Mechanoreceptors are sensory neurons that are active in response to mechanical stimuli. Outside forces cause a temporary physical deformity in the mechanoreceptor, which in turn initiates a nerve impulse in response to the stimulatory input. These outside forces generating stimulation include, but are not limited to touch, pressure, stretching, vibration, sound waves, and motion. There are four main types of mechanoreceptors, each responding to different types of mechanical stimuli: Pacinian corpuscles, Meissner’s corpuscles, Merkel cells, and Ruffini corpuscles. Other less specific forms of mechanoreceptors include muscle spindles and their function in the stretch-reflex arc, and various free nerve endings in the form of cutaneous receptors. Mechanoreceptors are known as low-threshold receptors due to their very high sensitivity, allowing even very weak mechanical stimulation of the skin to induce the generation of an action potential.
Pacinian corpuscles are deep touch and vibration receptors found primarily in the skin. They are relatively large in size, leading to the ability of scientists to isolate a single Pacinian corpuscle and study its properties and mechanism of sensory reception. The corpuscle reacts to physical deformity through creating a generator potential in the sensory neuron within the corpuscle, and the response of the generator potential is proportional to the degree of deformity; the greater the deformity, the greater the generator potential. If the generator potential reaches a certain threshold, it creates action potentials at the node of Ranvier of the sensory neuron, and thereby releases its action potential down the length of the axon, leading to a sensation of pressure being felt. With continuous pressure, the frequency of action potentials decreases and comes to a halt; this response is known as adaptation. Pacinian corpuscles are located in the subcutaneous tissue and in the deep layers of the interosseous membranes and mesenteries of the gastrointestinal tract.
Meissner’s corpuscles are often called “tactile corpuscles” due to the fact that they receive tactile information from stimuli. Meissner’s corpuscles are more sensitive to movement across the skin than Pacinian corpuscles. This is because after adaptation to a sustained stimulus, they are also activated when the stimulus is removed. Meissner’s corpuscles lie between the dermal papillae just beneath the epidermis in regions such as the palms, fingers, and soles of the feet.
Receptor-Building with Clay
Receptors, also called channels, are specialized proteins embedded within membranes of cells, particularly neurons. They act as gates through the membrane that selectively allow only certain molecules to travel through. The protein’s conformation (shape) exists in the closed state for the majority of the time. When a neurotransmitter or other chemical binds to the receptors located on the channels, the protein’s conformation changes to open, allowing molecules to pass through. This binding is crucial to ensuring the changes in membrane voltage so that neurons are able to transmit neural impulses. Without proper levels of neurotransmitters, neurons do not function properly. There are many different types of receptors for sensory function, including but not limited to mechanoreceptors (that activate when it is physically moved), olfactory sensory receptors (that activate when a volatile odorant binds to the receptor), and thermoreceptors (that activate when heat is applied). For this sidebar, the receptor being made is the most common type that is associated with an ion channel.
Three colors of clay (brown, yellow, red), waxed paper, tabletop, and paper towels
Take a walnut-sized piece of yellow clay and roll it on the waxed paper to protect the tabletop. Form the clay into a worm, approximately one-fourth of an inch in diameter. Cut this worm into three pieces, each an inch-and-a-quarter long. These pieces will represent one subunit of the channel. Clean your hands with the paper towels between each color of clay.
Repeat this process with brown clay, only cutting two pieces of clay. These pieces will represent the other subunit of the channel. Lay the subunits next to each other lengthwise, alternating colors. Gently push the pieces together so they will stick but not mix. Wrap the pieces into a hollow tube. This represents the channel.
Finally, take two pea-sized pieces of the red clay and roll them into balls. These pieces will represent neurotransmitters that act as an agonist for the channel. Stick the pieces of red clay on to the channel gently. This is a receptor-bound channel that is open, allowing molecules to pass through the channel to the other side of the cell membrane. Remove the red clay from the receptor and gently twist the channel so that it is closed. When the channel is not bound by a neurotransmitter, molecules cannot pass through.
Riannon C. Atwater
Merkel cells are transducers of pressure and are activated in response not only to the mechanical stimulus, but also to the edges, textures, and shapes of the objects that come in contact with the skin. Merkel cells are also specialized in their ability to differentiate in point localization and to discriminate between the applied pressures at two distinct points. Merkel cells do not undergo rapid adaptation to stimuli as Pacinian and Meissner’s corpuscles do; instead they will continue to generate the nerve impulse in the form of action potentials for as long as the stimulus remains in contact with the skin. Often, Merkel cells are found in the skin close to hairs. Merkel cells are located in the epidermis, primarily in the fingertips, lips, and external genitalia.
Ruffini corpuscles are responsible for the sense of deep pressure and the stretching of the skin. Like Merkel cells, Ruffini corpuscles are slow-adapting, spindle-shaped cells located deep in the skin, in tendons, and in ligaments. The functions of Ruffini corpuscles are much less well understood than those of the other mechanoreceptors, but it is believed that they respond to stretching, particularly through digit or limb movement.
Muscle spindles are responsible for reflexes in a mechanism known as proprioception, or the sense of relative position of body parts, and are activated in response to stretching; this is known as the stretch-reflex arc. The mechanism of nerve impulse generation is seen when the tendon of a muscle undergoes deformity, leading to the stretching of the muscle to which it is attached. This stretching of the muscle leads to the subsequent stretching of muscle spindles, which are made up of sensory nerve endings that are wrapped around spindle fibers. Stretching the spindle fibers sends a nerve impulse through the sensory neurons all the way up to the spinal cord, which then returns the stimulus through motor neurons to cause the contraction of the muscle, thus completing the stretch reflex.
See also: Meissner’s Corpuscles; Pacinian Corpuscles; Reflex; Sensory Receptors; Touch
Johnson, Kenneth O. (2001). The roles and functions of cutaneous mechanoreceptors. Current Opinion in Neurobiology, 11, 455—461. Retrieved from http://www.cns.nyu.edu/~david/courses/sm12/Readings/Johnson%20Curr%20Biol%202001.pdf
Purves, Dale, et al. (Eds.). (2001). Mechanoreceptors specialized to receive tactile information. Neuroscience (2nd ed.). Sunderland, MA: Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10895/
Wayne State University Department of Health Care Sciences. (n.d.). Receptor types and function. Somatosensory examination and evaluation study guide introduction. Retrieved from http://healthcaresciencesocw.wayne.edu/sensory/1_5.htm