Scientific Basis of Body Mapping
by T. Richard Nichols, Ph.D.
Departments of Physiology and Biomedical Engineering
Anatomical representations of the body are regular features of many parts of the brain. In the cerebral cortex, it has been known for a long time that cells in the primary motor and sensory areas are associated with different parts of the body, and that these cells are spatially arranged in such a way as to represent the anatomical correspondence of these parts. In the 19th century, the British neurologist John Hughlings Jackson noticed that certain epileptic patients would undergo seizures in which involuntary movements would progress along body parts in anatomical sequence (from toe to hip, for example).
On the basis of these careful observations, Hughlings Jackson proposed that the body is represented on the cortical surface in the appropriate spatial relationships. In later studies in which this “somatotopic” map was studied directly, it was found that the size of the representation of each area is related to the use and precision of movement of that area. More cortical “space” is devoted to the face, mouth and fingers than the trunk. Even more recent studies have shown that within these areas representing specific structures, such as the wrist or hand, individual muscles are represented in a number of places depending upon the type of movement to be performed. These maps occur in both motor areas and sensory areas, which communicate through pathways that link different parts of the cerebral cortex.
Recent research on rodents, non-human primates and human patients with neurological disorders has also shown that the representation of anatomy on the cortical surface is subject to considerable plasticity. In the cases of injury or overtraining, the cortical representation can change. In the case of amputation, the cortex representing the lost limb or limb segment will eventually come to represent neighboring portions of the body. In the case of damage to the cerebral cortex, such as occurs in a stroke, cortical areas near the damaged area can become associated with the affected body part.
The cortical maps described above pertain to portions of the cerebral cortex that are concerned with the execution of voluntary movements. These cortical areas communicate directly with the neurons that activate muscles. The mechanisms of voluntary movement require several prior stages of processing, however, including motor planning. Motor planning, which consists of the more abstract aspects such as programming movement sequences and motor strategies, occur in premotor areas that are less well understood than the executive motor areas described above that provide the last stages of information processing. These earlier stages of motor planning are closely linked to cortical sites of learning, memory, and the interpretation of the special sensory systems like the auditory system.
There is evidence that maps are present in these areas as well. These maps include representations of frequency and spatial localization of tones in the case of the auditory system. Presumably, spatial maps of the musculoskeletal system exist in the premotor areas as well. The use of the concept of “body map,” which was proposed by William Conable, is engaged at these more cognitive levels of processing. Conscious representations of the musculoskeletal system will influence motor learning and planning, and will have downstream effects on the cortical maps in the executive areas of primary motor cortex. Therefore, the details of the body map can influence cortical representation along the entire chain of information flow, from planning through execution.
The maps in the executive areas of the cortex that represent the anatomy of the body are clearly dependent upon the motor and sensory experiences of the individual. In the case of a highly trained artist such as a musician, it is expected that the cortical areas become reorganized in a way that reflects the motor planning practices of that individual. Cortical maps are sufficiently flexible that they can represent a wide range of motor behaviors. Some motor practices can, however, lead to pathological changes in the musculoskeletal system, such as tendonitis or carpal tunnel syndrome.
If movement is based on an inaccurate knowledge or perception about the anatomy of the body, then pathologic changes can result. These practices can lead to alterations in cortical representation, which can then become reinforcing of the faulty motor practice. Overtraining of one specific motor pattern can also lead to pathologic changes, such as focal dystonias, in the central nervous system. These conclusions underscore the importance of educating musicians in anatomy and physiology of the motor system so that practices that can lead to pathology in the musculoskeletal system can be avoided.
The basis of voluntary movement in the cortex, as well as in the cerebellum, basal ganglia and brainstem, is the focus of intense research at present. There are certain to be important breakthroughs in the knowledge about these mechanisms in health and disease in the near future. An excellent introduction to the mechanisms of voluntary movement, and the role played by maps can be found in: Schieber, MH, Voluntary descending control,” Chapter 33 of Fundamental Neuroscience, edited by Zigmond, Bloom, Landis, Roberts and Squire, Academic Press, 1999, pp. 931-950. A more general overview of mechanisms of voluntary motion can be found in Essentials of Neural Science and Behavior, by Kandel, Schwartz and Jessell, Appleton Lange.