Sunday, November 6, 2011

Extrapyramidal Motor System

Apart from the precentral area and corticospinal (pyramidal) tract, numerous other cortical areas and pathways control motor activity. They are collectively known as the extrapyramidal motor system, which consists of chains of multisynaptic neurons and is phylogenetically older than the corticospinal tract. This system is called "extra-pyramidal" since their fibers do not run through the medalla pyramids. In the narrower sense, the nerve cell nuclei that constitute the extrapyramidal motor system are the putamen and caudate nucleus, pallidum, subthalamic nucleus, red nucleus, and substantia nigra, which are known as basal ganglia. This group of nuclei is connected to other centers that are important for motor activity; however, these are integration centers rather than motor nuclei: cerebellum, thalamic nuclei, reticular formation, vestibular nuclei, and some cortical areas. They are collectively known as the extrapyramidal system in the broader sense.


When one limb is moved voluntarily, muscle groups of the other limbs and of the trunk are simultaneously activated so that balance and posture are maintained under the changed static conditions and the movement can be completed smoothly. The accompanying muscle activities, which are often nothing more than the increased tension or relaxation of certain muscle groups, are carried out involuntarily and are not experienced consciously. Without them, however, coordinated movement would be impossible. Such unconscious motions include associated movements (synkinesis) (such as arm movements while walking) as well as many movements that have been practiced for a long time and thus occur automatically. They are all under the control of the extrapyramidal system; this may be compared to a servomechanism that supports all voluntary movements in an autonomic way and without reaching the level of consciousness.