Living organisms have the ability to sense stimuli from the environment around them and respond in an appropriate manner. The sensory system deals with information being delivered to the nervous system by neurons that have receptors for various stimuli. A receptor is defined as either a specialized protein that detects chemical signals or as a specialized cell that detects environmental stimuli and generates a response. A stimulus is defined as a change in one or more conditions in the environment, both internal as well as external. A stimulus can be one of a variety of changes in the environment such as temperature, pressure, pain, or visible light, to name a few.
Sensory receptors are important because they provide the necessary information about the environment and allow the appropriate response to follow. They are involved in an animal’s, and particularly a human’s, sense of taste, smell, sight, touch, and hearing. Sensory receptors are also involved with an animal’s ability to breathe and move. Sensory receptors have different-sized receptive fields—the area or region where an external or internal stimulus will activate the neuron. In general, large receptive fields cover a large area but tend to have less perception while small receptive fields cover a small area and have strong perception. For example, in touch sensation, small receptor fields in the fingers allow for discriminative touch, giving the ability to sense detailed structures.
Neuron-Building with Clay
There are several different types of neurons such as unipolar (sensory neurons), bipolar (interneurons), multipolar (e.g., motor neurons, inhibitory neurons, etc.), and pyramidal cells (primary neurons of a specific brain circuit). You can make any of these types of neurons with clay; however, this sidebar focuses on the unipolar neurons that are the most common form of sensory neurons. Unipolar neurons, named for their appearance, are specialized neurons that act to bring sensory neural signals from the body to the central nervous system. The most common unipolar cells are found in the dorsal root ganglion, just outside of the dorsal spinal cord. Unipolar cells have a single input/output that come together at the soma. The dendrites (input) are branched at that location where it picks up the signal, and its axon may branch where it terminates in the spinal cord. This allows it to act on many neurons. Like most biological cells, unipolar neurons contain but are not limited to cell membrane, cytoplasm, nucleus, ribosomes, endoplasmic reticulum (ER), and mitochondria.
Four colors of clay (red, green, yellow, and blue), waxed paper, tabletop, paper towels, and toothpick
Roll a walnut-sized ball of red clay; this represents the cell membrane and cytoplasm together forming the cell body (soma). Flatten this into a quarter-inch-thick, two-inch-diameter pancake. Roll a pecan-sized ball into a long worm and a very short worm. Attach the middle of the long worm with the very short worm to one edge of the soma (making a T-shape). The long worm on one side represents the dendrites and the other side represents the axon. Create a few branches off both ends of the worm. Clean your hands each time you switch colors.
Flatten a pecan-sized ball of green clay to one inch in diameter and an eighth of an inch thick. This represents the ER. Place the clay on top of the soma, relatively in the center. With a toothpick, poke several small holes into the central section of the ER. These holes represent ribosomes, used to make proteins.
Slightly flatten one side of a peanut-sized ball of yellow clay. This represents the nucleus of the neuron. Nuclei house the genetic material. Place on top of the ER; the nucleus is not always in the center of the cell.
Finally, make four to five Tic-Tac-sized ovals. These are mitochondria, which generate energy for the cell. Place throughout the cytoplasm.
Jennifer L. Hellier
Types of Sensory Receptors
To date, sensory receptors have been categorized into five groups: mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors, and nociceptors. Some of these different receptor types are found throughout the body or in almost every sensory system.
Mechanoreceptors respond to a physical stimulus such as mechanical pressure or distortion. This mechanical change in position produces an electrical signal that is transmitted to the central nervous system. Mechanoreceptors are important for discerning between different sensations such as light touch, touch, positional change (balance), and pressure. Perhaps the most notable stimuli that mechanoreceptors respond to are sound waves (used for hearing) and proprioception.
Chemoreceptors are receptors that respond to chemical stimuli in the environment. The two prominent classes of chemoreceptors are those involved in olfactory and gustatory systems. Olfactory receptors are found in the membranes of olfactory sensory neurons located in the olfactory epithelium, which lines the insides of the nose. Volatile odor molecules in the environment enter the nose when breathed in and bind to receptors specific for that odor molecule. In the gustatory system, taste cells located in taste buds of the tongue operate in the same way as olfactory sensory neurons, except these respond to aqueous chemical stimuli.
Thermoreceptors are specialized sensory receptors that determine temperatures that are generally not painful (not too hot and not too cold). Thermoreceptors are considered to be free nerve endings or free nonspecialized endings. These receptors are found throughout the skin, cornea (the covering of the eye), and the bladder.
Photoreceptors are specialized sensory receptors that convert visible white light into the sense of vision. Specifically, these receptors absorb photons of light and transduce that absorption into a membrane potential, which activates the neuron. Photoreceptors are found in neurons located in the retina, which is the photosensitive lining of the back of the eye. There are two main types of photoreceptors: cones and rods.
Cone cells are short, wide, and have a tapering end. Cone cells need bright light to be activated and can absorb all short, medium, and long wavelengths of visible light, which represent different colors within the visible light spectrum. Rod cells are long and thin. Rod cells are activated with dim lighting, meaning they can absorb a single photon of light to be activated. This makes rod cells more sensitive to light compared to cone cells. Thus, rod cells are the main photoreceptor neurons used for night vision.
Pain receptors or nociceptors are specialized sensory receptors that determine when a stimulus is painful. These receptor types are free nerve endings or free nonspecialized endings. Nociceptors send their signal to the brain and spinal cord so that the body can respond appropriately, such as letting go of a hot handle. Nociceptors are found in all locations of the body, both internally (such as the gut, heart, joints, and muscles) and externally (like the cornea, mucosa, and skin—also known as cutaneous nociceptors). This is because pain is an important signal that should not be ignored.
Jennifer L. Hellier and Roberto Lopez
See also: Cones; Mechanoreceptors; Nociceptors; Rods; Taste Bud; Thermoreceptors
Abraira, Victoria E., & David D. Ginty. (2013). The sensory neurons of touch. Neuron, 79(4), 618—639.
Fetsch, Christopher R., Gregory C. DeAngelis, & Dora E. Angelaki. (2013). Bridging the gap between theories of sensory cue integration and the physiology of multisensory neurons. Nature Reviews: Neuroscience, 14(6), 429—442.
Kinnamon, Sue. (2013). Neurosensory transmission without a synapse: New perspectives on taste signaling. BMC Biology, 11, 42.