Everything about Touch totally explained
The
somatosensory system is a widespread and diverse
sensory system comprising of the receptors and processing centres to produce the
sensory modalities 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 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,
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.
At its simplest, the system works when a sensory
neurone is
triggered by a specific stimulus such as heat; this neurone 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
homunculus and is essential in the creation of
body image.
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)
neurones in the periphery (skin, muscle and organs for example), and further neurones within the
central nervous system.
General somatosensory pathway
A somatosensory pathway typically has three long neurons: primary, secondary and tertiary (or first, second, and third).
Periphery
In the periphery, the somatosensory system detects various stimuli by
sensory receptors, for example 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's 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.
Spinal cord
In the spinal cord, the somatosensory system 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 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.
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's 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's 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, although we don't 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.
Physiology
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.
Technology
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.
Further Information
Get more info on 'Touch'.
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