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GENERAL COMPONENTS OF SOMATOSENSORY SYSTEM 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 will typically have three long neurons[1]: primary, secondary and tertiary (or first, second, and third).
  1. 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).
  2. 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 system or the cerebellum.
  3. 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 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. So 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 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[3], 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.

Touch Deprivation

Touch deprivation is when someone experiences an excessive lack in the sense of touch, often during the development in infancy, affecting the wellness of a person. Touch deprivation in infants leads to many different issues later in life. It affects the behavioral, health and physiological development of a human. With only minimal research in the field of touch deprivation, there is only a short history of the research and effects. Touch, before research conducted, was seen as only a minor impact of the development of a person. But according to an article by Robert Hatfield, Ph.D. from the University of Cincinnati, in 1945-1947, that view began to fall apart. Premature infants and sick toddlers were dying unexpectedly, so Dr. Rene Spitz, the caregiver, searched for explanation to the deaths. Not until 1958-1962, in Harry Harlows research, was the mystery solved (Hatfield). The children were not being provided enough touch. Through Harlows research with monkey infants he was able to discover the great importance of touch. Research following by John Bowlby and Mary Salter Ainsworth, confirmed the impact touch has on the attachment theory (Hatfield). Robert Hatfield discusses the results of these experiments and stated, Affectionate touch vs. neglect or punishing touch is a central theme of Attachment Theory and much of this work may be viewed as the human research counterpart to the Harlow studies (Hatfield).

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. 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 m. Such vibrations are around 250 Hz, which is the optimal frequency sensitivity of Pacinian corpuscles. 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. senses