What do photoreceptors sense

All sensory receptors rely on one of these four capacities to detect changes in the environment, but may be tuned to detect specific characteristics of each to perform a specific sensory function. In some cases, the mechanism of action for a receptor is not clear.

For example, hygroreceptors that respond to changes in humidity and osmoreceptors that respond to the osmolarity of fluids may do so via a mechanosensory mechanism or may detect a chemical characteristic of the environment. Sensory receptors perform countless functions in our bodies. During vision, rod and cone photoreceptors respond to light intensity and color. During hearing, mechanoreceptors in hair cells of the inner ear detect vibrations conducted from the eardrum. During taste, sensory neurons in our taste buds detect chemical qualities of our foods including sweetness, bitterness, sourness, saltiness, and umami savory taste.

During smell, olfactory receptors recognize molecular features of wafting odors. During touch, mechanoreceptors in the skin and other tissues respond to variations in pressure. Adequate stimulus can be used to classify sensory receptors. Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:.

A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus. Learning Objectives Differentiate among the types of stimuli to which receptors respond.

Key Points Chemoreceptors detect the presence of chemicals. Thermoreceptors detect changes in temperature. Mechanoreceptors detect mechanical forces. Photoreceptors detect light during vision. A longitudinal section would appear similar however there would be no blind spot. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.

Here are schematic diagrams of the structure of the rods and cones:. This figure shows the variety in the shapes and sizes of receptors across and within species. Here is a summary of the properties and the differences in properties between the rods and cones:. If you look above at the schematic diagram of the rods and cones, you will see that in the outer segments of rods the cell membrane folds in and creates disks.

In the cones, the folds remain making multiple layers. The photopigment molecules reside in membranes of these disks and folds. They are embedded in the membranes as shown in the diagram below where the two horizontal lines represent a rod disk membrane either the membrane on the top or bottom of the disk and the circles represent the chain of amino acids that make up a rhodopsin molecule.

Rhodopsin is the photopigment in rods. Each amino acid, and the sequence of amino acids are encoded in the DNA. Each person possesses 23 pairs of chromosomes that encode the formation of proteins in sequences of DNA. The sequence for a particular protein is called a gene.

In recent years, researchers have identified the location and chemical sequence of the genes that encode the photopigments in the rods and cones. This figure shows the structure of the rhodopsin molecule.

The molecule forms 7 columns that are embedded in the disk membrane. Although not shown in this schematic, the columns are arranged in a circle like the planks of a barrel. Another molecule called a chromophore binds within this barrel. Each circle is an amino-acid which are the building blocks of proteins. Each amino acid is encoded by a sequence of three nucleic acids in the DNA.

Before identifying the genetic sequence of human rhodopsin, it was sequences in other animals. Here is shown the comparison between the bovine cow sequence and the human sequence. They are very similar with only a small number of differences the dark circles. Even when there is a difference it may not be functionally significant. The gene for human rhodopsin is located on chromosome 3. This figure shows the sequence for the S-cone pigment compared to that of rhodopsin.

The S-cone pigment gene is located on chromosome 7. Notice how different they are. This figure shows the sequence of the L- and M-cone pigments compared to each other. These pigments are very similar. Only those differences within the cell membrane can contribute to the differences in their spectral sensitivity. The M- and L- cone pigments are both encoded on the X chromosome in tandem. The 23rd pair of chromosomes determines gender. For females this pair is XX and for males this pair is XY.

We will return to this later on when we discuss color vision and color blindness. The Receptor Mosaic. This figure shows how the three cone types are arranged in the fovea.