ACCOMMODATION IN SPINAL MOTONEURONS OF THE CAT

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Abstract
Single spinal motoneurons of the cat were directly stimulated with linearly increasing currents flowing through an intracellular micropipette. Intensities of currents of varying slopes at the moment of spike generation were measured on the oscilloscopic records and compared with the rheobase. According to different courses of the threshold-latency curves, the motoneurons can be classified into two groups. In the first group of motoneurons, the threshold current intensity is progressively increased, to attain a certain final value, with decreasing rate of current rise. Electrical differentiation of the spike potentials reveals that they are initiated at the initial segment when the rise of current is rapid, and from the soma when the rise of current is slow enough. These motoneurons are characterized by relatively short after-hyperpolarization and considered to be phasic or fast motoneurons. The second group motoneurons have a threshold current rise. The spikes are initiated always at the initial segment. They are considered functionally to be tonic or slow motoneurons with the after-hyperpolarization of rather long duration. Deeper Nembutal anesthesia and reduced synaptic inflow to the motoneuron seem to favor the chance to encounter the first group motoneurons. Accomodation of the initial segment is in general faster than that of the soma, but the former may change its rate with ease depending on the environmental conditions. Two alternative interpretations of the threshold-latency curve of the first group motoneurons were proposed and their implications were discussed. The hypothesis admitting a transition zone between the initial segment and the soma seems to be preferable, being consistent with an assumption of constant threshold depolarization at fixed areas of the motoneuron.