The voluntary motricity is brought by a beam, i.e. a whole of nervous fibers located at the center known central nervous system. This beam, called pyramidal beam or cortico-spinal, takes its origin on the level of surface 4 (classification of the cerebral surfaces of Brodmann according to histological criteria).

This surface, still called surface préfrontale (because in the frontal lobe in front of the primary education furrow of Rolando), is consisted of the cellular bodies of pyramidal cells whose axons (neuronal prolongations) will take part in the formation of the known as pyramidal beam. The axons form in the beginning corona radiata, then the fibers undergo a torsion to pass in the internal capsule, a space of white substance located between thalamus and the lenticular core. Under the capsule, the fibers penetrate in the cerebral trunk, on the level of the mésencéphale and more precisely in the cerebral stalks. On the level of the medulla, 80% of fibers cross the line of centers to reach the side controlatéral, they will form the cross beam cortico-spinal in the posterior part of the side cord of the Spinal-cord. This explains why the attack of a surface 4 involves disorders in the hémicorps controlatéral. Incidentally, one describes fibers which do not cross in the bulb but go down on the same side to form the direct beam cortico-spinal in marrow, in contact with the ventral crack, in the former cord. These fibers end up articulating by synaptic connection with the cellular bodies of the motoneurones located in the former horn homolatérale for the crossed beam, controlatérale for the direct beam (there is thus crossing in marrow).

An attack of voluntary motricity, either of the surface préfrontale or of the pyramidal beam, is possible, and is thus responsible for a pyramidal syndrome. One can quote among the etiologies a AVC of an artery préfrontale for example. This pyramidal syndrome thus involves inter alia a reduction in the muscular force.