A comparison of the nerve impulses of mammalian non-medullated nerve fibres with those of the smallest diameter medullated fibres.

1. Filaments were dissected from the aortic nerve of cats and impulses recorded monophasically in vivo.2. The temporal characteristics of impulses of non-medullated fibres were compared with those of medullated fibres with conduction velocities between 4.5 and 12 m/sec. There were no obvious qualitative differences between the two types of fibres and they appeared to belong to a homogeneous sample.3. In non-medullated fibres the rise time, fall time and absolute refractory period varied inversely with conduction velocity, the relation being similar to that in the medullated fibres. There was practically no difference between the temporal characteristics of the fastest non-medullated fibres and those of the slowest medullated fibres, a result which might throw light on the question why fibres below 1 mu in diameter are not medullated.4. The mean blocking temperature of sixteen non-medullated fibres was 4.3 degrees C, a value that was significantly different from the mean blocking temperature of sixteen medullated fibres (6.5 degrees C).