HABITUATION OF SPINAL MOTONEURONS

The response of single splnal motoneurons decrements following repetitive stimulation is studied. The effects of varying the frequency, duration, and total number of stimuli are described. The decrement is shown to be a property of the flexor reflex. It is obtained only in flexor motoneurons following repetitive stimulation of flexor reflex afferents. The decrement probably occurs in the interneurons of the flexor reflex pathway, because the experimental results eliminate the primary afferents, the primary afferent terminals, and the motoneurons as the site of the decrement Two types of synaptic mechanisms for the decrement are discussed, and although the decrement bears a greater parametric similarity to post-tetanic potentiation than to low frequency depression, information sufficient to determine its exact synaptic mechanism is not yet available.