A thermoreceptor is the part of a neuron that detects changes in temperature. In mammals, these receptors are typically found in several tissues of the body, including the dermis and epidermis of the skin, the cornea, and the bladder. Thermoreceptors can respond to both absolute and relative changes and typically respond to a single directional change (warmer or colder). A receptor responding to warmth will send signals at a higher rate as the temperature warms, subsequently signaling at a lower rate if the temperature cools. A similar process occurs in reverse for receptors responding to cooling—for cooling receptors, the rate increases as the temperature decreases.
Currently, a lot of active research explores the mechanism by which thermoreceptors convert a temperature change into a signal (ie. an action potential that propagates electrically through the nervous system). Most contemporary research focuses on identifying and examining different temperature-sensitive proteins that modulate the flow of ions in and out of a neuron. As the flow of ions changes, the rate at which the neuron produces an action potential changes.
The subjective perception of heat or coolness in a specific area is an emergent property that arises from the combination of warming and cooling signals that the peripheral nervous system receives from each thermoreceptor. The nervous system processes this information via rapidly-changing thresholds and intermediary networks, and it integrates them to reduce the dimensionality to a subjective assessment.
Mechanoreceptors respond to pressure or distortion. There are four main types of mechanoreceptors: Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ruffini endings.
Pacinian corpuscles, also known as Lamellar corpuscles, are nerve endings responsible for the perception of pressure and vibration. These mechanoreceptors respond to sudden changes in the force applied to a portion of skin, and they may play a large role in detecting surface texture by detecting the rate at which skin is displaced as it moves across a surface. Unlike other mechanoreceptors, Pacinian corpuscles are larger and comprise a smaller number of the total mechanoreceptors present on the skin. Each corpuscle is approximately 1mm in length wrapped by a layer of connective tissue. Even a subtle change or deformation of the corpuscle triggers an action potential, opening pressure-sensitive ion channels along the axon membrane of the receptor. Fibrous tissue suspended in a gelatinous material then detect this pressure change. Movements across a textured surface with features smaller than a micrometer generate a frequency of 250 Hz, which is roughly the optimal vibrotactile sensitivity for Pacinian corpuscles.
Meissner’s corpuscles (tactile corpuscles) are responsible for sensitivity to light touch. They have the highest sensitivity when sensing vibrations between 10 and 50 Hz. Although Meissner’s corpuscles are located along many areas of the skin, they are concentrated in the fingertips and lips beneath the epidermis. Tactile corpuscles are encapsulated and unmyelinated with a typical length of 30-140 micrometers. Unlike other mechanoreceptors, the number of tactile corpuscles per square millimeter on the fingertips drops fourfold between ages 12 and 50. In spite of their high precision, tactile corpuscles can only detect if something is touching the skin at a particular location, and do not detect the extent of pressure or friction exerted.
Merkel’s discs, also known as tactile cells, Merkel cells, or Merkel-Ranvier cells, are oval receptor cells that enable the discrimination of shapes and textures. They are found in the skin and some parts of the mucosa, and are typically located at the bottom of sweat duct ridges of the epidermis. With an approximate diameter of 10 micrometers, they appear to augment the information obtained from other mechanoreceptors in close proximity. Although we still do not know the exact mechanism by which their feedback is integrated into a conscious interpretation of shape and texture, animal model experiments have demonstrated that their presence mediates the perception of fine spatial and tactile details.
Ruffini endings, also known as Bulbous corpuscles or Ruffini corpuscles, are found in the dermis tissue of humans. Responsible for the basis of mechanoreception, Ruffini endings are sensitive to the stretching of the skin, and contribute to the kinesthetic sense (body position and movement). At the fingertips, Ruffini corpuscles can be found at the highest density in the human body where they appear to be useful for monitoring the slip of objects along the surface of the skin, enabling efficient grasp and release coordination of the hands. They are also located in deep layers of the skin, even detecting the mechanical motion of joints with angular precision of up to 3 degrees. Ruffini endings can act as thermoreceptors over a long period of time, but they may be burned off in cases of deep burns. Although they can sometimes be classified as a thermoreceptor, their ability to detect temperature changes does not carry the same precision as thermoreceptors found in other layers, and their principle function provides measurements of body position.
Psychophysics of Haptics
Unlike vision and hearing, which operate on specific frequency ranges of light and sound respectively, the resolution of human skin can be difficult to quantify. Assuming that an individual can only pay attention to a single tactile stimulus at a time, we can roughly calculate the unique number of individual stimuli that may be presented each second through different haptic channels. The following calculations assume a human body to have 1.5 square meters of undamaged skin, but the values can be scaled up or down to accommodate for the typical range in surface area of 1.5–2.0 square meters.
The spatial resolution of temperature changes measured by the sensory system is limited. Further explorations of the psychophysics of thermoreceptors can elucidate the exact distribution of sensitivity throughout the body. Reasonably, 8 levels of coldness and hotness may be detected for every 500 square centimeters of the body, resulting in roughly 30 bytes per second of bandwidth.
Pressure & Vibration
With a spatial resolution of roughly 1 byte of information for every 10 square centimeters of skin surface area, 1500 bytes per second of bandwidth may be achieved with a combination of pressure and vibration.
With roughly 340 joints in the human body, proprioception can provide at least 680 bytes/second of information.
Slip & Texture
As a lower bound, the perception of texture can convey at least as much information as Braille; however, it is much harder to quantify a potential data transfer rate that may be achieved using the modality.
Ultimately, the true bandwidth of human tactile perception depends on our cognitive load – if we are focused only on understanding tactile stimuli (for example, while reading Braille) our “reading speed” or data transfer rate will be much faster, but these parameters describe a sensible lower bound for practical applications of haptic feedback. Since they assume the use of the entire human body (1.5 square meters of surface area), the figures must be scaled down to the exact size of a haptic display.