FURTHER OBSERVATIONS ON ISCHEMIC 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 studie d in five experiments on one subject. In each experiment, a well-isola ted AP of a low threshold MU was recorded by a needle electrode beginn ing 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 prec eded complete tendon reflex loss (min 25 of ischaemia). Low-threshold MUAPs were suppressed in two phases. During the first phase of MUAP sl owing, beginning with ischaemia onset up to min 17-22, MUAP amplitude decreased, while MUAP area remained approximately unchanged. During th e second phase, MUAP area as well as MUAP amplitude decreased, and in 2-5 min MUAP failed. More ischaemia-resistant high-threshold MUAPs wer e recorded after the population of low-threshold MUs was blocked. Some high-threshold MUAPs deteriorated similarly to low-threshold MUAPs. O thers were blocked abruptly, without any marked changes in shape. It i s likely that early extinction of low-threshold MUs contributes to the effect of the stretch reflex suppression during ischaemia. It is sugg ested that the pattern of MUAP changes during ischaemia in different M Us is related to their degree of aerobic metabolism.