Somatotopic
Somatotopic or somatotopia (derived from ancient Greek σῶμα soma = living body, corpse and τόπος topos = place, place, region, region, locality, space) is a structure of nerve tissue in the central nervous system (CNS) and peripheral nervous system that corresponds to the relative position of the body parts . In the CNS this division is found in different sections. It is most pronounced in the area of the somatosensitive and motor cerebral cortex. There it led to the name homunculus . The somatotopic structure is also referred to as topographical-genetic law, which determines the structure of the nerve substance of the CNS in addition to the functional structure of the nerve tracts in the so-called tracts or bundles of lines. It is therefore a neuroanatomical blueprint , as it is not only in humans, but also in various animal species, such as B. can be detected in primates .
Origin and development of the term
The assignment and correspondence of body regions and cerebral cortex fields had already been demanded in the 19th century by John Hughlings Jackson (1835-1911). Even before him, a “localization movement” had gripped many researchers in the natural sciences and the humanities. The term phrenology still bears witness to this today . The term somatotopic was coined by the neurosurgeon Wilder Penfield (1891–1976) around 1940 to 1950 to describe the motor and sensory centers in the area of the human cortex and the cerebellum in more detail. His knowledge was based on brain surgery on the open skull. Since then, other similar topical functional principles of the CNS have been described based on the principle of sensorimotor projection trajectories. These are the ones in Chap. 2.7 described somatotopic variants .
These variants initially refer to the classic five sensory areas, cf. in addition the terms tonotopia , retinotopia etc. Of the five classic sensory modalities , it is known that the skin and the sense of touch ascribed to it or the feeling for the term somatotopia was decisive. In accordance with the principle of cortical representations of sensory abilities as sensory centers to designate the relating to the sense of touch was cortical representation as somato sensory cortex named. The representation of the cortical homunculus , which can be traced back to the sense of touch, was consequently defined as the “ somato tope” representation.
At the level of the spinal cord, afferent pathways are known to be called sensitive and not sensory pathways.
The term “somatotopic”, which was “newly coined” from around 1940 to 1950, had and has achieved far-reaching significance for understanding how the brain works. The point-to-point arrangement of body representations, which maintains a largely structural or physical unit, fulfills higher-level integrative functions within the CNS. Especially in neurophysiology but also in psychology and psychosomatic medicine , the concept of integration plays an important role.
The term somatotopia, which is only related to the sense of touch, and the uniform functional principle of topical integration in the CNS that is clarified with it, would require a generic term for the corresponding aspects of the other sensory lines. - The term topical is also important in psychology . It appears to be useful as a generic term for the sensory modalities that are topically differently arranged in the CNS, especially from a physiological or empirical point of view. It would be appropriate for the neuronal processes in the sense of an anatomical topic. In the English language the term “place theory” is used in general terms. But the term somatotopia is often still used in the scientific literature as a generalization for the specific topical structure in the CNS as a generic term for other sensory performances than the various qualities of feeling.
The functional principle in the area of the CNS that can be recognized in the case of the somatotopic is not limited to the application within a sensory-motor system. Rather, this is just one of a multitude of feedback systems not only in the nervous system. The different somatosensory centers within the cerebellum and the cerebrum already make further integration of the various functional groups necessary. Somatotopic aspects also play a role in Gestalt psychology .
Recently, this integration of the nervous system has been made possible by network simulations such as B. Kohonen networks have been researched and made understandable. The structure of such a network according to very specific or very different characteristics and not just according to physical design principles plays a decisive role. The physical-shape structure in the case of somatotopia is only one of the examples for a design of Kohonen cards . This is generally done according to the principles of similarity, frequency and importance (relevance).
Anatomically confirmed somatotopics
Spinal cord
In the spinal cord , the somatotopic structure leads to a concentric or lamellar layered structure of the white matter around the central gray matter (butterfly figure). The conduction paths belonging to higher (e.g. cervical) sections of the spinal cord lie against the central gray layer, while the conduction paths belonging to lower (e.g. sacral) sections lie further outside. In contrast to this concentric somatotopic structure, the functional structure for pathways such as pressure, pain and temperature sensation is structured radially .
Pyramidal track
The fibers of the primary sensorimotor area emerging from the cerebral cortex form a wide, initially frontal, fan that quickly narrows when it enters the medullary bed of the hemisphere and reaches the inner capsule. This leads to a helical twisting of the entire fiberboard. Within the internal capsule are in the frontal aligned compartments of the precentral gyrus most initially below springing fibers after rotation rostral , the above springing most occipital . In the area of the cerebral thighs , the fibers in the frontally oriented fan of the primary motor area are located medially after the rotation, which initially originates furthest below . The fibers emerging from the top of the cerebral cortex form the lateral or lateral parts of the bundle.
Sensorimotor cortex
The sensorimotor cortex is the prime example of somatotopic structure. He coined the term homunculus. Since the corresponding sensorimotor structure goes hand in hand with perceptual and consciousness qualities, neuropsychological qualities are necessarily connected with it, see agnosias , body scheme , topics (psychology) , perceptual psychology . By shifting primary stimuli through the so-called sensory projection paths to secondary and tertiary association centers, new perceptual qualities arise. - The homunculus theories of perception are to be regarded both as an object of the history of philosophy and the history of psychology. It should also be noted that the figure for the body representation is partially incorrect or out of date, since a study has now shown that the human genitals are represented between the trunk and the legs and not below the toes in the longitudinal fissure.
Fossa Sylvii
In the depth of the Sylvian fossa there is a secondary motor region (field II) with an inverted somatotopic structure.
Cerebellar cortex
The cerebellum receives messages from practically all parts of the nervous system and can in turn exert an influence through its efferents.
Close somatotopic connections to the cerebrum could be demonstrated by means of evoked potentials. Such somatotopic representations have been experimentally found in the cerebellum in various animal species, such as B. also in primates, which is why a similar somatotopic is suspected in the human cerebellar cortex.
Segmental structure
A somatotopy can also be recognized in the segmental or radicular structure of the nervous supply to the skin ( dermatomes ) and muscles ( myotomes ). The segmental innervation refers to the sectionally structured neural supply of the body according to the embryonic connection of the ursal segments ( somites ) of the notochord with assigned (adjacent) sections of the spinal cord anlage, which is itself anatomically not structured. With radicular innervation is by individual spinal meant.
Somatotopic variants
Distortions and cracks of the homunculus
Archetype and image
The topics of the sensory projection centers presented so far refer to the human body and its external shape. This external shape can also be described as a somatic correlate or archetype and the shape of the body, which is still recognizable on somatotopic maps of the cortex - despite certain distortions and jumps - as a projection or image .
Ribbon-shaped figure
Distortions and jumps can be traced back to the fact that the functions in the brain , which can actually only be mapped spatially, are mapped in the form of a band on a cerebral turn and thus only one-dimensional, so to speak . So z. B. the tactile sensory qualities of the body in the primary projection centers of the postcentral gyrus represented band-shaped. This organization initially appears sensible because of the close topical comparison of sensory and motor areas in the sensorimotor cortex that is necessary . It is therefore not necessarily to be understood as a monosynaptic reflex arc . The sense of hearing is also projected in the form of a ribbon onto the transverse temporal gyri (Heschl's transverse turns). According to animal experiments, the basilar membrane is "rolled up" in all centers of the auditory pathway.
A leap in the form continuum exists z. B. between thumb and neck or between hand and head. In the cerebral convolutions of the postcentral gyrus and also the precentral gyrus - in contrast to the shape of the body that serves as the archetype - the neck connects directly to the thumb.
The hand, which is represented disproportionately in terms of size, can be seen as apparently distorted in its relations . The apparent distortion of individual body sections on the postcentral gyrus such as B. also the tongue is related to the topographically different differentiation of the tactile resolution . This was investigated experimentally as early as 1837 by Ernst Heinrich Weber using the caliper. The resolving power is greatest in the area of the tongue, lips and fingers. The varying degrees of representation in the cerebral cortex, corresponding to the different resolving power of the skin, is also referred to as the "fineness of the sensory› grain ‹". Body sections with a high tactile resolution are topically represented in the postcentral gyrus in an apparently disproportionate manner compared to the real human figure and thus appear topically distorted, cf. also Fig. of the homunculus.
This topic is thus based on the anatomical structure of the tactile sensory stimulus reception and the segmental structure of the nervous supply of the muscles, which can still be seen in the beginning. Other sense organs are based on different somatic qualities as the "archetype". This is particularly clear when these sensory qualities do not belong to the proprioceptive system ( exception ) , see also retinotopy and tonotopy .
Lateralization of the brain
The lateralization of the brain can also be viewed as a somatotopic variant. Both halves of the brain do not work independently of each other, but are connected by commissure tracts , especially in the corpus callosum . These pathways crossing the center line fulfill specific physiological tasks comparable to those of the association pathways . It can be assumed that both halves of the brain fulfill different functions, as do the primary cortical areas connected by association fibers . Only both hemispheres of the brain together fulfill integrative functions of different centers. In this respect, neural pathways that cross to the opposite side not only make a comparison with the opposite side, which is particularly important when one side is damaged, but also fulfill a specific task in the sense of complementarity , similar to the primarily somatosensory and primarily motor cortex ( sensorimotor cortex ). A specific example of the lack of cooperation between the two halves of the brain is the split brain symptoms.
Eyesight and somatotopic
The “ crosshairs ” built into the anatomical organization of vision as a geometrical archetype when seeing
or
spatial orientation and the origin of the four optic fibers.
This is also known as retinotopy .
In the case of vision , the structure of the nerve tracts is spatially based on a field of vision divided into 4 quadrants , a coordinate system of the image of the visible outside world that is initially to be thought of as abstract (“crosshairs”, see the illustration). Corresponding to the four quadrants of such an imaginary coordinate system, however, there are specifically four functionally different pathways from the eye to the brain, the so-called optic fibers. Similar to the sensorimotor cortical area, the visualization of the sensory stimuli in the primary sensory centers of the sulcus calcarinus ( area striata ) is band-shaped, even if it is represented two-dimensionally. H. as a two-dimensional image. The three-dimensional spatial vision is an achievement of the higher visual sensory centers, which arise from the fusion of the two two-dimensional images received in each half of the brain.
Hearing and somatotopics
With two harps standing next to each other, after plucking a string on one instrument, the same-tuned string of the other instrument will also vibrate.
Spiral-shaped geometric archetype when hearing
or
transformation of the acoustic wave band into a band-shaped somatic continuum through the organ of Corti .
This is also known as tonotopy .
In the case of hearing , the exact perception of a sound source including its localization is also a primarily spatial problem of orientation ( stereognosis or spatial hearing). In the context of somatotopics, however, it is also essential to recognize the pitch and the differences in pitch of a sound source. As the term pitch as such already suggests, hearing pitches is based on a spatial scheme, namely that of high and low tones. In the following it is shown that a spiral rolled up ribbon can be found in the organ of Corti as a geometrical archetype for the perception of the pitch.
The current state of perception theory in hearing has taken this into account. Hermann von Helmholtz (1821–1894) is the founder of the resonance theory of hearing (1863, 1870). This theory suggests that the basilar membrane of the snail ( organ of Corti ) plays the role of strings such as B. those of a harp (see Fig.), Which can be brought to resonance by appropriate external sound sources. With the idea of a stringed instrument, such a spatial scheme was adopted as the archetype for recognizing pitches. The approximately 24,000 fibrils in the basilar membrane, which run in a radial direction from the screw axis or the spindle (modiolus), were referred to as "hearing strings", the entire basilar membrane as a resonator replacement. These "listening strings" are shortest in the basal spiral and longest at the tip of the screw. This could z. For example, it can be explained that in the case of disease processes affecting the snail tip ( Helicotrema ), a bass deafness is observed. The exact measurable length ratios of these "strings" to the frequency ranges that are also measurable at very precise locations on the basilar membrane do not fit this theory. Even if this theory has since been refuted in individual parts, the one-place theory of hearing founded by Helmholtz still remains valid. The sound image theory by Ernst Julius Richard Ewald is also seen as outdated .
In the meantime it has been assumed that the vibrations transmitted through the stapes to the oval window of the snail generate a traveling wave, the energy of which is transmitted to different points of the basilar membrane according to the elasticity conditions for different wavelengths. The higher the frequency, the closer these points are - in accordance with resonance theory - to the screw base. Georg von Békésy (1899–1972) received the Nobel Prize in 1961 for this so-called dispersion theory of hearing (traveling wave theory) . The question arises as to how far the more recent theory of the cochlear amplifier is related to the fundamentals of somatotopia.
Several tonotopic maps in the human brain have been described using different neuroscientific techniques .
Neuroplasticity
The principle of neuroplasticity is to be seen as a variable in the natural somatotopic organization that is fundamentally required . This is understood as the ability of the brain to constantly adapt to the requirements of use. Nerve cells can therefore change their functions under changed external conditions. The principle of neuroplasticity is opposite to that of somatotopy or that of the specific localizability of brain functions. Neuroplasticity means that the performance of brain cells can be retrained, e.g. B. in phantom experiences, i.e. after injuries that change the topical continuum. But not only with injuries and loss of peripheral nervous supply areas such as B. loss of limbs shows the effect of neuroplasticity, even with central lesions such. B. with apoplexy this is the case. Neuroplasticity is a prerequisite for regaining lost neuronal abilities through new learning. This means that there is no rigid relation to specific brain functions and fixed brain maps. The principle of somatotopia is therefore not rigid, but plastic.
Individual evidence
- ↑ Norbert Boss (Ed.): Roche Lexicon Medicine. 2nd Edition. Hoffmann-La Roche and Urban & Schwarzenberg, Munich 1987, ISBN 3-541-13191-8 , p. 1592.
- ↑ Wilder Penfield , Theodore Rasmussen : The Cerebral Cortex of Man. A Clinical Study of Localization of Function . The Macmillan Comp., New York 1950.
- ^ Thure von Uexküll : Basic questions of psychosomatic medicine. Rowohlt, Reinbek bei Hamburg 1963, pp. 97f., 149, 219 ff. In this book, there is even explicit mention of an “integration space” in many places, see pages 128, 131, 224 f., 229ff., 234 f.
- ↑ Peter Duus: Neurological-topical diagnostics. Anatomy, physiology, clinic. Thieme, Stuttgart 1990, ISBN 3-13-535805-4 , p. 112. - There it says in the description of retinotopic facts before and after the crossing of the fibers of the optic nerve in the optic chiasm : “Despite the partial crossing, there is a strict somatotopic point -to-point arrangement up to the visual cortex retained. ”- The term retinotopy is nevertheless used elsewhere, namely on pages 367 and 373.
- ↑ Manfred Spitzer : Spirit on the Net. Models for learning, thinking and acting. Spektrum, Heidelberg 1996, ISBN 3-8274-0109-7 , p. 116.
- ↑ Alfred Benninghoff a. a .: Textbook of human anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, pp. 133f.
- ↑ Alfred Benninghoff a. a .: Textbook of human anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, p. 247ff.
- ↑ Christian Kell et al .: The Sensory Cortical Representation of the Human Penis: Revisiting Somatotopy in the Male Homunculus The Journal of Neuroscience, Frankfurt am Main 2005
- ↑ Alfred Benninghoff a. a .: Textbook of human anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, p. 250.
- ↑ Peter Duus: Neurological-topical diagnostics. 5th edition. Thieme, Stuttgart 1990, ISBN 3-13-535805-4 , p. 239.
- ^ W. Kahle: Pocket Atlas of Anatomy. Volume 3. Thieme, Stuttgart 1979.
- ↑ Norbert Boss (Ed.): Roche Lexicon Medicine. 2nd Edition. Hoffmann-La Roche and Urban & Schwarzenberg, Munich 1987, ISBN 3-541-13191-8 , pp. 1556, 1441.
- ↑ Peter R. Hofstätter (Ed.): Psychology. Fischer, Frankfurt am Main 1972, ISBN 3-436-01159-2 , keyword hearing sense, (ribbon-like representation of the continuum of pitches), p. 142
- ↑ Alfred Benninghoff a. a .: Textbook of human anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, chap. The acoustic system, p. 267
- ↑ Peter R. Hofstätter (Ed.): Psychology. Fischer, Frankfurt am Main 1972, ISBN 3-436-01159-2 , keyword brain (distortion of the homunculus), p. 133; Keyword skin senses (tactile resolution), p. 176
- ↑ Alfred Benninghoff a. a .: Textbook of human anatomy. Shown with preference given to functional relationships. Volume 3: Nervous System, Skin and Sensory Organs. Urban and Schwarzenberg, Munich 1964, chap. Die Sehbahn, p. 264f.
- ^ Hermann Voss , Robert Herrlinger : Taschenbuch der Anatomie. Volume III: nervous system, sensory system, skin system, increment system. Fischer Verlag, Jena 1964, p. 254.
- ↑ Helmut Ferner : Anatomy of the nervous system and the human sense organs. Reinhardt, Munich 1964, pp. 288f.
- ↑ Hofstätter (Ed.): Psychology. Fischer, Frankfurt am Main 1972, ISBN 3-436-01159-2 , keyword hearing sense, p. 143
- ^ CN Woolsey: Multiple auditory maps. (Cortical sensory organization, Volume 3.). Humana Press, Clifton, NJ 1982
- ^ Hermann Rein , Max Schneider : Human Physiology. 15th edition. Springer, Berlin 1964, p. 526
- ↑ Manfred Spitzer : Spirit on the Net. Models for learning, thinking and acting. Spektrum, Heidelberg 1996, ISBN 3-8274-0109-7 , pp. 148-182.