[Transformation of the structuro-functional organization of the motor neuron pool of the spinal cord during the postnatal ontogeny of dogs].

Data are presented concerning input-output characteristics of motoneuronal pool in puppies at various age periods (beginning from the first days of life) and distribution histograms of thresholds in motoneuronal populations. It was established that in early postnatal life, before realization of anti-gravitational reactions, low amplitude action potentials in the ventral root are recorded; threshold variation is rather insignificant accounting for the immediate maximum reflex reaction of motoneuronal pool. This is manifested in a rapid (relay) onset of plateau of input-output parameters. At this period, motoneurones of the pool are homogenous with respect to both the size and functional properties. During further postnatal life, relative amount of low amplitude elements of the pool decreases, modal value of threshold distribution being shifted to higher thresholds, indicating differentiation of motoneurones in the pool. After realization of antigravitational reactions, two plateaus in input-output parameters of the system are established together with corresponding two modal values in threshold distribution, indicating the onset of two main groups of motoneurones which differ in the size and functional properties.