Functional properties of motor units in motor neuron diseases and neuropathies.

The relationship between the size of single motor unit (MU) action potentials and their twitch properties was estimated in patients with spinal muscular atrophy (SMA, n = 5) and amyotrophic lateral sclerosis (ALS, n = 10), as well as in patients with peripheral nerve lesions (PNL, n = 9). The data obtained from these groups were compared to normal controls (n = 8). In controls, the single MU twitch force was highly correlated to the corresponding EMG potential size in terms of macro EMG area. An enlargement of MUs, due to collateral sprouting and reflected by increased potential size and twitch force, was found in regenerating PNL and in slowly progressing SMA. Both parameters were highly correlated which indicates a high functional quality of compensating mechanisms. However, in rapidly progressing forms of amyotrophic lateral sclerosis (ALS) this correlation is poor and reflects a disturbance of the contractile system. Contraction times and half relaxation times were not correlated in the different groups.