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Archived Newsletters 2010-08-11 Touch - as described in Anatomy and Physiology Touch - as described in Anatomy and Physiology - The somatosensory system is
a diverse sensory system comprising the receptors and processing centres to
produce the sensory modalities such as 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 (also,
more formally, tactition;
adjectival form: "tactile" or "somatosensory") 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, nociceptors, 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 activity in a
sensory neuron is triggered by a specific stimulus such as heat; this signal
eventually 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 point-to-point mapping of the body surfaces in the brain is called a
homunculus and is essential in the creation of a body image. This brain-surface
("cortical") map is not immutable, however. Dramatic shifts can occur
in response to stroke or injury.
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) neurons 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: primary, secondary and tertiary (or first, second, and third).
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. 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 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. Somatosensory information involved with proprioception and
posture also targets an entirely different part of the brain, the cerebellum.
Physiology
Initiation of "somatosensation" begins with
activation of a 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 µm. 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.
Technology
The new research area of haptic
technology can
provide touch sensation in virtual and real environments. This new discipline
has started to provide critical insights into touch capabilities.
Vibratese is the method of communication through touch.. |
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