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:''For the study of touching behaviour in humans, see [[Haptics]].''
 
:''For the study of touching behaviour in humans, see [[Haptics]].''
   
'''Somatic sensation''' consists of the various [[sensory receptors]] that trigger the experiences labelled as [[touch]] or [[pressure]], [[temperature]] (warm or cold), [[Pain and nociception|pain]] (including [[itch]] and tickle), and the sensations of muscle movement and joint position including [[posture]], [[locomotion|movement]], and facial expression (collectively also called [[proprioception]]).<BR>
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The '''somatosensory system''' is a diverse [[sensory system]] composed of the receptors and processing centres to produce the [[sensory modality|sensory modalities]] such as touch, [[temperature]], [[proprioception]] (body position), and [[nociception]] (pain). The [[sensory receptors]] cover the [[skin]] and [[epithelia]], [[skeletal muscle]]s, [[bone]]s and [[joint]]s, internal [[organ (anatomy)|organ]]s, and the [[cardiovascular system]]. While '''touch''' (also, more formally, '''tactition'''; [[adjectival form]]: "tactile" or "somatosensory") is considered one of the five traditional [[sense]]s, the impression of touch is formed from several modalities. In medicine, the colloquial term touch is usually replaced with '''somatic senses''' to better reflect the variety of mechanisms involved.
   
:A more complex concept comes into play when the term is used in reference to [[human]] beings. The sense of touch is mediated by the '''[[somatosensory system]]'''. '''[[Touch]]''' may simply be considered one of five human [[sense]]s; however, when a person touches something or somebody this gives rise to various [[feeling]]s: the perception of [[pressure]] (hence [[shape]], [[soft]]ness, [[texture]], [[oscillation|vibration]], etc.), relative [[temperature]] and sometimes [[Pain and nociception|pain]]. Thus the term "touch" is actually the combined term for several senses. In medicine, the colloquial term "touch" is usually replaced with '''somatic senses''', to better reflect the variety of mechanisms involved. <BR>
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The system reacts to diverse [[stimulus (physiology)|stimuli]] using different receptors: [[thermoreceptors]], [[nociceptors]], [[mechanoreceptors]] and [[chemoreceptors]]. Transmission of information from the receptors passes via [[sensory nerve]]s through tracts in the [[spinal cord]] and into the brain. Processing primarily occurs in the [[postcentral gyrus|primary somatosensory area]] in the [[parietal lobe]] of the [[cerebral cortex]].
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[[File:Sensory Homunculus.png|thumb|right|The [[cortical homunculus]] was devised by [[Wilder Penfield]]. ]]
The somatosenses include: Cutaneous ([[skin]]), [[Proprioception|kinesthesia]] (movement) and visceral (internal) senses, of which the first two are better known. Visceral senses have to do with sensory information from within the body, such as [[stomach ache]]s. <BR>
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At its simplest, the system works when activity in a sensory [[neuron]] is [[sensory transduction|triggered]] by a specific stimulus such as heat; this signal eventually passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location. The point-to-point mapping of the body surfaces in the brain is called a [[homunculus]] and is essential in the creation of a [[body image]]. This brain-surface ("cortical") map is not immutable, however. Dramatic shifts can occur in response to stroke or injury.
   
 
== Anatomy ==
 
== Anatomy ==
===Human brain localization===
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The somatosensory system is spread through all major parts of a [[mammal]]'s body (and other [[vertebrates]]). It consists both of [[sensory receptors]] and sensory ([[afferent]]) [[neurons]] in the periphery (skin, muscle and organs for example), to deeper neurones within the [[central nervous system]].
The <b>somatosensory system</b><ref>Nolte J.The Human Brain 5th ed. 2002. Mosby Inc, Missouri.</ref>consists of ascending pathways from the body to the [[postcentral gyrus]] in the cerebral cortex, namely the [[Dorsal_column-medial_lemniscus_pathway|Dorsal Column Medial Lemniscal]] pathway, the [[Ventral_spinothalamic_tract|Ventral Spinothalamic pathway]], [[Ventral_spinocerebellar_tract|ventral]] and [[Dorsal_spinocerebellar_tract|dorsal spinocerebellar tracts]].<BR>
 
   
The primary somatosensory area in the human cortex is located in the [[postcentral gyrus]] (Parietal Lobe). It is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus in this location. For the primary somatosensory cortex, this is called the sensory homunculus. Areas of this part of the [[human brain]] map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. This somatosensory map is termed the [[homunculus]].
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===General somatosensory pathway===
   
==Psychology==
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A somatosensory pathway will typically have three long neurons<ref>Saladin KS. Anatomy and Physiology 3rd edd. 2004. McGraw-Hill, New York.</ref>: primary, secondary and tertiary (or first, second, and third).
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* The '''first''' neuron always has its [[cell body]] in the [[dorsal root ganglion]] of the [[spinal nerve]] (if sensation is in head or neck, it will be the [[Trigeminal ganglion|trigeminal nerve ganglia]] or the ganglia of other sensory [[cranial nerves]]).
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* The '''second''' neuron has its [[cell body]] either in the spinal cord or in the brainstem. This neuron's ascending [[axons]] will cross ([[decussate]]) to the opposite side either in the [[spinal cord]] or in the [[brainstem]]. The [[axons]] of many of these [[neurones]] terminate in the [[thalamus]] (for example the [[ventral posterior nucleus]], VPN), others terminate in the [[Reticular_activating_system|reticular system]] or the [[cerebellum]].
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* In the case of touch and certain types of pain, the '''third''' neuron has its [[cell body]] in the VPN of the thalamus and ends in the [[postcentral gyrus]] of the [[parietal lobe]].
   
A somatosensory pathway typically has three long neurons<ref>Saladin KS. Anatomy and Physiology 3rd ed. 2004. McGraw-Hill, New York.</ref>: primary, secondary and tertiary (or first, second, and third).
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===Periphery===
* The <b>first</b> neuron always has its body in the dorsal root ganglion of the spinal nerve (if sensation is in head or neck, it will be the [[Trigeminal_ganglion|trigeminal nerve ganglia]] or ganglia of other sensory nerves).
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In the periphery, the somatosensory system detects various stimuli by [[sensory receptors]], e.g. by [[mechanoreceptors]] for tactile sensation and [[nociceptors]] for [[pain sensation]]. The sensory information (touch, pain, temperature etc.,) is then conveyed to the [[central nervous system]] by [[afferent]] [[neurones]]. There are a number of different types of [[afferent]] [[neurones]] which vary in their size, structure and properties. Generally there is a correlation between the type of [[sensory modality]] detected and the type of afferent neurone involved. For example, slow, thin, unmyelinated [[neurones]] conduct pain whereas faster, thicker, [[myelinated]] [[neurones]] conduct casual touch.
* The <b>second</b> neuron has its body either in the spinal cord or in the brainstem, and will cross (or decussate) to the opposite side and terminate in the thalamus. In the case of the somatosensory system, the pathways all terminate in the [[List_of_thalamic_nuclei|ventro-posterior nucleus]] of the thalamus.
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* The <b>third</b> neuron has its body in the VP nucleus of the thalamus and ends in the [[postcentral gyrus]] of the [[cerebral cortex]], which is the primary somatosensory area in the human cortex.
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===Spinal cord===
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In the spinal cord, the somatosensory system <ref>Nolte J.The Human Brain 5th ed. 2002. Mosby Inc, Missouri.</ref> includes ascending pathways from the body to the [[brain]]. One major target within the [[brain]] is the [[postcentral gyrus]] in the [[cerebral cortex]]. This is the target for neurons of the [[Dorsal column-medial lemniscus pathway|Dorsal Column Medial Lemniscal]] pathway and the [[Ventral spinothalamic tract|Ventral Spinothalamic pathway]]. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of [[Human position|posture]] are projected to the [[cerebellum]]. These include the [[Ventral spinocerebellar tract|ventral]] and [[dorsal spinocerebellar tract]]s. Another important target for [[afferent]] [[somatosensory]] [[neurons]] which enter the [[spinal cord]] are those neurons involved with local segmental [[reflexes]].
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===Brain===
  +
The primary somatosensory area in the human cortex is located in the [[postcentral gyrus]] of the [[parietal lobe]]. The postcentral gyrus is the location of the ''primary somatosensory area'', the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a [[homunculus]] at this location. For the primary somatosensory cortex, this is called the [[sensory homunculus]]. Areas of this part of the [[human brain]] map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Somatosensory information involved with [[proprioception]] and posture also targets an entirely different part of the brain, the [[cerebellum]].
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  +
==Physiology==
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Initiation of somatosensation begins with activation of a physical "receptor". These somatosensory receptors tend to lie in skin, organs or muscle. The structure of these receptors is broadly similar in all cases, consisting of either a "[[free nerve ending]]" or a nerve ending embedded in a specialised capsule. They can be activated by movement ([[mechanoreceptor]]), pressure ([[mechanoreceptor]]), chemical ([[chemoreceptor]]) and/or temperature. Another activation is by [[vibration]]s generated as a finger scans across a surface. This is the means by which we can sense fine textures in which the spatial scale is less than 200&nbsp;[[micrometre|µm]]. Such vibrations are around 250&nbsp;Hz, which is the optimal frequency sensitivity of [[Pacinian corpuscle]]s.<ref>Scheibert J, Leurent S, Prevost A, Debrégeas G. (2009). The role of fingerprints in the coding of tactile information probed with a biomimetic sensor. Science. 323(5920):1503-6. PMID 19179493 {{DOI|10.1126/science.1166467 }}</ref> In each case, the general principle of activation is similar; the stimulus causes [[depolarisation]] of the nerve ending and then an [[action potential]] is initiated. This [[action potential]] then (usually) travels inward towards the [[spinal cord]].
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==Diseases==
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A somatosensory deficiency may be caused by a [[peripheral neuropathy]] involving peripheral nerves of the somatosensory system.
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This may present as [[numbness]] or [[paresthesia]].
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Evaluation of any suspected disease of the somatosensory system is included in a [[Neurological_examination#Peripheral_nervous_system|neurological examination of the peripheral nervous system]]
   
 
==Technology==
 
==Technology==
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  +
The new research area of [[haptic technology]] can provide touch sensation in virtual and real environments. This new discipline has started to provide critical insights into touch capabilities. In the field of [[speech therapy]], tactile feedback has begun to be used to treat speech disorders.
   
The new research area of [[haptic]] technology allows to provide touch sensation in virtual and real environments. This exciting new area has started to provide critical insights into touch capabilities.<BR>
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==See also==
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* [[Allochiria]]
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* [[Cell signaling]]
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* [[Cutaneous sense]]
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* [[Mechanoreceptor]]
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* [[Molecular Cellular Cognition]]
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* [[Nociceptor]]
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* [[Muscle spindle]]
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* [[Proprioception]]
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* [[Special senses]]
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* [[Somatosensory Rehabilitation of Pain]]
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* [[Vibratese]], method of communication through touch
   
 
==References==
 
==References==
 
<references/>
 
<references/>
   
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  +
==Further reading==
 
Flanagan, J.R., Lederman, S.J. [http://brain.phgy.queensu.ca/flanagan/papers/FlaLed_NAT_01.pdf Neurobiology: Feeling bumps and holes], News and Views, Nature, 2001 Jul. 26;412(6845):389-91.
 
Flanagan, J.R., Lederman, S.J. [http://brain.phgy.queensu.ca/flanagan/papers/FlaLed_NAT_01.pdf Neurobiology: Feeling bumps and holes], News and Views, Nature, 2001 Jul. 26;412(6845):389-91.
   
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==See also==
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* [[Cell signaling]]
 
* [[Special senses]]
 
* [[Mechanoreceptor]]
 
* [[Molecular Cellular Cognition]]
 
* [[Nociceptor]]
 
* [[Muscle spindle]]
 
* [[Proprioception]]
 
   
 
==External links==
 
==External links==

Revision as of 11:58, November 20, 2011

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For the study of touching behaviour in humans, see Haptics.

The somatosensory system is a diverse sensory system composed of the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception (body position), and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch (also, more formally, tactition; adjectival form: "tactile" or "somatosensory") is considered one of the five traditional senses, the impression of touch is formed from several modalities. In medicine, the colloquial term touch is usually replaced with somatic senses to better reflect the variety of mechanisms involved.

The system reacts to diverse stimuli using different receptors: thermoreceptors, nociceptors, mechanoreceptors and chemoreceptors. Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cord and into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.

File:Sensory Homunculus.png
The cortical homunculus was devised by Wilder Penfield.

At its simplest, the system works when activity in a sensory neuron is triggered by a specific stimulus such as heat; this signal eventually passes to an area in the brain uniquely attributed to that area on the body—this allows the processed stimulus to be felt at the correct location. The point-to-point mapping of the body surfaces in the brain is called a homunculus and is essential in the creation of a body image. This brain-surface ("cortical") map is not immutable, however. Dramatic shifts can occur in response to stroke or injury.

Anatomy

The somatosensory system is spread through all major parts of a mammal's body (and other vertebrates). It consists both of sensory receptors and sensory (afferent) neurons in the periphery (skin, muscle and organs for example), to deeper neurones within the central nervous system.

General somatosensory pathway

A somatosensory pathway will typically have three long neurons[1]: primary, secondary and tertiary (or first, second, and third).

Periphery

In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g. by mechanoreceptors for tactile sensation and nociceptors for pain sensation. The sensory information (touch, pain, temperature etc.,) is then conveyed to the central nervous system by afferent neurones. There are a number of different types of afferent neurones which vary in their size, structure and properties. Generally there is a correlation between the type of sensory modality detected and the type of afferent neurone involved. For example, slow, thin, unmyelinated neurones conduct pain whereas faster, thicker, myelinated neurones conduct casual touch.

Spinal cord

In the spinal cord, the somatosensory system [2] includes ascending pathways from the body to the brain. One major target within the brain is the postcentral gyrus in the cerebral cortex. This is the target for neurons of the Dorsal Column Medial Lemniscal pathway and the Ventral Spinothalamic pathway. Note that many ascending somatosensory pathways include synapses in either the thalamus or the reticular formation before they reach the cortex. Other ascending pathways, particularly those involved with control of posture are projected to the cerebellum. These include the ventral and dorsal spinocerebellar tracts. Another important target for afferent somatosensory neurons which enter the spinal cord are those neurons involved with local segmental reflexes.

Brain

The primary somatosensory area in the human cortex is located in the postcentral gyrus of the parietal lobe. The postcentral gyrus is the location of the primary somatosensory area, the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space called a homunculus at this location. For the primary somatosensory cortex, this is called the sensory homunculus. Areas of this part of the human brain map to certain areas of the body, dependent on the amount or importance of somatosensory input from that area. For example, there is a large area of cortex devoted to sensation in the hands, while the back has a much smaller area. Somatosensory information involved with proprioception and posture also targets an entirely different part of the brain, the cerebellum.

Physiology

Initiation of somatosensation begins with activation of a physical "receptor". These somatosensory receptors tend to lie in skin, organs or muscle. The structure of these receptors is broadly similar in all cases, consisting of either a "free nerve ending" or a nerve ending embedded in a specialised capsule. They can be activated by movement (mechanoreceptor), pressure (mechanoreceptor), chemical (chemoreceptor) and/or temperature. Another activation is by vibrations generated as a finger scans across a surface. This is the means by which we can sense fine textures in which the spatial scale is less than 200 µm. Such vibrations are around 250 Hz, which is the optimal frequency sensitivity of Pacinian corpuscles.[3] In each case, the general principle of activation is similar; the stimulus causes depolarisation of the nerve ending and then an action potential is initiated. This action potential then (usually) travels inward towards the spinal cord.

Diseases

A somatosensory deficiency may be caused by a peripheral neuropathy involving peripheral nerves of the somatosensory system.

This may present as numbness or paresthesia.

Evaluation of any suspected disease of the somatosensory system is included in a neurological examination of the peripheral nervous system

Technology

The new research area of haptic technology can provide touch sensation in virtual and real environments. This new discipline has started to provide critical insights into touch capabilities. In the field of speech therapy, tactile feedback has begun to be used to treat speech disorders.

See also

References

  1. Saladin KS. Anatomy and Physiology 3rd edd. 2004. McGraw-Hill, New York.
  2. Nolte J.The Human Brain 5th ed. 2002. Mosby Inc, Missouri.
  3. Scheibert J, Leurent S, Prevost A, Debrégeas G. (2009). The role of fingerprints in the coding of tactile information probed with a biomimetic sensor. Science. 323(5920):1503-6. PMID 19179493
    1. REDIRECT Template:Doi


Further reading

Flanagan, J.R., Lederman, S.J. Neurobiology: Feeling bumps and holes, News and Views, Nature, 2001 Jul. 26;412(6845):389-91.

Hayward V, Astley OR, Cruz-Hernandez M, Grant D, Robles-De-La-Torre G. Haptic interfaces and devices. Sensor Review 24(1), pp. 16-29 (2004).

Robles-De-La-Torre G., Hayward V. Force Can OvercomFLARGUSe Object Geometry In the perception of Shape Through Active Touch. Nature 412 (6845):445-8 (2001).

Robles-De-La-Torre G. The Importance of the Sense of Touch in Virtual and Real Environments. IEEE Multimedia 13(3), Special issue on Haptic User Interfaces for Multimedia Systems, pp. 24-30 (2006).



External links

Nervous system - Sensory system - edit
Special sensesVisual system | Auditory system | Olfactory system | Gustatory system
Somatosensory systemNociception | Thermoreception | Vestibular system |
Mechanoreception (Pressure, Vibration & Proprioception) | Equilibrioception 
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