FUNCTIONAL SIGNIFICANCE OF CELL SIZE IN SPINAL MOTONEURONS.

SINCE THE BEGINNINGS OF NEUROHISTOLOGY it has been recognized that neurons within the central nervous system vary widely in size, but the functional significance of this basic observation has never emerged from the realm of speculation. The largest cells have surface areas which are at least 100, perhaps 1,000, times greater than those of the smallest cells. Correspondingly, the diameters of axons in the central and peripheral portions of the nervous system range from less than .25 p. to more than 20 c-c. This broad spectrum of physical dimensions invites a search for functional correlates. This is one of a series of studies on the problem of size as it relates to spinal motoneurons. The preceding papers (21, 25) were concerned chiefly with the peripheral part of the motoneuron and the muscle fibers it innervates. They provided experimental evidence that the diameter of a motor nerve fiber is related to the number of muscle fibers it supplies. This finding seemed to make good sense: if a motor fiber innervates many muscle fibers and forms a large motor unit, it must have sufficient axonal substance to give off a large number of terminals. The present paper is concerned with the central part of the motoneuron and the significance of its size in synaptic transmission. It asks whether the cell bodies (and dendrites) connected with large and small motor fibers have different functional properties which can be recognized by their discharge characteristics. In order to investigate this problem one must be able to distinguish the signals of a large motoneuron from those of a small one. This may be done by recording their action potentials from thin filaments of lumbar ventral roots. As Gasser (8) demonstrated, the amplitudes of nerve impulses recorded externally from peripheral nerves are directly related to the diameters of their fibers. If it may be assumed that the diameters of axons are also related to the sizes of their cell bodies, as scattered histo-