The Motor System Shares Some Organizational Principles with Sensory Systems
With most of the major components and pathways of the motor system now described, it appears that the motor system shares principles Oforganization common to other brain systems (e.g., sensory systems).
One such organizational principle is the existence of topographic maps of the body. As noted, there are organized Somatotopic maps of the body’s musculature in the motor cortices. Topographic organization also exists in many sensory systems, except it is the peripheral receptor surface that is topographically mapped. For example, CNS components of the somatosensory (touch) system, such as primary somatosensory cortex, contain an organized somato- topic map of the different regions of the skin surface.Two other principles of organization shared by the motor system and sensory systems are serial and parallel processing of nervous system information. In sensory systems, serial processing generally refers to the passage of information from the periphery to successively more rostral regions of the nervous system, in a serial fashion. For example, in the visual system, axons of cells in the retina synapse in the lateral geniculate nucleus of the thalamus, and these thalamic neurons in turn send their axons to synapse in primary visual cortex. Often, in serial processing within the sensory systems, the information collected at successively more rostral levels of the nervous system is organized into a more sophisticated form. Serial processing can also be observed in the motor system, although in a different direction; from more rostral regions to more caudal regions. The cortico-reticulospinal route is an example of this.
Parallel processing refers to the different pathways within a given sensory system operating in parallel, respectively, to carry qualitatively different forms of information. Again, using the somatosensory system as an example, there are separate pathways to cerebral cortex to carry information about gentle touch of the skin and about intense skin contact usually perceived as painful. In the motor system, an example of parallel processing is the respective control of the proximal antigravity musculature by one set of descending brainstem motor pathways (vestibulospinal, reticulospinal) and control of the distal flexor musculature by a different descending brainstem motor pathway (rubrospinal).
Undoubtedly, a combination of both serial and parallel processing is necessary for the integrated function of sensory, as well as motor systems.