Alpha- and gamma-motoneurons in the adult human spinal cord and somatic cranial nerve nuclei: the significance of dendroachitectonics studied by the Golgi method.

A modified Golgi method (Vaisamruat and Hess, '53) was found to give satisfactory impregnation of cell bodies and dendrites, but not of dendritic spines and axons, in adult human material fixed by immersion in formalin. Examination of the motor columns in the spinal cord intumescences and of the third and twelfth cranial nerve nuclei revealed four neuron types, based on dendritic field size and dendritic branching pattern. Two of these were recognized as alpha-motoneurons; one of them was seen only in the medial motor column of the spinal ventral horn, while the other was observed in the cranial motor nuclei as well as the spinal lateral motor column. Differences in somadendritic dimensions in this neuron type were thought to reflect motor unit size, and thus terminal axon field dimensions. Of the two types of gamma-motoneurons recognized in the spinal cord and oculomotor nucleus, one was a miniature version of the commoner type of alpha-motoneuron. On this basis, it is proposed that it may give rise to fusimotor axons with plate endings. The second type of gamma-motoneuron does not resemble any of the other motoneuron types, and its axons may therefore be thought to terminate in trail endings.