somatic sensation

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somatic sensation Sensations arising from the skin — such as touch, pressure, cold, warmth, and pain — and from the muscles, tendons, and joints — such as the position of the limbs and pain — are known as somatic sensations. Soma, the Greek word for body, refers to the whole of the body structure, apart from the germ cells (eggs and sperm). Sensations arising from the internal organs (the viscera), such as pain or the sense of fullness of the stomach or bladder, may therefore be included, although they are usually considered separately as visceral sensations. Pain arising from the viscera is often felt as though it comes from some part of the body surface or underlying tissue (referred pain).

All somatic sensations start with the excitation of sensory receptors located in the appropriate tissue — skin, muscle, joints etc. But we are not passive recipients of stimuli, and indeed the amount of information received in a passive way is severely limited. We, and other animals, actively explore objects to obtain information about them. Somatic sensation is intimately associated with movement (and also with resistance to movement). We use our fingers and also our tongue and lips to explore objects in order to identify their structure and form. Good examples of such ‘active touch’ include reading of Braille characters and the sorting and selecting of objects in a pocket, out of sight. Coins can be selected on the basis of size, shape, weight, and other distinguishing characteristics, such as the presence of a milled edge. Metal objects may be differentiated from non-metal ones on the basis of perceived temperature differences due to their different heat conducting properties, or by their weight relative to their size.

Modern experimental research on the mechanisms underlying somatic sensation (somatosensory mechanisms) began in the nineteenth century with psychophysical experiments on humans, supported by studies of the structure of sense organs in both human and animal tissues. Then, with the advent of electronic amplifiers, in the 1920s and 1930s the emphasis switched to animal experiments, where it continues to the present: electrical recordings of neural activity evoked by the stimulation of sensory receptors have been made from all parts of the nervous system concerned with somatic sensation. In the past thirty years, electrical recordings have also been made from peripheral nerves in conscious human subjects, and these important experiments have added enormously to our understanding. Most recently, advanced imaging techniques have been used to examine which parts of the brain are active during particular tactual tasks in awake humans.

The sensory receptors in the different tissues and organs are highly selective (or specific). Each type responds only to a particular stimulus, such as mechanical displacement, or cooling, or warming, or harmful stimuli, and not to more than one such kind of stimulus. These receptors, in turn, are connected through chains of nerve cells (neurons) to the somatosensory areas of the brain's cerebral cortex in such a way that the specificity of the information is maintained — the ascending information travels along parallel pathways that may be considered as ‘pure lines’.

The idea that sense organs are specific for particular stimuli and that their excitation leads to specific sensations was first clearly stated in 1811 by the Edinburgh anatomist Sir Charles Bell, but is more commonly attributed to Johannes Müller, who elaborated his Law of Specific Nerve Energies in 1826. Müller did not distinguish between the various sensations that can be elicited from the body, being more concerned with the special senses such as vision. Various workers established that cutaneous sensation (that arising from the skin) is punctate (spotty) in character, and attempts were made to identify particular structures (sensory receptors) at the sites of the sensory spots. These attempts were, at best, only partially successful, although the classical theory of von Frey (1852–1932) allots a particular type of receptor to each of the main cutaneous sensations (touch, cold, warmth, and pain). It was not until careful animal experiments were carried out that the situation clarified. A most important step forward was made by E. D. (later Lord) Adrian, the Cambridge physiologist who, in the 1920s, showed that there are specific sensory receptors in skin and muscle responding only to particular stimuli, and that these receptors transform the stimuli into trains of nerve impulses which are conducted into the central nervous system along peripheral nerve fibres. The analysis of the quality of a stimulus is therefore carried out by specific receptors, while information about stimulus intensity is carried by the frequency of the nerve impulses in the sensory nerve fibres. We now know that all mammalian species, including humans, have the same types of sensory receptors in skin, muscle, tendon, and joints. Remarkable experiments initiated by the Swedish neurophysiologists, K.- E. Hagbarth and Å. B. Vallbo, in the late 1960s, in which electrical recordings were made from single peripheral nerve fibres in conscious human subjects, have confirmed that humans have the same sensory receptors as animals such as the cat. In addition, by electrically stimulating the individual nerve fibres from which recordings were made, they were able to determine the conscious experience (sensation) which results from activation of a particular receptor type. Thus, in the skin there are separate receptors responding to touch, light pressure, hair movement, vibration, cooling, warming, and harmful (painful) stimuli. In muscle and tendon there are receptors responding to muscle length, muscle tension, and harmful stimuli, and in the joint capsule there are receptors monitoring joint position and also responding to harmful events, these latter being exaggerated in inflammatory conditions, mimicking arthritic disease.

The peripheral sensory apparatus, consisting of the sensory receptors and the nerve fibres which connect them to the central nervous system, is therefore responsible for establishing which kinds of stimuli we can respond to, for setting the sensitivity of the system, and for determining the intensity of stimulation. Furthermore, it is also largely responsible for sensory acuity of the different parts of the body, because certain parts contain a higher density of receptors than others. There are very high densities of cutaneous receptors on the tips of the fingers, the lips, and the tongue: the parts of the body surface at which the greatest spatial resolution of sensation can be made, and the parts which are actively used to explore objects.

The information carried by the peripheral nerve fibres enters the central nervous system either at the spinal cord or, for information from the head, at the brain stem. Here the various inputs from different receptors are distributed into separate sets of ascending channels (pathways or components of pathways) and passed on to the cerebral cortex. Because of the selective channelling of information from different receptor types into different ascending neuronal pathways, it is possible for damage to a particular pathway to produce a selective loss of sensations. For example, damage to part of the spinal cord (posterior or dorsal columns) leads to loss of vibration sense, whereas damage to another part (anterolateral columns) may lead to loss of temperature sense.

As the sensory information ascends to the cerebral cortex, considerable neuronal processing occurs at places where one set of nerve fibres connects with the next set of nerve cells in the chain, usually in clearly-defined parts of the nervous system called ‘nuclei’. The processing extracts information from the input and performs analyses on it, such as the enhancement of contrasts (e.g. detection of edges), the orientation of linear stimuli, and the direction of movement of moving stimuli. At each processing station there is the opportunity for certain parts of the information to be suppressed, as would be necessary for selective attention. The nociceptive information — information concerning harmful events — that ultimately gives rise to the sensation of pain is commonly suppressed, especially during activities that are highly charged with emotion, such as during sports activities or in battle.

Each of the central processing stations, including those in the cerebral cortex itself (cortical somatosensory areas), are organized such that they contain a map of the body which can be revealed by recording from the nerve cells. Adjacent nerve cells are excited from adjacent parts of the body. In this way the nervous system locates the position at which a stimulus is acting on the body. Damage to part of one of these sensory maps, for example in the cerebral cortex, will produce sensory changes (a loss or reduction in a particular sensation or group of sensations) localized to a particular part of the body. It is therefore possible for a clinician to determine where brain damage might be located, by testing sensation. Similarly, with special averaging techniques it is possible to record, from the human scalp, the electrical and, more recently, the magnetic activity evoked in localized areas of the brain following localized stimulation of the body surface.

Initial processing in the cerebral cortex takes place in the somatosensory cortical areas. In order to allow for more subtle analysis by the brain, the information is then passed on to motor areas (since active motion is important in active touch) and also to other parts of the cortex (parietal cortex), where higher-order analysis takes place and where information from other senses is received as well. Here, the analysis of spatial relations is important, as is the co-ordination of eye and hand movements. Damage to the parietal cortex, especially on the side of the brain not concerned with language, leads to impairment in the ability to deal with extrapersonal space, and the patient may even deny that the opposite side of his body exists. Conversely, phantom sensations of movement may occur following amputations.

Alan G. Brown

See also sensory receptors; visceral sensation.