Somatosensation is the set of sensory capacities dealing with bodily perception. This includes all skin sensation as well as proprioception and that of the internal organs. The main braches of the somatosensory system can be divided into the three main categories of cutaneous (skin) sensation, proprioception and kinesthesis as indicated below.

Tactile     Temperature     Pain
  \                    ‌                    /
Cutaneous Senses

Somatosensory System
/                                                           \
Proprioception                                          Kinesthesis

In terms of stimulus modality, there is a broad range of percepts that are interpreted by organisms. Human beings have a specific range of sensation that is possible for us to process, but certain animals have very different ranges. For example, migrating birds are able to sense the earth's magnetic field and similarly fish can detect present electrical fields.1 Such variations in perception can be accounted for by differences in somatosensation, or somatosensory systems.

Several different types of sensory receptors are employed during the processing of somatic percepts:

  • Cutaneous Senses:
  • Kinesthesis:
    • Mechanoreceptors: These deal with the kinestic motion of the body. Pacinian corpuscles add the capacities for touch perception in the muscles and the deeper layers of the skin, as well as around the joints. These receptors are also sensitive to pressure stimuli.
  • Proprioception: Proprioception is the sensory faculty of being aware of the position of limbs and the state of internal organs. Sensory information along these lines is conveyed in the same manner as kinesthetic information; the difference between the two of course being that kinesthetics deals with motion of the limbs, rather than their positioning.

These sensory receptors are not all the same; variations in axon (speed) type as well as density account for the differences in touch perception throughout the body. A good example of this is the differences in acuity of touch perception in the lips and fingers as opposed to the upper limbs and parts of the torso. This is due to the concentration of cutaneous receptors in certain areas; the tips of the fingers hold the highest density of said receptors in the human body.

Another central aspect to the somatosensory system is the primary sensory afferents. These cells are embedded in the skin, muscles, tendons, joints and deep tissues of the human body. All of them are grounded in the dorsal root ganglion and enter the spinal cord through the dorsal roots. After entering the spinal cord, afferents can branch in numerous directions, either heading to the brain's gray matter where further processing occurs or remaining local before travelling further up the nervous system. There are different types of afferents; each one encodes information in a different manner and terminate in a different layer of the dorsal horn so that the brain perceives stimuli as separate and distinct forms of sensation, such as pain, pressure and temperature. The types are as follows:

  • A-alpha: Afferents are large in diameter; they are myelinated and the fastest conducting of all types. They are associated with propioception and the mechanoreceptors.
  • A-beta: Afferents are medium in diameter; they are myelinated and fast conducting. They are also associated with the mechanoreceptors.
  • A-delta: Afferents are small in diameter; they are myelinated and slow conducting. They are associated with pain and temperature reception through nociceptors and thermoceptors.
  • C: Afferents are very small in diameter; they are unmyelinated and very slow conducting. They are associated with some temperature reception but mainly moderate sensations like itch.

Beyond considerations of the primary sensory afferents, there are two main pathways that carry sensory information to the brain. The first are the dorsal columns, which deal with carrying information about sensations of proprioception, touch, vibration and two point discrimination. Second are the spinothalamic tracts, which relay pain, temperature and some basic touch information.

Somatosensation, like any other sensory function, is dependent on many variables. Most important among those is the consumption of opiates like morphine and heroin; these drugs will inhibit all functioning of somatic receptors.


This is a basic outline of the very complex idea of somatic functioning.
See my sources for a more in-depth understanding of the faculties involved.

Sources:

1http://psy.ucsd.edu/~mgorman/Oct3001lecture.html
http://www.owlnet.rice.edu/~psyc362/lecturenotes/lec10a-outline.rtf
http://www.healthsci.utas.edu.au/neuro/chp310/lectures/somatosensory.html

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