Excitability and inhibitability of motoneurons of different sizes.

IN A RECENT PAPER (7) data were presented which indicate that during a gradually increasing stretch of an extensor muscle in a decerebrate cat the smallest alpha motoneurons of the muscle are the first to be discharged and that larger cells are recruited in order of increasing size. Some of the possible explanations for this relationship between size and order of recruitment were discussed briefly. It was concluded tentatively that the size or surface area of a motoneuron determines its excitability and hence its responsiveness to stretch-evoked impulses. If this conclusion is correct, we may infer that size is a prime determinant of excitability throughout the nervous system; if not, the size principle may apply only to motoneurons discharging in response to stretch. Fortunately, it is possible to test the proposed explanation experimentally. If size dictates excitability, the order in which a group of cells are recruited should be the same regardless of the source of the excitation and the particular circuits involved in mediating it. On the other hand, if the organization of the recruitment in input from , the stretch stretch receptors is responsible for the order of reflex, the order shoul .d change wi th d .ifferent types of excitatory input. Experiments were therefore designed in which the relative excitabilities of motoneurons were first determined by means of stretch reflexes and then retested by * means of crossed-extension reflexes, flexor reflexes, monosynaptic reflexes, and electrical “driving” from the muscle ner ve. The relative susceptibilities of neurons of different sizes to several varieties of inhibition were also investigated to ascertain whether inhibitibility bears some regular relation to cell size.