They are home to the axons of alpha motoneurons, which innervate extrafusal muscle fibbers. They are myelinated, with diameters of 12 to 30μ,and located in the spinal tray. The cell bodies of these efferent or motor axons also project to smooth and cardiac muscles located in the lateral horns of the spinal cord at T1-L2 levels. Those that do so are referred to pre-ganglionic sympathetic axons that terminate in the paravertebral ganglia and in the superior cervical. In the thoracic and lumbar segment of the spinal cord, the ventral roots also have pregranglionic sympathetic axons, situated at the sacral levels. The ventral and dorsal roots obey the Bell-Magendie law that afferent or sensory fibbers enter the spinal cord dorsally while the efferent or motor neurones project from the ventral spinal cord. Named after the anatomist Charles Bell (1774-1842), who first discovered it in 1811 (as well as being the first to distinguish between sensory and motor fibbers) and the physiologist François Magendie (1783-1855) See Alpha (α) motoneuron, Corticospinal tracts (CST), Dorsal roots, Dorsal root ganglia (DRG), Extrafusal muscle fibbers, Myelin, Spinal cord, Smooth (or involuntary) muscle, Spinal cord, Striated (or striped or voluntary) muscle, Ventral, Ventral horn