Neuronal and/or axonal hyperactivity and hyperexcitability is an important 
feature of motor neuron diseases. It results clinically in cramps and 
fasciculations. It is not specific to motor neuron diseases, and can occur in 
healthy subjects, as well as in various pathologies of the peripheral nervous 
system, including nerve hyperexcitability syndromes. Hyperexcitability plays an 
important and debated role in the pathophysiology of motor neuron diseases, 
especially in amyotrophic lateral sclerosis (ALS). The mechanisms causing 
hyperexcitability are not yet clearly identified. While most studies favor a 
distal axonal origin site of fasciculations, some of the fasciculations could be 
of cortical origin. The consequences of hyperexcitability are also discussed, 
whether it is rather protective or deleterious in the disease course. 
Fasciculations are depicted both clinically and using electromyogram, and more 
recently the interest of ultrasound has been highlighted. The importance of 
fasciculation potentials in the diagnosis of ALS led to changes in 
electrophysiological criteria at Awaji consensus conference. The contribution of 
these modifications to ALS diagnosis has been the subject of several studies. In 
clinical practice, it is necessary to distinguish fasciculations potentials of 
motor neuron disease from benign fasciculations. In most studies of 
fasciculation potentials in ALS, the presence of complex fasciculation 
potentials appears to be relevant for the diagnosis and the prognosis of the 
disease.