Motoneurones: morphology, cytology, and topographical organization

Abstract This chapter describes the morphology of individual gamma and (mainly) alpha motoneurones as well as the composition and localization of motoneuronal populations (pools) innervating different muscles and muscle portions. Within the ventral horn of the spinal cord, motoneuronal cell bodies for a given muscle lie within an elongated rostro-caudal ‘column’, and cells of different sizes and properties are generally intermingled. Each motoneurone has several dendrites, typically extending to distances of many cell body diameters in all directions. Reconstructions of dendritic trees are described and the relationships are analyzed between the dimensions of dendrites, sizes of cell bodies, and conduction velocities of motor axons. Furthermore, the possible relationships are discussed between various aspects of motoneuronal cytochemistry, morphological characteristics, and functional properties. Large motoneurones seem to be more vulnerable than smaller ones in various kinds of disease (e.g., poliomyelitis).