Spatial organization within rat motoneuron pools.

Topographical maps form the basis of the organization in many projections within the central nervous system, but in the neuromuscular system such detailed spatial organization has generally been assumed to be absent and indeed unnecessary for normal function (see, for example, ref. 1). However, there is some physiological evidence for a degree of spatial organization within the discrete, longitudinal motor columns which supply individual muscles. We have used horseradish peroxidase as a retrograde tracer to confirm the topographical relationship between the rostro-caudal location of motoneuron cell bodies and the antero-posterior motor unit distribution in the rat gluteus maximus muscle. We also provide evidence for a further axis of intracolumnar organization. The motor pools of the rat intercostal muscles, whose axons lie in a single, segmental nerve, have a ventro-dorsal axis in the ventral horn on which is mapped the proximo-distal position of the motor units. This suggests that during development, not only are motoneurons specified to innervate a particular muscle, but project within that muscle to a predictable location according to their position in the motoneuron pool. The presence of such topographical maps suggests that motoneurons are subject to greater developmental constraints than previously thought.