Piotrkiewicz, M.: Computer simulation study of the shape of motor unit action potential. Acta physiol, pol., 1988, 39(1): 35-45. Motor unit action potentials (MUAPs) of brachial biceps were simulated. A simulated MUAP was obtained as a sum of single fibre action potentials (SFAPs) from all the muscle fibres of a motor unit (MU). The influence of the following factors on MUAP shape for different kinds of recording electrode was studied: fibre density, neuromuscular jitter, temporal dispersion and electrode displacements. The simulation confirms that typical MUAPs recorded with needle electrodes from muscles of low fibre density such as brachial biceps are usually triphasic. Increased fibre density produces MUAPs of more complex shape and higher amplitude. Normal neuromuscular jitter is responsible for the variability of shape of subsequent potentials from the same MU as well as for electromyographic shimmer. Pathologic (increased) jitter makes the shapes of subsequent potentials unrecognizable. The influence of temporal dispersion is interconnected with other factors but rather of minor importance. The simulation shows how big changes in MUAP shape can be expected due to electrode displacements during single experiment or during estimation of MU territory.
Tetanic potentiation is a phenomenon, which expresses the ability of a motor unit (MU) to increase its force output in tetanic contractions above that predicted with an assumption of algebraic summation of single twitch responses. To quantify tetanic potentiation, a coefficient TPC (tetanic potentiation coefficient) was defined as a ratio of the areas below tetanic force recording corresponding to the single stimulus contribution and that of the single twitch. Single MUs (27 Slow, 71 Fast, Fatigue Resistant, and 47 Fast, Fatigable) were isolated from the rat medial gastrocnemius muscle (MG) by ventral root splitting. TPC value was rate-dependent, with the maximum TPCmax at a certain optimal rate. The largest values of the TPCmax were obtained in the weakest and most fatigue resistant (S and weak FR) MUs. The different manifestation of staircase effect, post-tetanic and tetanic potentiation in individual MU types indicates that these phenomena may be independent of each other. We suggest that these phenomena as well as the fatigue resistance should be reexamined with protocols adjusted to the MU optimal frequency.