Somatosensory system

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 neuron 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 a 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), to deeper 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).
 * 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 nerve ganglia or the ganglia of other sensory cranial nerves).
 * The second neuron has its cell body either in the spinal cord or in the brainstem. This neurones 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 system or the cerebellum.
 * 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.

Periphery
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 casual 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 are projected 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 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. Interestingly, one study showed somatosensory cortex was found to be 21% thicker in 24 migraine sufferers, on average than in 12 controls, 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.

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 then an action potential is initiated. This action potential then (usually) travels inward towards the spinal cord.

Technology
The new research area of haptic technology can provide touch sensation in virtual and real environments. This exciting new area has started to provide critical insights into touch capabilities.