Somatotopic arrangement

Somatotopik or somatotopy (derived from Ancient Greek σῶμα [ soma ] = living body, body and τόπος [ topos ] = Location, location, tract of land, area, place, room ) is one of the relative position of the body parts corresponding structure of nerve tissue in the central nervous system (CNS) and peripheral nervous system. In the CNS, this structure is found in different sections. It is most pronounced in the area of ​​somatosensory and motor cortex. There, it has led to the designation homunculus. It refers to the somatotopic structure as a topographic- genetic law that is determined not only by the functional organization of neural pathways in the so-called line bundles tract or the structure of the nervous tissue of the CNS. It thus is a neuroanatomical blueprint, as it can be demonstrated not only in humans but also in different animal species, such as primates.

Origin and development of the concept

The assignment and correlation of body regions and cortex areas was called for in the 19th century by John Hughlings Jackson ( 1835-1911 ). Even before he had taken a " localizing movement " many natural and humanities researchers. This is demonstrated by today the term phrenology. The term was Somatotopik by the neurosurgeon Wilder Penfield (1891-1976) coined around 1940 to 1950 to describe the motor and sensory centers in the area of the human cortex and cerebellum closer. His knowledge was based on brain surgery on the open skull. Since then, other topical principles of operation of the CNS have been described according to the principle of sensorimotor projection tracts. It is in the Cape. 2.7 described somatotopic variants.

These variants initially refer to the classical five senses areas cf. the names tonotopy, retinotopy, etc. Of the five classical sensory modalities known to the skin and attributed to her sense of touch and the sense of the term somatotopy was decisive. Accordance with the principle to describe cortical representations of sensory performance as sensory centers, also relating to the sense of touch cortical representation has been named as somatosensory cortex. The traced back to the sense of touch representation of the cortical homunculus was logical to the extent as " somatotopic " representation set.

At the level of the spinal cord afferent pathways are known to be designated as sensitive and not as sensory pathways.

The from about 1940 to 1950 " newly coined " the term Somatotopik had, and have had far-reaching implications for understanding the workings of the brain. The one broad gestaltliche or physical unit true point - to-point arrangement of body representations fulfilled parent integrative functions within the CNS. Especially in neurophysiology but also in psychology and psychosomatic medicine the concept of integration has an important role.

The related only to the sense of touch concept of somatotopy and clarified with him uniform operating principle of the topical integration in the CNS would require a preamble for the relevant aspects of the other sense lines. - Even in psychology, the term Topik is significant. He appears for the topically arranged differently in the CNS sensory modalities useful as a generic term especially under physiological or empirical point of view. It would be appropriate for the neural processes in the sense of an anatomical Topik. In the English language is spoken in general terms of "place theory". But often the term somatotopy is still generalizing used in scientific literature for the specific topical structure in the CNS as a generic term for other sensory abilities as the different qualities of feeling.

That in the case of Somatotopik to be recognized principle of operation in the CNS is not limited to the application within a sensory- motor system. Rather this is only one of a plurality of feedback systems not only in the nervous system. Already the different somatosensory centers within the cerebellum and the cerebrum make a further integration of the various functional circuits required. Also in the Gestalt psychology play a role somatotopical aspects.

Recently, this integration output of the nervous system by simulating networks such as Kohonen explored in more detail and made understandable. Here, the structure of such a network plays a crucial role according to very specific or very different characteristics and not only according to physical principles shape. The physically - gestaltliche structure in the case of somatotopy is only one example of a design of Kohonenkarten. This is generally done according to the principles of similarity, frequency and importance (relevance).

Anatomically confirmed Somatotopik

Spinal cord

In the spinal cord somatotopic structure leads to a concentric or lamellar layered structure of the white matter around the central gray matter ( butterfly figure). The associated to higher (eg cervical ) sections of the spinal cord pathways lie at the central gray layer corresponding to lower (eg, sacral ) sections pathways are further outside. In contrast to these concentric somatotopic outline the functional organization of pathways is organized as pressure, pain and temperature sensation radially.

Pyramidal tract

Emerging from the fibers of the sensorimotor cortex primary area form a broad frontal initially asked subjects that quickly narrows as it enters the medulla of the hemisphere and into the internal capsule. This results in a helical twist around the fiber board. Within the internal capsule initially springing in front aligned pockets of the precentral gyrus bottom-most fibers lie rostral to the rotation, the springing up most occipital. In the area of the cerebral peduncle are then aligned in the frontal compartments of the primary motor area initially farthest down springing fibers after rotation medially. The above springing in the cortex fibers form the lateral or lateral part of the bundle.

Sensorimotor cortex

The sensorimotor cortex is the prime example somatotopischer outline. He has coined the term homunculus. Since the corresponding sensory-motor structure with perception and qualities of consciousness goes hand in hand, so that neuropsychological qualities are necessarily linked, see agnosia, body scheme, Topik ( Psychology ), Cognitive Psychology. By moving stimuli by the so-called primary sensory projection pathways to secondary and tertiary association areas in each new perception qualities arise. - The Homunkulustheorien of perception are to be regarded both as the subject of the history of philosophy and the history of psychology. Should also be noted that the figure for Körperrepresentation here is partially incorrect or outdated, as is now occupied by a study that human genitals are represented between the torso and the legs, and not below the toes in the longitudinal fissure.

Fissure of Sylvius

In the depth of the fissure of Sylvius there is a secondary motor area ( Area II ) with reverse somatotopischer outline.

Cerebellar cortex

The cerebellum receives virtually all parts of the nervous system messages and in turn, may exert its influence efferent fibers.

By means of evoked potentials could be detected close somatotopic connections to the cerebrum. Such somatotopic representations in the cerebellum were experimentally in different animal species, such as, for example, in primates, demonstrated why a similar Somatotopik is suspected in the human cerebellar cortex.

Segmental breakdown

A somatotopy can also be seen in the segmental or radicular structure of the nervous supply of the skin ( dermatome ) and muscle ( myotome ). The segmental innervation refers to sections structured neural supply of the body according to the linking of the embryonic somites ( somites ) of the notochord with associated ( adjacent ) sections of the - even anatomically comprehensible not articulated - Rückenmarksanalage. With radicular innervation is meant by individual spinal nerves.

Somatotopic variants

Distortions and jumps of the homunculus

Original and copy

The topic that has been displayed the sensory centers of projection refers to the human body and its external shape. It is this outer shape as somatic correlate or archetype and the cortex to somatotopic maps - despite certain distortions and jumps - still recognizable shape of the body called a projection or image.

Tape Shaped Figure

Distortions and jumps are due to the fact that the exact enough, actually only spatially mappable functions in the brain on a gyrus come in strip form and thus to a certain extent in only one dimension to the picture. For example, the tactile sensation of the body are represented qualities postcentral band shape in the primary projection centers of the gyrus. This organization initially appears useful because of the necessary narrow topical comparison of sensory and motor areas in the sensorimotor cortex. However, it is therefore not necessarily the manner of a reflex arc monosynatischen conceive. Also, the sense of hearing is projected band shape to the temporal gyri transverse processes ( Heschlsche cross turns). According to animal experiments, the basilar membrane is " rolled up " in all the centers of the auditory pathway.

A jump of the continuum shape union consists, for example, between your thumb and neck or between hand and head. In the gyrus of the dentate gyrus and the precentral gyrus closes - deviating from the serving as the model body shape - the neck directly to the thumb.

As shown seemingly distorted in their relations one can view the represented disproportionately in size hand. The apparent distortion of individual body sections to the dentate gyrus, such as the tongue also is related to the different topographical differentiation of tactile resolution capability. This was investigated experimentally in 1837 by Ernst Heinrich Weber using the calipers. The resolving power is greatest in the area of the tongue, lips and fingers. The corresponding to the different resolution of the skin with varying degrees of representation in the cerebral cortex is also called a " fineness of sensory > grain <". Body sections with high tactile resolution are topically represented seemingly disproportionate in comparison to the real human form in the dentate postcentalis and thus appear distorted topically, see also Figure of the Homunculus.

This Topik thus based on the anatomical structure of the tactile sensory stimulus intake and still recognizable in approaches segmental breakdown nervous supply to the muscles. For other senses each other somatic qualities are as " archetype " is based. This is understood especially when these sense qualities not included in the proprioceptive system ( Exterozeption ), see → a retinotopy and tonotopy.

Lateralization of the brain

The lateralization of the brain is to be regarded as a somatotopic variant. Both halves of the brain are not working independently but are linked by Kommissurenbahnen one another, especially in the corpus callosum. This, the center line intersecting paths meet specific physiological roles similar to those of the association tracts. Indeed, it is likely that both hemispheres of the brain perform different functions as well as the connected by association fibers primary cortical areas. Together until both brain Häften meet integrative functions of different centers. To this extent, neuronal pathways, which cross to the opposite side, not just a match with the opposite side, which is particularly important when damage one side of meaning, but to fulfill a specific task in the sense of complementarity, similar to the primary somatosensory and primary motor cortex. ( sensorimotor cortex). Concrete example of the lack of cooperation of both hemispheres of the brain is the split-brain symptoms.

Vision and Somatotopik

In the vision the structure of the nerve pathways is spatially oriented in a split into 4 quadrants field, an initially abstract thinking to the coordinate system of the image of the visible outside world ( " crosshair ", see the figure). Corresponding to the four quadrants of such imaginary coordinate system there are in fact four functionally diverse pathways from the eye to the brain, called the optic fibers. Similar to the sensorimotor beef area is also the vision the picture of sensory stimuli in the primary sensory centers of the calcarine sulcus ( area striata ) ribbon-shaped, albeit represented in two dimensions, that is, as areal image. The three-dimensional spatial vision is a service of higher visual sensory centers, the result of fusing the two received in each hemisphere two-dimensional images.

Hearing and Somatotopik

When hearing the exact perception of a sound source, including their localization is likewise a primary spatial problem of the orientation ( stereognosis or a spatial hearing ). However, much in the context of the detection Somatotopik the pitch and the differences in the pitch of a sound source. As the term already suggests the pitch as such, is listening to pitches down a spatial schema based, namely that of high and low tones. Below it is shown that a spirally coiled ribbon can be found as a geometrical archetype for the perception of pitch in the organ of Corti.

The wore while listening to the account of the current state of the theory of perception. Hermann von Helmholtz (1821-1894) is the founder of the resonance theory of hearing (1863, 1870). This theory states that the basilar membrane of the cochlea ( organ of Corti ) the role of strings such as those of a harp ( see Figure ) assumes that may be brought by corresponding external sound sources targeted for resonance. With the introduction of a stringed instrument as a spatial scheme was adopted as the model of the recognition of pitches. Present in the basilar membrane in radiärer direction of the screw axis and the spindle ( modiolus ) extending approximately from 24,000 fibrils were referred to as " Hörsaiten ", the entire the basilar membrane as resonator replacement. This " Hörsaiten " are in the basal turn of the cochlea at the shortest and longest at the screw tip. This could be explained, for example, that when the screw tip in question ( helicotrema ) disease processes a bass deafness is observed. Not suitable for this theory are the exact measurable length ratios of these " strings " to also measurable belonged frequency ranges to each very precise locations of the basilar membrane. Although this theory has since been disproved in individual parts, such as founded by Helmholtz - A place theory of hearing shall nevertheless remain valid. Also as outdated the sound image theory of Ernst Julius Richard Ewald is considered.

Meanwhile, it was assumed that the data transmitted by the stirrup to the oval window generate vibrations of the screw a traveling wave whose energy is transferred to different parts of the basilar membrane in accordance with the elasticity ratios for various wavelengths. The higher the frequency, the closer these locations - in accordance with the resonance theory - to the screw base. For these so-called dispersion theory of hearing ( traveling wave theory) of 1961 Georg von Békésy ( 1899-1972 ) Nobel Prize. - It raises the question to what extent the recent theory of the cochlear amplifier is to the basics of somatotopy in conjunction.

With the help of different neuroscientific methods several tonotopic maps have been described in the human brain.

Neuroplasticity

The variable used in principle to demanding natural somatotopic organization the principle of neuroplasticity is to be considered. This is understood the brain's ability to constantly adapt to the requirements of use. Nerve cells can thus change their functions under changing external conditions. The principle of neuroplasticity is opposite to that of somatotopy or the specific localization of brain performance. Neuroplasticity means that the benefits of brain cells can be retrained, as for example in phantom experiences, so after injuries that change the topical continuum. But not only for injuries and loss of peripheral nervous coverage areas such as in loss of limbs shows the effect of neuroplasticity, even with central lesions, such as in apoplexy, this is the case. Neuroplasticity is a prerequisite to regain lost abilities through neural learning new again. This ie a rigid relatedness is not given to specific brain functions and firm brain maps. The principle of somatotopy is thus not rigid but plastic.