Somatosensory Systems


Sensations of touch, temperature, pain and movement of joints may be perceived from all parts of the body. Specialized receptor cells convert the stimuli producing such sensations into electrical signals that are processed in the somatosensory pathways of the nervous system.

Keywords: mechanoreceptors; touch; pain; temperature sensation; body maps

Figure 1.

When the fingertip moves over a textured surface (in this case a grating of alternating grooves and ridges) with a velocity v, low‐threshold mechanoreceptors are excited. Vertical tick marks represent action potentials propagating along the axon. These occur cyclically, corresponding to the cyclic compression and relaxation of the skin, and provide information about the texture. The box shows a drawing of a Meissner corpuscle. a, myelinated axon; b, branching axon terminals within corpuscle; s, Schwann cells; c, collagen fibres attaching to epidermis (e). (Adapted from Andres KH and von During M (1973). In: Iggo A (ed.) Handbook of Sensory Physiology. Somatosensory System, pp. 3–28. Berlin: Copyright 1973, Springer‐Verlag GmbH & Co.)

Figure 2.

Structure of the spinal cord. (a) The dermatomes on the front of the body. C, cervical (red); T, thoracic (blue); L, lumbar (green); S, sacral (yellow). (b) A cross‐section of the spinal cord at the L4 level. The central core of grey matter is surrounded by white matter. Large myelinated ‘touch fibres’ proceed mainly up the dorsal column to the brainstem. Smaller ‘temperature and nociceptive fibres’ synapse on cells in the dorsal horn, which in turn cross the spinal cord and send axons up the anterolateral system to the brainstem. The dorsal horn can be divided into laminae (I to V shown). DRG, dorsal root ganglion cell; DC, dorsal column; ALS, anterolateral system.

Figure 3.

Central somatosensory pathways. (a) The dorsal column system conveying high‐resolution tactile information. (b) The spinothalamic tract is the major component of the anterolateral system which conveys pain and temperature information. Cross‐sections are shown at three levels of the central nervous system; the spinal cord, the medulla of the brainstem and a section through the cerebral hemispheres showing the cortex and the thalamus (sections are not to scale). DRG, dorsal root ganglion cell; DC, dorsal column; DCN, dorsal column nuclei; ML, medial lemniscus; ALS, anterolateral system; SST, spinothalamic tract.

Figure 4.

A cross‐section through the postcentral gyrus showing the topographic representation of the body surface (often called the homunculus). Regions with high acuity, such as the fingertips and the lips, have a disproportionately large representation. (Adapted from Penfield W and Rasmussen T (1950) The Cerebral Cortex of Man: a Clinical Study of Localization of Function. New York: Macmillan.)

Figure 5.

Whisker barrel fields in the cerebral cortex (upper figures) and the corresponding whiskerpads (lower figures) for three strains of mice. In the normal mouse (NOR) the barrel pattern matches the whiskerpad pattern. In the H/H and MAP strains, bred for additional whiskerpads (shown shaded), the barrel field has corresponding additional barrels (outlined in red). (Adapted from Welker, E and Van der Loos, H (1986) Quantitative correlation between barrel‐field size and the sensory innervation of the whiskerpad: a comparative study in six strains of mice bred for different patterns of mystacial vibrissae. Journal of Neuroscience6: 3355–3373. Copyright 1986, Society for Neuroscience.)



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Further Reading

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Willis WD and Westlund KN (1997) Neuroanatomy of the pain system and of the pathways that modulate pain. Journal of Clinical Neurophysiology 14: 2–31.

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Goodwin, Antony W(Sep 2005) Somatosensory Systems. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0004082]