HOME WEB NEWS IMAGES CLASSIFIEDS YELLOW PAGESPOLLS - SURVEYS WIKI COUNTRIES PHOTOS US UK INDIA
Cities Stocks Weather Movies Coupons Travel Blogs Health Horoscope Maps ASIA EUROPE Classifieds Yellow Pages
|
 1
|
"Touch" redirects here. For other uses, see Touch (disambiguation).
For the study of touching behaviour in humans, see Haptics.
The somatosensory system is a sensory system that detects experiences labelled as touch or pressure, temperature (warm or cold), pain (including itch and tickle), as well as proprioception, which is the sensations of muscle movement and joint position including posture, movement, visceral (internal) senses and facial expression. Visceral senses have to do with sensory information from within the body, such as stomach aches.
Touch may be considered one of five human senses; however, when a person touches something or somebody this gives rise to various feelings: the perception of pressure (hence shape, softness, texture, vibration, etc.), relative temperature and sometimes 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.
Contents |
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) neurones in the periphery (skin, muscle and organs for example), and further neurones within the central nervous system.
A somatosensory pathway typically has three long neuronsSaladin KS. Anatomy and Physiology 3rd ed. 2004. McGraw-Hill, New York.: primary, secondary and tertiary (or first, second, and third).
In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g. by mechanoreceptors. 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. So for example slow, thin unmyelinated neurones conduct touch whereas faster, thicker, myelinated neurones conduct pain.
In the spinal cord, the somatosensory system Nolte J.The Human Brain 5th ed. 2002. Mosby Inc, Missouri. 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 neurones of the Dorsal Column Medial Lemniscal pathway and the Ventral Spinothalamic pathway. Note that many ascending 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 project to the cerebellum. These include the ventral and dorsal spinocerebellar tracts. Another important target for afferent somatosensory neurones which enter the spinal cord are those neurones involved with local segmental reflexes.
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 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. Interestingly, one study showed somatosensory cortex was found to be 21% thicker in 24 migraine sufferers, on average than in 12 controls"Thickening in the somatosensory cortex of patients with migraine." Alexandre F.M. DaSilva, Cristina Granziera, Josh Snyder, and Nouchine Hadjikhani. Neurology, Nov 2007; 69: 1990 - 1995. , although we do not yet know what the significance of this is. Somatosensory information involved with proprioception and posture also targets an entirely different part of the brain, the cerebellum.
Initiation of probably all "somatosensation" begins with activation of some sort of 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. In each case, the general principle of activation is similar; the stimulus causes depolarisation of the nerve ending and an action potential is inititated. This action potential then (usually) travels inward towards the spinal cord.
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.
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia
