Motor neuron columns in the lumbar spinal cord of the rat

The location in the rat spinal cord of motor neurons innervating 23 muscles or muscle groups of the hind limb has been determined by using retrograde transport of horseradish peroxidase. The motor neurons supplying a single muscle form a discrete longitudinal column in the lateral ventral horn. The columns extend through up to three adjacent spinal segments and their longitudinal location varies by as much as one segment in different animals. The relative positions of the columns supplying different muscles and their transverse location in the spinal cord are very consistent between individuals and a stereotaxic map has been constructed. This is briefly compared with descriptions from other species. Counts of the numbers of motor axons in seven different muscle nerves have been made with the aid of acetylcholin‐esterase histochemistry to identify the motor component. The numbers of motor neuron somata labelled with horseradish peroxidase are on average 70% of the axon counts.