Further observations on ischaemic suppression of motor units in human soleus muscle: A single case study

Changes in action potentials (APs) of human soleus motor units (MUs), that occur during ischaemia produced by thigh compression, were studied in five experiments on one subject. In each experiment, a well-isolated AP of a low threshold MU was recorded by a needle electrode beginning with ischaemia onset up to the final MU block. All registered low-threshold MUs responded to Achilles tendon jerks throughout their life cycle. The block of low-threshold MUAps either coincided with or preceded complete tendon reflex loss (min 25 of ischaemia). Low-threshold MUAPs were suppressed in two phases. During the first phase of MUAP slowing, beginning with ischaemia onset up to min 17-22, MUAP amplitude decreased, while MUAP area remained approximately unchanged. During the second phase, MUAP area as well as MUAP amplitude decreased, and in 2-5 min MUAP failed. More ischaemia-resistant high-threshold MUAPs were recorded after the population of low-threshold Mus was blocked. Some high-threshold MUAPs deteriorated similarly to low-threshold MUAPs. Others were blocked abruptly, without any marked changes in shape. It is likely that early extinction of low-threshold MUs contributes to the effect of the stretch reflex suppression during ischaemia. It is suggested that the pattern of MUAP changes during ischaemia in different MUs is related to their degree of aerobic metabolism.