Difference between revisions of "Touch" - New World Encyclopedia
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| − | + | [[Image:DSC 4050-MR-Braille.jpg|right|thumbnail|200px|The sense of touch is utilized in the system of reading for the blind known as Braille.]] | |
| − | + | '''Touch''' (or '''tangoreception''') is the physiological sense by which animals perceive an object in their environment when it comes into contact with the body surface. Touch is one of a variety of closely associated mechanisms or faculties (collectively known as ''the [[sense]]s'') by which a living organism receives information about its external or internal environment. | |
| − | + | There are two main types of sensory receptors related to touch: ''tactile hairs'' and ''subcutaneous receptors'' (receptors below the skin’s surface). Many animals, ranging from insects and other anthropods to birds and mammals, have hairs or hairlike projections richly supplied with nerves. Some hairs, such as [[whisker]]s (or ''vibrissae''), may be specially adapted in certain areas of the body to provide more sensitive tactile sensation. | |
| − | + | Sensory receptors below the skin are a second means of perceiving touch, which is a type of ''mechanoreception'', or sensitivity to mechanical stimuli. Like other sensory cells, the ''mechanosensors'' associated with touch convert physical stimuli into signals that are transmitted to specific areas of the [[central nervous system]] (i.e., the spinal cord and brain) for processing and interpretation. Stimuli create a temporary physical distortion in the membranes of these specialized receptors, causing ion channels to open and ultimately generating [[action potential]]s, which are the messages of the nervous system. Density of tactile sensors varies across the body, but tends to be particularly concentrated in organs of touch, such as an octopus's tentacles, a pig's snout, or or the fingertips of a human hand. | |
| − | + | Along with the senses of [[taste]], [[smell]], [[sight]], and [[hearing]], touch is an example of ''exteroreception'', which gives organisms complex feedback about their external environment. It allows organisms to obtain with a degree of precision points of tactile stimuli at the body's surface. This tactile perception is behind the system of [[Braille]], for example, enabling the blind to read raised type. However, other features of the animal often work in conjunction with touch receptors to enhance ''proprioreception'' – i.e., external feedback that helps the organism to understand its spacial position. | |
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| − | + | Despite its isolation for study, the sense of touch is very closely connected to other sensations received by the skin: the perception of [[pressure]] (hence [[shape]], [[soft]]ness, [[texture]], [[oscillation|vibration]], etc.), relative [[temperature]], and sometimes [[Pain and nociception|pain]], which are collectively known as the ''somatosensory system''. | |
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| − | + | ==The anatomy of touch== | |
| + | Objects touching an organism's skin generate diverse sensations because the skin is packed with a variety of specialized mechanosensors. | ||
| − | + | The outer layers of skin, especially hairless skin like lips and fingertips, contain coils of nerve endings enclosed in connective-tissue capsules. Known as ''Meissner’s corpuscles'', they respond to objects that touch the skin even lightly. These mechanosensors are notable because they are able to adapt readily. (In this context, ''adaptation'' is defined in as enabling an animal to ignore background or unchanging conditions while remaining sensitive to new changes or to new information. Thus, for example, a human perceives the sensation of fabric on skin when dressing in the morning but is not acutely aware of the feel of clothing on skin throughout the day.) | |
| − | + | Expanded-tip tactile sensors are also located in these outer regions. They differ from Meissner’s corpuscles in that they adapt only partly and slowly. Instead, they are useful for providing steady-state information about objects that touch the skin over long periods. | |
| − | Pacinian | + | ''Pacinian corpuscles'' sense pressure. Located deep in skin, they are made up of concentric layers of connective tissue that encapsulate an extension of a sensory neuron. These mechanosensors respond especially well to vibrations applied to the skin, but they also adapt rapidly to steady pressure. |
| − | Krause’s end bulb senses touch | + | Other receptors include ''Krause’s end bulb'', which senses touch, and ''Ruffini’s corpuscle'', which senses touch and pressure. |
==How touch works== | ==How touch works== | ||
| − | + | Most sensory cells are modified neurons. Sensors are specialized for particular types of stimuli - e.g., chemical, mechanical, and light. ''Mechanosensors'' are specialized cells that are sensitive to mechanical forces. The physical distortion of a mechanosensor’s plasma membrane causes ion channels to open and alters the resting potential of the cell, which in turn leads to the generation of action potentials. | |
| − | + | In general, the sensor possesses a membrane protein that detects the stimulus and responds by altering the flow of ions across the cell membrane; the resulting change in membrane potential causes the sensor to fire action potentials, the universal message system of the nervous system. The intensity of the stimulus is coded as the frequency of action potentials. Although the message is coded in the same form regardless of the stimulus, organisms perceive different sensations because messages from sensors arrive at different places in the central nervous system (CNS). | |
| − | + | ''Tactile hairs'' are an example of mechanosensors that are not neurons. From one surface, they have projections called ''stereocilia'', which, when bent, alter receptor proteins in the hair cell's plasma membrane. | |
| + | When they are bent in one direction, the receptor potential becomes more positive. In this case, the hair cell releases a neurotransmitter to the sensory neurons associated with it, and these neurons in turn send action potentials to the brain. | ||
| − | + | ==The functions of touch and associated senses== | |
| + | As mentioned above, the sense of touch allowed many vertebrates and invertebrates to localize with some precision points of tactile stimulation at the body surface. The long antennae of many [[arthropod]]s (e.g., crayfish) and the lengthened tactile hairs (vibrissae) on the snouts of nocturnally active mammals (such as cats and rats) enable them to sense objects in their immediate surroundings. | ||
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| + | The sense of touch also functions in aspects of ''proprioception'' such as orientation and equilibrium. For example, sensory contact with the ground often gives terrestrial animals information about their spatial position. Many invertebrates have equilibrium organs (known as ''statocysts'') that use hair cells to signal the position of the animal with respect to gravity. The mammalian inner ear also includes two organs of equilibrum that use hair cells to detect the body's position. Tactile hairs are found in the [[lateral line]] sensory system of fishes, a canal under the surface of the skin that provides information about movements of the fish through the water and about the moving objects that cause pressure waves in the surrounding water. | ||
| − | + | ==The technology of touch== | |
| − | + | ''Haptic'', from the [[Greek language|Greek]] αφή (''Haphe''), means pertaining to the sense of [[tactition|touch]]. ''Haptic technology'' refers to technology that interfaces the user via the sense of touch by applying forces, vibrations and/or motions to the user. This mechanical stimulation is used to create haptic virtual objects. Haptic technology has made it possible to investigate in detail how the human sense of touch works by allowing the creation of carefully-controlled haptic virtual objects. These objects are used to systematically probe human haptic capabilities. These new research tools contribute to our understanding of how touch and its underlying brain functions work. | |
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==References== | ==References== | ||
Revision as of 19:58, 21 October 2007
Touch (or tangoreception) is the physiological sense by which animals perceive an object in their environment when it comes into contact with the body surface. Touch is one of a variety of closely associated mechanisms or faculties (collectively known as the senses) by which a living organism receives information about its external or internal environment.
There are two main types of sensory receptors related to touch: tactile hairs and subcutaneous receptors (receptors below the skin’s surface). Many animals, ranging from insects and other anthropods to birds and mammals, have hairs or hairlike projections richly supplied with nerves. Some hairs, such as whiskers (or vibrissae), may be specially adapted in certain areas of the body to provide more sensitive tactile sensation.
Sensory receptors below the skin are a second means of perceiving touch, which is a type of mechanoreception, or sensitivity to mechanical stimuli. Like other sensory cells, the mechanosensors associated with touch convert physical stimuli into signals that are transmitted to specific areas of the central nervous system (i.e., the spinal cord and brain) for processing and interpretation. Stimuli create a temporary physical distortion in the membranes of these specialized receptors, causing ion channels to open and ultimately generating action potentials, which are the messages of the nervous system. Density of tactile sensors varies across the body, but tends to be particularly concentrated in organs of touch, such as an octopus's tentacles, a pig's snout, or or the fingertips of a human hand.
Along with the senses of taste, smell, sight, and hearing, touch is an example of exteroreception, which gives organisms complex feedback about their external environment. It allows organisms to obtain with a degree of precision points of tactile stimuli at the body's surface. This tactile perception is behind the system of Braille, for example, enabling the blind to read raised type. However, other features of the animal often work in conjunction with touch receptors to enhance proprioreception – i.e., external feedback that helps the organism to understand its spacial position.
Despite its isolation for study, the sense of touch is very closely connected to other sensations received by the skin: the perception of pressure (hence shape, softness, texture, vibration, etc.), relative temperature, and sometimes pain, which are collectively known as the somatosensory system.
The anatomy of touch
Objects touching an organism's skin generate diverse sensations because the skin is packed with a variety of specialized mechanosensors.
The outer layers of skin, especially hairless skin like lips and fingertips, contain coils of nerve endings enclosed in connective-tissue capsules. Known as Meissner’s corpuscles, they respond to objects that touch the skin even lightly. These mechanosensors are notable because they are able to adapt readily. (In this context, adaptation is defined in as enabling an animal to ignore background or unchanging conditions while remaining sensitive to new changes or to new information. Thus, for example, a human perceives the sensation of fabric on skin when dressing in the morning but is not acutely aware of the feel of clothing on skin throughout the day.)
Expanded-tip tactile sensors are also located in these outer regions. They differ from Meissner’s corpuscles in that they adapt only partly and slowly. Instead, they are useful for providing steady-state information about objects that touch the skin over long periods.
Pacinian corpuscles sense pressure. Located deep in skin, they are made up of concentric layers of connective tissue that encapsulate an extension of a sensory neuron. These mechanosensors respond especially well to vibrations applied to the skin, but they also adapt rapidly to steady pressure.
Other receptors include Krause’s end bulb, which senses touch, and Ruffini’s corpuscle, which senses touch and pressure.
How touch works
Most sensory cells are modified neurons. Sensors are specialized for particular types of stimuli - e.g., chemical, mechanical, and light. Mechanosensors are specialized cells that are sensitive to mechanical forces. The physical distortion of a mechanosensor’s plasma membrane causes ion channels to open and alters the resting potential of the cell, which in turn leads to the generation of action potentials.
In general, the sensor possesses a membrane protein that detects the stimulus and responds by altering the flow of ions across the cell membrane; the resulting change in membrane potential causes the sensor to fire action potentials, the universal message system of the nervous system. The intensity of the stimulus is coded as the frequency of action potentials. Although the message is coded in the same form regardless of the stimulus, organisms perceive different sensations because messages from sensors arrive at different places in the central nervous system (CNS).
Tactile hairs are an example of mechanosensors that are not neurons. From one surface, they have projections called stereocilia, which, when bent, alter receptor proteins in the hair cell's plasma membrane. When they are bent in one direction, the receptor potential becomes more positive. In this case, the hair cell releases a neurotransmitter to the sensory neurons associated with it, and these neurons in turn send action potentials to the brain.
The functions of touch and associated senses
As mentioned above, the sense of touch allowed many vertebrates and invertebrates to localize with some precision points of tactile stimulation at the body surface. The long antennae of many arthropods (e.g., crayfish) and the lengthened tactile hairs (vibrissae) on the snouts of nocturnally active mammals (such as cats and rats) enable them to sense objects in their immediate surroundings.
The sense of touch also functions in aspects of proprioception such as orientation and equilibrium. For example, sensory contact with the ground often gives terrestrial animals information about their spatial position. Many invertebrates have equilibrium organs (known as statocysts) that use hair cells to signal the position of the animal with respect to gravity. The mammalian inner ear also includes two organs of equilibrum that use hair cells to detect the body's position. Tactile hairs are found in the lateral line sensory system of fishes, a canal under the surface of the skin that provides information about movements of the fish through the water and about the moving objects that cause pressure waves in the surrounding water.
The technology of touch
Haptic, from the Greek αφή (Haphe), means pertaining to the sense of touch. Haptic technology refers to technology that interfaces the user via the sense of touch by applying forces, vibrations and/or motions to the user. This mechanical stimulation is used to create haptic virtual objects. Haptic technology has made it possible to investigate in detail how the human sense of touch works by allowing the creation of carefully-controlled haptic virtual objects. These objects are used to systematically probe human haptic capabilities. These new research tools contribute to our understanding of how touch and its underlying brain functions work.
ReferencesISBN links support NWE through referral fees
- Flanagan, J.R. and S.J. Lederman. 2001. Neurobiology: Feeling bumps and holes. Nature 412(6845):389-91.
- Hayward, V., Astley, O.R., Cruz-Hernandez, M., Grant, D., and G. Robles-De-La-Torre. 2004. Haptic interfaces and devices. Sensor Review 24(1):16-29.
- Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer. ISBN 0716766728.
- Robles-De-La-Torre, G. and V. Hayward. 2001. Force Can Overcome Object Geometry in the Perception of Shape through Active Touch. Nature 412(6845):445-8.
- Robles-De-La-Torre, G. 2006. The Importance of the Sense of Touch in Virtual and Real Environments. IEEE Multimedia 13(3):24-30.
External links
| Nervous system: Sensory systems/sense | |
|---|---|
| Special senses | Visual system/Visual perception • Auditory system/Hearing • Olfactory system/Olfaction • Gustatory system/Taste |
| Somatosensory system | Nociception • Thermoreception • Vestibular system • Mechanoreception (Pressure, Vibration, Proprioception) |
| Other | Sensory receptor |
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