closely spaced points.

Cone cells, therefore, provide a high visual acuity.