Component of muscle spindle discharge related to arterial pulse

In human subjects microelectrode recordings were made from 25 muscle spindle afferents from the pretibial muscles. 2. The spike discharges of three endings were locked in time to the arterial pulse. With 17 of the remaining endings, there was a significant pulse-related modulation of discharge rate. For these 20 endings the latency to the onset of the pulse-related influence was 200-310 ms. 3. The time course of the modulation of discharge rate was similar to that of arterial blood flow, as estimated using a Doppler flowmeter. With four endings occlusion of blood flow using a sphygmomanometer cuff reduced any modulation. 4. For five endings the contribution by the arterial modulation to the variance of discharge of the ending was 3-54%. For the population of endings there was no significant relationship between the depth of modulation and coefficient of variation. 5. It is concluded that the arterial pulse can be significant contributor to the variability of muscle spindle discharge. The pulsatile effects seen in the responses of single afferents are unlikely to be eliminated in the summed activity forming the population response. This could constitute a limitation of the information capacity of the population of muscle spindle afferents.