- Somatosensory system
The somatosensory system is a widespread and diverse
sensory systemcomprising the receptors and processing centres to produce the sensory modalities touch, temperature, proprioception(body position), and nociception(pain). The sensory receptorscover the skinand epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While touch 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, mechanoreceptorsand chemoreceptors. Transmission of information from the receptors passes via sensory nerves through tracts in the spinal cordand into the brain. Processing primarily occurs in the primary somatosensory area in the parietal lobeof the cerebral cortex.
At its simplest, the system works when a sensory
neuronis triggered by a specific stimulus such as heat; this neuron 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 mapping of the body surfaces in the brain is called a homunculusand is essential in the creation of a body image.
The somatosensory system is spread through all major parts of a
mammal's body (and other vertebrates). It consists both of sensory receptorsand sensory ( afferent) neuronesin the periphery (skin, muscle and organs for example), to deeper neurones within the central nervous system.
General somatosensory pathway
A somatosensory pathway typically has two long neurons [Saladin KS. Anatomy and Physiology 3rd ed. 2004. McGraw-Hill, New York.] : primary, secondary and tertiary (or first, second, and third).
* The first neuron always has its
cell bodyin the dorsal root ganglionof the spinal nerve(if sensation is in head or neck, it will be the trigeminal nerve ganglia or the ganglia of other sensory cranial nerves).
* The second neuron has its
cell bodyeither in the spinal cord or in the brainstem. This neuron's ascending axonswill cross ( decussate) to the opposite side either in the spinal cordor in the brainstem. The axonsof many of these neuronesterminate in the thalamus(for example the ventral posterior nucleus, VPN), others terminate in the reticular systemor the cerebellum.
* In the case of touch and certain types of pain, the third neuron has its
cell bodyin the VPN of the thalamus and ends in the postcentral gyrusof the parietal lobe.
In the periphery, the somatosensory system detects various stimuli by
sensory receptors, e.g. by mechanoreceptorsfor tactile sensation and nociceptorsfor pain sensation. The sensory information (touch, pain, temperature etc.,) is then conveyed to the central nervous systemby afferent neurones. There are a number of different types of afferent neuroneswhich vary in their size, structure and properties. Generally there is a correlation between the type of sensory modalitydetected and the type of afferent neurone involved. So for example slow, thin unmyelinated neuronesconduct casual touch whereas faster, thicker, myelinated neuronesconduct 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 brainis the postcentral gyrusin 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 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 postureare projected to the cerebellum. These include the ventral and dorsal spinocerebellar tracts. Another important target for afferent somatosensory neuroneswhich enter the spinal cordare those neurones involved with local segmental reflexes.
The primary somatosensory area in the human cortex is located in the
postcentral gyrusof 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 homunculusat this location. For the primary somatosensory cortex, this is called the sensory homunculus. Areas of this part of the human brainmap 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 proprioceptionand 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 depolarisationof the nerve ending and then an action potentialis initiated. This action potentialthen (usually) travels inward towards the spinal cord.
The new research area of
haptic technologycan provide touch sensation in virtual and real environments. This new discipline has started to provide critical insights into touch capabilities.
Molecular Cellular Cognition
*cite book |title=Medical Physiology |coauthors= Walter F. Boron |author=Emile L. Boulpaep |authorlink=Emile Boulpaep |year=2003 |publisher=Saunders |isbn=0-7216-3256-4 |pages=352-358
*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.
*Hayward V, Astley OR, Cruz-Hernandez M, Grant D, Robles-De-La-Torre G. [http://www.roblesdelatorre.com/gabriel/VH-OA-MC-DG-GR-04.pdf Haptic interfaces and devices] . Sensor Review 24(1), pp. 16-29 (2004).
*Robles-De-La-Torre G., Hayward V. [http://www.roblesdelatorre.com/gabriel/GR-VH-Nature2001.pdf Force Can Overcome Object Geometry In the perception of Shape Through Active Touch] . Nature 412 (6845):445-8 (2001).
*Robles-De-La-Torre G. [http://www.roblesdelatorre.com/gabriel/GR-IEEE-MM-2006.pdf The Importance of the Sense of Touch in Virtual and Real Environments] . IEEE Multimedia 13(3), Special issue on Haptic NO User Interfaces for Multimedia Systems, pp. 24-30 (2006).
* [http://www.informaworld.com/csmr 'Somatosensory & Motor research'] [http://www.informahealthcare.com (Informa Healthcare)]
* [http://hwr.nici.kun.nl/~miami/taxonomy/node21.html Overview]
* [http://www.sirinet.net/~jgjohnso/senses.html Somatic vs. Special senses]
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